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Abstract
This work addresses the physics of a plasma which has two electron populations with very different temperatures. Electron cyclotron resonance heating is used to heat the electrons and produce a cold collisional component(1-20eV) and a hot collisionless component$(\sp\sim$1keV). Striking changes are observed depending on the ratio of these two electron populations. This ratio can be controlled by changing the rf power and neutral pressure or by looking at different times during the plasma decay. The hot electrons are lost much faster than the cold electrons, with decay times of tens of microseconds as opposed to milliseconds. The experiments were performed on a single cell mirror device, LAMEX.
One result is that the background plasma can be stabilized against the interchange mode by raising the hot electron fraction above a threshold value. The important point is that it is the hot electron density rather than the beta which does the stabilizing. The stabilization is due to the rigidity of the fast electrons to ExB movements caused by the electric field of the interchange mode. This mechanism, termed charge uncovering, is not restricted to electrons, but only requires that the particles have a high drift velocity compared to the phase velocity of the wave. Energetic ions could work just as well.
The second finding is that the hot electrons are rapidly lost due to two high frequency instabilities, the hot electron interchange and the compressional Alfven mode. One of these is always active, though never at the same time. The hot electron interchange is unstable when the hot electron fraction is too high and the compressional Alfven mode is unstable when the hot electron fraction is too low. This does not necessarily rule out a stable window of operation, however, since the plasma exists in two distinct states, one having a high hot electron fraction and one having a low hot electron fraction. If it were possible to fix the hot electron fraction at a value intermediate between these two states a stable regime may result.
