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  • 1
    Electronic Resource
    Electronic Resource
    Power crowbar with an air-core transformer (1985)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 56 (1985), S. 1465-1466 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An air-core power crowbar transformer has been successfuly applied to a passively crowbarred theta-pinch plasma physics experiment with a large (1-MA), fast-rising (0.5-μs) solenoid current. Without the power crowbar, the current falls exponentially from its peak value with a decay time of 20 μs. With the power crowbar, the current remains at its peak value for ∼30 μs before falling. As described here, it can be built with materials that are lightweight and inexpensive. For applications similar to this one, these two features make the air-core power crowbar transformer simpler and less expensive than one with a ferrite or ferromagnetic core.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138508
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  • 2
    Electronic Resource
    Electronic Resource
    Experimental beta limits of symmetric linear heliac configurations (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2718-2732 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Helically symmetric heliac equilibria [H. P. Furth, Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] are formed on the High Beta Q Machine (HBQM) [C. M. Greenfield, Phys. Fluids B 2, 133 (1990)] by using a fast-rising central conductor (hardcore) current in conjunction with a shock-heated l=1 stellarator configuration. The equilibria are found to possess a high global beta and the plasma pressure is approximately a flux-surface quantity. Under the effects of plasma, the magnetic well is found to deepen and the rotational transform is greatly increased and becomes highly sheared, owing to plasma currents induced by the fast-rising hardcore current. In the second phase of the experiment, the equilibrium fields of the symmetric heliac are lowered while maintaining the same shock heating in an attempt to raise the global beta. No substantial change in global beta is seen, indicating that the configuration forms at the beta limit in the shock-heated HBQM, and that the plasma beta seen in the first phase of the experiment is the symmetric heliac beta limit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870599
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  • 3
    Electronic Resource
    Electronic Resource
    Alfvén-wave heating of a high-beta plasma column (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 392-403 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Alfvén-wave heating experiments are performed on the High Beta Q Machine (HBQM) [Phys. Fluids 25, 262 (1982)] straight plasma column. An investigation is made of Alfvén-wave energy absorption by the Alfvén-resonant layer at which the local shear Alfvén velocity is equal to the phase velocity of the driven plasma standing wave. The radial position of the resonance in the plasma is defined by the equilibrium magnetic field diamagnetic profile and nearly constant electron number density profile. The experiment uses a 250 MW ringing LC circuit in the 615–720 kHz frequency range with the capacitors driving the HBQM compression coil segments that are connected to form a 2.0 m long m=±1 antenna configuration with axial wave number kz=15.7 m−1. Experiments were conducted on both hydrogen and deuterium plasmas. In the hydrogen case the Alfvén resonance is predicted to lie inside the plasma. Internal magnetic field probes were used to measure both the rf and equilibrium magnetic fields. The rf heating produces a larger change in the hydrogen plasma axial magnetic field profile that is centered around r=6 cm, near the Alfvén-resonance location. The rf B˜θ signal shows a phase reversal at the same radius. Other measurements show a 25% increase in plasma pressure for hydrogen. The antenna resistance increases from 0.06 Ω vacuum value to approximately 0.2 Ω in the presence of hydrogen plasma with a 31% absorption efficiency. The rf-induced effects in the deuterium plasma are 20%–50% smaller, agreeing with estimates based on collisional viscous damping, or collisonless model with finite ion cyclotron and Larmor radius effects. In the hydrogen case both these models predict insufficient energy absorption to explain observed heating. However, a magnetofluid calculation taking account of the singularity at the Alfvén-resonant layer reproduces the value of the antenna resistance for the hydrogen case.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860289
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  • 4
    Electronic Resource
    Electronic Resource
    Experimental studies of linear high-beta heliac plasma configurations (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 133-142 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The formation and quasi-equilibrium of a high-beta heliac plasma are investigated in the High-Beta Q Machine [Phys. Fluids 30, 2885 (1987)], a linear high-beta l=1 stellarator with an internal current carrying conductor (hardcore). The hardcore current rise time is varied from 9 μsec to smaller values comparable with that of the main compression field (450 nsec). Flux contours and plasma pressure calculated from internal magnetic-probe measurements are used to distinguish between nearly axisymmetric plasma confined near the hardcore and the heliac plasma confined near the magnetic axis. For the shorter hardcore rise times, the axisymmetric plasma pressure becomes small compared to that of the heliac. Further analysis of the data allows calculation of the currents flowing in the plasma, the rotational transform, and the magnetic well depth. Appreciable axial current is observed, consistent with induction by the changing magnetic fields during the heliac formation. The observed relationship between the axial current and rotational transform is confirmed by computer modeling using the hase magnetohydrodynamic equilibrium code [Nucl. Fusion 23, 1061 (1983)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859525
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  • 5
    Electronic Resource
    Electronic Resource
    Reconnection studies in field-reversed configurations (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 3142-3153 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Internal magnetic field measurements are used to study the reconnection process near the ends of a low-compression field-reversed theta pinch. The measurements indicate that the addition of independently driven magnetic mirror coils at the ends are effective in inducing rapid reconnection and in minimizing plasma losses out the ends. Fast reconnection leading to the formation of a chain of magnetic islands along the length of the coil is observed on a time scale of a 100 nsec. These islands coalesce on a time scale of 1–2 μsec. Measured values of the resistivity are two orders of magnitude greater than classical during the implosion phase. The resistivity during the equilibrium phase is four times the classical value. Axial contraction and the n=2 rotational instability are not observed to occur in these experiments. Sudden termination of the configurations is thought to be caused by rapid loss of the internal field. This flux dissipation may be a consequence of electron cooling by the crossing of a radiation barrier. An oxygen impurity concentration of 2% is enough to account for the power dissipated during the collapse.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865356
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental studies of the equilibrium of a linear high-beta l=1 stellarator (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 2885-2892 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A high-beta linear l=1 stellarator plasma is produced in a low-compression 3 m theta pinch whose compression coil (22 cm i.d.) is modified to have a helical offset of 2 cm and an axial helical period of 40 cm. Internal probes are used to determine the spatial structure and temporal behavior of the magnetic fields, plasma pressure, and current. The results are well described by ideal magnetodydrodynamic equilibrium theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866053
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  • 7
    Electronic Resource
    Electronic Resource
    Excitation and damping of compressional hydromagnetic waves on a magnetized, high-pressure plasma column (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 3036-3051 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Azimuthally symmetric (m=0) radio-frequency (rf) waves for zero and for finite axial wavenumber k are investigated in a low-compression linear theta pinch. The (k=0) modes occur spontaneously following the implosion phase of the discharge. For the k≠0 modes, a novel 100 MW, 1 MHz current drive is used to excite the plasma column in the vicinity of the lowest fast magnetoacoustic mode at various filling pressures. Phases, amplitudes, and radial mode structure are studied for both the k=0 modes and the externally driven (k≠0) modes. In the first case, the damping is determined from the e-folding time of the decaying oscillations. In the latter case, the phases and amplitudes indicate a broad resonance structure, from which we extract the damping constant. The energy deposition of the externally driven rf wave leads to a radial expansion of the plasma column, as observed by axial interferometry and by excluded flux measurements. These experimental results are compared with the predictions of two MHD-like ion-kinetic models. The characteristic and resonant frequencies, as well as the oscillatory radial mode structure, can be understood within the ideal MHD description. It is found that the viscous kinetic model overestimates the observed k=0 damping by at least an order of magnitude, while both this and the ion Landau-damping model (with and without ion-cyclotron effects) underestimate the (k≠0) damping by at least an order of magnitude. The experimentally observed damping and wave-energy deposition are consistent with the magnitude of the rf oscillations. The efficiency of the rf energy deposition is at least 27%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865465
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  • 8
    Electronic Resource
    Electronic Resource
    Control of toroidal-like asymmetries in a heliac (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1567-1575 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple modification to the heliac coil configuration [A. H. Boozer et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] is described that reduces the toroidal perturbation to the magnetic field structure and significantly restores the helical symmetry. This is accomplished by shifting the internal current-carrying conductor (hardcore) of the heliac radially inward from its normal position at the geometric center of the external l=1 coils. It is shown by computations that a large-aspect-ratio toroidal heliac is similar to a helically symmetric linear heliac in which the hardcore has been shifted. This similarity is seen in the analytic form of the magnetic potential, the flux surface shape, and in the harmonic spectra of the field lines. It is then shown that the toroidicity perturbations can be reduced in a toroidal heliac by combining these effects—using a radially inward hardcore shift in the toroidal configuration. In the experiments reported here the toroidal-like effects are created in the linear High Beta Q Machine (HBQM) heliac [C. M. Greenfield et al., Phys. Fluids B 2, 133 (1990)] by shifting the hardcore off the geometric center of the l=1 coils. The induced toroidal-like effects are seen from an asymmetry in the axial excluded flux; however, no decrease in global beta is seen as compared to the unshifted symmetric case (G. G. Spanjers, Ph.D. dissertation, University of Washington, 1992) indicating the absence of large-scale magnetohydrodynamic instabilities at high beta.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860896
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  • 9
    Electronic Resource
    Electronic Resource
    Experimental studies of the dynamics of a linear high-beta stellarator (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 3821-3824 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a linear l=1 stellarator [Phys. Fluids 30, 2885 (1987)] internal probes are used to measure the structure and temporal behavior of the plasma and magnetic field. At lower densities (and higher temperatures) diffuse-profile equilibria are produced that are perturbed by a k≈0 oscillation. At higher densities (and lower temperatures) sharp-boundary plasmas are produced that are unstable with an exponentially growing m=1, k≈0 mode. Finite Larmor radius (FLR) effects are adduced to explain the absence of the instability in the oscillatory regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866419
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  • 10
    Electronic Resource
    Electronic Resource
    Formation of fast "notched'' current waveforms through a high inductance (1991)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2338-2341 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A fast "notch'' current has been produced on the (4 μH) hardcore central conductor [C. M. Greenfield, M. E. Koepke, and F. L. Ribe, Phys. Fluids B 2, 133 (1990)] of the high beta Q machine, a 2.6 m theta pinch [S. O. Knox, H. Meuth, E. Sevillano, and F. L. Ribe, 3rd IEEE International Pulsed Power Conf., 1981, IEEE Publ. 81 CH1662/6, paper 3.1]. With the notch circuitry, the current can be slowly (τ1/4 = 14 μs) brought to a crowbarred dc value (20 kA) and then quickly (τ1/4 = 1.3 μs) "notched'' to a different value (typically either 0 kA or twice the dc value) and then quickly returned to the dc value. The use of a new inductively loaded spark gap switch eliminates extraneous ringing in the final crowbarred current waveform. As described here, by driving the hardcore circuit with two isolated capacitor banks, and a voltage stepup transformer, the notch current is created using spark gaps and ignitrons for switching, resulting in an inexpensive and technically simple circuit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142513
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