Mitigation of radio frequency sheaths through magnetic field-aligned ICRF antenna design

One of the primary challenges of auxiliary heating of tokamaks in the ion cyclotron range of frequencies (ICRF) is the reduction of impurities associated with ICRF operation. On Alcator C-Mod, a new magnetic field-aligned antenna was optimized for magnetic flux coupling, power handling, and minimized integrated parallel electric field (E∥). Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E∥ in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E∥ is more modest, suggesting 3-D geometrical effects are important. Using finite element method simulations with a 3-D toroidal cold plasma model, multiple antenna phases were analyzed: [0, π, 0, π], [0, 0, π, π], [0, π, π, 0], [0, 0, 0, 0], [0, π/6, 0, π/6], and [0, π/2, π, 3π/2]. In each case, the field-aligned antenna had reduced integrated E∥ relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing.