Abstract
Fast-wave heating will be a major heating scheme on ITER, as it can heat ions directly and is relatively
unaffected by the large machine size unlike neutral beams. However, fast-wave interactions with the plasma edge can
lead to deleterious effects such as, in the case of the high-harmonic fast-wave (HHFW) system on NSTX, large losses of
fast-wave power in the scrape off layer (SOL) under certain conditions. In such scenarios, a large fraction of the lost
HHFW power is deposited on the upper and lower divertors in bright spiral shapes. The responsible mechanism(s) has
not yet been identified but may include fast-wave propagation in the scrape off layer, parametric decay instability, and
RF currents driven by the antenna reactive fields. Understanding and mitigating these losses is important not only for
improving the heating and current-drive on NSTX-Upgrade but also for understanding fast-wave propagation across the
SOL in any fast-wave system. This talk summarizes experimental results demonstrating that the flow of lost HHFW
power to the divertor regions largely follows the open SOL magnetic field lines. This lost power flux is relatively large
close to both the antenna and the last closed flux surface with a reduced level in between, so the loss mechanism cannot
be localized to the antenna. At the same time, significant losses also occur along field lines connected to the inboard edge
of the bottom antenna plate. The power lost within the spirals is roughly estimated, showing that these field-aligned
losses to the divertor are significant but may not account for the total HHFW loss. To elucidate the role of the onset layer
for perpendicular fast-wave propagation with regards to fast-wave propagation in the SOL, a cylindrical cold-plasma
model is being developed. This model, in addition to advanced RF codes such as TORIC and AORSA, is aimed at
identifying the underlying mechanism(s) behind these SOL losses, to minimize their effects in NSTX-U, and to predict
their importance in ITER.
