May 8-10

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Author: Samuel J Frank
Requested Type: Consider for Invited
Submitted: 2023-03-24 14:42:13

Co-authors: J.C. Wright, P.T. Bonoli

Contact Info:
MIT
77 Massachusetts Avenue
Cambridge, MA   02139
USA

Abstract Text:
The lower-hybrid current drive (LHCD) spectral gap problem has existed since the first radiofrequency current drive experiments in tokamaks. The spectral gap refers to situations where the lower-hybrid (LH) wave’s launch phase velocity is much greater than phase velocity where significant Landau damping occurs. Under these conditions, the LH wave should be weakly damped and produce inefficient current drive. Despite this, the past 40 years of LHCD experiments show LHCD can efficiently drive current when a spectral gap is present. These observations are thought to be the result of the spectral gap closure by mechanisms which downshift the LH wave’s phase velocity such as turbulence, toroidal magnetic geometry, and diffraction. However, raytracing simulations used to predict LHCD usually include only toroidal geometry effects, omitting other spectral gap closure mechanisms, and often cannot replicate the LHCD efficiencies and profiles measured in experiments. A frequently cited source of the error in the raytracing simulations is the lack of “full-wave” effects like diffraction that can contribute to gap closure, but this claim has never been thoroughly investigated. Here, we tested the full-wave gap closure hypothesis using groundbreaking simulations of non-Maxwellian LHCD; coupling TORLH, a massively parallel semi-spectral full-wave code, to the CQL3D Fokker-Planck code self-consistently. The TORLH/CQL3D results were compared to GENRAY/CQL3D raytracing simulations, and in most situations, raytracing accurately reproduced full-wave simulation results. Full-wave effects usually did not meaningfully contribute to the spectral gap closure process. We demonstrate that this result is actually in good agreement with the predictions made by analytic theory, and LHCD’s interactions with the plasma in the edge region provide more robust explanation for spectral gap closure.

Supported by SciDAC Contract DE-SC0018090 and Department of Energy Grant DEFG02-91ER54109

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