0.351 micron Laser Beam propagation in High-temperature Plasmas

Abstract

A study of the laser-plasma interaction processes have been performed in plasmas that are created to emulate the plasma conditions in indirect drive inertial confinement fusion targets. The plasma emulator is produced in a gas-filled hohlraum; a blue 351-nm laser beam propagates along the axis of the hohlraum interacting with a high-temperature (Te = 3.5 keV), dense (ne = 5 × 10 20 cm −3 ), long-scale length (L ∼ 2 mm) plasma. Experiments at these conditions have demonstrated that the interaction beam produces less than 1% total backscatter resulting in transmission greater than 90% for laser intensities less than I < 2 × 10 15 W-cm −2 . The bulk plasma conditions have been independently characterized using Thomson scattering where the peak electron temperatures are shown to scale with the hohlraum heater beam energy in the range from 2 keV to 3.5 keV. This feature has allowed us to determine the thresholds for both backscattering and filamentation instabilities; the former measured with absolutely calibrated full aperture backscatter and near backscatter diagnostics and the latter with a transmitted beam diagnostics. A plasma length scaling is also investigated extending our measurements to 4-mm long high-temperature plasmas. At intensities I < 5 × 10 14 W-cm −2 , greater than 80% of the energy in the laser is transmitted through a 5-mm long, high-temperature (Te > 2.5 keV) high-density (ne = 5 × 10 20 W-cm −3 ) plasma. Comparing the experimental results with detailed gain calculations for the onset of significant laser scattering processes shows a stimulated Brillouin scattering threshold (R=10%) for a linear gain of 15; these high temperature, low density experiments produce plasma conditions comparable to those along the outer beams in ignition hohlraum designs. By increasing the gas fill density (ne = 10 21 cm −3 ) in these targets, the inner beam ignition hohlraum conditions are accessed. In this case, stimulated Raman scattering dominates the backscattering processes and we show that scattering is small for gains less than 20 which can be achieved through proper choice of the laser beam intensity. The first three-dimensional (3D) simulations of a high power 0.351µm laser beam propagating through a high-temperature hohlraum plasma are also reported. We show that 3D linear kinetic modeling of Stimulated Brillouin scattering reproduces quantitatively the experimental measurements, provided it is coupled to detailed hydrodynamics simulation and a realistic description of the laser beam from its millimeter-size envelop down to the micron scale speckles. These simulations accurately predict the strong reduction of SBS measured when polarization smoothing is used.

Laser propagation measurements in long-scale-length underdense plasmas relevant to magnetized liner inertial fusion

“…These experiments show that, when LPI is not significant, RMHD codes such as HYDRA [6] can accurately model laser energy deposition [7], [8]. However, at MagLIF-relevant scale-lengths and parameters, there are no reported experiments to identify the target and drive conditions where undesirable LPI become important, and thus hydrodynamic simulations have never been validated for MagLIF preheat physics.…”