Recent upgrades to many of the diagnostic systems on DIII-D (Luxon J.L. 2002 Nucl. Fusion42 614) such as the CO2 interferometer, far-infrared scattering, beam-emission spectroscopy (BES), and quadrature reflectometer have significantly extended their capabilities and made possible the experimental study of Alfvén eigenmodes (AEs) through observation of the AE induced density perturbation. Measurements have revealed the presence of several different classes of AEs in DIII-D discharges including the toroidal Alfvén eigenmode (TAE), reverse shear AE (RSAE or Alfvén cascade) and ellipticity induced Alfvén eigenmode. Based on a simple model for the RSAE frequency, a sensitive diagnostic for the evolution of the minimum magnetic safety factor (qmin) is presented and results are compared with motional Stark effect (MSE) measurements. Strong localization of high toroidal mode number RSAEs to regions near the minimum of the magnetic safety factor is exhibited on the CO2 interferometer and BES measurements. Based on this observation, a method for providing constraints on the radial location of qmin is demonstrated and a favourable comparison to MSE measurements is made. Detailed measurements of TAEs using a new all-digital large bandwidth two-colour CO2 interferometer system show a strong asymmetry between vertical and radial viewing interferometer chords confirming previously reported results. Additionally, effects related to line-integrated observations are clearly illustrated by comparison to local BES measurements and potential issues related to this are discussed.
Internal amplitude, structure and identification of compressional and global Alfvén eigenmodes in NSTX