Initial results from the NSTX Real-Time Velocity diagnostic

A new diagnostic for fast measurements of plasma rotation through active charge-exchange recombination spectroscopy (CHERS) was installed on NSTX. The diagnostic infers toroidal rotation from carbon ions undergoing charge-exchange with neutrals from a heating Neutral Beam (NB). Each of the 4 channels, distributed along the outer major radius, includes active views intercepting the NB and background views missing the beam. Estimated uncertainties in the measured velocity are $<$5{\%} at the maximum sampling rate of 5000 Hz (or $<$1{\%} at 1000 Hz), to be compared with $<$0.5{\%} and 100 Hz of the main NSTX CHERS system. Signals are acquired on 2 CCD detectors, each controlled by a dedicated PC. Spectra are fitted in real-time through a C++ processing code and velocities are made available to the Plasma Control System for future implementation of feedback on velocity. Results from the initial operation during the 2011 run are discussed, emphasizing the fast dynamics of toroidal rotation, $e.g.$ during L-H mode transition and breaking caused by instabilities and by externally-imposed magnetic perturbations.