Velocity Profile Shapes in Alcator C-Mod Plasmas

Abstract

Abstract Toroidal rotation velocity spatial profiles (r/a $<$ 0.8) have been obtained from C-Mod over a wide
range of operational conditions, including H-mode, I-mode, ICRF-heated L-mode and Ohmic L-mode (LOC and SOC), and in
plasmas with ITBs, LH wave injection and MCFD. Peaked, flat and hollow rotation profiles have been
observed. In H- and I-mode plasmas, generally with co-current
peaked profiles, the peaking is correlated with temperature profile peaking, and both increase with
toroidal magnetic field (decrease with $\rho_*$). Any dependence on density peaking is unclear.
For Ohmic L-mode discharges, with LOC, the velocity profiles are usually
flat and most often directed co-current, while with SOC the profiles are hollow, mostly co-current at the edge
and counter-current in the core. Both of these Ohmic rotation states exist with matched density and temperature
profiles (and gradients), indicating that neither gradient is relevant during rotation reversals. For plasmas
with LH wave injection and discharges with ITBs, the velocity profiles are hollow while the density and temperature
profiles exhibit substantial peaking. Broadly speaking for all operational regimes, there is no unifying ordering of the
velocity gradient with plasma parameters.