Type-I ELM precursor modes in JET

Abstract

Clear low frequency (typically 5–25 kHz) coherent type-I ELM precursor modes have been identified in JET. They are detected through various diagnostics, especially in the ECE and Mirnov signals, but also on the SXR cameras and the multichannel O-mode edge reflectometer. The modes propagate in the direction of the ion-diamagnetic drift and are usually accompanied by a slight rise in the divertor Dα-emission. The precursors show no radial phase inversions and are localized to a few centimetres inside the separatrix, in the pedestal region. The time by which the precursor onset precedes the ELM event varies greatly, and ranges from 0.2 ms up to several tens of milliseconds, though the timescale has no effect on the characteristics of the precursors. During their lifetime, the precursor amplitude is often seen to fluctuate, and the frequency and dominant n-number of the precursors can change rapidly, typically within 1 ms. The range of toroidal precursor mode numbers n which have been observed is 1–13. Parametric studies show that n seems to be mainly prescribed by a combination of the electron pedestal pressure and the (normalized) electron pedestal collisionality , with increasing leading to higher n-numbers. Above a certain edge collisionality (roughly ) the precursors seem to be absent, indicating that a sufficiently high edge current is important to destabilize the precursors. The low-n precursors are known to be external kinks (also known on JET as Outer Modes), while experimental findings and their comparison with stability calculations suggest that the precursor modes with higher n are not pure external kinks but coupled ballooning–kink modes. Several hundred discharges have been analysed, and, in discharges with low to moderate edge collisionality, the precursors could be detected prior to most type-I ELMs, while prior to type-III ELMs the modes were not encountered. In spite of their regular occurrence, there is no evidence of the precursor mode growth rate rapidly accelerating before the ELM, indicating that type-I ELMs on JET are not triggered by these modes.