Particle-in-cell Simulations of Raman Laser Amplification in Preformed Plasmas
Authors: Daniel S. Clark and Nathaniel J. Fisch
Date of PPPL Report: June 2003
Published in: Physics of Plasmas 10 (December 2003) 4848-4855.
Two critical issues in the amplification of laser pulses by backward Raman scattering in plasma slabs are the saturation mechanism of the amplification effect (which determines the maximum attainable output intensity of a Raman amplifier) and the optimal plasma density for amplification. Previous investigations [V.M. Malkin, et al., Phys. Rev. Lett., 82 (22):4448-4451, 1999] identified forward Raman scattering and modulational instabilities of the amplifying seed as the likely saturation mechanisms and lead to an estimated unfocused output intensities of 1017 W/cm2. The optimal density for amplification is determined by the competing constraints of minimizing the plasma density so as to minimize the growth rate of the instabilities leading to saturation but also maintaining the plasma sufficiently dense that the driven Langmuir wave responsible for backscattering does not break prematurely. Here, particle-in-cell code are simulations presented which verify that saturation of backward Raman amplification does occur at intensities of ~1017 W/cm2 by forward Raman scattering and modulational instabilities. The optimal density for amplification in a plasma with the representative temperature of Te = 200 eV is also shown in these simulations to be intermediate between the cold plasma wave-breaking density and the density limit found by assuming a water bag electron distribution function.