Interpretation of Mode Frequency Sweeping in JET and NSTX
Fast frequency chirping, on the millisecond time scale, has been observed in many tokamak experiments observing Alfvénic activity. The cause is generally attributed to the presence of energetic particles that produce a kinetic drive for instability that then nonlinearly form hole and/or clump phase space structures, that must sweep in frequency to balance the intrinsic dissipation present from the background plasma. A general theory, independent of a specific system, has been developed to describe this dynamics, and when applicable the theory gives quantitative information about intrinsic plasma characteristics. Until recently the observed fast chirping modes in tokamaks were found to be associated with fishbones and TAE's. Within the last two years fast chirping has been observed in the excitation of other modes: the Geodesic Acoustic Mode (GAM) in JET and the Compressional Alfven wave (CAE) in NSTX. In addition an experimental effort on NSTX is attempting to alter the nature of the observed chirping by introducing additional rf heating and thereby test the hypothesis that the observed chirping phenomenon is associated with the formation of phase space structures.