Damping and destabilization of drift waves by electron-neutral collisions

The effects of electron-neutral collisions on drift waves in a plasma confined by a magnetic quadrupole are investigated. A new formula for the growth rate is derived which includes nondetrapping as well as detrapping collisions and is not based on the small ion-Larmor-radius approximation. A numerical study has been made of the fundamental drift mode and the next harmonic which have one and two full wavelengths around a closed magnetic field line, respectively. The more numerous non-detrapping collisions are found to have no significant effect on the growth rate of the fundamental mode observed in the UMIST quadrupole. Electron-neutral collisions dampen the next harmonic drift mode for axial wave numbers of the order of 1 cm-1 but tend to destabilize it at somewhat larger or smaller wave numbers. (author)