Plasma profiles and flows in the high-field side scrape-off layer in Alcator C-Mod

Abstract

A novel, magnetically-driven swing probe was recently installed near the midplane on the high-field side SOL in Alcator C-Mod. The probe collects plasma from co- and counter-current directions during its respective 0–90° and 90–180° of motion, thus providing profiles of density, electron temperature and plasma flow parallel to magnetic field lines (Mach number, M∥) up to the separatrix. Results are reported from discharges with different magnetic topologies: lower single-null, upper single-null, and double-null. In single-null, a strong parallel flow (∣M∥∣∼1) is detected, which is always directed from the low- to high-field SOL. In double-null discharges, e-folding lengths in the high-field SOL are a factor of ∼4 shorter than the low-field SOL. Thus, plasma appears to ‘fill-in’ the high-field SOL in single-null plasmas, not by cross-field transport but by parallel flow from the low-field SOL – a picture consistent with a very strong ballooning-like component to the cross-field transport.