EFFECT OF ELECTRON COLLISIONS ON ION-ACOUSTIC-WAVES AND HEAT-FLOW

The damping rate of ion-acoustic waves in a plasma is calculated by nu merically solving the electron Fokker-Planck and cold-ion fluid equati ons for arbitrary electron collisionality klambda(ei) and charge numbe r Z The damping rate reaches a maximUM at klambda(ei) approximately (Z m(e)/m(i))1/2, as predicted by fluid theory, but then remains above fl uid-theory predictions for klambda(ei) > (Zm(e)/m(i))1/2. This enhance ment is most significant for high-Z plasmas, where the thermalization due to electron-electron (e-e) collisions is least effective. For klam bda(ei) much greater than 1, the damping approaches the collisionless Landau limit. The isotropic-Rosenbluth-potential approximation for e-e collisions gives rise to errors of up to 10% in the damping rates. A further approximation that involves adjusting the e-i angular scatteri ng collision strength to simulate the contribution from e-e collisions is found to be similarly accurate. In the high-Z limit, there is a st rong reduction in the effective thermal conductivity kappa relative to the classical Spitzer-Harm value kappa(SH) for klambda(ei) > 10(-4). For low-Z plasmas, this reduction only becomes significant for klambda (ei) > 10(-2). By introducing a spatially modulated inverse-bremsstrah lung heating source and solving for the steady-state distribution func tion, a further reduction in the value of kappa/kappa(SH) is obtained.