(TH/3-1) Interpretation of Mode Frequency Sweeping in JET and NSTX

H.L. Berk1), C. J. Boswell2), D. Borba3), B. N. Breizman1), A. C. A. Figueiredo4), E. D. Fredrickson5), N. N. Gorelenkov5), R. W. Harvey6), W. W. Heidbrink7), T. Johnson8), S. S. Medley5), M. F. F. Nave4), S. D. Pinches4), E. Ruskov7), S. E. Sharapov8), R. G. L. Vann9)
1) Institute for Fusion Studies, University of Texas at Austin, Austin, TX, United States of America
2) Plasma Science and Fusion Center, MIT, Cambridge, MA 02139, United States of America
3) EFDA Close Support Unit, Culham Science Centre, OX14 3DB, United Kingdom
4) Centro de Fusão Nuclear, Associação EURATOM/IST, Instituto Superior Tcnico, Lisboa, Portugal
5) Princeton Plasma Physics Laboratory, Princeton, NJ, United States of America
6) Comp X, Del Mar, CA, United States of America
7 University of California, Irvine; Irvine, CA, United States of America
8) Alfvén Laboratory, KTH, Euratom-VR Association, Sweden
9) University of York, Heslington, York, YO105DD, United Kingdom

Abstract.  Fast frequency chirping, on the millisecond time scale, has been observed in many tokamak experiments observing Alfvénic activity. The cause is generally attributed to energetic particles that produce a kinetic drive for instability that then form holes and clump phase space structures, that must sweep in frequency to balance the intrinsic dissipation present in the background plasma. A general theory, independent of a specific system, has been developed to describe this dynamics. Within the past few years fast chirping has been observed in the excitation of the geodesic acoustic mode in JET and compressional Alfvén 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 phenomena is associated with the formation of phase space structures. The GAM oscillations on JET manifest itself as repeated and prolonged n=0 frequency sweeping behavior which can persist during the entire discharge. This particular excitation demonstrates that the problem of zonal flow can strongly correlate with the formation of kinetic phase space structures. On NSTX fast chirping of the (CAE) is due to the excitation of an ion cyclotron resonance and this signal may be explained by the application of the aforementioned chirping theory. In previous experiments on the Columbia dipole experiment, Terella, chirping signals were quenched by applying rf heating, with the explanation that the heating destroyed the phase space structures. A similar experiment has been attempted to destroy chirping signals observed on NSTX, with negative results at the large rf heating did not change the chirping characteristics. Analysis shows that there may not have been enough power supplied to alter the phase space structures.

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