EXPERIMENTAL STUDIES OF STIMULATED RAMAN-SCATTERING IN REACTOR-SIZE, LASER-PRODUCED PLASMAS

In this paper the results of experiments designed to provide information regarding the scaling of the stimulated Raman scattering (SRS) instability in long-scale-length, inertial-confinement-fusion-reactor-size plasmas are described. Reactor-scale plasma conditions were experimentally simulated by exploding thin CH foils with nine beams of the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)]. Using a one-dimensional hydro model, the target and irradiation parameters were chosen to produce the largest-possible scale length plasma consistent with the energy and pulse width capabilities of Nova. The SRS emissions, driven by both the nine plasma-production beams and a tightly focused and delayed tenth beam, were recorded using an angularly resolved photodiode array and a temporally resolved optical spectrometer. These measurements verified the production of a reactor-size plasma (L/lambda almost-equal-to 5000) and indicated that the absolute levels of SRS backscatter detected in the present experiments have not increased relative to those measured in previous experiments despite a calculated sevenfold increase in the plasma density-gradient scale length. Inverse bremsstrahlung absorption of the scattered light is believed to be the dominant mechanism for this apparent saturation and, in this parameter regime, may enhance the effective thermal coupling between the incident laser and plasma densities below 0.15n(crit) where absorption of the incident laser light is typically weak.