ICRF Antenna Coupling and Wave Propagation in a Tokamak Plasma

Citation

Greene, Glenn Joel (1984) ICRF Antenna Coupling and Wave Propagation in a Tokamak Plasma. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ppan-gp26. https://resolver.caltech.edu/CaltechETD:etd-11102005-144119

Abstract

A variety of experiments are reported pertaining to the excitation, propagation, and damping of waves in the ion cyclotron range of frequencies (ICRF) in the Caltech Research Tokamak.

Complex impedance studies on five different RF antennas addressed the nature of the anomalous density-dependent background loading observed previously in several laboratories. A model was proposed which successfully explained many of the observed impedance characteristics solely in terms of particle collection and rectification through the plasma sheath surrounding the antenna electrode. Peaks were observed on the input resistance of the shielded antennas and were coincident with toroidal eigenmode production; their magnitude was explained by a simple coupling theory.

The toroidal eigenmodes were studied in detail with magnetic field probes. The mode dispersion curves in density-frequency space were mapped out and the results compared with various theoretical models. A surprising result was that all of the antennas, both magnetic and electric in nature, coupled to the eigenmodes with comparable efficiency with respect to the antenna excitation current. Wave damping was investigated and found to be considerably higher than predicted by a variety of physical mechanisms. A numerical model of the wave equations permitting an arbitrary radial density profile was developed, and a possible mechanism for enhanced cyclotron damping due to density perturbations was proposed. Toroidal modes were identified using phase measurements between pairs of magnetic probes; they were found to have m = 1 poloidal character and low integral toroidal mode numbers, in accordance with theoretical predictions.

A new approach to the study of ICRF wave propagation was investigated: wave-packets were launched and their propagation was followed around the tokamak using magnetic probes. This technique avoided the dominant effect of the eigenmode resonances because it observed propagation on a time scale short compared to the formation time for the modes. The transit time of the packets around the machine yielded the toroidal group velocity, and the results of the experiments were compared with several theoretical models. The inclusion of a vacuum layer at the plasma edge was useful in explaining some of the observations.

Finally, a plasma-compatible Rogowski current probe was developed and used to observe, for the first time, RF particle current in a tokamak plasma. The diagnostic permitted investigation of the spatial form of the RF current driven in the edge plasma by the electric field antennas. The results dramatically showed that the current from these antennas flows largely along the toroidal field lines. This highly localized current distribution suggests a mechanism for the good coupling to the eigenmode fields observed with these antennas.

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