Unusual face of radiation friction: enhancing production of longitudinal plasma waves

We study the penetration of ultraintense circularly polarized laser pulses into a thick subcritical plasma layer with accounting for radiation friction. We show that radiation pressure is enhanced by radiation friction in the direction transverse to the laser pulse propagation, and that for stronger and longer laser pulses this mechanism dominates over the ordinary ponderomotive pressure, thus resulting in a stronger charge separation than anticipated previously. We give estimates of the effect and compare them with the results of 1D and 2D PIC simulations.