Effects of magnetic field distribution on power absorption through second harmonic electron cyclotron resonance heating in mirror machines

Y.Tatematsu, T.Saito, I.Katanuma, T.Cho

Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan

Microwaves of 28 GHz are injected in the central and the plug/barrier cells for creating plasma confining potential and raising electron temperature in the GAMMA 10 tandem mirror. Here, fundamental and second harmonic electron cyclotron resonance heating (ECRH) are used. In order to find the best configuration of magnetic field distribution for the power absorption through the second harmonic ECRH, absorption rate and its spatial distribution of the heating microwave power are investigated in detail.
Power absorption of microwave is evaluated by ray tracing calculations for three different magnetic field distributions, which are those in the central, the barrier and the plug regions in GAMMA 10. Power absorption coefficient depends on plasma density, electron temperature and magnetic field distributions along the rays. Plasma density in the central cell is about ten times larger than that in the barrier and plug cell, and the scale length of the magnetic field in the central resonance region is much longer than that in the plug region. Thus, the absorption rate is much larger in the central cell than in the plug/barrier cell for the second harmonic ECRH.
Difference of the magnetic field scale length much affects heating efficiency. Absorption region is narrow along the magnetic axis for plug-ECRH, where local heating is realized. On the other hand, the scale length is so long that the beam width determines the resonance length along the axis for central-ECRH. Heating efficiency for the long scale length is favorable, but it makes difficult to control radial distribution of microwave power absorption. It is found that the absorption rate is very sensitive to the variation in the central cell magnetic field, because the position where the resonance condition is satisfied far shifts against a slight variation of magnetic field strength.
Optical depth of the microwave is proportional to square root of the on-axis electron temperature Te for rays with almost constant magnetic field strength and is proportional to Te for the rays with a large magnetic field gradient. This relation can be explained with analytic formula of ECR-absorption.