A study of reversed shear Alfvén eigenmodes in Alcator C-Mod with phase contrast imaging

Abstract

Shear Alfvén waves are a fundamental mode of magnetized plasma oscillation and may exist in tokamak plasmas as eigenmodes with a global structure and discrete frequencies. The inhomogeneity of the plasma profiles in conjunction with tokamak geometry tends to focus the Alfvén waves in regions of near uniformity defined by local extrema in the Alfvén continuum, a quantity which describes the local Alfvén resonance frequency for particular mode numbers. Modes excited in the vicinity of these near uniform regions may be weakly damped and excited by energetic ions. The reversed shear Alfvén eigenmode (RSAE), localized deep in the plasma core, is typically associated with a minimum in q in reversed magnetic shear configurations. RSAEs have proven especially useful for MHD spectroscopy, that is the inference of plasma equilibrium properties through their frequency spectra, due to their high sensitivity to the value of qmin. Reversed shear equilibria during the current ramp-up phase and sawtoothing phase are studied through the spatial and temporal characteristics of the RSAEs. Analysis of the spatial structures of RSAEs measured by phase contrast imaging, and interpreted with a synthetic diagnostic using numerical results from the ideal MHD code NOVA, provides constraints on the evolution of qmi,. Additional observations including mode coupling, mode tunneling, and minimum frequency scaling are discussed in light of theoretical and numerical models, with commentary on possible future experiments.