B. Stefanoska (P): Characterization of edge turbulence during ELM suppression by resonant magnetic perturbations in ASDEX Upgrade

• Bojana Stefanoska (IPP Garching-E2 (von HGW))

ELMs pose a major challenge for future fusion reactors due to their potentially unsustainable heat loads on plasma-facing components. Resonant magnetic perturbations (RMPs) can mitigate or suppress ELMs, but the underlying physics of RMP-induced suppression remains unclear. Previous ASDEX Upgrade experiments have shown that the application of RMPs coincides with the onset of a characteristic turbulence regime at the plasma edge, leading to enhanced radial particle transport and a significant reduction of the pedestal density. However, the driving instability, the nonlinear saturation mechanism, and the structure of this turbulence remain open questions. Addressing these open questions experimentally and through collaborations with gyrokinetic modelling groups is the central aim of the PhD project As a first step, the ECE-Imaging diagnostic at ASDEX Upgrade, providing high-resolution, 2D measurements of electron temperature fluctuations, was upgraded and brought back into operation after several years of inactivity. The system was operational during the second half of the 2025 campaign, enabling measurements in several scenarios with an emphasis on ELM-free regimes. Although the planned RMP ELM-suppression experiments could not be fully explored due to operational constraints, the renewed ECEI system delivered new insights into the quasi-coherent mode in QCE and EDA discharges. In parallel, systematic multi-diagnostic analysis of turbulence in previous AUG RMP discharges is ongoing with the aim to identify how RMP-driven turbulence depends on plasma parameters and to inform experimental strategies for the upcoming campaign.

L. van Ham (P): Modelling the effect of magnetic fields on particles in the Wendelstein 7-X neutral beam boxes

• Lucas van Ham (IPP Bereich Greifswald)

Neutral Beam Injection (NBI) is an essential tool for achieving high performance plasmas in the Wendelstein 7-X (W7-X) stellarator. NBI beams are observed to shift when the main magnetic field of W7-X is active, suggesting that stray magnetic fields penetrate the NBI system. This phenomenon potentially restricts NBI operation as it may lead to overheating of certain NBI components. The effect of these stray fields on particle trajectories inside the neutral beam box must be investigated to identify any potential issues for NBI operation, to make the NBI system more reliable, and to develop new magnetic shielding geometries. In this contribution, the development of a computational model to reproduce phenomena inside the NBI system is presented. The model incorporates the finite-element magnetostatics code MUMAT for determining magnetic fields due to magnetized materials, the Monte-Carlo code BEAMS3D for following particles through the resulting fields, and is planned to be expanded with a module handling charge-exchange inside the neutralizer. Application of the model to the W7-X NBI system has produced results which has qualitatively agreed with experimental observations. We discuss ongoing work on quantitatively comparing the model with experimental data from Hall Effect Sensors, infrared imaging, and ion dump calorimetry at W7-X. Lastly, we will briefly discuss the application of the expanded model to develop a basic design of a novel NBI beamline.