Use of burn-through membranes for laser imprint mitigation*

Our previous experiments have found that high-Z coating is highly effective at reducing laser imprint when it is pre-expanded. On Nike KrF laser (λ=248nm) at NRL this is accomplished using a smoothed, low intensity laser prepulse [Karasik et al, PRL. 114, 085001 (2015)] . On Omega EP, we used an externally generated low level soft x-ray prepulse of ~10 J/cm2 to accomplish the pre-expansion due to lack of suitable laser prepulse [Karasik et al., Phys. Plasmas 28, 032710 (2021) ] . The prepulse was generated using an x-ray converter foil(s) with additional beam(s). We are now able to generate the required prepulse using a burn-through membrane in the path of the drive beams in order to avoid use of auxiliary targets and beams. In this approach, the laser first interacts with the thin Au-coated membrane generating a prepulse on the inner target; after which the membrane and the coating are given time to expand. The membrane becomes under dense and transmits the subsequent laser pulse which then drives the inner target. Measurements of the membrane and coating expansion using 4ω probe, soft x-ray emission using an NRL transmission grating spectrometer, as well as areal mass radiography demonstrating successful imprint mitigation will be presented.