New Configurations for High-Field Tokamak ICRF Antenna Couplers

High-field tokamaks offer a cheaper and faster path to fusion power. At the higher magnetic fields ICRH auxiliary heating is the primary proven, and previously fielded, method of heating such tokamaks. However, ICRH auxiliary power also carries a significant disadvantage in terms of its higher rate of high- Z (metallic) impurity generation, which poison the fusion plasma. Recent studies of the antenna coupling in high-field tokamaks hint at a new regime of antenna coupling behavior, available for the first time, in high-field tokamaks. In particular, the higher field and densities closer to the wave launching structures, result in a more "beamö-like radiation pattern from the antenna. This more ôbeamö-like coupling is reminiscent of the direct waveguide coupling from LH and ECRH systems, which do not suffer from the impurity production disadvantage. We propose to look carefully at this behavior, and investigate new design configurations which might take advantage of this recently observed behavior, and thus might reduce or eliminate the primary disadvantage of ICRH, at least for high-field tokamaks. In order to perform these studies, we propose to use our plasma modeling software that includes the ability to simulate ICRF and non-linear RF sheaths, which are at the origin of impurity production. We will further study the previously observed ôbeamö-like coupling properties of ICRH antennas in High- Field Tokamaks, with simulation, modeling, and theoretical analysis. We will look for geometries which are more radiator-like, that might involve simpler geometries, less prone to impurity production. This will include looking carefully at the role of the Faraday screens, and will include analysis of RF sheaths for each of these geometries. Also, we will look at the role of density fluctuations on antenna performance, for classical strap-like configurations, and new configurations which result from this research project. This includes looking carefully at the potential to generate slow waves and surface waves, which might reduce the effectiveness of the antennas. Commercial applications from the Phase I work will focus on the development of new ICRH antenna configurations for the commercial fusion projects. This also includes development of modeling and simulation techniques of value to these efforts. In Phase II, commercial development of the sheath model, which is also of interest for the semiconductor fabrication industry, is an additional goal of this project.