Theory of a parametric instability excited by two pump waves

Abstract

The threshold of a parametric instability in which the frequency of the excited Langmuir waves is exactly the arithmetic mean of two pump frequencies, is calculated. The kinetic equation is used to describe the ion dynamics. The threshold is infinitely high when the difference between the two pump frequencies is exactly double the frequency of ion-acoustic waves if pump waves of equal intensity are assumed. For very large values of T_e/T_i the threshold reaches its lowest values when the difference frequency is slightly more or slightly less than twice the resonant frequency of the ion-acoustic waves.