Interchange stabilization of an axisymmetric single cell mirror using high-frequency electric fields

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Abstract
Stabilization of interchange instabilities in the Phaedrus tandem mirror [Phys. Rev. Lett. 5 1, 1955 (1983)] through the use of externally applied rf at a single frequency is demonstrated over a wide range of frequency (1.03Ωi <ωi). Here rf stabilized plasmas are reported with ion temperature up to 550 eV and beta up to 8%. Calculations of the rf electric fields show that the dominant mode is the m=+1 fast wave. When the rf frequency is near Ωi the calculated ponderomotive forces on ions and electrons are shown to be comparable. For ω>1.3Ωi it is shown that the force acts primarily on electrons. Stable plasmas are only achieved when the net radial ponderomotive force is calculated to be stabilizing and comparable to the curvature force. Results are also reported for rf applied simultaneously at two frequencies, one which is stabilizing and one which is destabilizing. Abrupt changes in stability are observed as the rf power is increased whenever one of the rf frequencies is effective at plasma heating.