Direct measurements of counter-streaming flows in a low-shear stellarator magnetic island topology

Abstract

We report on the first experimental verification of theoretically predicted multiple bundles of counter-streaming plasma flows in the island-divertor scrape-off layer (SOL) of the stellarator Wendelstein 7-X. In the standard toroidal field direction (counter-clockwise when looking from the top) experiments, the overall structure of the SOL flows, such as flow directions, the number of flow bundles and the magnitude of the flow velocities, are consistent with numerical predictions obtained with EMC3-EIRENE. However, the modelling does not predict changes of the flow patterns with reversal of the magnetic field direction, which are experimentally observed. This indicates that additional relevant physics, such as particle drifts, will need to be incorporated into the numerical model to better describe the whole stellarator scrape-off layer behaviour.

Coherence imaging spectroscopy at Wendelstein 7-X for impurity flow measurements

“…in ∆Φ c ), as shown in Fig.8(b). Once ∆Φ Zn is applied, like in equation (4), the expected counter-streaming flow pattern [17] becomes once more visible, as shown in Fig.8(c). This implies that the effect introduced by the lamp is possible to correct despite the temperature dependence of the phase.…”

“…The C III photo-emission coefficient peaks for electron temperatures of 10-20 eV, values typically found in the SOL of W7-X [34,35]. Moreover, the magnetic islands are characterized, for standard plasma conditions, by strong pressure gradients along the field lines that enhance the parallel particles flows [17,36]. Accordingly, flows in the magnetic islands are expected to be the major contributors to the measurements.…”

“…In the next sections, the toroidal system will be use as main example, as its typical flow pattern has been already explained in [17]. Flow measurements performed with the vertical system are expected to be topic of future physics…”

“…The integration over multiple regions of different flows and temperatures is translated in a complex superposition of different spectral components. The effective inferred single velocity value is in general not simply the line averaged velocity, as typically assumed in simulations [17]. Consequently, some kind of localization is necessary for carrying the geometrical analysis.…”

“…This new level of accuracy, together with the 2D flow imaging capability of CIS, allowed to gain new information about the effects of the complex 3D geometry of W7-X. In particular, the counter-streaming flows induced by the overall low-shear magnetic island topology have been successfully measured and compared with dedicated EMC3-EIRENE numerical simulations for the first time [17]. Moreover, the behavior of the intrinsic impurities was observed under different plasma scenarios, e.g.…”