Magnetic compression and electron cyclotron heating of a low temperature, lithium plasma

Magnetic mirror compression and electron cyclotron heating have been investigated in a high field, simple mirror, compression device (midplane field rises from 200 G to 50 kG in 0.5 msec, mirror ratio = 1.4), using a contact ionization lithium source (n ≈ 108−1010 cm−3, Te = Ti ≈ 0.2 ev) to produce the initial plasma. Experimental results are: (1) For magnetic mirror compression, the plasma is macroscopically unstable, and, although Ti ≈ 10 eV, confinement is destroyed with ∼200 μsec, for all ranges of initial plasma density and neutral pressure. A set of rate equations for electron and ion density and temperature and ambipolar potential predicts classical confinement times of the order of milliseconds. (2) For short pulse, electron cyclotron heating (250 kW, 0.25 μsec, 8.95 GHz pulse) followed by magnetic compression, Te ≈ 45 keV, Ti ≈ 10 eV and the plasma is stably confined, provided the neutral gas pressure exceeds ∼2 × 10−5 Torr. The observed stability is in rough agreement with the Guest-Beasley criterion for stabilization of flute modes by flow of cold plasma to the conducting chamber walls. The confinement time is in agreement with previous rate equation calculations of hot electron plasma confinement in magnetic mirrors.