High resolution MHD mode structure measurements via multichannel reflectometery in NSTX*

MHD activity [e.g. Alfv\'{e}n eigenmodes (AE) and tearing modes] plays a critical role in many aspects of plasma performance. AEs, for instance, can significantly impact fast-ion transport in neutral beam heated plasmas. The investigation of MHD activity in NSTX has been aided by an array of fixed-frequency quadrature reflectometers used to determine the radial density perturbation structure of a variety of modes. Recently, the array was upgraded from 5 to 16 channels. The maximum frequency, which was 50 GHz, was also increased, so it now spans 30 -- 75 GHz ($n_{cutoff}$ = 1.1 -- 7.0 x 10$^{19}$ m$^{-3}$ in O-mode). The upgrade improves radial resolution in structure measurements, while also allowing access to higher density plasmas, including, in particular, H-mode plasmas. Structure measurements highlighting these capabilities are illustrated for a variety of MHD modes, including tearing modes, energetic particle modes and internal kinks (f$\sim $1~--~25 kHz), reverse shear and toroidicity-induced AEs (f$\sim $50 --~200 kHz) and compressional and global AEs (f$\sim $0.5 --~2.5 MHz).