Continuation of the Application of Parallel Pic Simulations to Laser and Electron Transport through Plasma under Condition Relevant to ICF and HEDS
The UCLA simulation group develops and uses state-of-the-art simulation tools to study laser and beam interactions in high energy density plasmas. These tools run extremely efficiently on more than 100,000 processing cores. Understanding how a laser interacts with a high energy density plasma and the related generation of energetic electrons is essential for the success of the National Ignition Facility and of Inertial Fusion Energy and it is a of fundamental importance to the nonlinear optics of plasmas.
A laser propagating in plasma can effectively couple to plasma waves through instabilities named stimulated Raman scattering, two-plasmon decay, and the high-frequency hybrid instability. These are extremely complicated processes because they involve waves coupling to waves, particles surfing on waves, and the highly dispersive nature of plasma waves in density gradients. An ultra-intense laser provides pressures on a plasma in excess 100 Billion Atmospheres and it wiggles electrons at energies in excess of 100 Mega Electron Volts. Due to the complexity of these interactions, computer simulations that track individual trajectories of plasma particles are necessary to make sense of it all.
Using our massively parallel particle-in-cell codes, we discovered that packets of plasma waves behave very differently than their idealized cousins which are infinitely long and wide. They erode away and localize due to complex wave-particle interactions which can profoundly alter the reflectivity of the laser. We discovered that higher than expected electron energies occur when the laser scatters both backward and forward and this scattered light rescatters, generating a spectrum of plasma wave packets. Electrons first surf on the slower wave and then progressively bootstrap their way onto the faster waves. We found that for parameters expected in the future for inertial fusion energy that extremely strong absorption into energetic electrons can be generated near the so-called quarter critical density. We also found a new absorption mechanism for an intense laser impinging a sharp solid density plasma interface and studied how these electrons are transported forward.