Impurity transport studies at Wendelstein 7-X by means of x-ray imaging spectrometer measurements

Abstract

This paper reports on the effect of on-and off-axis heating power deposition on the impurity confinement in purely electron cyclotron resonance heated He plasmas on the stellarator Wendelstein 7-X. Therefore, impurity transport times τ I have been determined after Fe impurity injections by laser ablations and monitoring the temporal impurity emissivities by the x-ray imaging spectrometer HR-XIS. A significant increase of τ I has been observed when changing the power deposition from on-to off-axis heating with energy confinement times τ E being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power P ECRH and electron density n e has been investigated by keeping the heating power deposition on-axis. The observed τ I scaling compares well to known τ I scaling laws observed in other machines. A comparison of τ I and τ E yields an averaged ratio of τ E /τ I =1.3 and transport times in the range of τ I =40-130 ms and τ E =40-190 ms. Comparing those absolute values to neoclassical predictions supports the recently observed nature of anomalous transport in Wendelstein 7-X, given within the up to now investigated operational parameters.

The importance of the classical channel in the impurity transport of optimized stellarators

“…Recent experimental studies by Langenberg et al. (2018) and Geiger et al. (2019) already point strongly in that direction.…”

Effects of collisions on impurity transport driven by electrostatic modes

“…Recent measurement in the stellarator Wendelstein 7-X indicate that the impurity transport is dominated by turbulent diffusion (Langenberg et al 2018;Geiger et al 2019). Diffusion coefficients two orders of magnitude larger than those calculated from collisional transport have been measured for iron impurities (Geiger et al 2019), and the impurity confinement time appears to be insensitive to the impurity charge number (Langenberg et al 2020) -in contradiction to predictions for collisional transport (Helander & Sigmar 2005).…”

Effects of collisions on impurity transport driven by electrostatic modes

“…, where J 0 is the zeroth order Bessel function of the first kind, and Ω z = ZeB/m z . In (1) and throughout the rest of this paper, gradients are taken with λ = v 2 ⊥ /(Bv 2 ) and v fixed. Given a solution to (1), we calculate the quasi-linear impurity flux using 5…”

“…Recent measurement in the stellarator Wendelstein 7-X indicate that the impurity transport is dominated by turbulent diffusion 1,2 . Diffusion coefficients two orders of magnitude larger than those calculated from collisional transport have been measured for iron impurities 2 , and the impurity confinement time appears to be insensitive to the impurity charge number 3 -in contradiction to predictions for collisional transport 4 .…”