Full wave simulations of fast wave efficiency and power losses in the scrape-off layer of tokamak plasmas in mid/high harmonic and minority heating regimes^*

Abstract

Abstract. Several experiments on different machines and in different fast wave (FW) heating regimes, such as hydrogen minority heating and high harmonic fast waves, have found strong interaction between radio-frequency (RF) waves and the scrape-off layer (SOL) region. This paper examines the propagation and the power loss in the SOL by using the full wave code AORSA, in which the edge plasma beyond the last closed flux surface (LCFS) is included in the solution domain and a collisional damping parameter is used as a proxy to represent the real, and most likely nonlinear, damping processes. 3D AORSA results for National Spherical Torus eXperiment (NSTX), where a full antenna spectrum is reconstructed, are shown confirming the same behavior found for a single toroidal mode results in Bertelli et al Nucl. Fusion, 54 083004, 2014, namely, a strong transition to higher SOL power losses (driven by the RF field) has been found when the FW cut-off is removed from in front of the antenna by increasing the edge density. Additionally, full wave simulations have been extended for "conventional" tokamaks with higher aspect ratios, such as the DIII-D, Alcator C-Mod, and EAST devices. DIII-D results show similar behavior found in NSTX and NSTX-U, consistent with previous DIII-D experimental observations. In contrast, a different behavior has been found for C-Mod and EAST, which operate in the minority heating regime unlike NSTX/NSTX-U and DIII-D, which operate in the mid/high harmonic regime.

Filament-assisted mode conversion in magnetized plasmas

“…It also naturally explains the observed origin of the parallel RF power flow from the SOL in front of the antenna rather than from the antenna surface itself 26 . It does not contradict any of the existing proposed explanations: Wavefilament bound states could act in synergy with the "opening" of the boundary plasma to FW propagation 27 , and with sheath rectification and the subsequent parallel spread of DC bias via DC current transport [28][29][30] . In future work, we will use Finite Element calculations to ascertain to what extent the claims in this paper depend on the unrealistic assumptions made in the Mie solution, such as non-circular filament cross sections, steep but continuous density gradients at the filament interface, and nonconstant background density.…”

The importance of realistic plasma filament waveforms for the study of resonant wave-filament interactions in tokamak edge plasmas

“…The origin of the spurious parallel RF power flows in NSTX, which end up on the divertors, is observed 8,34 to be the entire SOL in front of the antenna, stretching from the antenna to the last closed flux surface. Proposed hypotheses to explain this spurious power redirection include both the hypothesis that it is due to propagating Fast Waves in the edge plasma 35 , and the hypothesis that it is due to propagating Slow Waves in the edge plasma 36 . It is not clear that there is a region in the NSTX SOL where the Slow Wave is propagating at all, e.g.…”

Role of Radio Frequency and Microwaves in Magnetic Fusion Plasma Research

“…Then, the study of the coupling of RF power to the edge plasmas through the evanescent layer can be modeled to optimize the power coupling to the plasma. The coupled RF power at the edge of the plasma needs to propagate in a fast-wave form until it reaches the resonance layers [18]; then, the fast wave is converted into electrostatic waves (such as the IBW) to transfer the wave energy to the particles. The modeling for overall wave propagation has been developed so as to understand the heating process while using a full-wave solution.…”

“…The ion heating with RF is not limited to the second ion cyclotron harmonic or hybrid resonance frequencies. The application of higher harmonic frequencies has been successfully demonstrated in a device with a lower magnetic field, such as NSTX [18]. Even higher-RF frequencies (approximately 500 MHz), as a form of helicon waves, is challenged in KSTAR [19] and DIII-D.…”