PARAMETRIC DECAYS OF A CIRCULARLY-POLARIZED ELECTROMAGNETIC-WAVE IN AN ELECTRON-POSITRON PLASMA

Parametric decays of a circularly polarized electromagnetic wave in an electron-positron unmagnetized plasma are studied. Like in the case o f a plane polarized wave, there are two distinctive situations. One in which upsilon(s)/c<c/upsilon(p) (upsilon(s) is the electroacoustic sp eed, c the speed of light, and upsilon(p) is the phase velocity of the electromagnetic wave), and the other situation when upsilon(s)/c grea ter than or equal to c/upsilon(p). In the first case, there is an ordi nary decay instability and two modulational instabilities. One of the modulational instabilities is a resonant instability, and the other is a nonresonant, essentially electromagnetic instability in which the p ump wave decays into two sideband waves. In the second situation (upsi lon(s)/c greater than or equal to c/upsilon(p)), there are two modulat ional instabilities similar to the previous situation, but as the inte nsity of the pump wave increases, the resonant modulational instabilit y disappears and only the nonresonant electromagnetic instability rema ins. The effect of Landau damping on the electroacoustic modes is also studied. This effect is simulated through a collisional term in the f luid equations. (C) 1998 American Institute of Physics. [S1070-664X(98 )00509-6].