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A new ion cyclotron resonance heating system has been installed on the Model C stellarator. Use of low-power rf during the entire Ohmic heating pulse shows that the density for optimum coupling is a factor of 1.5-2 greater than that calculated from a simple theory. The variation of coupling with density and plasma radius is found to agree with theory. An anomalous second maximum in the coupling curve is found. At 1 MW power levels, little energy is transmitted around the ∪ bends by ion cyclotron waves, and it is necessary to have magnetic beaches adjacent to the Stix coil. Best heating is obtained with the least amount of magnetic beach. Use of a larger axial field gradient results in rapid loss of density under the Stix coil, with subsequent decrease in coupling and heating. The maximum ion temperature obtained in the ∪ bends is ∼ 550 eV, uniform to within 20%. A model is proposed in which cooling is dominated by particle losses and charge exchange, the particle loss rate being dependent on kTe.
Rothman et al. (1969) studied this question.