Threshold Of Parametric Instability Research Articles

On the threshold of parametric instability of spin waves in a film with a rough surface

The threshold of parametric excitation of spin waves in a ferrite film with a rough surface is calculated. The influence of the surface on the threshold is twofold: (i) the normal component of the wave vector of a spin wave is quantized and (ii) surface roughnesses scatter a spin wave elastically. The elastic scattering conserves energy of spin waves and mixes phases of spin waves with different wave vectors. The threshold of parametric excitation hth depends on the relationship between the film thickness d and the mean free path l. The threshold in a thick film (d≫l) is close to that in an unbounded sample of ferromagnet hth0: (hth/hth0)2 = 1 + (πl/d)2. In a thin film (d≪l) the threshold is close to that in a randomly inhomogeneous ferromagnet (hth/hth0)2 ∼ γs/γ, where γ is the spin-wave damping and γs is the frequency of elastic scattering of spin waves on surface roughnesses.

Threshold of Lower Hybrid Parametric Instability in a Modulated Electron-Beam–Plasma System

An experimental study of parametric instability driven by a beam modulation near the lower hybrid frequency is presented. If the modulation voltage exceeds a threshold value, the pump wave appears to decay into an ion acoustic wave and a lower hybrid wave. The threshold electron drift speed is found to be of the order of the ion acoustic speed as predicted by a theory of parametric instability in an inhomogeneous plasma.

Parametric instability thresholds and their control

It is shown that the “swelling” of the laser electric field must be included in threshold estimates for the electrostatic instabilities excited by a laser near the critical density. Random amplitude modulation of the laser is found to lead to an increased instability threshold.