Measurements of ion and neutral atom flows and temperatures in the inner and outer midplane scrape-off layers of the Alcator C-Mod Tokamak

Abstract

Measurements of toroidal flow velocities in the midplane of the inner and outer scrape-off layers (SOL) of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas are made using a high-resolution spectrograph. Doppler shifts of the line emissions from either He+ at 468.6 nm or neutral deuterium at 656.1 nm from two local gas puffs are observed along radially separated, toroidally viewing chords to obtain the radial profiles of the toroidal ion and neutral flow velocities and of the temperatures in the midplane of the inner and outer SOL’s. In these experiments, the ion vertical drift due to the toroidal magnetic field gradient (ion B×∇B drift) is directed towards the lower X-point, and an outer SOL plasma flow with a velocity in the range of 5–10 km/s in the direction of the plasma current is observed at the midplane far above the divertor plates. Plasma flow in a similar direction with similar velocities is observed in the inner SOL midplane. This observation suggests a toroidal rotation of the edge plasma in a co-current direction, consistent with an Er×Bθ motion that would arise from a radially outward electric field. Interestingly, the neutral atoms in these views are observed to flow in the opposite directions in the inner and outer SOL’s, i.e., the neutrals flow in the plasma current direction in the inner SOL and in a direction opposite to the plasma current in the outer SOL. Also, the neutrals move much slower (velocities ∼2–4 km/s) in the outer SOL compared to the inner SOL (velocities ∼4–10 km/s). Temperatures determined from the Doppler broadening of the spectral lines are found to be in the range of ∼15–25 eV for the He+ ions in the inner and outer SOL’s. Much lower temperatures of ⩽2–5 eV are indicated for the deuterium atoms in both SOL’s.