Theoretical interpretation of frequency sweeping observations in the Mega-Amp Spherical Tokamak

Frequency sweeping (chirping) of high frequency magnetohydrodynamic modes is widely observed in tokamak plasmas. In this paper observations of chirping in neutral-beam-heated plasmas in the Mega-Amp Spherical Tokamak (MAST) [A. Sykes, R. J. Akers, L. C. Appel et al., Nucl. Fusion 41, 1423 (2001)] are considered, and it is shown that these may be interpreted using the Berk–Breizman augmentation of the Vlasov–Maxwell equations. This model includes an energetic particle source: it leads not only to a single chirp but also to a series of bursting events. This repetitious behavior is characteristic of the chirping seen in experiments such as MAST. The similarity between features in velocity space and features in frequency space reinforces the theory that hole-clump pair formation is responsible for the observed frequency sweeping.