Observation of Modes at Frequencies Near the Second Alfvin Gap in TFTR

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Modes has been observed in the frequency range of the second Alfvenic gap in H-minority ICRF heated plasmas in TFTR. This observation is surprising in that the second gap is generally considered to be small in circular cross section plasmas. The mode is inferred to be a core mode, i.e., localized in some sense within the q=1 surface. This follows from the observation that the time dependence of the mode frequency is consistent with the changes in the central density, with the appearance of the mode in the latter part of the sawtooth period when the central fast ion beta ... continued below

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6 p.

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E., Fredrickson.; Van Dam, J.W.; Budny, R.V.; Darrow, D.; Fu, G.Y.; Hosea, J. et al. April 1, 1999.

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Modes has been observed in the frequency range of the second Alfvenic gap in H-minority ICRF heated plasmas in TFTR. This observation is surprising in that the second gap is generally considered to be small in circular cross section plasmas. The mode is inferred to be a core mode, i.e., localized in some sense within the q=1 surface. This follows from the observation that the time dependence of the mode frequency is consistent with the changes in the central density, with the appearance of the mode in the latter part of the sawtooth period when the central fast ion beta has peaked up, and with the direction of propagation, the last of these being explained by a hollow first ion beta profile, which is only present in the core region. The modes are generally not observed during on-axis H-minority heating, but commonly observed during off-axis heating on the high field side (with the resonant layer outside the q=1 surface). A model has been proposed that the beta of the fast ions opens the second gap, allowing instability. For TFTR parameters, the model predicts a gap width of approximately 10 kHz, which is 2.5% of the second gap frequency. If the backwards mode propagation is due to a hollow fast ion profile (as indicated in the TRANSP calculations), then instability due to wave-particle resonance at the magnetic curvature precessional frequency can occur only if the precessional frequency is reversed--which can indeed be the case for off-axis heating on the high field side. Thus, trapped fast ion pressure effects seem to explain several of the observed features of these second gap fluctuations.

Physical Description

6 p.

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INIS; OSTI as DE00005907

Medium: P; Size: 6 pages

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  • 13 Topical Conference on Applications of Radio Frequency Power to Plasma, Annapolis, MD (US), 04/12/1999--04/14/1999

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  • Report No.: Cfpaper-4019
  • Grant Number: AC02-76CH03073
  • Office of Scientific & Technical Information Report Number: 5907
  • Archival Resource Key: ark:/67531/metadc691388

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  • April 1, 1999

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  • Aug. 14, 2015, 8:43 a.m.

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  • April 11, 2017, 1:08 p.m.

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E., Fredrickson.; Van Dam, J.W.; Budny, R.V.; Darrow, D.; Fu, G.Y.; Hosea, J. et al. Observation of Modes at Frequencies Near the Second Alfvin Gap in TFTR, article, April 1, 1999; Princeton, New Jersey. (digital.library.unt.edu/ark:/67531/metadc691388/: accessed September 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.