Speaker
Description
A novel spectroscopic technique has been used to perform the direct measurement of impurity poloidal flow in the DIII-D divertor. Particle dynamics in the open-field-line scrape-off-layer (SOL) and divertor, to which both parallel and perpendicular transport contribute, are critical 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 cross-field velocity (< 100 m/s) has remained elusive, despite its expected essential role in the dynamics.
Here, we report the first direct poloidal-flow measurement in the DIII-D upper-outer divertor using Flowmetry via Absorption-Cell-Enhanced Spectroscopy (FACES). To achieve 100 m/s resolution, systematic uncertainties, such as the temperature and humidity drifts, must be compensated. In the FACES technique, a Te2 absorption cell inserted in front of the entrance slit of high-resolution spectrometer embeds molecular absorption lines into the Doppler-broadened impurity emission line from the divertor. These molecular lines serve as an in-situ wavelength ruler. This in-situ calibration enables us to achieve 50 m/s resolution, which is an order-of-magnitude improvement over conventional spectroscopy.
We have measured the flow speed of C²⁺ ions from the C III (465 nm) line with FACES. The data reveal C²⁺ ions streaming from the target toward the X-point at 0.3 - 2.0 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.
This flow direction is consistent with the impurity particle balance: because the C2+ ion source should be located near the target plate and the sink should be upstream, the C2+ ions must flow from the target toward the main plasma. On the other hand, friction with the downward main-ion flow should be large enough to push the impurity ions back to the target plate. A simple one-dimensional model does not account for the large (> 1 km/s) poloidal flow of the impurity ions, indicating the two-dimensional structure must be considered.
In the presentation, we will discuss the relation to the parallel flow and its possible drivers, including frictional coupling to neutral recycling flows, poloidal electric-field, and flow reversal, with the aid of numerical modelling.
*Work supported by the US DOE under contracts DE-FC02-04ER54698 and DE-AC05-00OR22725.