Study of fast-electron transport in laser-illuminated spherical targets

The transport and scattering of fast electrons created by the two-plasmon-decay instability are studied by comparing the hard x-ray signal from two identically irradiated targets: a 1-mm-diam solid Cu sphere and a 1-mm-diam solid CH sphere, both coated with a 15 μm layer of CH. Comparing the results with Monte Carlo code simulations shows the role of scattering in the higher-Z Cu target. We find evidence that the fast electrons are created with a wide angular divergence and that higher-energy electrons transmitted through the target are reflected back into the target. Because of scattering, the fast-electron energy deposition (preheat) in Cu is about half that in CH, namely, ∼0.15% of the laser energy for Cu as compared with ∼0.30% for CH. Embedded high-Z layers in imploding fusion targets, because of the scattering, could provide protection against preheat.