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- Papp, Gergely, 1985, et al.
(författare)
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Low frequency sawtooth precursor in ASDEX Upgrade
- 2011
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Ingår i: Proceedings of 5th IAEA Technical Meeting on the Theory of Plasma Instabilities. ; , s. B4.1-
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Konferensbidrag (refereegranskat)abstract
- The present paper describes the precursor activity observed in the ASDEX Upgrade tokamakbefore pronounced sawtooth crashes in various neutral beam heated plasmas, utilizing the soft X-raydiagnostic. Besides the well-known (m, n) = (1, 1) internal kink mode and its harmonics, a Low FrequencySawtooth Precursor (LFSP) mode is studied in detail. Indications of a second, lower frequency sawtoothprecursor have been reported on JET and HT-7 as well. Throughout the studied sawtooth crashes, thepower of the lower frequency mode rose by several orders of magnitude just before the crash with a growthrate of 400 1/s, which is shown to be consistent with the growth rate of a resistive core mode. Besidesits temporal behaviour, its spatial structure was estimated with a wavelet based method used on SXRmeasurements, and the most likely value was found to be (m, n) = (1, 1). Power modulation of this modeis found to correlate with the power modulation of the (1,1) kink mode in the quasi-stationary intervals,and significant bicoherence was measured, both indicating non-linear interaction. The frequency ratio ofthe two modes was calculated with an instantaneous frequency following algorithm and was found to bein the 0.5-0.7 range. The LFSP is expected to play a role in the partial magnetic reconnection process,hence every sawtooth crash model involving such reconnection may be affected by the existence of theLFSP.
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