Fluctuation Measurements of Neoclassical Tearing Mode using Imaging Neutral Particle Analyzer on DIII-D

An upgrade to the Imaging Neutral Particle Analyzer (INPA) on DIII-D has been installed to add higher temporal resolution (1 MS/s) measurements using an array of photomultiplier tubes (PMTs) that view several key areas of energetic particle (EP) phase space. The new detectors measured EP transport resulting from a 5 cm wide Neoclassical Tearing Mode (NTM) that locked to the vessel wall. All three PMTs that detected the mode measured the m/n=2/1 fundamental, and a 3/2 harmonic was also picked up by one detector. Analysis of the data shows that the signal comes from charge exchange events near the edge of the plasma, where the INPA can detect a range of pitches from -0.75 ≤ v_‖/v ≤ -0.5 across the separatrix. Contributions to the signal come from both the prompt transport of the neutral beam ions as well as the slowing down distribution over an energy range from 10 keV to 50 keV. Modelling of the fast ion and edge neutral densities from TRANSP show the signal from the slowing down distribution is stronger for orbits with high pitch. Tracing of orbits through the equilibrium fields shows that the prompt signal is stronger for orbits with lower magnitude pitch.