Nonlinear electron heating in ultrahigh-intensity-laser endash plasma interaction

The interaction of an ultraintense laser pulse with an overcritical collisionless plasma at normal incidence is investigated with 1.5 dimensional particle-in-cell simulations. Laser absorption and hot electron energy are reported for a large range of intensities and plasma densities. We observe a strong dependence of the electron temperature on the plasma density profile, and a transition between two different heating mechanisms when the gradient length is varied. For sharp edged profiles, the electron temperature is well below the laser ponderomotive potential. copyright 1997 The American Physical Society