Geometrical resonances of helicon waves in an axially bounded plasma

The absorption of a low-power rf signal from various shell antennas was measured in a short plasma column produced by a microwave electron cyclotron resonance source. Both azimuthally asymmetric (straight and helical) and symmetric (m = 0) antennas as well as antennas with rotating field were provided to compare their efficiencies. A multi-peaked dependence of the load resistance on density, which is intrinsic for any antenna, was shown to result from the excitation of axially standing helicon modes. The magnitudes of the absorption peaks were shown to substantially depend on both the antenna design and its position relative to the plasma column. The double half-turn and m = 0 antennas were measured to have a low excitation efficiency. The left- and right-hand helical antennas were found to be of equal impedance. The excitation efficiency of the phased antenna was considerably higher in the case of generation of the right-hand (n = +1) polarized waves. Increasing input power stimulated a transition to a self-sustained helicon discharge which could exist in discrete modes with densities corresponding to the absorption peaks. Gas pressure and matching conditions were shown to substantially affect the antenna coupling to various modes and absorption efficiency. Measured results on the antenna load impedance were compared with computations performed with two alternative models. The model regarding the excitation of the Trivelpiece-Gould (TG) waves yields a reasonable agreement with experimental data. The absorption mechanism is argued to originate in the mode conversion of helicon waves into TG waves. Results obtained with the other model. which neglects the TG waves, were found to be strongly contrary to the measurements.