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Experimental study of second harmonic electron cyclotron resonance heating in an axisymmetric end mirror or a tandem mirror

Second harmonic electron cyclotron resonance heating has been used for the production of hot electrons in an axisymmetric end mirror of the GAMMA 10 tandem mirror. Heating power is injected in the linearly polarized extraordinary mode into a limited area of the mirror field for the purpose of localized heating and temperature control. A ray tracing code and a Fokker-Planck code are used for a preparatory consideration of the heating configuration as well as for an interpretation of the experimental results. The path of the injected power flow is analysed with a simple model and the localized heating is confirmed. The electron temperature is controlled and the plasma in which the hot electrons form a substantial fraction of the total density (up to 0.8) is maintained macroscopically stable. The hot electron production rate is found to depend on the magnetic field strength, the target plasma density, the gas feed rate and the heating power. Several key factors that are important for successful results to be achieved are found. Experimental observations indicate that, essentially, the concept of quasi-linear diffusion of electrons in velocity space holds. It is also found that warm electrons play an important role in microwave absorption as well as in efficient hot electron production.