Dynamics of a laser-induced relativistic electron beam inside a solid dielectric

Two-frame interferometry and shadowgraphy were used to investigate the dynamics of interaction of a powerful laser (UNR Leopard 2x10$^{18}$ W/cm$^{2}$, 0.5ps, 1057nm) with a glass target. The two-frame laser diagnostic reveals an ionization wave propagating inside the glass with half the speed of light. The interferometry delineates regions of ionization and excitation inside the glass target. A ``fountain effect'' of fast electrons inside the solid dielectric has been observed for the first time: a radially compact electron beam with sub-light speed fans out from the axis of the original beam, heading back to the target surface. Comparison with French ($\sim $10$^{19}$W/cm$^{2})$ and UK ($\sim $10$^{17}$W/cm$^{2})$ experiments implies a logarithmic dependence of the ionization depth with the laser intensity. Relativistic electron beam dynamics stemming from intense laser-glass interaction is a critical concern for the NIF ``fast ignition'' concept.