Resonant decrease of the parametric instability threshold by the pump frequency modulation

The pump wave frequency modulation, as well as the plasma nonstationarity, are well known to stabilize the parametric decay instabilities because of the mismatch of the decay condition for wave frequencies w0(t) = w1(t) + w2(t). The plasma inhomogeneity also increases the decay instability threshold due to convective losses of parametrically excited waves from the narrow interaction region in the vicinity of the so called decay point xd, where the decay condition k0(x) = k1(x) + k2(x) for the wave numbers is fulfilled. In the present paper the simultaneous influence of these two factors on the decay instability of the lower hybrid wave in magnetized plasma is studied. The strong increase of the amplitude of decay waves is observed for the narrow range of the pump frequency modulation rates. The drift of decay point xd = xd(t) 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. The suppression of convective losses is proved to take place under these conditions. The dependence of amplitude of the decay waves on pump power and on the range of variation of pump frequency is studied for this resonance.