Authors: R. Perkins

Abstract: Efficient high harmonic fast-wave (HHFW) heating on the National Spherical Torux eXperiment Upgrade would facilitate experiments in turbulence, transport, fast-ion studies, and more. However, previous HHFW operation on NSTX exhibited a large loss of fast-wave power to the divertor along scrape-o layer (SOL) field lines. It was postulated that the RF field amplitude is being enhanced in the SOL due to cavity-like modes, and that these enhanced fields are driving sheath losses through RF rectification. As part of ongoing work to conrm this hypothesis, we have developed a cylindrical cold-plasma model to identify and understand instances where a substantial fraction of wave power remains confined to the plasma periphery. We previously identified a peculiar class of modes, named annulus resonances, that conduct approximately half of their wave power in the periphery and can also account for a significant fraction of the total wave power. Here, we study the influence of annulus resonances on full three-dimensional reconstructions. We nd instances where the annulus resonant modes dominate the specrum and trap over half of the total wave power in the edge. The work is part of an ongoing effort to determine the mechanism underlying these SOL losses on NSTX, identify optimal conditions for operation on NSTX-Upgrade, and check whether this mechanism could be present in future machines such as ITER.

Submitted to: Nuclear Fusion
_________________________________________________________________________________________________

Download PPPL-5347 2.2 MB (32 pp)
_________________________________________________________________________________________________