LPI comparison of foam and gas-filled hohlraum experiments on LMJ*

We report results from two shots on LMJ that studied the effect of replacing a substantial fraction of the fill gas with a 1 mg/cc SiO₂ aerogel foam. The first shot was a control, which used a standard C₅H₁₂ gas-filled hohlraum with a gas fill density of 0.5 mg/cc. The second shot used a hohlraum with a 0.25 mg/cc C₅H₁₂ gas fill in addition to a cylindrical 1 mg/cc SiO₂ aerogel foam insert that radially spanned the width of the hohlraum. The mean electron hohlraum density was equal for both the foam and control shots

 

The foam insert was placed at the same axial position as where the outer beams strike the hohlraum wall in order to tamp gold bubble expansion. A reduction in gold bubble size was predicted in simulations and observed in experimental X-ray images. A smaller gold bubble allows better propagation of the inner beams to the waist. This improves symmetry, which ultimately enlarges the ICF design space.

 

We assess backscatter in the foam and control shots by performing time-dependent LPI simulations with the code pF3D. We compare the pF3D results to steady state gain calculations along rays using the code FLIP. The results show good agreement between simulations and experiments.

Both foam and gas shots used quad-splitting on the inner beams. We also discuss how quad-splitting is a powerful tool in mitigating LPI on LMJ.