On the Use of Reduced Mass Targets to Obtain High Temperature Solid Density Materials using Petawatt Lasers for Astrophysical Applications

Summary form only given. Laboratory experiments that employ petawatt lasers are rapidly approaching parameter regimes once thought to be the exclusive domain of compact astrophysical objects. In fact, experiments and simulations of petawatt lasers impinging on solid density targets have always shown that some fraction (usually ~10-50%) of laser energy couples to hot electrons in the target, which can be in excess of 50 Joules. Up to this point, targets have usually been macroscopic foils (millimeter-sized slab targets) or large cones, with small wires at the end. In hopes of simulating conditions thought to be present in the higher atmospheres of neutron stars, we have designed and fielded reduced mass targets (RMTs) that are of order 100 mum times 100 mum times 5 mum. These targets were designed to achieve hotter material and radiation temperatures than previous macroscopic targets. Results from a recent campaign consisting of a series of RMT's on the Vulcan laser at Rutherford Appleton Laboratory (RAL) suggest that this is indeed the case. We present the results of spectroscopic measurements taken at RAL for copper RMT's tamped by 1 micron of aluminum on either side, and compare them to detailed particle-in-cell, hydro, and atomic physics computer simulations. By comparing them with identical targets, with the exception that the target sizes are increased to 400 mum times 400 mum and 1 mm times 1 mm, we find clear evidence for enhanced heating in the case of the smallest targets, where we infer nearly homogenous temperatures of over 200 eV for solid copper. Various explanations for the high temperatures will be explored, and applications of these results to EOS measurements and astrophysics will be discussed