Fusion Plasma Diagnostics with mm‐Waves

Plasma turbulence, and edge density fluctuations in particular, can under certain conditions broaden the cross-section of injected microwave beams significantly. This can be a severe problem for applications relying on well-localized deposition of the microwave power, like the control of MHD instabilities. Here we investigate this broadening mechanism as a function of fluctuation level, background density and propagation length in a fusion-relevant scenario using two numerical codes, the fullwave code IPF-FDMC and the novel wave kinetic equation solver WKBeam. The latter treats the effects of fluctuations using a statistical approach, based on an iterative solution of the scattering problem (Born approximation). The full-wave simulations are used to benchmark this approach. The Born approximation is shown to be valid over a large parameter range, including ITER-relevant scenarios.

Section: Numericsmentioning

confidence: 98%

Microwave beam broadening due to turbulent plasma density fluctuations within the limit of the Born approximation and beyond

Köhn

Guidi

Holzhauer

et al. 2018

“…2, Doppler reflectometer measurement locations are in the E r minimum (ρ pol ≈ 0.995). Due to the density gradient, they have a radial extent of roughly 2 mm [37]. Figure 9 shows probability density functions (PDFs) of the turbulence amplitude at different k ⊥ .…”

Section: B Dependence On Structure Sizementioning

confidence: 99%

The I-mode confinement regime at ASDEX Upgrade: global properties and characterization of strongly intermittent density fluctuations

Happel¹,

Mänz²,

Ryter³

et al. 2016

Properties of the I-mode confinement regime on the ASDEX Upgrade tokamak are summarized. A weak dependence of the power threshold for the L-I transition on the toroidal magnetic field strength is found. During improved confinement, the edge radial electric field well deepens. Stability calculations show that the I-mode pedestal is peeling-ballooning stable. Turbulence investigations reveal strongly intermittent density fluctuations linked to the weakly coherent mode in the confined plasma, which become stronger as the confinement quality increases. Across all investigated structure sizes (k ⊥ ≈ 5 -12 cm −1 , with k ⊥ the perpendicular wavenumber of turbulent density fluctuations), the intermittent turbulence bursts are observed. Comparison with bolometry data shows that they move poloidally toward the X-point and finally end up in the divertor. This might be indicative that they play a role in inhibiting the density profile growth, such that no pedestal is formed in the edge density profile.

“…In general, a ray travelling through a plasma along a path s from position s 1 to s 2 will experience a change in intensity or, equivalently, radiation temperature T rad . In the Rayleigh-Jeans regime ( ω ≪ kT e ) relevant for microwaves, this change is described by [20,21] T rad (s 2 , ω) = T rad (s 1 , ω)e −τω(s1,s2)…”

Section: Raytracing In An Optically Thin Plasma With Wall Reflectionsmentioning

confidence: 99%

Modeling the electron cyclotron emission below the fundamental resonance in ITER

Rasmussen

Stejner

Figini

et al. 2019

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.