Theory of electron cyclotron heating in the AFT torsatron

Abstract

The heating of torsatron devices by electron cyclotron waves has been investigated using ray tracing techniques. The geometry of the magnetic field and mod-B contours in torsatrons is very different from that found in tokamaks or in most conventional stellarators. The sensitivities of electron cyclotron heating to antenna position, antenna orientation, wave polarization and magnetic field strength are therefore correspondingly different. These sensitivities are identified and quantitative calculations of wave propagation, absorption and radial heating profiles are carried out to develop a systematic picture of heating in these devices. A simple model is introduced which statistically treats the effect of power reflected from the walls. Specific ray tracing calculations were performed using the RAYS geometrical optics code for the Advanced Toroidal Facility torsatron now under construction at Oak Ridge National Laboratory. Significant power absorption (up to 100% of incident wave power) can be obtained with an optimally placed and oriented beam. This is the case when the resonance is at the saddle point in the magnetic field. These results can be achieved through either ordinary mode waves at the fundamental resonance (high or low field launch) or extraordinary mode waves at the second harmonic resonance (low field launch). These results are quite sensitive to antenna placement, antenna orientation and the magnetic field level. Depending on experimental constraints (such as high field or inside accessibility), other considerations can come into play; for example, there are theoretical advantages to launching the waves horizontally in the equatorial plane. The qualitative results from the wall reflection model indicate that polarization control and maximization of the first-pass absorption are more important when the central plasma is resonant near the fundamental resonance thanwhen it is resonant at the second harmonic. Finally, a comparison of the calculations with data from the Heliotron-E torsatron is made and found to be in qualitative agreement.