New insights on divertor parallel flows, ExB drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak \`a Configuration Variable

TL;DR

This study uses 2D Langmuir probe measurements in the TCV tokamak to analyze divertor flows, drifts, and fluctuations, revealing how baffles influence detachment and particle flux behavior.

Contribution

It provides new detailed insights into divertor particle balance, flow contributions, and fluctuation behavior using in situ 2D measurements, especially regarding the effects of divertor baffles.

Findings

Baffles increase neutral pressure and facilitate detachment.

Poloidal flux from ExB drifts can dominate parallel flows.

Fluctuations peak near the X-point during detachment.

Abstract

In-situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuations behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and ExB drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, such that the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.