Laser light backscatter from intermediate and high Z plasmas

Abstract

In experiments at the Omega Laser Facility, stimulated Brillouin backscatter (SBS) from gasbags filled with Krypton and Xenon gases was unexpectedly 10 times lower than from CO 2 -filled gasbags with similar electron densities. The SBS backscatter was a 1-5 % for both 527nm and 351nm interaction beams at an intensity of ∼ 10 15 W/cm 2 . The SRS backscatter was less than 1%. The 351nm interaction beam is below the threshold for filamentation and all the SBS occurs in the density plateau between the blast waves. Inverse bremsstrahlung absorption of the incident and SBS light account for the lower reflectivity from Kr than from CO 2 . The 527nm interaction beam filaments in the blowoff plasma before the beam propagates through the blast wave where it is strongly absorbed. Thus, most of the 527nm SBS occurs in the flowing plasma outside the blast waves. Nonlinear processes that limit the acoustic wave amplitudes to 1% of electron density model the SBS reflectivity well in both CO 2 and Krypton plasmas . * Electronic address: berger5@llnl.gov

Role of hydrodynamics simulations in laser-plasma interaction predictive capability

“…While the SRS spectrum showed qualitative agreement with linear theory, a quantitative assessment of reflectivity versus gain was not possible. The hydrodynamics of gasbags is largely dominated by the inward-running blast waves driven by the initial explosion of the bag membrane by the laser [21], and the 2ω laser is strongly absorbed in the high-density peaks. The SRS backscattered light is also strongly absorbed, so small errors in the calculated density of the blast wave peaks result in very large errors in the measured SRS and in the predicted gain.…”

Modeling of two-dimensional effects in hot spot relaxation in laser-produced plasmas

“…Controlling the propagation of high-power laser beams through large-scale underdense plasmas is a crucial point in the context of inertial confinement fusion ͑ICF͒. Controlling the growth rate and saturation level of parametric instabilities such as filamentation 1 and stimulated scattering [2][3][4] has been a challenging issue for several tens of years. In the indirect drive approach, 5 the growth of plasma instabilities may induce beam spreading, deflection, and energy losses leading to a significantly deteriorated target performance.…”