Simulation of an I-mode pedestal relaxation event on ASDEX Upgrade using BOUT++ code

The I-mode is a promising tokamak operational regime characterized by a high energy confinement and the absence of type-I edge localized modes (ELMs). However, I-mode plasmas occasionally exhibit small ELM-like events known as pedestal relaxation events (PREs), transiently elevating the heat flux on the divertor targets. These PREs have been observed on ASDEX Upgrade (AUG) and Alcator C-Mod. In this work, BOUT++ simulation using the AUG experimental equilibrium and profiles are conducted. The three-field simulation yields a stable outcome, indicating that the PRE profile is peeling and ballooning (P-B) mode stable. In the six-field nonlinear simulations, an I-mode PRE was successfully reproduced in both a qualitative and near-quantitative sense, with PRE characteristics, including time scales, weakly coherent mode (WCM) frequency, and four eigenfrequencies of precursor oscillations (75, 50, 35 and 16 kHz), all exhibiting excellent agreement with experimental observations. Based on the dominant toroidal mode numbers, the entire evolution can be divided into three phases. The first phase is dominated by drift-wave, exhibiting clear WCM characteristics. During the second phase, when the collapse begins to develop, cross-phase analysis reveals a value close to between the potential and electron temperature perturbations, indicating that the interchange mode acts as the direct trigger of the PRE. Further analysis of turbulence and transport confirms that the triggering region is located within the area of WCM turbulence. This work proposes a physical picture of the PRE in which drift-wave turbulence evolves into interchange modes, ultimately leading to a pedestal collapse.