2D coherence imaging measurements of C²⁺ ion temperatures in the divertor of Wendelstein 7-X

Abstract

For the first time, 2D ion temperature values are derived from coherence imaging spectroscopy (CIS) fringe contrast measurements by taking Zeeman line broadening effects into account during the analysis procedure of a spatial-heterodyne CIS instrument. This allowed 2D images of C 2+ ion temperatures (T i ) across the 3D-shaped island divertor of the Wendelstein 7-X stellarator. Ion temperatures ranging from 10 to 20 eV are observed for the C 2+ impurity species in the region above the divertor targets. During the transition from the attached to the detached plasma state, the C 2+ radiation zone moves from close to the divertor target towards the last closed flux surface. Within this radiation zone, C 2+ temperature does not decrease significantly. Experimentally, the coherence imaging measurements were cross-calibrated at one poloidal cross-section using a high resolution Echelle spectrometer, that shared its sightlines with the coherence imaging diagnostic. The spectra demonstrated that, apart from Doppler broadening, the Zeeman effect significantly contributes to the spectral line broadening and cannot be neglected when analyzing the CIS contrast data for T i extraction in the edge and scrape-off-layer of Wendelstein 7-X (W7-X), due to the relatively low temperatures (T i < 100 eV) and high magnetic fields (B ≈ 2.5 T).

Effect of magnetic geometry on the energy partition between ions and electrons in the scrape-off layer of magnetic fusion devices

“…A 2D coherence imaging spectroscopy measurements indicate that there is a C 2+ radiation zone moving from the region close to the divertor target towards LCFS during the transition from the attached to the detached plasma state. Though the electron temperature decreased going into the detachment, the C 2+ ion temperature was only slightly reduced within the receding C 2+ layer [26].…”

Impurity temperatures measured via line shape analysis in the island scrape-off-layer of Wendelstein 7-X

“…This C III multiplet is one of the most intense impurity emission lines in the SOL of W7-X. Usually, in attached and detached conditions, the C 2+ temperature in front of the divertor is fitted between T i ≈ 10-15 eV [11], as can be seen in the left image of figure 13. Since the S/N ratio is in the medium range, the uncertainty of the C 2+ temperature is ∆T i = ±2 eV (cf table 1).…”

“…This accuracy is sufficient for e.g. observing whether the radiation is close to the targets (as it is usual in attached conditions) or close to the last-closed flux surface (LCFS) during detached conditions, as was investigated in [11]. This is possible because the distance between the targets and the LCFS varies between 17 cm (LOS 1) and 9 cm (LOS 10) along the LOS in the divertor area.…”

“…The individual fine-structure splitting of each spectral transition is altered by the Zeeman effect, which is caused by the confining magnetic field of W7-X (B ≈ 2.5 T on axis). In the SOL, the effective line broadening by Zeeman splitting is of the same order as the Doppler broadening for many spectral lines [11]. This is why Zeeman-broadened spectra are rich in additional features, which may help for identification of the transitions, and have to be considered for accurately analyzing the spectral line shapes.…”

First measurements with a Coherence Imaging Charge Exchange Recombination Spectroscopy (CICERS) diagnostic at Wendelstein 7-X

“…(ii) ζ accounts for the different broadening mechanisms of the spectral lineshape. It can be described as [14,15]:…”