Electron Cyclotron Heating Modification of Alfvén Eigenmode Activity in the DIII-D and ASDEX-Upgrade Tokamaks*

Joint experiments on the DIII-D and ASDEX-Upgrade tokamaks demonstrate that electron cyclotron heating (ECH) can have a dramatic effect on Alfvén eigenmode activity in fusion plasmas, with the actual outcome depending sensitively on ECH deposition location, discharge conditions and fast ion source.  The most common effect in DIII-D plasmas is a shift in the observed modes from a mix of reversed shear Alfvén eigenmodes (RSAEs) and toroidicity induced Alfvén eigenmodes (TAEs) to a spectrum of only weak TAEs when ECH is deposited near the shear reversal point. Experiments in AUG have found a similarly large impact on beam driven AEs including examples with the complete stabilization of both RSAEs and TAEs by ECH.  The ECH impact on RSAEs results primarily from an increase in the local electron temperature and its gradient at q_min, which causes an increase in both the geodesic acoustic frequency and RSAE frequency to a point where the RSAE has a much reduced frequency sweep range or is no longer an eigenmode of the system.  A simple q-evolution model that includes these effects on the RSAE frequency sweep is in agreement with measurements and captures the relative balance of TAE or RSAE-like modes in a broad range of discharges. Additionally, MHD and gyrofluid calculations confirm this interpretation and show both modification of plasma pressure and pressure gradient play an important role in altering RSAE activity. Recent AUG experiments have extended these results to include the impact of ECH on TAEs driven by ion cyclotron heating accelerated ions.  In these experiments detailed analysis showed ECH actually increased TAE amplitudes due to a locally enhanced slowing down time / fast ion pressure.  The ECH modification of AE activity is of practical importance as both devices have significantly reduced fast ion transport with weaker modes.