2D measurements of parallel counter-streaming flows in the W7-X scrape-off layer for attached and detached plasmas

Abstract

Investigations of particle parallel flow velocities have been carried out for the scrape-off layer (SOL) of the Wendelstein 7-X (W7-X) stellarator, in order to gain insights on the SOL transport properties during attached and detached plasma scenarios. The experimental evidence is based on the coherence imaging spectroscopy (CIS) diagnostic, able to measure 2D impurity emission intensity and flow velocity. The impurity monitored by CIS is C2+, characterized by a line-emission intensity observed to be linearly proportional to the total plasma radiated power in both attached and detached plasmas. The related C2+ velocity shows a strong dependence on the line-averaged electron density while remaining insensitive to the input power. During attached plasmas, the velocity increases with increasing line-averaged density. The tendency reverses in the transition to and during detachment, in which the velocity decreases by at least a factor of 2. The sharp drop in velocity, together with a rise in line-emission intensity, is reliably correlated to the detachment transition and can therefore be used as one of its signatures. The impurity flow velocity appears to be well coupled with the main ions’ one, thus implying the dominant role of impurity-main ion friction in the parallelimpurity transport dynamics. In view of this SOL impurity transport regime, the CIS measurement results are here interpreted with the help of EMC3-Eirene simulations, and their major trends are already explainable with a simple 1D fluid model.

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…During the detachment phase, the CIS diagnostic measures consistently decreased flow velocities, observed for the entire counter-streaming flow pattern typical of the magnetic island chain as visible in figure 17. This velocity drop is considered to be a consequence of the decrease in power available for ionization in the SOL [66]. A similar flow pattern has also been simulated [20] for W7-AS, where the confined plasma volume was surrounded by nine islands forming the island divertor.…”

“…the so-called friction dominated impurity transport regime [20]. Under this assumption, the CIS results are explained with a simple 1D fluid model, in which the ionization source term plays a central role, suggesting that the ionization source is the main contributor to the particle dynamics in the region where the measurements take place [66].…”

Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

“…Detailed two-dimensional patterns of particle flow parallel to the magnetic field have now been measured in the SOL of W7-X using a coherence imaging spectroscopy (CIS) diagnostic [29,30,33,34]. Multiple counter-streaming C 2+ impurity flows are seen-and in most cases closely agree with numerical predictions.…”

“…[28]), thanks to measurements from a suite of diagnostics (e.g. [29][30][31][32]) as well as further modeling. The discharge of figure 1 approached steady-state in terms of the overall particle inventory, including the substantial inventory of hydrogen in the mostly graphite-based plasmafacing components, and toward the end of the discharge, the measured rate of particle exhaust through the divertor turbopumps was almost equal to the actively injected particle rate, as seen in figure 2.…”

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

“…Especially during detachment, EMC3-Eirene simulations indicate a radiation belt around the entire LCFS [19]. During attachment, filtered camera data show C 2+ radiation usually dominant in the divertor area close to the pumping gap, and much less intense in the islands [34].…”