Transport of laser generated hot electrons through vacuum and low density foam

A 1.06-μm laser focused to a 30-μm diameter was used to irradiate vacuum gap and 0.03g/cc foam targets with 80 to 130-J, 1-ns and 25 to 50-J, 250-ps pulses, respectively. The vacuum gap targets, fabricated with 11-μm polystyrene followed by 20 to 250-μm vacuum gaps followed by 13-μm Cu, were used to study gap-closure and space-charge-limit phenomena. While the low density foam targets, fabricated with layers of 1-μm parylene, 1-μm Fe, 50 to 400-μm foam and 13-μm Cu, were used to study electron transport and spreading. For both types of targets, the transported electrons incident upon the Cu foil produced Cu Kα x-rays that were used to indicate the number of transported electrons. For the foam targets, hot electrons incident upon the Fe foil produced Fe Kα x-rays, which indicate the initial electron production. An array of seven PIN detectors filtered respectively by Sc, Cr, Fe, Co, Ni, Se, and Sn foils were used to measure the x-ray spectrum. The spatial distribution of transported electrons was obtained for both types of targets by comparing x-ray images obtained from two pinhole cameras; one used an Ni filter, which transmitted the Cu Kα x-rays produced by the hot electrons, and the other used a Co filter, which did not transmit the Cu Kα x-rays. For the foam targets, the camera using the Co filter produced an image of the Fe Kα x-rays and, thus, was used to determine the size of the hot electron spot produced in the initial parylene layer