Study of a collisionless, curvature and rotationally driven, trapped particle instability

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Abstract
The Columbia Linear Machine [Plasma Phys. 2 4, 185 (1982)] has been redesigned to enter the parameter regime of the collisionless, curvature driven, trapped particle instability. The changes that were made reduced the neutral collisionality by over a factor of 2 and the Coulomb collisionality by an order of magnitude, and increased the curvature drive by nearly an order of magnitude. With the new machine configuration, a strong (ñ/N≳60%), coherent, m=1 fluctuation is observed which is identified as a collisionless, curvature and rotationally driven, trapped particle instability. The mode is localized strongly to the region of trapping. The radial structure of the mode is broad and the mode exhibits magnetohydrodynamiclike body motion of the plasma column. Furthermore, the mode displays the expected parametric dependences of increasing mode amplitude with increasing mirror ratio and rf power and decreasing mirror cell length. A theory whose predictions are in fair agreement with the experiment has been developed for the mode in the presence of a dc radial electric field. Preliminary experiments designed to test the concept of charge overshoot stabilization of trapped particle modes show results consistent with that idea. A measurement of the growth rate of the instability has been made and is in fair agreement with the theoretical prediction.