Absorption in a Hot Plasma near the Second Harmonic of the Electron Cyclotron Frequency

Analysis is presented of 4-mm microwave propagation in the extraordinary mode (for propagation angles of 76°, 90°, and 104° to the external magnetic field), near the second harmonic of the electron cyclotron frequency, across reasonably uniform, highly ionized cylindrical plasma columns (approximately 3, 4, and 5 in. in diameter) confined in a homogeneous (0.05%) magnetic field in the Model C stellarator. Reasonably good agreement with the general predictions of the conventional collisionless hot-plasma theory is obtained. In particular, the measured values of the width, the depth, and the strength of the experimentally detected absorption resonances fall within ±50% of the corresponding theoretically expected values. For these experiments the electron densities are typically in the range of 1011 to 1013 per cm3; the electron temperatures are in the range of 5-35 eV; and the electron collision frequencies ν are such that ν/ω ≲ 5 × 10−5, where ω is the angular frequency of the incident microwave beam.