Excitation of Beta-induced Alfven-acoustic eigenmodes and q-profile MHD spectroscopy in NSTX.

We report on observations and interpretations of a new class of global MHD eigenmode solutions arising in gaps in the low frequency Alfv\'{e}n -acoustic continuum below the geodesic acoustic mode (GAM) frequency. These modes have been just reported [N. N. Gorelenkov {\em et.al.}, Phys. Letters A, doi:10.1016/j.physleta.2007.05.113 (2007)] are the result of coupling of the Alfv\'{e}n and acoustic continua due to geodesic curvature. We show good quantitative comparison of the theory predictions with recent NSTX experiments on the observations of these modes, referred to as Beta-induced Alfv\'{e}n - Acoustic Eigenmodes (BAAE). They exist near the extrema points of the Alfv\'{e}n - acoustic continuum and can sweep up in frequency from zero value in the plasma frame as q-profile relaxes. We show that the measurements of BAAE frequency can be used to infer $q_{min}$ in both reversed and monotonic q-profile plasmas. In NSTX experiments we see a correlation of the MSE measured q-profile evolution and BAAE instabilities. TRANSP code is used to understand q-profile evolution by comparing it to classical predictions.