Measurement of poloidal velocity on the National Spherical Torus Experiment (invited)

A diagnostic suite has been developed to measure the impurity poloidal flow using charge exchange recombination spectroscopy on the National Spherical Torus Experiment. Toroidal and poloidal viewing systems measure all the quantities required to determine the radial electric field. Two sets of up/down symmetric poloidal views are used to measure both the active emission in the plane of the neutral heating beams and the background emission in a radial plane away from the neutral beams. Differential velocity measurements isolate the line-integrated poloidal velocity from apparent flows due to the energy-dependent charge exchange cross section. Six f/1.8 spectrometers measure 276 spectra to obtain 75 active and 63 background channels every 10 ms. The local measurements from a similar midplane toroidal viewing system are mapped into two dimensions to allow the inversion of poloidal line-integrated measurements to obtain local poloidal velocity profiles. The radial resolution after inversion is 0.6-1.8 cm from the plasma edge to the center. (C) 2010 American Institute of Physics. [doi:10.1063/1.3485027]