Phase measurements of fast wave toroidal eigenmodes

An experiment is discussed where phase-resolved techniques were used to investigate the resonances, or eigenmodes, formed when a radiofrequency wave (the "fast" Alfven wave) propagates in a toroidal plasma. A resonant structure is formed, where the parameters of the plasma and the rf excitation frequency plus the physical dimensions of the torus determine where the eigenmodes occur. The experiments were performed on a research tokamak using magnetic probes as the principal wave diagnostic. The signals from these probes were then sent through phase comparators to yield the desired information. It was found that an antenna structure (essentially a coupling loop) which was not insulated from direct contact with the plasma did not launch fast wave eigenmodes. However, the addition of a dielectric layer (glass was used) between the antenna structure and the plasma enabled the excitation of a variety of eigenmodes. The excited modes were then analyzed using the phase comparator circuitry to determine the toroidal and azimuthal mode numbers. The predominant azimuthal mode number was found to be m = +1 and the toroidal mode number was estimated to be either N = 3 or N = 4. The amplitude of the wave vs the location of the second harmonic ion cyclotron resonance layer was also investigated, as was the increased loading resistance seen by the antenna due to the occurrence of an eigenmode.