Low frequency sawtooth precursor in ASDEX Upgrade

Paper i proceeding, 2011

The present paper describes the precursor activity observed in the ASDEX Upgrade tokamak before pronounced sawtooth crashes in various neutral beam heated plasmas, utilizing the soft X-ray diagnostic. Besides the well-known (m, n) = (1, 1) internal kink mode and its harmonics, a Low Frequency Sawtooth Precursor (LFSP) mode is studied in detail. Indications of a second, lower frequency sawtooth precursor have been reported on JET and HT-7 as well. Throughout the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash with a growth rate of 400 1/s, which is shown to be consistent with the growth rate of a resistive core mode. Besides its temporal behaviour, its spatial structure was estimated with a wavelet based method used on SXR measurements, and the most likely value was found to be (m, n) = (1, 1). Power modulation of this mode is 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 of the two modes was calculated with an instantaneous frequency following algorithm and was found to be in 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 the LFSP.