Effect of density fluctuations on ECCD in ITER and TCV

Abstract

Abstract. Density fluctuations near the edge of tokamak plasmas can affect the propagation of electron cyclotron (EC) waves. In the present paper, the EC wave propagation in a fluctuating equilibrium is determined using the ray-tracing code C3PO. The evolution of the electron distribution function is calculated self-consistently with the EC wave damping using the 3-D Fokker-Planck solver LUKE. The cumulative effect of fluctuations results in a significant broadening of the current profile combined with a fluctuating power deposition profile. This mechanism improves the simulation of fully non-inductive EC discharges in the TCV tokamaks. Predictive simulations for ITER show that density fluctuations could make the stabilization of NTMs in ITER more challenging.

Experimental Measurement of ECH Deposition Broadening: Beyond Anomalous Transport

“…Past ECH transport work has often used a rectangular radial deposition function for analytic simplicity [17], but ray-tracing TORAY-GA, as well as C3PO/LUKE code from TCV [7], predict a smoother deposition. In lieu of LUKE/C3PO simulations for DIII-D, which are in progress,this work relies a series of trial blurred P ECH (r) source functions.…”

“…Anomalous transport has been credited for the discrepancy between ray tracing predictions of ECCD profiles and experimentally measured profiles, which are spatially broader [5] [6]. Work at TCV on ECCD [7] casts doubt on anomalous transport as a primary driver of broadening. Certain experimental signatures are expected from the fast electrons generated by ECCD.…”

“…Results from Decker on TCV showing the ECH source function's range of variation over time due to edge fluctuations. Sufficiently rapid shifts compared to observation frequencies cause an effectively broader source function [7]/ tersection of the RF beam with the EC resonance on an edge turbulence timescale, τ < 1ms. Fig.…”

“…ECCD profiles predicted for ITER from the code C3PO/LUKE in the presence of modest edge fluctuation levels σ f ∝ñ e /n e . Reprinted with kind permission of The European Physical Journal [7] ments have also seen indications of broadening on the 110 GHz 2nd harmonic X-mode gyrotron system. Modulated ECH transport experiments by Gentle [10] found a hysteresis effect in critical gradient behavior, but this is minimal if P ECH (r) deposition is taken as ∼ ×2 wider than predicted by TORAY-GA.…”

Finding evidence for density fluctuation effects on electron cyclotron heating deposition profiles on DIII-D

“…The physics of ECH deposition has been studied on a variety of machines [1,2] and the use of predictive ray tracing is viewed as accurate when including a modeled fast radial diffusion factor [3]. However work on TCV [4], as well as theoretical work [1] and past results on DIII-D [5] suggest that ! fluctuations near the edge of a tokamak modify the RF waves passing through them, shifting and broadening deposition in the plasma, with potential deleterious consequences for ECCD mode suppression in ITER [4].…”