Internal amplitude, structure and identification of compressional and global Alfvén eigenmodes in NSTX

Abstract

Fast-ions (e.g. fusion alphas and neutral beam ions) will excite a wide range of instabilities in ITER and a Fusion Nuclear Science Facility device. Among the possible instabilities are high frequency Alfvén eigenmodes (AEs) excited through Doppler-shifted cyclotron resonance with beam ions. High frequency AEs cause fast-ion transport, correlate with enhanced electron thermal transport and are postulated to contribute to ion heating. These high frequency modes have historically been identified as a mixture of compressional (CAE) and global (GAE) Alfvén eigenmodes, but distinguishing between the CAEs and GAEs has sometimes proven difficult. Identification is essential for understanding the extent of their effect, since the two types of modes have very different effects on resonant particle orbits. The effect on plasma performance of high frequency AEs is investigated in NSTX, facilitated by a recently upgraded array of 16 fixed-frequency quadrature reflectometers. Detailed measurements of high frequency AE amplitude and eigenmode structure were obtained in a high power (6 MW), beam-heated H-mode plasma that is very similar to those in which high frequency AE activity is shown to correlate with enhanced electron thermal transport. These measurements, which extend from the plasma edge to deep in the core, can be used in modelling the effects of the modes on electron thermal transport. The observed modes are identified by comparison of their frequency and measured toroidal mode numbers with local Alfvén dispersion relations. The modes identified as CAEs have higher frequencies (predominantly f> ∼600 kHz) and smaller toroidal mode numbers (|n| ⩽ 5) than the GAEs (predominantly f < ∼600 kHz, n = −6 to −8). Also, they are strongly core localized, in contrast with the GAEs, which also peak towards the plasma centre but have much broader radial extent.