Low frequency sawtooth precursor in ASDEX Upgrade

Paper in 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.