Poloidally and radially resolved parallel D⁺ velocity measurements in the DIII-D boundary and comparison to neoclassical computations

First measurements of the D⁺ parallel velocity, V _∥D⁺, in L-mode discharges in the DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak boundary region at two poloidal locations, θ∼0° and θ∼255°, made using Mach probes, feature a peak with velocities of up to 80 km/s at the midplane last closed flux surface (LCFS), as high as ten times the charge exchange recombination C⁶⁺ toroidal velocity, V_φC⁶⁺, in the same location. The V_∥D⁺ profiles are very asymmetric poloidally, by a factor of 8-10, and feature a local peak at the midplane. This peak, 1-2 cm wide, is located at or just inside the LCFS, and it suggests a large source of momentum in that location. This momentum source is quantified at ∼0.31 N m by using a simple momentum transport model. This is the most accurate measurement of the effects of so called "intrinsic" edge momentum source to date. The V_∥D⁺ measurements are quantitatively consistent with a purely neoclassical computational modeling of V_∥D⁺ by the code NEO [E. A. Belli and J. Candy, Plasma Phys. Controlled Fusion 50, 095010 (2008)], using V_φC ⁶⁺ as input, for ρ ∼0.7-0.95 at the two poloidal locations, where V_∥D⁺ measurements exist. The midplane NEO-calculated V_∥D⁺ grows larger than V _∥C⁶⁺ in the steeper edge gradient region and trends to agreement with the probe-measured V D⁺ data near ρ ∼1, where the local V_∥D⁺ velocity peak exists. The measurements and computations were made in OH and L-mode discharges on an upper single null, with ion ∇B_T drift away from the divertor. The rotating layer finding is similar in auxiliary heated discharges with and without external momentum input, except that at higher density the edge velocity weakens.