Resonant decrease of the parametric instability threshold by pump frequency modulation

Abstract

The pump wave frequency modulation, as well as the plasma non-stationarity, are well known to stabilize the parametric decay instabilities due to violation of the decay condition for wave frequencies. The plasma inhomogeneity has a similar effect associated with the decay condition for the wavenumbers. In the present paper the simultaneous influence of these two factors on the decay instability of the Trivelpiece-Gould wave in a magnetized plasma is studied. A strong increase of the amplitude of decay waves is observed in a narrow range of pump frequency modulation rates. The decay point motion due to the pump frequency modulation is shown to be responsible for this effect, which occurs when the decay point velocity is equal to the group velocity of one of the parametrically driven waves. Elimination of convective losses is proved to occur under these conditions. The dependence of the decay wave amplitude on pump power and on the pump frequency modulation time is studied for this resonance.