Quantifying electron cyclotron power deposition broadening in DIII-D and the potential consequences for the ITER EC system

Abstract

The injection of electron cyclotron (EC) waves fulfills a number of important tasks in nuclear fusion devices for which detailed knowledge of the spatial power deposition profile is critical. This deposition profile is commonly determined using forward models such as beam or ray tracing. Recent numerical and experimental studies have shown that small-angle scattering of the EC beam as it passes through the turbulent plasma edge can cause significant broadening of the effective deposition profile, leading to considerable underestimation of the deposition width by forward methods. However, traditional inverse methods to determine the deposition profile from measurements overestimate the deposition profile width due to transport broadening. In this work, we implement three novel methods to resolve the EC power deposition profile from measurements that counteract transport broadening by simultaneously resolving transport and power deposition. We validate their assumptions and compare the results from these methods to the traditional break-in-slope method as well as to the TORAY ray-tracing code in a set of DIII-D discharges spanning five different confinement modes. We show that the four different inverse methods, novel and established, paint a consistent picture of deposition broadening. Specifically, we show that the measured power deposition profile is between 1.6 and 3.6 times wider than the TORAY profiles. Moreover, we show the considerable consequences that this level of broadening can have for ITER.

Impact of microwave beam scattering by density fluctuations on the electron–cyclotron power deposition profile in tokamaks

“…This broadening can hamper the accuracy of the EC power deposition localization, and the underlying mechanisms must be understood to improve the predictive capabilities of the numerical tools. Recent work on DIII-D [5] has shown that the experimentally reconstructed EC power deposition profile is broader than the one obtained using forward ray-tracing modeling by a factor 1. 6-3.6.…”

Experimental and numerical investigations of electron transport enhancement by electron-cyclotron plasma-wave interaction in tokamaks

“…Recently, this perturbative technique has been applied in DIII-D to infer the EC power deposition profile from electron temperature measurements, comparing different profile inversion methods [27]. It has been shown that, regardless of the underlying mechanism, the measured EC power deposition profile is broader by a factor 1.6-3.6 with respect to forward ray-tracing modeling.…”

Broadening of microwave heating beams in the DIII-D tokamak by edge turbulence

“…The beam size at the resonance is much larger, in small parts due to the divergence of the beam and mostly due to the additional broadening by turbulence (see this paper). Furthermore, given the relative position of launcher and target on the mostly vertical resonance layer and the nearly perpendicular launch of 110 GHz at 2 T in DIII-D (as illustrated in figure 1), the absorption layer thickness aligns along flux surfaces minimizing the impact of deposition width on beam size [23].…”