ALBERT

Notizen: An investigation of the boundary effects on the operation of a relativistic backward wave oscillator (BWO) is presented. Analytical expressions for dispersion relation, global instability, and start oscillation current are obtained for the case of weak ripple of the slow wave structure. Numerical results show a periodic behavior of the start oscillation current with the length of the device. Results are also obtained depicting the variation of the start current with reflection coefficient and beam energy. A comparison of the results is made with those of earlier work [Phys. Fluids B 4, 2293 (1992)]. © 1995 American Institute of Physics.

Notizen: Turbulent diffusion modeling for the applied-B diode is considered. It is proposed that the simulation can be improved by considering the instability structure. (AIP)

Notizen: From numerical solutions of two colliding electromagnetic solitary waves, it is shown that their shapes are modified, and a trailing wake of plasma oscillations is produced by the collision. They therefore cannot be classified as solitons. For sufficiently large relative velocities, however, it is shown that the modifications to the vector and scalar potentials, as well as to the wake due to the collision, are small. © 1995 American Institute of Physics.

Notizen: In a free-electron laser (FEL) with a long wiggler, an additional focusing field is required to keep beam particles from diverging. An additional long scale transverse motion induced by the focusing field modulates the longitudinal motion and sometimes destroys the axial phase relation between the beam particles and radiation field, even if the beam is successfully transmitted. In order to investigate such a delicate phase relation under a complex wiggler configuration, a systematic perturbation method that employs noncanonical variables and a Lie transformation developed by Cary and Littlejohn [Ann. Phys. 151, 1 (1983)] is presented. The method is applied to a focusing wiggler, which has a parabolically curved pole face. Radial space charge force is taken into account in the analysis and the maximum current that can propagate the wiggler is estimated for an electron beam whose energy is around 1 MeV. The effect of the beam current on the axial phase relation is also investigated and found to be small. The present noncanonical approach is found to be transparent and comprehensive for this kind of problem, compared with the standard canonical perturbation method and applicable to arbitrarily complex wigglers. © 1995 American Institute of Physics.

Notizen: The effect of massive charged dust grains on plasma–wall interaction is investigated using the electrostatic probe model for the dust charging process. It is shown that the dust grains can have a substantial effect on the behavior of the electrostatic potential and particle densities near the wall. © 1995 American Institute of Physics.

Notizen: The primary electron beam in the plasma wake-field accelerator is studied, using multidimensional particle simulation, for cases in which a shaped electron beam is used, with length L(approximately-greater-than)λp, where λp is the plasma wavelength. Excellent agreement is found between radial equilibrium conditions and simulation results. Wake amplitudes are significantly reduced when the beam is initialized in a nonequilibrium state. The electron-hose and the transverse two-stream instabilities are observed to cause transverse deflections of the beam, with the transverse two-stream instability having a lower growth rate. Operation in the electron-hose regime can be avoided by reducing the beam density. © 1995 American Institute of Physics.

Notizen: In modern pulsed power systems the electric field stresses at metal surfaces in vacuum transmission lines are so high that negative surfaces are space-charge-limited electron emitters. These electrons do not cause unacceptable losses because magnetic fields due to system currents result in net motion parallel to the electrodes. It has been known for several years that a parameter known as flow impedance is useful for describing these flows. Flow impedance is a measure of the separation between the anode and the mean position of the electron cloud, and it will be shown in this paper that in many situations flow impedance depends upon the geometry of the transmission line upstream of the point of interest. It can be remarkably independent of other considerations such as line currents and voltage. For this reason flow impedance is a valuable design parameter. Models of impedance transitions and voltage adders using flow impedance will be developed. Results of these models will be compared to two-dimensional, time-dependent, particle-in-cell simulations. © 1995 American Institute of Physics.

Notizen: A Monte Carlo code has been developed which efficiently calculates plasma currents by finding the deviation, fˆ, of the exact distribution function, f, from the Maxwellian, FM(ψ,H), with ψ the toroidal magnetic flux enclosed by a pressure surface and H the Hamiltonian. The particles in the simulation are followed with a traditional Monte Carlo scheme consisting of an orbit step in which new values for the positions and momenta are obtained, and a collision step in which a Monte Carlo equivalent of the Lorentz operator is applied to change the pitch of each particle. The δf code was used to study the aspect ratio and collisionality dependence of the bootstrap and two Fourier components of the Pfirsch–Schlüter current. The δf code is about 104 times as efficient as other Monte Carlo techniques used to calculate currents in plasmas. © 1995 American Institute of Physics.

Notizen: The problem of helicity injection and current drive due to ion-cyclotron waves in a bounded inhomogeneous plasma is reexamined. It is shown that Hall term contributions (which had been neglected earlier in the work of Mett and Taylor [Phys. Fluids B 4, 73 (1992)]), even in the limit of small corrections of the order of ω/Ωi (ω and Ωi are the wave and the ion-cyclotron frequencies, respectively), improve substantially the parallel electromotive force (EMF) for the resonance layer. © 1995 American Institute of Physics.

Notizen: Fast magnetic compression, where the compression time is shorter than the ion gyroperiod, is illustrated by a simple model. In this model the Vlasov–Ampère equation gives a Bernstein–Greene–Kruskal (BGK) stationary state at the so-called zero-β limit. It is possible to achieve hundreds of keV ion energy by the fast compression heating, because there is very little shielding from the plasma currents in the heating phase. Strong shielding appears when the compression has generated strongly phase-correlated oscillations of the ion orbits. Thresholds of plasma β for the appearance of ion stochasticity and ion trapping are estimated. Parameters in a 500 keV ion energy case are presented. © 1995 American Institute of Physics.

Notizen: The laws of Ohm, Fourier, and Navier–Stokes for an ionized gas are obtained from a simple method that combines the features of the methods of Chapman–Enskog and Grad. The transport coefficients are easily determined through inversions of second-order and fourth-order tensors. This work is based on the Boltzmann equation for ionized gases, where (i) the preponderant number of collisions are those that lead to small deflections corresponding to distant encounters; and (ii) a cutoff at the Debye length has to be considered. © 1995 American Institute of Physics.

Notizen: Parametric instabilities of Langmuir waves in strong magnetic fields such that Ωe(approximately-greater-than)ωe were investigated, where Ωe(ωe) denotes the electron cyclotron (plasma) frequency. It has been well established that as a plasma is more strongly magnetized, the parametric instabilities of Langmuir waves become more field aligned and thus one dimensional. Here, it is demonstrated that this trend continues only up to Ωe/ωe=1, and that above this value, most Langmuir waves decay to relatively short wavelength oblique-Langmuir and ion waves that are damped rapidly. These new parametric instabilities have growth rates greater than those of the conventional modulational and decay instabilities. Consequently, the scenario of strong Langmuir turbulence in which Langmuir solitons play a major role may not apply to the strong magnetic-field regime. In the weak Langmuir turbulence regime these decay instabilities make the turbulence more isotropic. Therefore, it is concluded that there occurs a type of phase transition in magnetized Langmuir turbulence at Ωe/ωe∼1. © 1995 American Institute of Physics.

Notizen: Transverse modulation instabilities for single and double electron cyclotron waves in finite and infinite media are investigated. This research shows that there is a intensity threshold that depends on the strength of external magnetic field. The growth rate of instability decreases as the external magnetic field strength increases. In finite media, the threshold for absolute transverse modulational instability of counterpropagating waves is increased by the external magnetic field in the small transverse wave number region. However, in large wave number region, the threshold depends scarcely on the external magnetic field. © 1995 American Institute of Physics.

Notizen: Experiments are described on the interaction of a weak warm beam with a broad spectrum of unstable waves on a traveling wave tube. The wave–particle interactions are similar to those in beam–plasma systems, and are traditionally described by quasilinear theory. The precise wave evolution is obtained by launching a specified waveform, allowing it to interact with the beam, and analyzing the received waveform. Significant mode coupling is observed, resulting in saturated waves correlated less than 0.5 with their launch values. Experimentally, each wave is separated into a component proportional to the launch amplitude and a component due solely to mode coupling. The measured properties of these separate components agree quantitatively with a four-wave coupling model. Strongest coupling is observed between waves whose wave numbers match within about an inverse turbulent trapping length. In the linear growth regime, the measured ensemble-averaged wave growth rates and beam velocity diffusion rates agree reasonably with quasilinear and resonance-broadening theory; in the nonlinear regime near saturation, the discrepancies become larger. © 1995 American Institute of Physics.

Notizen: A two-dimensional [2d(x,y)] fluid code has been developed to explore nonlocal dissipative drift-wave turbulence and anomalous transport. In order to obtain steady-state turbulence, the y-averaged fluctuating density 〈n〉 has been forced to be zero in simulations, thus the difficulty of choosing proper sources and sinks in turbulence simulation codes has been avoided. If Ln(very-much-greater-than)Lc or Lαlc(very-much-greater-than)Lc, where Ln is the density gradient scale length, Lc the turbulence correlation length Lc, and Lαlc the adiabaticity-layer width, it has been shown that "local'' turbulence simulations give reasonable results. However, for Ln∼Lc, or Lαlc∼Lc "local'' turbulence codes are found to overestimate the flux. For a family of hyperbolic tangent background density profiles, n0(x)=nm−n1 tanh[(2x−Lx)/2Δn] with n1〈0.5nm, it has been demonstrated that the nonlocality of the turbulence leads to a transition from local gyro-Bohm (Dlocal(approximately-equal-to)7.6(Te/eB)[ρs/Ln(x)] [αlc(x)/0.01]−1/3), where αlc(x)=α(x)/κ(x)〈1, to nonlocal gyro-Bohm transport scaling [Dnonlocal(approximately-equal-to)7.6(Te/eB)(n1ρs/nmΔn) (αnlc/0.01)−1/3(Δn/40ρs)2/5 for αnlc(x)=α/κmax〈1, κ(x)=ρs/Ln(x) and α=k2(parallel)χe]. For the case Φ0(x)=−n0(x) with the model hyperbolic tangent density profiles n0(x), velocity shear increases the turbulence flux by 230% and the root-mean-square (RMS) fluctuating density by 36%. Otherwise, for Φ0(x)=n0(x), the turbulence flux is reduced by 71% and the RMS value of fluctuating density is decreased by 31% by velocity shear effects. © 1995 American Institute of Physics.

Notizen: The ion flow velocity entering the Debye sheath is constrained to satisfy the Bohm criterion: velocity must be equal to or greater than sonic. The ion flow velocity entering the magnetic presheath (which arises upstream of the Debye sheath when a magnetic field, oblique to the solid surface is present) is constrained to satisfy the Chodura criterion: velocity-parallel to B must be equal to or greater than sonic. The Bohm criterion, in its marginal form where v=cs, is associated with a "gentle entry'' into the sheath with Δne=Δni, while v(approximately-greater-than)cs corresponds to ||Δne||(approximately-greater-than)||Δni||. The magnetic presheath, in contrast with the Debye sheath, is a region of quasineutrality, and thus only Δne=Δni is permitted. This might appear to prohibit supersonic flow parallel to B at the entrance to the magnetic presheath, which the analysis of Chodura and Riemann has found to be permitted, in fact. This apparent contradiction is resolved by analyzing the magnetic presheath region to bring out more explicitly the role played by inertia in the E×B direction, thus confirming that such supersonic flow is permitted. © 1995 American Institute of Physics.

Notizen: The ion-acoustic solitons are investigated in three-component plasmas, whose constituents are electrons, positrons, and singly charged ions. It is found that the presence of the positron component in such a multispecies plasma can result in reduction of the ion-acoustic soliton amplitudes. © 1995 American Institute of Physics.

Notizen: The bootstrap current Jb=−IcP'B/〈B2〉 and χi= (square root of)2NνiMiTi(Ic/e)2〈1/B2〉/||∇ψ ||2 are valid for both the banana and the Pfirsch–Schlüter regimes for any finite value of the collision frequency at a radius where the local aspect ratio A approaches unity. Here, I=RBt with R the major radius, Bt the toroidal magnetic field strength, and the prime denoting the derivative with respect to the poloidal flux ψ. Thus, the bootstrap current does not vanish, even in the collisional regime, when A approaches unity. The physical reason for this dramatic result is that the magnitudes of the electron and ion parallel viscosity approach infinity as A approaches unity. This also indicates that the conventional theory underestimates the magnitude of bootstrap current in an ultralow-aspect-ratio tokamak. © 1995 American Institute of Physics.

Notizen: Negative bias is applied to a mesh placed in front of an endplate of the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)]. A bias voltage of around 10 V results in an increase of the potential difference by an order of kV in the end region. The increment is about two times the temperature of end-loss electrons and most of that stems from the sheath potential drop in front of the endplate. Provided that almost all the secondary electrons emitted from endplate are reflected, these results agree with the prediction of a potential model for the open magnetic field configuration [J. Phys Soc. Jpn. 61, 3153 (1992)]. © 1995 American Institute of Physics.

Notizen: The properties of small-amplitude, modified-electron acoustic solitary waves in an unmagnetized plasma consisting of relativistic drifting electrons and nondrifting thermal ions are investigated. The influences of relativistic effects and ion inertia on the propagation of such waves are examined. It is found that rarefactive types of soliton solutions exist only when the electron drift velocity is less than the ion thermal velocity. © 1995 American Institute of Physics.

Notizen: The invariant manifold reduction is applied to the problem of nonlinear tearing mode evolution in a plane plasma slab, and the resulting reduced description compared with existing numerical simulation results. The reduced description is found to give a significant degree of insight into the mode competition processes responsible for the major features of the nonlinear evolution, and even provide a degree of quantitative accuracy. © 1995 American Institute of Physics.

Notizen: An effect called rotational pumping by the authors (by analogy with magnetic pumping) causes a slow damping of the m=1 diocotron mode in non-neutral plasmas. In a frame centered on the plasma and rotating at the diocotron mode frequency, the end confinement potentials are nonaxisymmetric. As a flux tube of plasma undergoes E×B drift rotation about the center of the column, the length of the tube oscillates about some mean value, and this produces a corresponding oscillation in T(parallel). In turn, the collisional relaxation of T(parallel) toward T⊥ produces a slow dissipation of electrostatic energy into heat and a consequent radial expansion (cross-field transport) of the plasma. Since the canonical angular momentum is conserved, the displacement of the column off axis must decrease as the plasma expands. In the limit where the axial bounce frequency of an electron is large compared to its E×B drift rotation frequency theory predicts the damping rate γ=−2κ2ν⊥,(parallel) (r2p/R2w)(λ2D/L20)/(1−r2p R2w), where κ is a numerical constant, λD is the Debye length, Rw is the radius of the cylindrical conducting wall, rp is the effective plasma radius, L0 is the mean length of the plasma, and ν⊥,(parallel) is the equipartition rate. A novel aspect of this theory is that the magnetic field strength enters only through ν⊥,(parallel). As the field strength is increased, the damping rate is nearly independent of the field strength until the regime of strong magnetization is reached [i.e., Ωc(approximately-greater-than)v¯/b=(kT)3/2/(square root of)me2], and then the damping rate drops off dramatically. This signature has been observed in recent experiments. For completeness, the theory is extended to the regime where the bounce frequency is comparable to the rotation frequency, and bounce-rotation resonances are included. © 1995 American Institute of Physics.

Notizen: On the basis of the spontaneous reconnection model, computer simulations study the physical mechanism by which magnetic energy, initially stored in a current sheet system, is released into plasma energies. For the uniform resistivity model, the Sweet–Parker mechanism is eventually set up with the diffusion region becoming longer with time. It is the Ohmic heating ηJ2 in the diffusion region that plays the dominant role in releasing the magnetic energy. Attached to the diffusion region, a long plasmoid is formed and propagates like a large-amplitude Alfvén pulse, where the generator and motor effects are canceled along the plasmoid boundary. For the anomalous resistivity model, the fast reconnection mechanism is eventually set up with the diffusion region remaining to be localized near an X neutral point. It is the powerful motor effect [u⋅(J×B)(approximately-greater-than)0] along the slow shock layers that drastically releases the stored magnetic energy. A large-scale plasmoid distinctly swells, so that the ambient magnetic fields are compressed (by the generator effect), and the enhanced magnetic energy is then reduced by the strong motor effect in the backward half of the plasmoid. The slow shocks extend with time from near the X point, leading to a drastic catastrophe for the overall magnetic field system. © 1995 American Institute of Physics.

Notizen: A simplified two-dimensional nonlinear fluid model is developed to elucidate the basic mechanism of the formation of multifaceted asymmetric radiation from the edge of tokamak plasmas (MARFE). In the framework of a mixed Eulerian–Lagrangian description, the problem is reduced to a reaction-diffusion-type equation with nonlocality, which obeys the constraints of length constancy and mass conservation along the magnetic field. With sufficient radiative cooling, this model predicts formation of MARFE-like plasma condensations from a variety of initial conditions. © 1995 American Institute of Physics.

Notizen: Linear and nonlinear dynamics of various toroidicity induced global Alfvén eigenmodes is investigated by means of an initial value numerical code. The code uses the reduced magnetohydrodynamic (MHD) model expanded to the third order in the inverse aspect ratio parameter ε (ε ≡ a/R0, with a the minor radius of the plasma column and R0 the major radius of the vacuum chamber). Nonlinear saturation is shown to occur due to the excitation of a perturbation to the nonlinear mode structure, which dissipates energy on very short scales. In particular, the nonlinear saturation of the Toroidal Alfvén Eigenmodes (TAE) and of the Resistive Periodic Shear Alfvén Eigenmodes (RPSAE) due to MHD nonlinearities is investigated and compared with analytical predictions. © 1995 American Institute of Physics.

Notizen: Density fluctuations in low-collisionality, low-beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0.85) increases from ñ/n∼1% when the density profile is flat to ñ/n∼3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped-electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellarator field; they suggest that trapped-electron instabilities may play a role in constraining the shape of the density profile in ATF, but have little effect on global energy confinement. © 1995 American Institute of Physics.

Notizen: The linear stability of reversed field pinches (RFP's) to current driven modes is investigated including the effects of resistive walls at a finite distance from the plasma and mode rotation. A comprehensive study is made of the stability to ideal and resistive modes and the implications for long-pulse operation of RFP's are discussed. Stability to resistive modes of poloidal mode number m=1 requires conducting walls at a distance b/a〈1.11 (a is the plasma and b the wall radius) for optimally peaked current profiles. Stability to m=0 requires very close-fitting conductors, b/a〈1.03. The requirements on wall times and mode rotation frequencies posed by these modes are discussed. Nonresonant m=1 modes cannot be stabilized by resistive walls and require active feedback. © 1995 American Institute of Physics.

Notizen: A weakly relativistic theory of current drive by parallel acceleration of electrons in toroidal geometry is presented. Analytic expressions of efficiency are obtained for superthermal and subthermal electrons. By using a few adjustable parameters for intermediate energies, an accurate formula for the efficiency is obtained by fitting numerical results. This includes trapping effects and is valid for all frequencies up to the lower hybrid range of frequencies. It is found that for low-frequency fast-wave current drive, the relativistic and nonrelativistic efficiencies agree to within 15% up to 500 keV, although the relativistic corrections to power absorption and current density can be much larger separately. Relativistic effects on the efficiency become prominent at lower energies for Landau damping. © 1995 American Institute of Physics.

Notizen: A self-adjoint form of linearized Coulomb collision operator for energetic ions is obtained. Background electron and ion species are assumed to have Maxwellian distributions with different temperatures. The result can be used in the kinetic investigations of collisional effects on the energetic ion behavior, including variational studies of the energetic ion Fokker–Planck equation. © 1995 American Institute of Physics.

Notizen: The system of equations governing the axisymmetric toroidal equilibrium of a non-neutral plasma is shown to become hyperbolic when the poloidal velocity of the plasma exceeds a sound speed. © 1995 American Institute of Physics.

Notizen: The formation and maintenance of an electrostatic potential well by injecting electrons into a quasispherical cusped magnetic confinement geometry is studied experimentally, as a function of plasma fill density and of the energy and current of the injected electrons. A model is developed to analyze the experiment. It is found that the potential is comparable to the energy of the injected electrons at low density, and decreases as an increasing density of cold plasma fills the device because of ionization or wall bombardment. Implications for fusion based on electrostatic/magnetic confinement are discussed. © 1995 American Institute of Physics.

Notizen: A possible new driving mechanism for poloidal plasma rotation in the scrape-off layer (SOL) of a tokamak is proposed. This driving mechanism results from charge exchange between "cold'' hydrogen atoms from the wall and "hot'' plasma ions in the SOL, where the consequent loss of microscopic angular momentum is compensated by an increase of the macroscopic rotation of the edge plasma. Balancing the increase by viscous damping, inherent to the toroidal geometry of the tokamak, gives values for the poloidal rotation velocity, which are in good agreement with experimental results. © 1995 American Institute of Physics.

Notizen: A translation experiment of field-reversed configuration (FRC) plasma is performed on the FIX machine [Shiokawa and Goto, Phys. Fluids B 5, 534 (1993)]. The translated FRC bounces between magnetic mirror fields at both ends of a confinement region. The plasma loses some of its axial kinetic energy when it is reflected by the magnetic mirror field, and eventually settles down in the confinement region. In this reflection process, the plasma temperature rises significantly. Such plasma rethermalization has been observed in OCT-L1 experiments [Ito et al., Phys. Fluids 30, 168 (1987)], but rarely in FRX-C/T experiments [Rej et al., Phys. Fluids 29, 852 (1986)]. It is found that the rethermalization depends on the relation between the plasma temperature and the translation velocity. The rethermalization occurs only in the case where the translation velocity exceeds the sound velocity. This result implies the rethermalization is caused by a shock wave induced within the FRC when the plasma is reflected by the magnetic mirror field. © 1995 American Institute of Physics.

Notizen: During lower hybrid current drive on the Princeton Beta Experiment Modification PBX-M [Bell et al., Phys. Fluids B 2, 1271 (1990)], suprathermal electrons in the 30–150 keV range are generated. These electrons emit hard x-ray bremsstrahlung in collisions with plasma ions; the radiation creates images in a hard x-ray pinhole camera. In order to interpret the hard x-ray images, a computer simulation code has been written; the "pbxray'' code, which is described in this paper. The code represents an extension of the Stevens code [Stevens et al., Nucl. Fusion 25, 1529 (1985)], which calculates the free–free and free–bound radiation for non-Maxwellian relativistic electron tail distributions. The pbxray code provides the chord integration in the bean-shaped plasma geometry on the PBX-M tokamak and integrates over photon energy. The simulations show that the location of the suprathermal electrons can be determined with an accuracy of approximately 2 cm in the plasma. In particular, discharges are analyzed whose characteristic "hollow'' images indicate off-axis lower hybrid current drive. A comparison of images taken with different absorber foils reveals that the suprathermal electrons have less than 150 keV parallel energy for the hollow discharges. © 1995 American Institute of Physics.

Notizen: This work investigates the current–voltage characteristics of a highly negatively biased, isolated probe in the plasma wake of a much larger, unbiased object. The system is investigated for both flowing and stationary plasmas. For the stationary plasma case, spherical probe theory and a simple correction factor to account for the shadow of the plate adequately explain the results. The introduction of a flowing plasma leads to a much more complicated situation. In this case, since the potential field is asymmetric and the ions no longer conserve angular momentum, spherical probe theory is inadequate. The current–voltage characteristics take a much more complicated form than in the stationary case. Indeed, the simulations show that the current–voltage characteristics of varying axial probe positions cross each other. The high complexity of the ion orbits makes theoretical predictions of where the crossover occurs difficult. However, the potential structure around the probe and the ion trajectories are analyzed to explain the current–voltage characteristics. These results show regions can exist in energy–angular momentum space which cause incoming ions to orbit the probe and then escape the simulation. The fraction of particles which do orbit and then escape as a function of probe potential and position determines the shape of the current–voltage characteristic. © 1995 American Institute of Physics.

Notizen: The accelerating and focusing wake fields that can be excited by a short laser pulse in a hollow underdense plasma are examined. The evacuated channel in the plasma serves as an optical fiber to guide the laser pulse over many Rayleigh lengths. Wake fields excited by plasma current at the edge of the channel extend to the center where they may be used for ultrahigh gradient acceleration of particles over long distances. The wake field and equilibrium laser profiles are found analytically and compared to two-dimensional (2-D) particle-in-cell (PIC) simulations. Laser propagation is simulated over more than ten Rayleigh lengths. The accelerating gradients on the axis of a channel of radius c/ωp are of order of one-half of the gradients in a uniform plasma. For present high-power lasers, multi-GeV/m gradients are predicted. © 1995 American Institute of Physics.

Notizen: A theoretical framework is established for the description of anchored field-line distortion by a flowing resistive magnetofluid. The Hall effect is included and significantly influences field-line deformation in two dimensions when the electron magnetization parameter, ωce/νe(approximately-greater-than)1. Moreover, the formalism described herein provides for a direct and local magnetic-field determination of the effective electron collisionality, which may then be gauged against microturbulence descriptions of enhanced transport. © 1995 American Institute of Physics.

Notizen: A nonlinear equation for the evolution of a Langmuir harmonic in a wide spectrum is derived in a consistent manner. This equation substitutes the usual quasilinear growth rate of the weak plasma turbulence theory. The derivation is based on the determination of the resonant particle distribution through the solution of the exact nonlinear equations of motion in a given field, composed of all effectively acting harmonics. The method requires a slow variation of the harmonic amplitudes over the nonlinear characteristic time of resonant interaction, and small enough amplitudes for the averaging method to be applicable. Two nonlinear terms appear in this equation: one is proportional to the particular harmonic considered, the other is a contribution from the whole effective spectrum around this harmonic. From this last term, it follows that a wave with negligible initial amplitude can grow spontaneously through the nonlinear wave–particle interaction. © 1995 American Institute of Physics.

Notizen: An amplitude equation for an unstable mode in a collisionless plasma is derived from the dynamics on the unstable manifold of the equilibrium F0(v). The mode eigenvalue arises from a simple zero of the dielectric εk(z); as the linear growth rate γ vanishes, the eigenvalue merges with the continuous spectrum on the imaginary axis and disappears. The evolution of the mode amplitude ρ(t) is studied using an expansion in ρ. As γ→0+, the expansion coefficients diverge, but these singularities are absorbed by rescaling the amplitude: ρ(t)≡γ2r(γt). This renders the theory finite and also indicates that the electric field exhibits trapping scaling E∼γ2. These singularities and scalings are independent of the specific F0(v) considered. The asymptotic dynamics of r(τ) can depend on F0 only through exp iξ where dεk/dz=||εk'||exp−iξ/2. Similar results also hold for the electric field and distribution function. © 1995 American Institute of Physics.

Notizen: We study the relationship between measures invariant for a piecewise expanding transformation τ of a compact metric space endowed with a underlying measure and measures invariant for an iterated function system Tτ, generated by inverse branches of τ. The main result says that the τ-invariant absolutely continuous measure μ is also Tτ invariant if and only if τ is absolutely continuously conjugated with a piecewise linear transformation. Measures of maximal entropy and general equilibrium states are also discussed. © 1995 American Institute of Physics.

Notizen: A center manifold reduction and numerical calculations are used to demonstrate the presence of limit cycles, two-tori, and multistability in the damped harmonic oscillator with delayed negative feedback. This model is the prototype of a mechanical system operating with delayed feedback. Complex dynamics are thus seen to arise in very plausible and commonly occurring mechanical and neuromechanical feedback systems. © 1995 American Institute of Physics.

Notizen: An experimental thermal pulse combustor and a differential equation model of this device are shown to exhibit chaotic behavior under certain conditions. Chaos arises in the model by means of a progression of period-doubling bifurcations that occur when operating parameters such as combustor wall temperature or air/fuel flow are adjusted to push the system toward flameout. Bifurcation sequences have not yet been reproduced experimentally, but similarities are demonstrated between the dynamic features of pressure fluctuations in the model and experiment. Correlation dimension, Kolmogorov entropy, and projections of reconstructed attractors using chaotic time series analysis are demonstrated to be useful in classifying dynamical behavior of the experimental combustor and for comparison of test data to the model results. Ways to improve the model are suggested. © 1995 American Institute of Physics.

Notizen: We present a mathematical framework for the theory of a synchronization phenomenon for dynamical systems discovered by Pecora and Carroll [Phys. Rev. Lett. 64, 821–824 (1990)]. From this perspective, we can synchronize, using a single coordinate, an open dense set of linear systems. We use our insights to synchronize nonlinear systems which were not previously recognized as being synchronizable. © 1995 American Institute of Physics.

Notizen: It has been suggested that the normal irregular beating of the heart is a manifestation of deterministically chaotic dynamics. Evidence proffered in support of this hypothesis includes a 1/f-like power spectrum, a small noninteger correlation dimension, and self-similarity of the time series. The major cause of the normal fluctuations in heart rate is the impingement of several neural and hormonal control systems upon the sinoatrial node, the natural pacemaker of the heart. However, intrinsic fluctuations of beat rate can be seen in the isolated node, devoid of all neural and hormonal inputs, and even in a single cell isolated from the node. The electrical activity in such a single cell is generated by ions flowing through discrete channels in the cell membrane.We decided to test the hypothesis that the fluctuations in beat rate in a single cell might be due to the fluctuations in the activity of this population of single channels. We thus assemble a model consisting of 6000 channels and probe its dynamics. Each channel has one or more gates, all of which must be open to allow current to flow through the channel. Since these gates are thought to open and close in a random manner, we model each gate by a Markov process, assigning a pseudorandom number to each gate every time that it changes state from open to closed or vice versa. This number, in conjunction with the classical voltage-dependent Hodgkin–Huxley-like rate constants that control the speed with which a gate will open or close, then determines when that gate will next change state. We also employ a second method that is much more efficient computationally, in which one computes the lifetime of the ensemble of 6000 channels. We show that the Monte Carlo model has behavior consistent with the hypothesis that the irregular beating seen experimentally in single nodal cells is due to the (pseudo)random opening and closing of single channels. However, since the pseudorandom number generator used in the simulations is deterministic, one cannot state that the activity in the model is random (or stochastic). Thus, it would be premature to claim that the irregularity of beating in a single nodal cell is accounted for by the stochastic behavior of a population of a few thousand single channels lying in the membrane of the cell. Finally, we consider some implications of our work for the naturally occurring in situ fluctuations in heart rate ("heart rate variability''). © 1995 American Institute of Physics.

Notizen: Dynamical diseases are characterized by sudden changes in the qualitative dynamics of physiological processes, leading to abnormal dynamics and disease. Thus, there is a natural matching between the mathematical field of nonlinear dynamics and medicine. This paper summarizes advances in the study of dynamical disease with emphasis on a NATO Advanced Research Worshop held in Mont Tremblant, Québec, Canada in February 1994. We describe the international effort currently underway to identify dynamical diseases and to study these diseases from a perspective of nonlinear dynamics. Linear and nonlinear time series analysis combined with analysis of bifurcations in dynamics are being used to help understand mechanisms of pathological rhythms and offer the promise for better diagnostic and therapeutic techniques. © 1995 American Institute of Physics.

Notizen: Breathing is regulated by a central neural oscillator that produces rhythmic output to the respiratory muscles. Pathological disturbances in rhythm (dysrhythmias) are observed in the breathing pattern of children and adults with neurological and cardiopulmonary diseases. The mechanisms responsible for genesis of respiratory dysrhythmias are poorly understood. The present studies take a novel approach to this problem. The basic postulate is that the rhythm of the respiratory oscillator can be altered by a variety of stimuli. When the oscillator recovers its rhythm after such perturbations, its phase may be reset relative to the original rhythm. The amount of phase resetting is dependent upon stimulus parameters and the level of respiratory drive. The long-range hypothesis is that respiratory dysrhythmias can be induced by stimuli that impinge upon or arise within the respiratory oscillator with certain combinations of strength and timing relative to the respiratory cycle. Animal studies were performed in anesthetized or decerebrate preparations. Neural respiratory rhythmicity is represented by phrenic nerve activity, allowing use of open-loop experimental conditions which avoid negative chemical feedback associated with changes in ventilation.In animal experiments, respiratory dysrhythmias can be induced by stimuli having specific combinations of strength and timing. Newborn animals readily exhibit spontaneous dysrhythmias which become more prominent at lower respiratory drives. In human subjects, swallowing was studied as a physiological perturbation of respiratory rhythm, causing a pattern of phase resetting that is characterized topologically as type 0. Computational studies of the Bonhoeffer–van der Pol (BvP) equations, whose qualitative behavior is representative of many excitable systems, supports a unified interpretation of these experimental findings. Rhythmicity is observed when the BvP model exhibits recurrent periods of excitation alternating with refractory periods. The same system can be perturbed to a state in which amplitude of oscillation is attenuated or abolished. We have characterized critical perturbations which induce transitions between these two states, giving rise to patterns of dysrhythmic activity that are similar to those seen in the experiments. We illustrate the importance of noise in initiation and termination of rhythm, comparable to normal respiratory rhythm intermixed with spontaneous dysrhythmias. In the BvP system the incidence and duration of dysrhythmia is shown to be strongly influenced by the level of noise. These studies should lead to greater understanding of rhythmicity and integrative responses of the respiratory control system, and provide insight into disturbances in control mechanisms that cause apnea and aspiration in clinical disease states. © 1995 American Institute of Physics.

Notizen: Interaction of the immune system with a target population of, e.g., bacteria, viruses, antigens, or tumor cells must be considered as a dynamic process. We describe this process by a system of two ordinary differential equations. Although the model is strongly idealized it demonstrates how the combination of a few proposed nonlinear interaction rules between the immune system and its targets are able to generate a considerable variety of different kinds of immune responses, many of which are observed both experimentally and clinically. In particular, solutions of the model equations correspond to states described by immunologists as "virgin state,'' "immune state'' and "state of tolerance.'' The model successfully replicates the so-called primary and secondary response. Moreover, it predicts the existence of a threshold level for the amount of pathogen germs or of transplanted tumor cells below which the host is able to eliminate the infectious organism or to reject the tumor graft. We also find a long time coexistence of targets and immune competent cells including damped and undamped oscillations of both. Plausibly the model explains that if the number of transformed cells or pathogens exeeds definable values (poor antigenicity, high reproduction rate) the immune system fails to keep the disease under control. On the other hand, the model predicts apparently paradoxical situations including an increased chance of target survival despite enhanced immune activity or therapeutically achieved target reduction. A further obviously paradoxical behavior consists of a positive effect for the patient up to a complete cure by adding an additional target challenge where the benefit of the additional targets depends strongly on the time point and on their amount. Under periodically pulsed stimulation the model may show a chaotic time behavior of both target growth and immune response. © 1995 American Institute of Physics.

Notizen: In the heart, the AV node is the primary conduction pathway between the atria and ventricles and subserves an important function by virtue of its rate-dependent properties. Cell clusters isolated from the rabbit atrioventricular (AV) node beat with a stable rhythm (cycle length: 300–520 ms) and are characterized by slow action potential upstroke velocities (7 to 30 V/s). The goal of this study is to better characterize the phase resetting and the rhythms during periodic stimulation of this slow inward current system. Single or periodic depolarizing pulses (20 ms in duration) were injected into AV nodal cell clusters using glass microelectrodes. Phase resetting curves of both strong, weak as well as discontinuous types were obtained by applying single current pulses of different intensities and latencies following every ten action potentials. Graded responses were elicited in a wide range of stimulus phases and amplitudes. A single premature stimulus caused a transient prolongation of the cycle length. Sustained periodic stimulation, at rates faster than the intrinsic beat rate, resulted in various N:M (stimulus frequency: action potential frequency) entrainment rhythms as well as periodic or irregular changes in action potential morphology. The changes in action potential characteristics were evaluated by computing the area under the action potential trace and above a fixed threshold (−45 mV). We show that the variations in action potential morphology play a major role in the onset of complicated dynamics observed in this experimental preparation. In this context, the prediction of entrainment rhythms using techniques based on the iteration of phase resetting curves (PRCs) is inadequate since the PRC does not carry information directly related to the changes in action potential morphology. This study demonstrates the need to consider graded events which, though not propagated, have important implications in the understanding of dynamical diseases of the heart. © 1995 American Institute of Physics.

Notizen: A model for the pulsatile release of luteinizing hormone releasing hormone (LHRH) can be reduced to a FitzHugh–Nagumo model subject to regular and quasiregular (i.e., with slight random variation in the interstimulus interval), discrete-time stimulation. The relationship of output pulse frequency (OPF) to stimulus frequency is compared between the excitable and oscillatory forms of the model and discussed in the context of results from other pulse-driven model systems. Some examples of the changes in OPF caused by quasiregular and purely Poissonian stimuli are given for the excitable case. The unstimulated system frequently interacts with the stimulation in such a complex manner that the OPF bears little resemblance to the frequency of stimulation or of the unstimulated system. Furthermore, the inability of the oscillatory form of the model to allow complete suppression of output pulses for moderate stimulation frequencies suggests that the LHRH system can be more appropriately described by the excitable form of the model. © 1995 American Institute of Physics.

Notizen: Experimental evidence has shown a plethora of short-term fluctuations in patients with Parkinson's disease. We investigate these transitory events using the concept of dynamical disease. Several examples of short-term fluctuations in tremor are analyzed, and in two cases, other systemic variables (i.e., respiration and blood pressure) are examined as well. A model for tremor, based on negative feedback with delays is proposed, and the transient events are simulated. The theoretical implications of the model suggest that interactions between the central and peripheral loops, as well as interactions between the control loops and other systemic signals, can give rise to transitory events in tremor, both in the pathological and in the normal case. © 1995 American Institute of Physics.

Notizen: The original sheath inverse bremsstrahlung model [P. J. Catto and R. M. More, Phys. Fluids 20, 704 (1977)] is modified by including the v×B term in the equation of motion, as the evanescent magnetic field in an overdense plasma is greater than the corresponding electric field. It is shown that the present results are significantly different from those derived without the v×B term. The v×B term is also important in interpreting the absorption mechanism. If the v×B term were neglected, the absorption of the light would be incorrectly interpreted as an increase in the transverse components of the canonical momentum, in the case of a normally incident laser light. It is also shown that both the sheath inverse bremsstrahlung and the anomalous skin effect are limiting cases of the same collisionless absorption mechanism. Results from particle-in-cell (PIC) plasma simulations are compared with the absorption coefficient calculated from the linear theory. Finally, the effects of finite density gradients are investigated by PIC simulations. © 1995 American Institute of Physics.

Notizen: Current sheet formation and magnetic reconnection in a two-dimensional coronal loop with an X-type neutral line are simulated numerically using compressible, resistive magnetohydrodynamic equations. Numerical results in the linear and nonlinear regimes are shown to be in good agreement with a recent analytical theory [X. Wang and A. Bhattacharjee, Astrophys. J. 420, 415 (1994)]. The topological constraint imposed by helicity-conserving reconnection is discussed. It is found numerically that helicity-conserving reconnection causes the initial X-point structure of the loop to change to Y points, with current sheets at the separatrices encompassing the Y points. Implications for observations are discussed. © 1995 American Institute of Physics.

Notizen: Theoretical investigation is made into ion sound propagation and damping in strongly coupled plasmas near the Wigner crystallization of the ion subsystem. Sound velocity grows with the strengthening of the coupling, and sound decay is due to ion viscosity. The transition from the collisional mechanism of viscosity to the vacancial one proceeds along the Wigner crystallization of the ion subsystem. As a result, a sharp increase in viscosity occurs. Viscoelastic sound replaces hydrodynamic viscous sound. The damping of the viscoelastic sound is determined by Landau damping in the electron subsystem. © 1995 American Institute of Physics.

Notizen: A two-dimensional Boussinesq fluid with nonlinear interaction between Rayleigh–Bénard convection and an external magnetic field is investigated numerically and analytically. A simplified model consisting of a fifth-order system of nonlinear ordinary differential equations with five parameters is introduced and integrated numerically in certain parameter regions. Various types of bifurcations from periodic solutions are found numerically: period-doubling bifurcation, heteroclinic bifurcation, intermittency, and saddle-node bifurcation. A normal form equation is also derived from the fifth-order system, and center manifold theory is applied to it. An expression for the renormalized Holmes–Melnikov boundary for the evaluation of the numerical results is given. It is shown from the normal form equation that each property of the two phase portraits described by the Duffing equation and the van der Pol equation emanates from one common attractor in the five-dimensional space of the fifth-order system. © 1995 American Institute of Physics.

Notizen: Self-consistent equations for ion density, envelope electric fields, and charge variations of dust particles are obtained for a dusty plasma. A new type of modulational instability of Langmuir waves related to dust charging processes is found. © 1995 American Institute of Physics.

Notizen: A numerical procedure has been developed for the self-consistent simulation of the nonlinear interaction of energetic particles with discrete collective modes in the presence of a particle source and dissipation. A bump-on-tail instability model is chosen for these simulations. The model presents a kinetic nonlinear treatment of the wave–particle interaction within a Hamiltonian formalism. A mapping technique has been used in this model in order to assess the long time behavior of the system. Depending on the parameter range, the model shows either a steady-state mode saturation or quasiperiodic nonlinear bursts of the wave energy. It is demonstrated that the mode saturation level as well as the burst parameters scale with the drive in accordance with the analytical predictions. The threshold for the resonance overlap condition and particle global diffusion in the phase space are quantified. For the pulsating regime, it is shown that when γL(approximately-greater-than)0.16 ΔΩ, where γL is the linear growth rate for the unperturbed system and ΔΩ is the frequency separation of neighboring resonances, overlap occurs together with an amplification of the free energy release compared to what is expected with the saturation of nonoverlapping modes. The effect of particle losses on the wave excitation is included in the model, which illustrates in a qualitative way the bursting collective losses of fast ions/alpha particles due to Alfvén instabilities. © 1995 American Institute of Physics.

Notizen: A toroidal two-fluid model allowing for arbitrary magnetic field curvature is used to describe a contaminated plasma consisting of two ion species. A higher-order ballooning mode is identified in the high- and low-α regimes, and coexists with the ordinary ballooning mode in the medium-α regime (α=ballooning parameter). In the low-α regime the temperature-gradient-driven ηi mode is present. The impurities are, in general, found to be stabilizing—lowering the growth rate and narrowing the instability regime. Comparisons are made with ideal magnetohydrodynamics (MHD) and with a semilocal approximation of the eigenfunction, which are found to give a higher growth rate. © 1995 American Institute of Physics.

Notizen: A number of experiments have attempted, with varying degrees of success, to influence the low to high confinement mode (L–H) transition through the application of an external flow shear forcing mechanism. In order to theoretically describe this type of experiment, an external torque is included in the phase transition model of Diamond et al. [Phys. Rev. Lett. 72, 2965 (1994)] for the L–H bifurcation. These equations exhibit a bifurcation between an L-mode fixed point and an H-mode fixed point with variation of a critical parameter, corresponding to the edge gradient, across a threshold value. With the addition of the external torque [from biased limiter, probe, or radio frequency (rf) wave], the character of the transition changes in a manner analogous to the addition of an external magnetic field to a ferromagnetic material. In addition to the change in the transition dynamics, there is a marked decrease in the threshold level. In this simple model, the decrease in threshold level is proportional to the applied torque to the two-thirds power, in approximate functional agreement with that seen by some experiments. © 1995 American Institute of Physics.

Notizen: The amount of interpenetration and stagnation in counterstreaming laser-produced plasmas is investigated via a kinetic model and a multifluid model in one-dimensional planar geometry. Each of these models can evolve multiple ion species in the self-consistent electrostatic field with Coulomb collisional interaction. The two approaches are applied to various colliding plasma systems, ranging from simple homogeneous slowing-down and temperature-equilibration problems to colliding plasmas characteristic of laser irradiated thin foils and thick disks. In particular, direct comparisons are made between the kinetic formulation and the multifluid model with various approximations for the multifluid collisional coupling coefficients. © 1995 American Institute of Physics.

Notizen: An analytical model of two-dimensional collisionless reconnection in an X-type magnetic geometry is presented. The conversion of magnetic energy to the kinetic energy of accelerated ions takes place in the vicinity of the neutral line. The structure of this dissipation region and the magnetic energy release rate have been investigated both for linear and nonlinear regimes of collisionless reconnection. A simple model of global reconnection flow has been constructed, assuming an incompressible ideal magnetohydrodynamics approximation outside the dissipation region. The corresponding scaling for the external Mach number (Me) is found, which predicts a maximum reconnection rate Me=1/2R˜m −1/2, where R˜m≈(Le/λi)2(very-much-greater-than)1 is the effective magnetic Reynolds number for collisionless reconnection (Le is the global size of the system and λi is the ion inertial skin depth). © 1995 American Institute of Physics.

Notizen: Rate equations for the coupled evolution of density and temperature in nonideal partially ionized plasmas are presented. The equations are derived from generalized quantum kinetic equations that include many-particle effects, such as screening, self-energy and lowering of the binding energy. The equations are solved for a three-component plasma. The influence of many-body effects on the density–temperature relaxation is demonstrated. © 1995 American Institute of Physics.

Notizen: Edge electrostatic fluctuations, in the Extrap T1 reversed-field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing-mode-related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed-field pinch configuration. © 1995 American Institute of Physics.

Notizen: The investigation by means of particle simulations of the process whereby magnetic island structures tend to merge into larger units is extended to include the effects of electron dynamics and the in-plane components of the currents. With the ions marginally unmagnetized (ρi/λ∼1, where ρi is the asymptotic ion gyroradius and λ is the current sheet half-thickness), the electrons reduce the linear growth rate by some 20%, but they do not alter the basic coalescence physics. During the nonlinear stage of the pairwise merging process, a quadrupole out-of-the-plane magnetic field structure with peak amplitude ∼0.1B0 is formed, but this field collapses after the merging is completed. In the parameter regime accessible to the simulations [flux bundle size L/(c/ωpi)(approximately-greater-than)3 and mass ratio Mi/me≤36], there is no indication of any significant change in the reconnection rate that could be associated with whistler-mediated reconnection. The sequential merging of 4 islands into one large island leads to the formation of a vacuum X point and substantial particle energization. © 1995 American Institute of Physics.

Notizen: Shear Alfvén waves can be driven unstable by hot particles such as alpha particles in an ignited fusion device or hot ions in existing devices. Motivated by rather collisional Wendelstein 7 Advanced Stellarator (W7-AS) [Phys. Rev. Lett. 72, 1220 (1994)] beam-driven global Alfvén instability experiments, the effect of electron and ion collisions on these modes has been examined. Collisions broaden and suppress the peak associated with Landau effects. This broadening makes ion damping more important, while the electron damping is suppressed. Additional resistive effects provide increased damping for the main part of the spectrum, which can have a rather high phase velocity. Of more general interest is the fact that collisional and collisionless resistivity has a numerically stabilizing effect that is known to be important for nonlinear resistive magnetohydrodynamics (MHD). This can preclude the need for introducing and testing the sensitivity to similar ad hoc effects. Numerical and analytic results for both a particle-conserving Krook collision operator and a Lorentz (pitch angle) collision operator are compared and contrasted. © 1995 American Institute of Physics.

Notizen: A system of equations describing the nonlinear coupling of high frequency electron Bernstein (EB) and upper hybrid (UH) waves near harmonics of the electron cyclotron frequency with low frequency lower hybrid (LH) waves in a homogeneous, weakly magnetized, and weakly collisional plasma is derived. The EB and UH modes are described by a single second order equation, taking into account the interaction with low frequency density fluctuations. The ponderomotive force of the high frequency oscillations increases near the cyclotron harmonics due to the resonance with the electron motion. The obtained equations are used to study the parametric decay of an infinite wavelength electromagnetic pump wave into EB or UH waves and LH waves. The threshold electric fields are sufficiently low to be exceeded in high frequency ionospheric modification experiments. However, the instability cannot be excited for pump frequencies near the cyclotron harmonics. For the decay into EB waves, the resulting forbidden frequency range depends on the harmonic number in a power law manner, consistent with observations of stimulated electromagnetic emissions in ionospheric modification experiments. Further, for sufficiently high pump electric fields the instability is also suppressed, when the frequency mismatch around the eigenfrequencies at which the interaction can occur is of the order of the frequency separation between the EB and UH modes near the cyclotron harmonics. © 1995 American Institute of Physics.

Notizen: The electron density, the electron kinetic energy, the particle motion, and electric fields in a coaxial positive-polarity plasma opening switch (POS) were studied using spectroscopic diagnostics. A gaseous source that injects the plasma radially outward from inside the inner POS electrode was developed. The plasma was locally seeded with various species, desired for the various measurements allowing for axial, radial, and azimuthal resolutions both prior to and during the 180 ns long current pulse. The electron density was determined from particle ionization times and the electron energy from line intensities and time dependent collisional-radiative calculations. Fluctuating electric fields were studied from Stark broadening. The ion velocity distributions were obtained from emission-line Doppler broadenings and shifts. The early ion motion, the relatively low ion velocities and the nearly linear velocity dependence on the ion charge-to-mass ratio, leads to the conclusion that the magnetic field penetrates the plasma early in the pulse. The ion velocity dependence on the axial location were thus used to infer the time dependent axial distribution of the magnetic field, indicating the formation of a relatively high current density at the load-side edge of the plasma. This is expected to cause plasma acceleration towards the load, found to be supported by charge-collector measurements. The fast magnetic field penetration could be explained by mechanisms based on the Hall effect. © 1995 American Institute of Physics.

Notizen: The application of plasma physics to the manufacturing and processing of materials may be the new frontier of our discipline. Already partially ionized discharges are used in industry, and the performance of plasmas has a large commercial and technological impact. However, the science of low-temperature plasmas is not as well developed as that of high-temperature, collisionless plasmas. In this paper several major areas of application are described and examples of forefront problems in each are given. The underlying thesis is that gas discharges have evolved beyond a black art, and that intellectually challenging problems with elegant solutions can be found. © 1995 American Institute of Physics.

Notizen: Ion plasma waves—purely electrostatic ion waves with a wavelength of order of the electron Debye length and frequency of the order of the ion plasma frequency—have long been known in theory but have proven difficult to detect experimentally. The difficulties stemmed from the techniques used to produce the plasma and to drive and detect the waves. In the work reported here, these problems were overcome by using resonant laser scattering to detect ion plasma waves in a multiply ionized, laser-produced plasma. This nonetheless required careful experimental design to minimize frequency smearing of the scattered signal by plasma gradients. The plasma was extensively characterized, allowing comparison of the theoretical dispersion relation with the wave data. The agreement of these two provides conclusive proof of the detection of ion plasma waves. © 1995 American Institute of Physics.

Notizen: Progress toward a detailed understanding of the transport in full-volume gyrokinetic simulations of tokamaks is described. The transition between the two asymptotic regimes (large and small) of scaling of the heat flux with system size a/ρg reported earlier is explained, along with the approximate size at which the transition occurs. The larger systems have transport close to that predicted by the simple standard estimates for transport by drift-wave turbulence (viz., Bohm or gyro-Bohm) in scaling with a/ρg, temperature, magnetic field, ion mass, safety factor, and minor radius, but lying much closer to Bohm, which seems the result better supported theoretically. The characteristic downshift in the 〈kθ〉 spectrum observed previously in going from the linear to the turbulent phase is consistent with the numerically inferred coupling coefficients Mkpq of a reduced description of the system. An explanation of the downshift is given from the resemblance of the reduced system to the Hasegawa–Mima or Terry–Horton systems. These manifest an analogous downshift in slab geometry, and have Mkpq resembling those inferred from the gyrokinetic (GK) data. © 1995 American Institute of Physics.

Notizen: The first experimental evidence of Alfvén Wave Current Drive (AWCD) in a tokamak is shown. In a low-density experiment, an estimated 20–35 kA out of 65 kA total current, or 30%–55% of the total current has been driven. The estimated efficiency for current driven per unit RF input power is approximately ICD/PRF≈0.2 A/W, which is near the predicted efficiency, and corresponds to the commonly used figure of merit, neR0ICD/PRF≈0.4×1018 A m−2 W−1, where ne is plasma density and R0 is the major radius. The significant 30%–40% drop in loop voltage observed cannot be explained by any plausible increase in electron temperature Te, or decrease in inductive plasma energy, or changes in plasma resistivity. Independently measured loop voltage, Te, effective ionic charge Zeff, and plasma inductance and resistance are all consistent with this conclusion. © 1995 American Institute of Physics.

Notizen: The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (〈200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core. © 1995 American Institute of Physics.

Notizen: A geometric estimate for the size of the bootstrap current has been implemented in a plasma equilibrium code and applied to the Modular Helias-like Heliac (MHH) stellarator [Nucl. Fusion 34, 881 (1994)]. The predicted change in the rotational transform of the MHH due to the bootstrap current has been brought down to an acceptable level, and this has resulted in one of the smallest stellarator reactor candidates to be discovered so far. © 1995 American Institute of Physics.

Notizen: A simple, heuristic model for the early nonlinear phase of the Rayleigh–Taylor instability (RTI) in thin-cylindrical-shell Z-pinch implosions has been developed. This model is based on the fact that, as the field–plasma interface is deformed, there is a component of the applied force that acts to move mass from the low mass per unit area bubble region into the higher mass per unit area spike region. The resulting reduced mass per unit area of the bubble causes it to be preferentially accelerated ahead of the spike. The pinch begins to radiate as the bubble mass first reaches the axis, and it continues to radiate while the mass that is entrained within the spikes and within unperturbed parts of the shell also arrives on axis. This model relates the time at which the bubble arrives on axis to an initial wavelength and amplitude of a single mode of the RTI. Then, by comparing this to the time at which the unperturbed mass reaches the axis, one estimates pinch thermalization time, a quantity that is determined experimentally. Experimental data, together with analytic models, have been used to choose appropriate initial wavelength and amplitude both for foils and for certain gas puff implosions. By noting that thermalization time is a weak function of these parameters, it is argued that one may use the same values for an extrapolative study of qualitatively similar implosions. © 1995 American Institute of Physics.

Notizen: It is now well known that space plasmas frequently contain particle components that exhibit high, or superthermal, energy tails with approximate power law distributions in velocity space. Such nonthermal distributions, with overabundances of fast particles, can be better fitted, for supra- and superthermal velocities, by generalized Lorentzian or kappa distributions, than by Maxwellians or one of their variants. Employing the kappa distribution, with real values of the spectral index κ, in place of the Maxwellian we introduce a new plasma dispersion function expected to be of significant importance in kinetic theoretical studies of waves in space plasmas. It is demonstrated that this function is proportional to Gauss' hypergeometric function 2F1[1,2κ+2;κ+2;z] enabling the well-established theory of the hypergeometric function to be used to manipulate dispersion relations. The reduction, for integer values of κ, to the less general so-called modified plasma dispersion function [Phys. Fluids B 3, 1835 (1991)] is demonstrated. An example illustrating the use of the function is presented. © 1995 American Institute of Physics.

Notizen: Measurements of bremsstrahlung down to 7 keV emitted from a 7.25 GHz electron cyclotron resonance discharge and the deconvolution of the spectra using a newly developed integration method yield the exact electron distribution function. Two separate electron populations with temperatures about 3 and 25 keV and with densities about (0.2...4)108 cm−3 were recorded. First, results are obtained for the behavior of these populations belonging to the mixture (argon–oxygen–hydrogen). Gas mixing increases the densities of these populations, but hardly changes the temperatures. © 1995 American Institute of Physics.

Notizen: Computer modeling has become a powerful tool for exploring the physics of plasmas. Early computers could handle only relatively simple models but nevertheless showed that these devices could shed a lot of light on the complex physics of plasmas. This capability has proved not only valuable to research but also is becoming an important teaching tool; modeling allows students to experience in concrete ways plasma phenomena which are otherwise presented only abstractly. Present-day plasma models combined with parallel computing provide sufficient power that numerical modeling of laboratory experiments on complex devices has become possible. Two examples of simulations are discussed in some detail: The "Beat Wave Accelerator'' and the "Numerical Tokamak.'' © 1995 American Institute of Physics.

Notizen: After many years of fusion research, the conditions needed for a D–T fusion reactor have been approached on the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. For the first time the unique phenomena present in a D–T plasma are now being studied in a laboratory plasma.The first magnetic fusion experiments to study plasmas using nearly equal concentrations of deuterium and tritium have been carried out on TFTR. At present the maximum fusion power of 10.7 MW, using 39.5 MW of neutral-beam heating, in a supershot discharge and 6.7 MW in a high-βp discharge following a current rampdown. The fusion power density in a core of the plasma is ≈2.8 MW m−3, exceeding that expected in the International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] at 1500 MW total fusion power. The energy confinement time, τE, is observed to increase in D–T, relative to D plasmas, by 20% and the ni(0) Ti(0) τE product by 55%. The improvement in thermal confinement is caused primarily by a decrease in ion heat conductivity in both supershot and limiter-H-mode discharges. Extensive lithium pellet injection increased the confinement time to 0.27 s and enabled higher current operation in both supershot and high-βp discharges. Ion cyclotron range of frequencies (ICRF) heating of a D–T plasma, using the second harmonic of tritium, has been demonstrated. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP [Nucl. Fusion 34, 1247 (1994)] simulations. Initial measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from He gas puffing experiments. The loss of alpha particles to a detector at the bottom of the vessel is well described by the first-orbit loss mechanism. No loss due to alpha-particle-driven instabilities has yet been observed. D–T experiments on TFTR will continue to explore the assumptions of the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor. © 1995 American Institute of Physics.

Notizen: A new theory for electromagnetic radiation emission with wave frequency at the plasma frequency and/or its harmonic (known as the plasma emission) is presented. According to the new theory, the radiation emission takes place as a result of combined effects of the excitation of electrostatic waves by a nonlinear beam instability, and the concurrent conversion into electromagnetic waves by a nonlinear mode coupling process. The underlying physical mechanism for the nonlinear beam instability is an interaction of energetic electron beam with enhanced ion-acoustic or Langmuir turbulence. The turbulence, which is treated as intrinsic in the present analysis, also gives rise to the nonlinear mode coupling process between the electrostatic and electromagnetic modes. That is, the two processes (nonlinear instability and mode coupling) occur concomitantly. An important aspect of the present theory is that the effective growth rate associated with the radiation is comparable to the usual beam–plasma (or bump-in-tail) instability growth rate, which makes the radiation emission process very efficient, a feature very different from any standard theory. © 1995 American Institute of Physics.

Notizen: A special method is presented to construct exact time-dependent solutions of the two-dimensional ideal magnetohydrodynamic (MHD) equations for which plasma elements experience no acceleration. The momentum equation then contains the time merely parametrically and assumes the structure of an equilibrium equation. For a special form of the pressure profile p(A), for which the corresponding quasi-equilibrium equation is a completely integrable non-linear elliptic equation that is invariant under conformal transformations, these invariance properties are then used to determine the possible time-dependences of the solutions. Contrary to the common use of the term quasi-equilibrium arbitrarily large plasma velocities are allowed in the present treatment. In polar coordinates, the time evolution turns out to be self-similar in the radial coordinate, but it is in general not self-similar in the azimuthal coordinate. The adiabatic exponent of the plasma is found to be equal to one, which means that the plasma is isothermal. Explicit examples of solutions are discussed. © 1995 American Institute of Physics.

Notizen: The results of laboratory experiments at the University of California at Irvine are presented in which multidimensional ion velocity distributions in the presence of radio-frequency (RF) waves and a spatially divergent external magnetic field are observed. A plasma volume is subjected to either local or nonlocal electrostatic turbulence, which in turn is responsible for accelerating the ions transverse to the confining magnetic field. The ions flow away from the source of turbulence in a spatially decreasing magnetic field, where the μ ∇B force and magnetic-moment conservation work to distort the heated distribution. Laser-induced-fluorescence (LIF) signals, measured downstream from the plasma source with the aid of optical tomography techniques, reveal substantial ion heating and conic formation. © 1995 American Institute of Physics.

Notizen: The self-consistent nonlinear evolution and saturation of the dynamo, including the back reaction of the magnetic field on the flow through the Lorentz J×B force, is investigated via simulation of the fully compressible magnetohydrodynamic (MHD) equations. The saturated state is found to be highly turbulent. The energy in the saturated magnetic field is only a small fraction of the kinetic energy in the flow which drives the dynamo. However, as the collision frequency decreases and the Reynolds number R increases, the ratio of magnetic to kinetic energy in the saturated state increases gradually. The nonlinear viscosity generated by the turbulent fluctuations rises rapidly relative to the collisional viscosity as R increases, such that the total transport of momentum remains virtually unchanged as the collisional viscosity is reduced. The scale lengths of the magnetic and velocity fluctuations both decrease as R increases, so that the scale size of the magnetic field remains comparable to the scale size of the flow. © 1995 American Institute of Physics.

Notizen: Modes localized at a plasma–vacuum interface are studied in plane slab equilibria by using resistive magnetohydrodynamics. Such modes are unstable whenever the current density and the magnetic field are not perpendicular to each other in the interface. The perturbations have no radial nodes but large mode numbers in the other two directions. The instability occurs for a wide range of angles between the nodal lines and the magnetic field lines in the interface and does not depend on the presence of a mode resonant surface. © 1995 American Institute of Physics.

Notizen: The propagation of holes, solitons, and rarefaction waves along the axis of a magnetized pure electron plasma column is described. The time dependence of the radially averaged density perturbation produced by the nonlinear waves is measured at several locations along the plasma column for a wide range of plasma parameters. The rarefaction waves are studied by measuring the free expansion of the plasma into a vacuum. A new hydrodynamic theory is described that quantitatively predicts the free expansion measurements. The rarefaction is initially characterized by a self-similar plasma flow, resulting in a perturbed density and velocity without a characteristic length scale. The electron solitons show a small increase in propagation speed with increasing amplitude and exhibit electron bursts. The holes show a decrease in propagation speed with increasing amplitude. Collisions between holes and solitons show that these objects pass through each other undisturbed, except for a small offset. © 1995 American Institute of Physics.

Notizen: The effects of uniform rotation on the dynamics, equilibria and stability of cylindrically symmetric, radiating Z-pinch plasmas are studied. Rotation changes the Bennett and Pease–Braginskii equilibria qualitatively, eliminating radiative collapse for both quasisteady and dynamic plasmas. In particular, a steady rotating plasma column can support any current above the Pease–Braginskii value, with Ohmic heating balanced by radiative losses. Stabilizing effect of rotation on the m=0 mode of Rayleigh–Taylor instability of a hollow plasma shell was found for long perturbation wavelengths. © 1995 American Institute of Physics.

Notizen: The loop voltage response in the low-frequency current drive experiments is analyzed in order to extract information about the current drive profile and efficiency. © 1995 American Institute of Physics.

Notizen: The flow of low collisionality plasma down a magnetic hill during electron cyclotron resonance heating is analyzed using a particle-in-cell code that follows both the electron and ion guiding centers. The analysis is applied to the formation of an ion beam by plasma expansion in a magnetic nozzle, of interest for plasma thrusters and other applications. The ambipolar electric potential that accelerates the ions is enhanced by effects of the magnetic forces on the electrons that are heated perpendicular to the magnetic field. Comparison with flow in the presence of isotropic heating demonstrates an increase in the efficiency of the particle and energy fluxes down the field relative to those that flow up the field to a wall at a large mirror ratio. The increases are reduced somewhat if particles recycle near the wall, but are still significant.

Notizen: Nonlocal effects are observed in a number of transient transport studies in tokamaks and stellarators. In this paper some consequences of nonlocality are discussed on the basis of a heuristic model for the electron heat diffusivity (χe). The main consequence is the presence of an additional ("missing'') heating power term (p˜χ) in the heat transport equation. Suitable combinations of p˜χ and modulated power profiles should give rise to detectable effects in modulated heating experiments such as those planned on the Frascati Tokamak Upgrade (FTU) [R. Andreani et al., in Fusion Technology, Proceedings of the 16th Symposium, London, 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (Elsevier Science, Amsterdam, 1991), Vol. 1, p. 218]. Failure to properly include p˜χ effects (if present) can lead to an overestimate of χe from heat wave propagation analysis. © 1995 American Institute of Physics.

Notizen: The weakly nonlinear stage of the ablative Rayleigh–Taylor instability has been investigated by expanding the one-fluid and one-temperature equations to second order. Mode coupling of linear growing perturbations with wave numbers kA and kB and corresponding growth rates γA and γB excite a long-wavelength perturbation with wave number k0=kA−kB. Time evolution of the excited perturbation has been studied for two cases, the first having no initial perturbation with the wave number k0 and the second having a finite perturbation. In both cases, parts of the excited perturbation are initially convected out from an ablation surface toward the first and second sonic points. In the latter case, the perturbation grows initially with a linear growth rate and later with the growth rate γA+γB. Mass flows across an ablation surface, which determine deformation of the surface, are investigated in detail. Calculated mass flows to first and second order are found to be quite different. © 1995 American Institute of Physics.

Notizen: The effects of scattering are assessed for energetic ions of interest as ρR, yield, and/or implosion symmetry diagnostics. Because the Coulomb logarithm between such energetic ions ((approximately-greater-than)1 MeV) and the "field'' ions is always of order 10 or larger, scattering effects are shown to be small. In contrast the Coulomb logarithm between energetic ions and "field'' electrons is typically of order 5; but here the large mass ratio (104) between ion and electron precludes the possibility of ion scattering. Consequently the continuous-slowing-down stopping power and the linear-energy-transfer (LET) stopping power are very close. The consequences of this near identity in the stopping powers is exploited in order to calculate ρR over a range of plasma parameters of relevance to inertial confinement fusion (ICF) plasmas. © 1995 American Institute of Physics.

Notizen: The nonlinear evolution of the Rayleigh–Taylor instability from multimode initial perturbations is studied by two complementary approaches. First, a statistical-mechanics bubble-merger model is presented, that enables determination of the late-time scaling properties based on single-mode and two-bubble interaction physics. The results for Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) bubble and spike front penetrations are given, as well as scaling laws for mixing under a time-dependent driving acceleration. The second approach is a modal model, in which nonlinear mode coupling and saturation are included in an equation for effective modes that describe the mixed region. The importance of mode coupling in the generation of large structure that dominates the late stage evolution, and the relative importance of long-wavelength components in the initial perturbation spectra on the late-stage evolution are studied. Finally, multimode RT instability in three dimensions is studied by both full-scale simulations and the modal model. The effect and late-stage memory loss of different aspect ratios in the initial perturbation are demonstrated. © 1995 American Institute of Physics.

Notizen: Stimulated Brillouin backscatter from large scale-length gas-filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te(approximately-greater-than)3 keV), and gradient scale lengths (Ln∼2 mm, Lv(approximately-greater-than)6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas-filled balloons with polyimide walls (gasbags) and gas-filled hohlraums. Detailed characterization using x-ray imaging and x-ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time-resolved SBS backscatter from these targets is 〈3% for conditions similar to ignition target designs. © 1995 American Institute of Physics.

Notizen: Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x-ray irradiation, and to use a low-Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions. © 1995 American Institute of Physics.

Notizen: Hall magnetohydrodynamic (MHD) theory has been used to understand and describe a variety of space and laboratory plasma phenomena. Generally speaking, the theory is applicable to phenomena occurring on length scales shorter than an ion inertial length and time scales shorter than an ion cyclotron period. The theory has been successfully applied to structuring of sub-Alfvénic plasma expansions, and to rapid magnetic field transport in plasma opening switches. An overview of the underlying physics associated with the Hall term, and a brief description of recent research on the application of Hall MHD theory to space and laboratory processes is presented. © 1995 American Institute of Physics.

Notizen: Good radiation drive symmetry will be crucial for achieving ignition in laboratory inertial fusion experiments. The indirect-drive inertial confinement fusion (ICF) method utilizes the soft x-ray field in a radiation-containing cavity, or hohlraum, to help achieve a high degree of symmetry. Achievement of the conditions necessary for ignition and gain will require experimental fine tuning of the drive symmetry. In order to make tuning possible, a significant effort has been devoted to developing symmetry measurement techniques. These techniques have been applied to a series of experiments that give a graphic picture of the symmetry conditions in the complex hohlraum environment. These experiments have been compared with detailed, fully integrated theoretical modeling. The ultimate goal of this work is the detailed understanding of symmetry conditions and the methods for their control. Comparison with experiments provides crucial benchmarking for the modeling—a key element in planning for ignition. © 1995 American Institute of Physics.

Notizen: Recent low light level monochrome television observations obtained from the ground and from the space shuttle, and low light level color and monochrome television images obtained from aboard jet aircraft, have shown that intense lightning in mesoscale thunderstorm systems may excite at least two distinct types of optical emissions that together span the space between the tops of some thunderstorms and the ionosphere. The first of these emissions, dubbed "sprites,'' are luminous red structures that typically span the altitude range 60–90 km, often with faint bluish tendrils dangling below. A second, rarer, type of luminous emission are "blue jets'' that appear to spurt upward out of the anvil top in narrow cones to altitudes of 40–50 km at speeds of ∼100 km/s. In this paper the principal observational characteristics of sprites and jets are presented, and several proposed production mechanisms are reviewed. © 1995 American Institute of Physics.

Notizen: A series of experiments were conducted using cryogenic deuterium targets to study fundamental physics and implosion dynamics with the GEKKO XII glass laser system [IEEE J. Quantum Electron. QE-17, 1639 (1981)]. Preheat sources were found to be due to a shock wave and hot electrons. A new method to measure the fuel ρR using proton spectra was employed. Measured in detail were the implosion dynamics of cryogenic deuterium foam with a plastic ablator and a CH shell with a controlled pressure of deuterium gas targets. Under current experimental conditions sources of nonuniformity were discussed in terms of Rayleigh–Taylor instability. © 1995 American Institute of Physics.

Notizen: Measurements and analyses show that the 13 TW, pulsed Hermes-III [J. J. Ramirez, Proceedings of the 1989 Particle Accelerator Conference, Chicago, 1989 (IEEE, New York, 1989), p. 1446] electron beam, with current near the Alfvén limit, has two regimes of stable transport: a low-pressure window (between ∼1 and ∼100 mTorr) that is dominated by propagation in the semicollisionless IFR (ion-focused regime), and a high-pressure window (between ∼1 and ∼100 Torr) that is dominated by propagation in the resistive CDR (collision-dominated regime). Below ∼1 mTorr, there is insufficient ionization to confine the beam; between the windows, the two-stream and hollowing instabilities disrupt propagation; and above ∼100 Torr, the resistive hose instability degrades propagation. © 1995 American Institute of Physics.

Notizen: Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse-velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion-cyclotron range of frequencies that results from the coupling of the magnetic-field-aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well-known current-driven electrostatic ion-cyclotron mode and this inhomogeneous energy–density-driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse-velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion-cyclotron frequency at small values of the magnetic-field-aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition. © 1995 American Institute of Physics.