Propagation of Laser-Driven Relativistic Electron Beam inside Solid Dielectric

Laser probing diagnostics shadowgraphy, interferometry and polarimetry was used for comprehensive characterization of ionization wave dynamics inside glass target induced by laser-driven relativistic electron beam. Experiment was done using 50-TW Leopard laser at University of Nevada Reno. It has been shown that for laser flax $\sim $2$\times $10$^{18}$W/cm$^{2}$ hemispheric ionization wave propagates with c/3 speed has smooth electron density distribution, absorbing probing green beam in 2-10 times. Maximum of free-electron density inside glass target is $\sim $2x10$^{19}$cm$^{-3}$, which correspond to ionization $\sim $0.1{\%}. Magnetic and electric fields do not exceed $\sim $15 kG and $\sim $1 MV/cm. Electron temperature has hot-ring structure with maximum 0.1-0.5 eV. The topology of the interference phase shift shows the signature of the ``fountain effect'', a narrow electron beam that fans out from the propagation axis and heads back to the target surface. Two-dimensional PIC-simulations demonstrate radial spreading of fast electrons by self-consistent electrostatic fields. The very low ionization, $\sim $0.1{\%}, observed after the heating pulse suggests a fast recombination at the sub-ps time scale. Work was supported by the DOE/NNSA under UNR grant DE-FC52-06NA27616 and grant DE-PS02-08ER08-16.