A reduced global Alfven eigenmodes model for Mirnov array data on the H-1NF heliac

Abstract

We extend a reduced-dimension stellarator ideal-MHD normal-mode model to include a vacuum region, and describe magnetic fluctuations in the H-1 heliac. We apply the reduced model to a magnetic field configuration with high density and compute the two lowest frequency global Alfvén eigenmodes (GAEs): (m, n) = (4, 5) and (7, 9). The poloidal mode number, predicted frequency and radial attenuation profile are then compared with measurements from a poloidal Mirnov array. Of the two candidates, the (7, 9) mode has a closer match in frequency and radial attenuation profile, and its narrower radial eigenfunction is a better match to recent optical-emission measurements [18]. Measurements of the temporal evolution of the mode are also consistent with the inferred Alfven frequency scaling at the radial localization of the (7, 9) mode. Combined, these observations suggest the measured fluctuation is a (7, 9) GAE. A wider benefit of our work includes the potential of a reduced-dimension normal-mode MHD model, with negligible computational needs, to characterize and identify mode activity in real time based on frequency, phase information, internal measurements and vacuum region attenuation. Such a model, which is likely to improve in accuracy for devices with simpler configuration such as a tokamak, may be useful as a tool for real time MHD spectroscopy, and to predict and control global eigenmodes associated with fast ion loss in burning plasmas.