Effect of impurity density gradients on low-frequency electrostatic fluctuations

For a plasma with impurities, the density scale lengths for electrons and bulk ions become different. Here it is shown that for drift-wave-type instabilities, the electron density scale length arises in the equations of motion (to lowest order in ion gyroradius), not the density scale length of the bulk ions, as is commonly believed. Thus, when comparing experiments with theoretical predictions, it is appropriate to take the drift frequency as ω*eω*e= -(Ti/Te)ω*i = -(kθcTe/eB)d ln n0e/dr, and the parameter ηi≡d ln Ti/d ln n0e (that is, with ne replacing the more commonly used ni in these terms)