Poloidal flow measurement of impurity ions in DIII-D divertor

A novel spectroscopic technique has been used to perform the direct measurement of impurity poloidal flow in the DIII-D divertor. Understanding cross-field transport in the low-temperature open-field-line scrape-off-layer (SOL) and divertor is essential for predicting heat and particle loads on plasma-facing components. While the parallel component of impurity flow is routinely determined from Doppler shifts, quantifying the much smaller poloidal velocity (< 100 m s⁻¹) has remained elusive. We report the first direct poloidal-flow measurement in the DIII-D upper-outer divertor using Flowmetry via Absorption-Cell-Enhanced Spectroscopy (FACES). By inserting a Te₂ absorption cell in front of the entrance slit of high-resolution spectrometer, and analyzing the C III (465 nm) line, we reduce the uncertainty in the line centroid to ≲ 50 m s⁻¹—an order-of-magnitude improvement over conventional spectroscopy. The data reveal C²⁺ ions streaming from the target toward the X-point at 0.3 - 1.5 km s⁻¹, opposite to the direction expected from main-ion parallel flow, ∇B drift, and E × B drift due to the sheath electric field. The possible drivers for this flow are investigated with the aid of numerical modeling, including frictional coupling to neutrals and poloidal electric fields.



*Work supported by the US DOE under contracts DE-FC02-04ER54698 and DE-AC05-00OR22725.