ION-ACOUSTIC-WAVE INSTABILITY DRIVEN BY DRIFTING ELECTRONS IN A GENERALIZED LORENTZIAN DISTRIBUTION

A generalized Lorentzian (kappa) particle distribution function is useful for modelling plasma distributions with a high-energy tail that typically occur in space. The modified plasma dispersion function Z(kappa)*(xi) is employed to study the instability of ion-acoustic waves driven by electron drift in a hot isotropic unmagnetized plasma modelled by a kappa distribution. The real and imaginary parts of the wave frequency omega-0 + i-gamma are obtained as functions of the normalized wavenumber k-lambda(D), where lambda(D) is the electron Debye length. Marginal stability conditions for instability are obtained for different ion-to-electron temperature ratios. The results for a kappa distribution are compared with the classical results for a Maxwellian. In all cases studied the ion-acoustic waves are strongly damped at short wavelengths, k-lambda(D) much greater than 1, but they can be destabilized at long wavelengths. The instability for both the kappa and Maxwellian distributions can be quenched by increasing the ion-electron temperature ratio T(i)/T(e). However, both the marginally unstable electron drift velocities and the growth rates of unstable waves can differ significantly between a generalized Lorentzian and a Maxwellian plasma; these differences are also influenced by the value of T(i)/T(e).