Experimental Investigation of the Collective Raman Scattering of Multiple Laser Beams in Inhomogeneous Plasmas

Abstract

Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.

Theory and measurements of convective Raman side scatter in inertial confinement fusion experiments

“…Likewise, a similar calculation of the absolute threshold including EPW damping for the conditions of Ref. [13] shows that the single-quad intensity in these experiments was also several orders of magnitude below the absolute threshold. This is due to the EPW damping for typical ICF conditions which pushes the absolute instability threshold (as derived in Ref.…”

“…While recent work (Refs. [11,13]) brought attention to collective processes for Raman side-scatter (where multiple lasers collectively drive a shared EPW or scattered light wave), these observations show that the instability can also exist as a single-quad process.…”

“…The SRS side-scatter signals observed in our experiments as well as those from Refs. [13] should rather be interpreted using a convective gain expression like the one we derived.…”

“…exponentially growing in time at a fixed spatial location, rapidly reaching saturation via pump depletion or other non-linear processes) in low density regions below n c /4, the quarter-critical density for the laser wavelength; Raman side-scatter was therefore initially anticipated to be particularly threatening for ICF, but its observation has remained rather elusive in experiments and thus generated less interest after the 1980's. However, this process has recently been the subject of renewed investigations and interest in the ICF and laser-plasma communities: it was indeed suggested as the generating mechanism for the observation of suprathermal electron "beaming" in indirect-drive ICF experiments (as a collective process via a shared EPW) [11,12] , and was also identified in experiments using foam targets at the Omega facility (also as a collective process, via a shared scattered light wave) [13]. However, the laser intensities in most of these experiments were in fact orders of magnitude below the absolute instability threshold for side-scatter, if the EPW damping is accounted for.…”

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments

“…SRS entails the decay of a laser light wave into an electron plasma wave and a scatteredlight wave at densities at or below one quarter of the critical density of the laser, while TPD is the decay of a laser light wave into two electrostatic plasma waves (plasmons) near the quartercritical density. Previous studies of SRS and TPD have examined single-beam thresholds, 9,10 quantified suprathermal electron production, 6,11,12 explored collective multibeam processes, [13][14][15][16][17][18] and investigated the spatial properties of TPD 19 and the angular distribution of the resulting hot electrons 20 -an important consideration when computing preheat. SRS imposes serious constraints on ignition designs in the indirect-drive approach to ICF because of the high single-beam intensities and large volumes of quasi-homogeneous plasma that are required when using gas-filled hohlraums.…”