Observations of toroidicity-induced Alfven eigenmodes in a reversed field pinch plasma

High frequency peaks in the spectra of magnetic field signals have been detected at the edge of Extrap-T2R [P. R. Brunsell, H. Bergsåker, M. Cecconello, J. R. Drake, R. M. Gravestijn, A. Hedqvist, and J.-A. Malmberg, Plasma Phys. Controlled Fusion, 43, 1457 (2001)]. The measured fluctuation is found to be mainly polarized along the toroidal direction, with high toroidal periodicity n and Alfv́nic scaling (f∝B mi ni). Calculations for a reversed field pinch plasma predict the existence of an edge resonant, high frequency, high- n number toroidicity-induced Alfv́n eigenmode with the observed frequency scaling. In addition, gas puffing experiments show that edge density fluctuations are responsible for the rapid changes of mode frequency. Finally a coupling with the electron drift turbulence is proposed as drive mechanism for the eigenmode. © 2005 American Institute of Physics.