Delayed shock breakout from aluminum samples in a halfraum due to hot electron beaming from the LEH window

We have shot gas filled Au halfraums at the National Ignition Facility with a sample on the back wall. With Au samples we measure radiation burn-through to assess opacity and equation of state at 300 eV. The Dante instrument measures halfraum drive via x-ray emission through the laser entrance hole (LEH). To more accurately assess the drive at the sample position, we measured shock breakout from Al samples. The shock broke out 1 nsec later than expectations based on a drive that matched the Dante. We attribute that discrepancy to hot electrons beaming from the LEH window, and preheating the back surface of the Al samples. This causes motion of that back surface, which delays the shock breakout. We quantify this effect by measuring the time resolved hard x-rays produced by those hot electrons, and analytically calculate a 6 eV preheat. Simulations show that precisely such an amount leads to the observed 1 nsec delay in shock breakout. Future experiments will eliminate this preheat by using a small pre-pulse to blow down the LEH window before it is illuminated by the high power pulse.