Gyrofluid simulation of an I-mode pedestal relaxation event

The I-mode is an edge-localized mode (ELM)-free confinement regime showing enhanced heat confinement. Nevertheless, also in I-mode, events similar to ELMs—albeit less harmful to the divertor—can occur, relaxing the pedestal gradients. A global electromagnetic gyrofluid simulation of such a pedestal relaxation event (PRE) in I-mode conditions is presented and qualitatively compared with previous measurements of PREs in ASDEX Upgrade [Silvagni et al., Nucl. Fusion 60, 126028 (2020)]. The gyrofluid simulation shows a precursor-oscillation of the characteristic instability of the I-mode, the weakly coherent mode (WCM). Prior to the PRE, the drive of the WCM-precursor changes from drift-wave to interchange. A sudden jump in cross-phase between electron temperature and plasma potential fluctuations at the WCM scale leads to the transport enhancement of the PRE.