Propagation and spreading of unstable perturbations in electron-hole plasmas subject to crossed fields

Abstract

A model is developed for electrostatic drift instabilities which arise in inhomogeneous electron-hole plasmas subject to crossed fieldsE ₀⊥B ₀. The instabilities are initiated by gradients in the equilibrium plasma densityn ₀. Using two-fluid magnetohydrodynamics and linear perturbation theory the dispersion relation of local density oscillations is calculated for arbitrary inhomogeneous equilibrium distributions and plasma densities. For cases where∇n ₀⊥B ₀ andn ⁺₀ ≪n ^-₀ it is found that the propagation direction of maximal gain,$\hat k_m$, is the bisectrix of the angle between (−∇n ₀) and (E ₀ ×B ₀) and that stable and unstable configurations are distinguished by the angle between∇Φ ₀ and∇n ₀. A local density perturbation built by superpositions of the plane waves, and initially chosen radially symmetric, broadens unisotropically. In the direction transverse to$\hat k_m$ the broadening is anomaleously enhanced, as compared to the broadening by diffusion in the stable case. The results are referred to experimental observations of low-frequency instabilities reported in [1].