Absolute and convective nature of the modulational and Raman instabilitiesin the relativistic regime

The nature (convective or absolute) of the modulational and Raman instabili ties is investigated in the relativistic regime, for a large amplitude elec tromagnetic wave propagating in a plasma of arbitrary density. The present paper extends previous results by Guerin [Phys. Plasmas 2, 2807 (1995)]. St ability diagrams are obtained and compared to the stability properties foun d by a classical temporal stability analysis. It is shown that the modulati onal and the (forward and backward) Raman instabilities are individually co nvective. However, the Raman instability is absolute for large density and high intensity when the forward and backward Raman branches have merged int o a hybrid Raman branch. Near quarter critical density, this hybrid branch is destabilized and is absolute from the threshold. The plasma equilibrium is convectively unstable between quarter critical and critical density at l ow intensity. (C) 2001 American Institute of Physics.