Energetics of multiple-ion species hohlraum plasmas

Abstract

A study of the laser-plasma interaction processes has been performed in multiple-ion species hohlraum plasmas at conditions similar to those expected in indirect drive inertial confinement fusion targets. Gas-filled hohlraums with electron densities of 5.5×1020 and 9×1020cm−3 are heated by 14.3kJ of laser energy (wavelength 351nm) to electron temperatures of 3keV and backscattered laser light is measured. Landau damping of the ion acoustic waves is increased by adding hydrogen to a CO2 or CF4 gas. Stimulated Brillouin backscattering of a 351nm probe beam is found to decrease monotonically with increasing Landau damping, accompanied by a comparable increase in the transmission. More efficient energy coupling into the hohlraum by suppression of backscatter from the heater beams results in an increased hohlraum radiation temperature, showing that multiple-ion species plasmas improve the overall hohlraum energetics. The reduction in backscatter is reproduced by linear gain calculations as well as detailed full-scale three-dimensional laser-plasma interaction simulations, demonstrating that Landau damping is the controlling damping mechanism in inertial confinement fusion relevant high-electron temperature plasmas. These findings have led to the inclusion of multiple-ion species plasmas in the hohlraum point design for upcoming ignition campaigns at the National Ignition Facility.