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  • 1
    Electronic Resource
    Electronic Resource
    Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2076-2082 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to 3×1020 W cm−2 were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to 3×1013 protons with kTion∼several MeV representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller than the full back surface area. Any ion emission from the front of the targets is much less than from the rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of order MV per micron, which apparently accelerate the ions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874030
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  • 2
    Electronic Resource
    Electronic Resource
    The longitudinal wall impedance instability in a heavy-ion fusion driver (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 3398-3409 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For more than ten years [J. Bisognano, I. Haber, L. Smith, IEEE Trans. Nucl. Sci. NS-30, 2501 (1983)], the longitudinal wall impedance instability was thought to be a serious threat to the success of heavy-ion driven inertial confinement fusion. This instability is a "resistive wall" instability, driven by the impedance of the induction modules used to accelerate the beam. Early estimates of the instability growth rate predicted tens of e-folds due to the instability which would modulate the current and increase the longitudinal momentum spread and prevent focusing the ion beam on the small spot needed at the target. We have simulated this instability using an r−z particle-in-cell code which includes a model for the module impedance. These simulations, using driver parameters, show that growth due to the instability is smaller than in previous calculations. We have seen that growth is mainly limited to one head to tail transit by a space-charge wave. In addition, the capacitive component of the module impedance, which was neglected in the early work of Lee [E. P. Lee, Proc. Linear Accelerator Conference, (UCRL-86452), Santa Fe, NM, 1981] significantly reduces the growth rate. We have also included in the simulation intermittently applied axial confining fields which are thought to be the major source of perturbations to seed the longitudinal instability. Simulations show the beam can adjust to a systematic error in the longitudinal confining fields while a random error excites the most unstable wavelength of the instability. These simulations show that the longitudinal instability must be taken into account in a driver design, but it is not the major factor it was once thought to be. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.365035
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  • 3
    Electronic Resource
    Electronic Resource
    Particle-in-cell simulations of ultra intense laser pulses propagating through overdense plasma for fast-ignitor and radiography applications : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2041-2047 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Zohar (two-dimensions, particle-in-cell) [C. K. Birdsall and A. B. Langdon, Plasma Physics via Computer Simulation (McGraw–Hill, New York, 1985)] simulations of ultra intense laser beams boring into overdense plasmas whose parameters are guided by the fast-ignitor concept and radiography applications are presented. Complex low frequency magnetic field structures, narrow channel formation, and beam deflection are all evident. Particle tracking diagnostics elucidate the nature of the currents that produce and interact with these static magnetic fields which are larger than 109 G for simulations at 1021 W/cm2 in a 50nc plasma. Tracking electron orbits provides a more complete understanding of the hot electron generation as the short pulse, high intensity laser penetrates overdense plasma. Particles which constitute the current in the narrow channel are partially confined by the low frequency magnetic field. In contrast, the return current particles on the outside of the channel are defocused by the high magnetic field and move away from the channel.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873496
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  • 4
    Electronic Resource
    Electronic Resource
    Absorption of laser light in overdense plasmas by sheath inverse bremsstrahlung (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3146-3154 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871146
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  • 5
    Electronic Resource
    Electronic Resource
    Evolution of the Weibel instability in relativistically hot electron–positron plasmas (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3059-3077 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Analytical and numerical studies of the evolution of the Weibel instability in relativistically hot electron–positron plasmas are presented. Appropriate perturbations on the electromagnetic fields and the particle orbits, corresponding to a single unstable mode, are determined analytically and used as initial conditions in the numerical simulations to excite a single unstable mode. A simple estimate of the saturation amplitude is also obtained analytically. Numerical simulations are carried out when a single unstable mode is favorably excited. Comparisons of the simulation results with the analytical ones show very good agreement. Also observed in the simulations are mode competition, mode suppression, and the difference in the long-term evolution between the magnetized and unmagnetized plasmas. For relativistic unmagnetized plasmas, energy-like global constraints, which are conservation laws in addition to the conservation of energy and momentum, are derived. Numerical simulations of the multimode evolution are described. Simulation results show growth in electromagnetic energy in the early stage, a narrowing in the bandwidth and a shift in the peak of the spectrum to longer wavelength in the subsequent evolution, and a decrease in the temperature anisotropy. In a simulation for an unmagnetized plasma, it is observed that the system reaches a steady state halfway through the simulation. In contrast, the peak of the spectrum continues to shift to lower wave number k, and the temperature anisotropy continues to decrease during the entire simulation for a magnetized plasma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870498
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  • 6
    Electronic Resource
    Electronic Resource
    Resonantly excited nonlinear ion waves (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 956-977 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One- and two-dimensional simulations and supporting analysis of nonlinear ion acoustic waves as might be associated with the saturation of stimulated Brillouin backscattering (SBBS) are presented. To simulate ion wave phenomena efficiently, while retaining a fully kinetic representation of the ions, a Boltzmann fluid model is used for the electrons, and a particle-in-cell representation is used for the ions. Poisson's equation is solved in order to retain space-charge effects. We derive a new dispersion relation describing the parametric instability of ion waves, evidence for which is observed in our simulations. One- and two-dimensional simulations of plasma with either initially cold or warm ions (and multi-species ions) exhibit a complex interplay of phenomena that influence the time evolution and relaxation of the amplitude of the excited ion wave: ion trapping, wave steepening, acceleration, heating and tail formation in the ion velocity distribution, parametric decay into longer wavelength ion waves, modulational and filamentation instabilities, and induced scattering by ions. The additional degrees of freedom in two dimensions allow for a more rapid relaxation of the primary ion wave. One-dimensional electrostatic simulations with externally driven ion waves agree qualitatively with electromagnetic simulations in one dimension in which the ponderomotive driving potential is computed self-consistently by solving a Schroedinger-like equation for the electromagnetic waves and calculating the low-frequency ponderomotive force on the electrons. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872187
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  • 7
    Electronic Resource
    Electronic Resource
    Mechanisms for collisionless absorption of light waves obliquely incident on overdense plasmas with steep density gradients (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2702-2709 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For p-polarized laser light obliquely incident on overdense plasmas with steep density gradients, a new collisionless absorption mechanism (sheath-transit absorption) is studied analytically and numerically. Complementary to Brunel's "not-so-resonant'' resonant absorption, and to the conventional resonant absorption, the sheath-transit absorption is most effective for steep density gradients and when the light pressure is less than the plasma pressure. It is also shown that the assumption of instantaneous particle reflection, usually a reasonable assumption for the normal incidence case, is invalid for the p-polarized oblique incident case. A test-particle model which provides a simple physical picture of the sheath-transit absorption is presented. Absorption coefficients obtained from the test-particle model agree reasonably well with those from particle-in-cell (PIC) simulations. The transition from the resonant absorption to the sheath-transit absorption as the density gradient steepens is demonstrated by PIC simulations with a wide range of density gradients. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871527
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  • 8
    Electronic Resource
    Electronic Resource
    Laser filamentation in a thermally unstable plasma (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 175-178 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The propagation of laser light in a thermally unstable plasma attributable to potential imbalances in the local heating and cooling rates caused by density or temperature perturbations is considered. The linear growth rates of laser filamentation and thermal instability are governed by a single fifth-order dispersion relation. The behavior of different modes in various limits is discussed in detail and examples of potential applications to laser–target interactions are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866166
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  • 9
    Electronic Resource
    Electronic Resource
    Weibel instability in relativistically hot magnetized electron–positron plasmas (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3369-3387 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A linear stability analysis is carried out for the Weibel instability in relativistic magnetized electron–positron-pair plasmas, with the propagation direction parallel to the background magnetic field. The instability in the ultrarelativistic regime, with the typical Lorentz factor γ much greater than unity, is emphasized for its relevance to astrophysical sources of synchrotron radiation. Detailed stability properties are examined, in the ultrarelativistic regime, for two model distribution functions, the water-bag distribution function, and a smooth distribution function. The dispersion relations are obtained in closed analytic forms for both distribution functions. The necessary and sufficient conditions for instability are determined when the temperature along the background magnetic field is cold (T(parallel)=0). The dispersion relations are solved numerically with T(parallel)≠0 over a wide range of system parameters to determine the detailed dependence of the instability on the strength of the background magnetic field and the temperature anisotropy. The present analysis shows that both a decrease in temperature anisotropy and an increase in the background magnetic field can cause a significant decrease in growth rate. For the smooth distribution function, it is found that, for a given plasma density, the system stabilizes completely when the background magnetic field is stronger than the moderate threshold value [(ωp±/ωc±)2≤2/π], corresponding to T(parallel)=0. As the temperature anisotropy decreases, the threshold magnetic field decreases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860631
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  • 10
    Electronic Resource
    Electronic Resource
    Dynamics of ponderomotive self-focusing in plasmas (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 766-775 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The space-time evolution of nonlinear self-focusing of a coherent electromagnetic beam in a plasma is calculated. Parameters are considered for which the dominant nonlinearity is the ponderomotive force and the plasma response is hydrostatic. A simple nonlinear wave equation with a saturable, exponential nonlinearity is presented and solved in the paraxial limit. Nondimensional scaled variables are introduced, and both steady-state and time-dependent numerical solutions of the scaled equation are calculated in the limit of cylindrical symmetry. Self-focusing can be important both for high-power lasers in inertial-confinement-fusion applications and for heating of magnetically confined plasmas with intense, pulsed free-electron lasers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859872
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