Modeling the electron cyclotron emission below the fundamental resonance in ITER

Abstract

The electron cyclotron emission (ECE) in fusion devices is nontrivial to model in detail at frequencies well below the fundamental resonance where the plasma is optically thin. However, doing so is important for evaluating the background for microwave diagnostics operating in this frequency range.Here we present a general framework for estimating the ECE levels of fusion plasmas at such frequencies using ensemble-averaging of rays traced through many randomized wall reflections. This enables us to account for the overall vacuum vessel geometry, self-consistently include cross-polarization, and quantify the statistical uncertainty on the resulting ECE spectra. Applying this to ITER conditions, we find simulated ECE levels that increase strongly with frequency and plasma temperature in the considered range of 55-75 GHz. At frequencies smaller than 70 GHz, we predict an X-mode ECE level below 100 eV in the ITER baseline plasma scenario, but with corresponding intensities reaching keV levels in the hotter hybrid plasma scenario. Benchmarking against the SPECE raytracing code reveals good agreement under relevant conditions, and the predicted strength of Xmode to O-mode conversion induced by wall reflections is consistent with estimates from existing fusion devices. We discuss possible implications of our findings for ITER microwave diagnostics such as ECE, reflectometry, and collective Thomson scattering.

Inference of α-particle density profiles from ITER collective Thomson scattering

“…Each receiver is labelled in the following according to the location of the corresponding scattering volume, as illustrated in Figure 3. Note that the measurement volume of Receiver 1 is located on the high-field side of the plasma, and that all receiver views, in particular those close to the plasma edge, are subject to significant refraction due to the proximity of the X-mode L-cutoff [13]. Refraction also acts in the toroidal direction, but this effect is similar for the probe and receiver ray paths, thus maintaining the overlap factors.…”

“…Here we will as default split this into five shorter segments of ∆t = 20 ms, allowing us to average the fit results pertaining to those individual segments and derive a statistical uncertainty, and hence characteristic accuracy, on the results for a 100 ms measurement period. For P b , detailed ECE modelling [13,56] suggests that the ITER ECE background in the baseline scenario should not exceed ∼ 10 eV at the relevant frequencies for realistic reflectivities of the first wall. Here we take P b = 100 eV across all frequencies as a deliberately conservative choice.…”

“…In addition, we set P b = 1 eV and σ sys = σ sd = 0. The motivation for this is partly to remove these sources of uncertainty in this initial test case, and partly that the ECE background at the relevant frequencies might well be negligible [13], while systematic noise contributions can potentially be brought under control and suppressed by a suitable choice of frequency channels. By minimizing these sources of noise, we can isolate any discrepancies between the fitted and true fast-ion densities to three main effects, namely frequency-dependent refraction, degeneracy of the functional dependence on n α and n NBI , and using an analytical model to fit the simulated α-particle contribution.…”

“…As the ITER CTS diagnostic will operate at 60 GHz, below the fundamental electron cyclotron resonance at the nominal ITER field of B t = 5.3 T, evaluation of the anticipated diagnostic background is not straightforward and is associated with some uncertainty. To test the impact of a much higher ECE background, which could potentially be relevant in high-T e ( 30 keV) plasma scenarios [13], we have repeated fit series 2 assuming P b = 2 keV, but with all other parameters unaltered. In the case of 50 frequency channels, this raises the typical statistical noise level from 0.5 eV (our adopted noise floor) to 2.0 eV per channel, and the ECE background then clearly dominates the noise at most frequencies.…”

“…In this context, we note that analysis of CTS measurements in ITER will differ from that of existing CTS implementations in two important ways. First, the diagnostic at ITER will operate in a frequency range of 55-65 GHz in X-mode polarization, below the fundamental electron cyclotron resonance and close to the X-mode L-cutoff in the plasma [4,5,13]. This implies that refraction can play a significant role in distorting the measured CTS spectra.…”