Unstable Precession under the Influence of Drag Forces

Coherent precession of plasma particles in ▿B is generally subjected to drag forces associated with viscous or resistive energy dissipation in background plasma. The dissipated energy can be supplied by continual displacement of the processing entity along — ▿B. Precession in mirror machines therefore tends to be unstable against drag, while the converse is true for minimum-B geometries. A heuristic mechanical model is first described. The precession of the E-layer is next shown to be drag-unstable if one uses the ``super-particle model,'' which does not allow for a finite spread in particle precession frequencies. Similar results are obtained for ordinary mirror-machine plasmas in the two limits that are ordinarily considered stable by virtue of the high-density and low-density finite-Larmor-radius conditions.