Mitigation of RF sheaths through design and implementation of magnetic field-aligned ICRF antenna

Abstract

In ITER and in eventual reactors, enhanced impurity confinement due to internal transport barriers (ITBs) and H-mode operation establishes a very low tolerance for high-Z impurities [1]. Experiments have shown that impurity accumulation increases as power in the ion cyclotron range of frequencies (ICRF) is increased [2]. As a result, one of the primary challenges of ICRF heating is the reduction or elimination of impurities introduced into the plasma during ICRF operation, particularly for tokamaks with high-Z plasma facing components (PFCs). Plasma impurities associated with ICRF auxiliary heating are universally observed [3, 4, 5, 6]. However, the underlying physics of ICRF-specific impurity generation is not well understood, and observations of impurity characteristics differ among various tokamak experiments. Several methods have been proposed to reduce ICRF-specific impurity characteristics: low-Z PFC coatings such as boronization [7]; toroidal phasing of antenna straps [3]; and alignment of antenna Faraday screen elements with the total magnetic field [8]. On Alcator C-Mod we have designed a new magnetic field-aligned ICRF antenna to minimize ICRF-specific impurity characteristics. The field-aligned antenna is rotated 100 from horizontal, such that the antenna straps are perpendicular to the total magnetic field at the edge for a typical plasma discharge (BT ~ 5.4 T, 1, ~ 1 MA). ICRFinduced E-parallel is a likely candidate for producing enhanced sheath voltages that lead to greatly increased sputtering of material surfaces and enhanced impurity edge transport. Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E-parallel in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E-parallel is more modest, suggesting 3-D geometrical effects are important. Multiple antenna phases were analyzed for the field-aligned antenna using finite element method with a 3-D toroidal cold plasma model. In each case, the field-aligned antenna had reduced integrated E-parallel relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing. Initial results suggest that the field-aligned antenna operation results in fewer impurities in the plasma than conventional antennas.