Local Measurement of helical flows in the Single Helical Axis state

Results from local flow measurements in helical RFP plasmas challenge previous assumptions that a global m=1 poloidal flow profile is the dominant flow structure. Charge Exchange Recombination Spectroscopy (CHERS) provides the first core-localized measurements of the 3D ion flow structure in Single Helical Axis (SHAx) plasmas. In high current and low density RFP plasmas, the island associated with the innermost resonant tearing mode can grow to sufficient width to envelop the magnetic axis, resulting in a helical equilibrium. The orientation of the helical structure relative to the fixed CHERS diagnostic is controlled using an RMP. Toroidal flow measurements are dominantly m=1 throughout the plasma; however, the axisymmetric component of the flow has two characteristic regions, a flat flow profile in the core, and a highly sheared flow profile outboard from r/a ~ 0.3. Poloidal flows have a small m=1 structure in the core, while the mid-radius has a larger, more complicated structure, with inboard/outboard asymmetry and a dominantly m=2 structure at outboard r/a=0.62. These observations are compared with results from toroidal NIMROD computations with limited toroidal periodicity, which also produce complicated flow patterns in SHAX-like conditions.