Measurements of the complex plasma loading impedance of the magnetosonic cavity modes

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Abstract
The complex plasma loading impedance of the fast magnetosonic wave in the Caltech tokamak has been investigated at low power (between 10 and 300 W). The magnetosonic cavity modes are observed both as voltage maxima at the output of a six‐turn receiving probe, and as maxima in the input resistance of the transmitting antenna. When the real and imaginary parts of the measured plasma loading impedance are plotted in the complex impedance plane, the resulting curves are approximately circles, indicating a resonance phenomenon. The observed plasma loading resistance at the various cavity modes is as high as 3 to 4 times the basic antenna resistance (∼0.4 Ω). This means that efficient energy coupling into the tokamak and low losses in the antenna system are possible. The estimated cavity Q’s are between 400 and 700.