Convective Raman Amplification of Light Pulses Causing Kinetic Inflation in Inertial Fusion Plasmas

TL;DR

This study uses particle-in-cell simulations to investigate convective Raman amplification and kinetic inflation of light pulses in inertial fusion plasmas, revealing large-amplitude scattering bursts driven by seed modifications.

Contribution

It demonstrates the role of seed pulse characteristics in triggering kinetic inflation and large-amplitude scattering in Raman amplification within inertial fusion plasmas.

Findings

Large-amplitude scattering bursts occur due to kinetic inflation.

Seed wavelength and intensity influence inflation onset.

Oscillations in reflectivity are observed at specific frequency differences.

Abstract

We perform 1D particle-in-cell (PIC) simulations using OSIRIS, which model a short-duration (~500/{\omega}0 FWHM) scattered light seed pulse in the presence of a constant counter-propagating pump laser with an intensity far below the absolute instability threshold. The seed undergoes linear convective Raman amplification and dominates over fluctuations due to particle discreteness. Our simulation results are in good agreement with results from a coupled mode solver when we take into account special relativity and the use of finite size PIC simulation particles. We present linear gain spectra including both effects. Extending the PIC simulations past when the seed exits the simulation domain reveals bursts of large-amplitude scattering in many cases, which does not occur in simulations without the seed pulse. These bursts can have amplitudes several times greater than the amplified seed pulse, and we demonstrate that this large-amplitude scattering is the result of kinetic inflation by examining trapped particle orbits. This large-amplitude scattering is caused by the seed modifying the distribution function earlier in the simulation. We perform some simulations with longer duration seeds, which lead to parts of the seeds undergoing kinetic inflation and reaching amplitudes several times more than the steady-state linear theory results. Simulations with continuous seeds demonstrate that the onset of inflation depends on seed wavelength and incident intensity, and we observe oscillations in the reflectivity at a frequency equal to the difference between the seed frequency and the frequency at which the inflationary SRS grows.