Simultaneous measurement of viscosity and flow velocity in Texas Experimental Tokamak-Upgrade (TEXT-U) edge plasmas by using a Visco-Mach probe

Abstract

Shear viscosity [η≡α×(mass density)×(diffusivity)] and parallel flow velocity [M∞×(Te+Ti)/mi] of tokamak edge plasma were simultaneously measured in the Texas Experimental Tokamak-Upgrade (TEXT-U) tokamak by using a “Visco-Mach” probe with the iteration method (VMPI) [K-S. Chung, Nucl. Fusion 34, 1213 (1994)]. The VMPI was composed of two Mach probes measuring two ratios of ion saturation current densities, with the small Mach probe located within the free presheath generated by the large Mach probe (LMP). Radial variations of the normalized shear viscosity, α, and parallel Mach number, M∞, for typical discharges were deduced from two measured ratios of ion saturation current densities, and plasma density from ion saturation currents by LMP. Here α varied from 0.7 to 1.3 with an estimated error of ±40%, and M∞ from 0 to 0.2 for 0<r−rl<4.5 cm, where rl=27 cm is the limiter radius. Also, α and M∞ increased with the radial direction, while cross-field diffusivity (D⊥) was approximately constant. Here D⊥ is obtained as ∼3DB, where a typical Bohm diffusivity (DB) is ∼1 m2/s.