Plasma instabilities in high electric fields

We analyze nonequilibrium screening with nonequilibrium Green function techniques. By employing the generalized Kadanoff-Baym ansatz to relate the correlation function to the nonequilibrium distribution function, the latter of which is assumed to be a shifted Maxwellian, an analytically tractable expression is derived for the nonequilibrium dielectric function ε(k,ω). For certain values of momenta K and frequency, ωImε(K,ω) becomes negative, implying a plasma instability. This new instability exists only for strong electric fields, underlining its nonequilibrium origin.