Effect of laser produced plasma bubbles and wall expansion on laser beams propagation inside gas filled halfraum cavity

We present results of an experiment, performed at the Omega Laser Facility, to characterize the effect of bubbles of plasma, produced by ns “outer” laser beams, on the propagation of delayed “inner” beams inside methane gas filled hohlraum targets. Outer and inner refer to the angle of propagation of beams with the hohlraum axis. Halfraums with a “rugby-like” shape were used to enhance this effect, as the incidence angle of the inner relative to the cavity wall is wider in this geometry, which contributes to a larger interaction length. Hard x-ray imager and a broad-band spectrometer were used to get information on plasma dynamics and emission. The results show that the hard x-ray emission produced by the inner, while propagating through the expanding plasma, increases linearly with the bubble size independently of the gas pressure: this is an interesting measurement, as in an implosion experiment, this energy deposition that is difficult to reproduce by simulation, would be a source of illumination asymmetry of the capsule. Experimental results are then compared to those from the three-dimensional radiation hydrodynamics code TROLL. The measured dynamic and x-ray emission of the plasma bubbles, produced by the outer, are correctly reproduced by the simulations, but only when using a simple model that modifies the laser energy absorption within the expanding plasma wall. Yet, discrepancies between experimental and simulated results on inner beams' interactions with bubbles persist. Two hypotheses are presented to explain these differences.