Alcator C-Mod Ion Cyclotron Antenna Performance

One of the keys to successful ion cyclotron range of frequency (ICRF) heating is antenna performance. In Alcator C-Mod, we have investigated the compatibility of high power ICRF antennas with high performance plasmas and all high-Z plasma facing components, to provide operational information for future devices such as ITER. Boronization appears to be critical to control plasma radiation but it can be quickly eroded particularly in the presence of ICRF. Here we present circumstantial evidence that suggests RF- enhanced sheaths on flux tubes passing near the antennas terminating on the top of the outer divertor are the most likely erosion mechanism. In addition, antenna operation without a Faraday screen was found to degrade antenna performance through increased impurity production local to the antenna. Analysis suggests that this impurity source might be reduced or mitigated by modifying the antenna strap design. At high neutral pressure, the antenna voltage handling degraded rapidly above a threshold, so-called neutral pressure limit. This degradation was confirmed by benchtop experiments to be related to discharge formation in the vacuum coaxial lines of the antenna feedlines. Reducing the secondary emission coefficient below unity for these regions was shown in tests to substantially raise this limit. 1.0 Introduction Ion cyclotron range of frequency (ICRF) heating is expected to be an important auxiliary heating source for ITER and future fusion reactors. Critical to ICRF utilization is antenna performance and a number of issues can limit the antenna performance including poor voltage and power handling, impurity production, strong RF plasma edge interactions, poor RF coupling, and localized heating of the antenna structure. In this paper, antenna performance will refer to the increase in stored energy per input power which is of course affected by confinement and impurities as well as RF heating efficiency. Alcator C-Mod has developed a set of experimental tools and capabilities that enable unique ICRF compatibility studies. C-Mod's molybdenum plasma facing components (PFCs) allow operational experience with high Z PFCs useful for predicting ITER and reactor situations where tungsten is planned. Utilizing electron cyclotron (EC) resonance discharges for application of the boronization coating allows the boron deposition to be applied over small ranges of major radius. Since the PFC surfaces are molybdenum, the boron coatings can also be removed more readily than for carbon PFCs. The localized nature of the boronization affords an opportunity to identify whether the boron coating must cover all PFCs, localize where the boron coating is most effective, and characterize its lifetime. Further, we have a flexible ICRF system that enables comparisons between antennas. In this paper, we will report results from high power operation with metallic PFC's, Faraday screen-less operation, and an investigation into a mechanism limiting the antenna voltage at high neutral pressure. 2.0 Experimental Description