High spatial sampling global mode structure measurements via multichannel reflectometry in NSTX

Global modes - including kinks and tearing modes (f <∼ 50 kHz), toroidicity-induced Alfvén eigenmodes (TAE; f ∼ 50-250 kHz) and global and compressional Alfvén eigenmodes (GAE and CAE; f >∼ 400 kHz) - play critical roles in many aspects of plasma performance. Their investigation on NSTX is aided by an array of fixed-frequency quadrature reflectometers used to determine their radial density perturbation structure. The array has been recently upgraded to 16 channels spanning 30-75 GHz (n _cutoff = (1.1-6.9) × 10¹⁹ m^-3 in O-mode), improving spatial sampling and access to the core of H-mode plasmas. The upgrade has yielded significant new results that advance the understanding of global modes in NSTX. The GAE and CAE structures have been measured for the first time in the core of an NSTX high-power (6 MW) beam-heated H-mode plasma. The CAE structure is strongly core-localized, which has important implications for electron thermal transport. The TAE structure has been measured with greatly improved spatial sampling, and measurements of the TAE phase, the first in NSTX, show strong radial variation near the midplane, indicating radial propagation caused by non-ideal MHD effects. Finally, the tearing mode structure measurements provide unambiguous evidence of coupling to an external kink.