Experimental study of fast electron propagation in compressed matter

We report on experimental results of the fast electron transport in compressed plasmas, created by laser-induced shock propagation in both cylindrical and planar geometry. Two experiments were carried out. The first one was based on the compression of a polyimide cylinder filled with foams of three different initial densities (rho(0)). X-ray and proton radiographies of the target during the compression coupled with hydrodynamic simulations show that the obtained core densities and temperatures range from 2 to 11 g/cm(3) and from 30 to 120 eV, respectively. By studying the K-shell fluorescence from dopant atoms inside the target and from tracer layers situated at both front and rear side of the target it has been possible to investigate the fast electron propagation. The results show that Cu K(alpha) yield emitted by the target rear side foil decreases with increasing compression, independently of rho(0). An electron collimation can also be observed for certain experimental conditions where a convergent resistivity gradient interacts with the fast electron beam. The second experiment was performed in a planar geometry with a compressing shock counter-propagative to the fast electron beam. In this case the areal density rho z seen by the electrons is constant during the compression in such a way that changes in the fast electron range should be ascribed to collective mechanisms. The study of the K(alpha) fluorescence, from buried fluorescent layers of different atomic numbers, shows that the electrons with energy <75 key are more affected by resistive losses in compressed compared to non-compressed targets. These two experiments were part of the Experimental Fusion Validation Program of the HiPER project. (C) 2010 Elsevier B.V. All rights reserved.