Effects of ion-ion collisions and inhomogeneity in two-dimensional kinetic ion simulations of stimulated Brillouin backscattering
- University of California, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551 (United States)
Two-dimensional simulations with the BZOHAR [B. I. Cohen, B. F. Lasinski, A. B. Langdon, and E. A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability, including the effects of ion-ion collisions and inhomogeneity. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models the collisions are functions of the local ion temperature and density, but the collisions have no velocity dependence in the first model. In the second model the collisions are also functions of the energy of the ion that is being scattered so as to represent a more physical Fokker-Planck collision operator. Collisions decorrelate the ions from the acoustic waves in SBS, which disrupts ion trapping in the acoustic wave. Nevertheless, ion trapping leading to a hot ion tail and two-dimensional physics that allows the SBS ion waves to nonlinearly scatter, remain important saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities decrease with increasing collisionality in the simulations for velocity-independent collisions and very weakly decrease for the range of Fokker-Planck collisionality considered. SBS backscatter in the presence of a spatially nonuniform plasma flow is also investigated. Simulations show that, depending on the sign of the spatial gradient of the flow relative to the backscatter, ion trapping effects that produce a nonlinear frequency shift can enhance (autoresonance) reflectivities relative to anti-autoresonant configurations, in agreement with theoretical arguments.
- OSTI ID:
- 20782514
- Journal Information:
- Physics of Plasmas, Vol. 13, Issue 2; Other Information: DOI: 10.1063/1.2168405; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BACKSCATTERING
BRILLOUIN EFFECT
ELECTRON TEMPERATURE
ELECTRONS
FOKKER-PLANCK EQUATION
GAIN
ION ACOUSTIC WAVES
ION DENSITY
ION TEMPERATURE
ION-ION COLLISIONS
IONS
NONLINEAR PROBLEMS
PLASMA
PLASMA DENSITY
PLASMA INSTABILITY
PLASMA SIMULATION
REFLECTIVITY
SOUND WAVES
TRAPPING
TWO-DIMENSIONAL CALCULATIONS