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 of a plane polarized wave, there are two distinctive situations. One in which upsilon(s)/c<c/upsilon(p) (upsilon(s) is the electroacoustic speed, c the speed of light, and upsilon(p) is the phase velocity of the electromagnetic wave), and the other situation when upsilon(s)/c greater than or equal to c/upsilon(p). In the first case, there is an ordinary 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 pump wave decays into two sideband waves. In the second situation (upsilon(s)/c greater than or equal to c/upsilon(p)), there are two modulational instabilities similar to the previous situation, but as the intensity of the pump wave increases, the resonant modulational instability disappears and only the nonresonant electromagnetic instability remains. The effect of Landau damping on the electroacoustic modes is also studied. This effect is simulated through a collisional term in the fluid equations. (C) 1998 American Institute of Physics. [S1070-664X(98)00509-6].