Imaging Neutral Particle Analyzer Measurements of Alfvén Eigenmode (AE) Induced Fast Ion Transport in DIII-D

A new imaging neutral particle analyzer (INPA), which provides unprecedented energy-resolved radial profiles of confined fast ions, has been fielded on the DIII-D tokamak. The INPA measures charge-exchanged energetic neutrals by viewing an “active” neutral beam through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightlines. The incident neutrals are ionized by stripping foils and the local tokamak magnetic field acts as a magnetic spectrometer to disperse ions on the scintillator. A fast camera provides 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA clearly shows fast ion transport in localized regions of phase space due to AEs. Measurements in reversed magnetic shear current ramp experiments have revealed for the first time a large preferential transport of stagnation orbits over passing orbits at nearby radii. The data also show a large net outflow across the plasma midplane during strong AE activity while at lower amplitude, redistribution occurs from the core to larger radii.