Resonant heating by microwave power has been used to produce high β plasmas with electron temperatures near 1 MeV. Typically plasmas are produced with ωpe∼ωce. Further experimental heating studies described here have shown that a large increase in stored plasma energy is produced by microwave power with a frequency higher than the cold-electron resonance frequency. This increase, caused by off-resonance heating, is attributed both to stochastic heating and to the control of an instability through changes in the electron distribution function. Alternatively, a decrease in the stored plasma energy is produced by microwave power at a frequency below the cold-electron resonance frequency. This effect is attributed in part to enhanced diffusion into the loss cone. However, a small fraction of the plasma is heated to high energies.
A high-0 hot-electron plasma was studied in an asymmetric magnetic mirror device with a variable cant angle. The plasma was produced by microwave heating at a frequency corresponding to cold-electron-cyclotron resonance together with a higher frequency suitable for upper-off-resonance heating. The position of the high-0 plasma annulus was studied as a function of cant angle and was found to correspond approximately to the position of a midplane modulus-B contour. High-0 effects did modify the spatial location of the losses of energetic electrons into the loss cone but had little effect on the coldplasma losses. Destabilization was not observed when the line-tying of the plasma centre was reduced with glass end plates. An asymmetry of the cold-plasma loss with respect to the equatorial plane was observed at small cant angles and an electric field model was conjectured to explain this behaviour.
The sequence of experiments resulting in the development of steady-state dense hot-electron plasmas is briefly described. These plasmas are produced in magnetic-mirror machines by radiation at the electron-cyclotron frequency. The electron-cyclotron plasma with the greatest stored energy to date has a volume of ∼ 50 liters, an electron “temperature” of 120 keV, a density of 4 — 7 × 1011/cm3, an d an average beta of ∼ 0.4. This plasma is created in the EPA Facility by a 50-kW c.w. 10.6-gHz microwave power source. The construction and operation of the machine are briefly discussed. The analysis and interpretation of the bremsstrahlung and diamagnetic measurements employed to determine these parameters are given. Diamagnetic and particle decay measurements on a smaller machine, the Physics Test Facility, are also described.
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