ISSN:
1089-7666
Source:
AIP Digital Archive
Topics:
Physics
Notes:
An experimental study of the velocity-space diffusion of electrons in an electron cyclotron resonance heated (ECRH) mirror plasma, in the presence of microunstable whistler radio frequency (rf) emission, is presented. The dominant loss mechanism for hot electrons, with temperatures Th ∼400 keV, is end loss produced by rf diffusion into the mirror loss cone. In a case with 4.5 kW of applied power, this loss limits the stored energy to 120 J with a hot electron energy confinement time τEh ∼40 msec. The corresponding value associated with collisional scattering is 320 msec. Whistler microinstability rf induces up to 25% of the endloss. The hot electron temperature is limited by rf-induced end loss of high-energy electrons, and decreases with increasing rf power in strong diffusion regimes. Measurements of collisional loss agree with standard scattering theory. Weaker diffusion is seen in experiments in which the vacuum chamber walls are lined with microwave absorber than in experiments with reflecting walls. The presence of absorber produces two effects: (1) a smaller resonant region in velocity space due to a narrower spatial rf profile, and (2) a decreased level of random wall-reflected rf fields, which could lead to an enhancement of super-adiabatic effects. Other superadiabatic effects, however, such as the existence of a "warm'' electron component with Tw∼Es and a decrease of the heating rate as Th rises above Es, where Es∼30 keV is the calculated limit to fully stochastic diffusion, are not seen in the experiment.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.859011
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