Precise reconstruction of plasma equilibrium using Alfvén eigenmode spectroscopy

Precise reconstruction of plasma equilibria is essential to the successful operation of a nuclear fusion device; however, the accuracy of the reconstructions is often compromised by large uncertainties in safety factor (q) profiles in the plasma core. On the other hand, reverse-shear Alfvén eigenmodes (RSAE) are often destabilized in advanced plasma scenarios with reversed magnetic shear, and their presence contains important information about plasma equilibrium, this is known as ‘AE spectroscopy’. The radial mode structure of RSAEs peaks near q_min, the minimum q, and mode frequency is sensitive to the evolution of q_min [1]. To routinely extract q_min information from RSAE activity, a model is developed for the best-fitting of experimentally-measured RSAE frequencies to theoretically-predicted ones with minimum human interference. The accuracy of EFIT reconstructions [2] is largely improved by inclusion of RSAE-derived constraints on the radial location and the temporal evolution of q_min .