Non-Spitzer return currents in intense laser-plasma interactions

The propagation of an intense (1019Wcm−2) laser-produced hot electron distribution through overdense, long scale-length plasma is modeled using the one-dimensional relativistic electron Vlasov-Fokker-Planck code KALOS [Bell et al., Plasma Phys. Control. Fusion 48, R37 (2006)]. The initial density profile is chosen to be relevant to the coronal region in fast ignition fusion scenarios in which the density rises from around 10ncr to solid density over 35μm. Under these conditions, the return current transport is intermediate between that of collisional Spitzer transport characterized by strong resistivity and that of collisionless transport characterized by electron inertia. It is shown that the isotropic component of the distribution function of the return-current-carrying electrons becomes doubly peaked. Strong anisotropic pressure inhomogeneities can drive additional contributions to the return current not considered in Spitzer’s transport equations. As a result the electric field can become an order-of-magnitude greater than its classical (Spitzer) value.