Speaker
Description
Runaway electrons (REs) generated during plasma disruptions in tokamak reactors present a challenge due to their capacity to induce severe damage to the plasma facing components (PFCs) [1]. Investigations on the Joint European Torus (JET), operating with the metallic wall (formerly known as ITER Like Wall – ILW) [2] between 2010 and 2023 demonstrated the damaging effect of REs on beryllium (Be) main chamber limiters [3], raising concerns about reactor integrity and operational reliability. Despite the high risk of these events damaging the PFCs, a limited number of experiments have been carried out and results reported.
In the final operational phase of JET, during December 2023, additional experimental sessions were conducted with the focus on RE production, mitigation, impact on PFCs and implications for reactor operations. For this purpose, REs were deliberately triggered using argon (Ar) massive gas injection (MGI), enabling controlled studies of their triggering mechanisms and impact [4]. In the following intervention, high resolution photography of in-vessel components revealed a new toroidal distribution map of the damage induced to the Be inner limiters by these events. Infrared (IR) wide-angle camera assessments, combined with temperature data from thermocouples embedded in Be limiters, revealed that two inner limiters, located approximately 120° apart in the toroidal direction, were impacted by the REs with surface damages in the range of 30 to 40 cm2. Moreover, the analysis showed that for one limiter the damage was primarily from a single RE event, whereas the other showed cumulative damage, resulting from multiple RE impacts. In-vessel Laser Induced Breakdown Spectroscopy (LIBS) [5] with an average spot size of 150 to 200 µm in diameter was performed on both limiters to assess fuel retention and material composition in the damaged and non-damaged areas. During the JET 2024 intervention, damaged tiles from these two limiters were removed for detailed post-mortem analysis. Consequently, a comparative analysis of single event and multi-event RE damage on the Be limiters will be presented, highlighting the morphological and structural properties modifications, material property changes, molten Be behaviour dynamics and the implication to the fuel retention and release.
[1] S. Ratynskaia et al, Plasma Phys. Control. Fusion (2025) DOI 10.1088/1361-6587/ae1c6c
[2] G. F. Matthews et al, Phys. Scr. (2007) T128 137
[3] I. Jepu et al, Nucl. Fusion 64 (2024) 106047 (15pp)
[4] C. Reux et al, Nucl. Fusion 55 (2015) 093013
[5] J. Likonen et al, Nucl. Mater Energy 45 (2025) 102021