UNMITIGATED PLASMA DISRUPTIONS DAMAGE EFFECT TO THE BERYLLIUM LIMITERS IN JET FUSION DEVICE

• I. JEPU
• A. WIDDOWSON
• G.F. MATTHEWS
• J.P. COAD
• J. LIKONEN
• S. BREZINSEK
• M. RUBEL
• G. PINTSUK
• P. PETERSSON
• E. FORTUNA-ZALESNA
• J. GRZONKA
• C. POROSNICU
• O. POMPILIAN
• S. SILBURN
• E. ALVES
• N. CATARINO
• R.A. PITTS
• L. CHEN
• S. RATYNSKAIA AND JET CONTRIBUTORS**

1. INTRODUCTION

Future generation tokamaks, including ITER, will strive for better confinement, which means a higher fusion plasma current to achieve a good Fusion Efficiency Factor value greater than 10 [1]. One of the major threats with this scenario is a potential for increased damage of first wall components due to unmitigated plasma disruptions and production of Runaway Electrons (RE). Two direct consequences of disruptions and REs are:   i) high thermal loads with fast melting and melt motion due to electromagnetic forces as previously seen toroidally distributed in JET [2] and TEXTOR [3] fusion devices and ii) localised melting from RE impact as reported in JET [4]. Aside from damage to components, the production of droplets arising from melting may act as a dust source.  In this contribution, an overview of the melt damage to beryllium first wall components as a direct consequence of unmitigated plasma disruptions and REs in JET ITER-Like Wall is presented. Such beryllium components provide a unique source of material for characterising the impact of melt damage on components and for benchmarking melt damage modelling in ITER. This work combines new findings on layer melting, molten material movement and material loss from the upper dump plates (UDP), as a continuation of the findings presented in [2] part of the 1^st type of damage i), and new unique data on the inner wall beryllium limiters (IWGL) changes as a direct consequence of the REs, part of the 2^nd damage scenario ii).

1. Beryllium melting under high heat loads in unmitigated disruptions

The migration of the beryllium layer along the tile surface during a disruption phenomenon was caused by the j × B forces acting on the molten surface, as evidenced by the damages induced on the UDP beam of tiles and described in [2]. An additional photographic survey was conducted on the entire beam of tiles of the UDPs. The results of the survey show new evidence that molten material can bridge gaps between tiles. Castellation bridging previously reported within a single tile, is also observed across neighbouring tiles (Fig.1a, b), where the gaps between them are larger, with a significant number of resolidified beryllium droplets found in between the DP tiles towards the low field side (Fig. 1c). This shows that long range pathways for the gradual migration of melted material from the high field side to the low field side of the UDP beam is possible. When melted beryllium ultimately reaches the last tile in the beam (DP8), an inversed Be ‘waterfall’ forms and droplets splash onto the outer vessel wall. This photographic survey provides further evidence to support the theory that the inversed Be ‘waterfall’ structure observed in the previous study may consist of beryllium which has migrated along the UPD beam and is not just arising from the immediate melt zone. This supports the findings on mass loss of UDP tiles presented in [2], whereby ~18.2 g (toroidal extrapolation to 64 beams) of Be was estimated to be removed from a high field side UPD tile location (DP2) could partially contribute to the mass of 42 g of beryllium estimated to make up the inverse waterfall structures at the end of the UDP beams.  Another potential mechanism for mass loss from the UDP is “Be rain” following melting. A survey of infrared camera videos has identified 25 pulses with post disruption “Be rain” in the ILW operating period 2013-2014. The impact of “Be rain” on mass loss changes in the UDP will be considered.

1. Runaway electron damage on beryllium plasma facing components

The magnitude of the impact of RE damage was first noted in JET during in vessel inspection surveys periodically carried out and later by high-resolution images taken during a shutdown period. One RE-damaged tile retrieved from JET has been analysed to characterise the damage and benchmark modelling for ITER. A photograph of the damaged tile is shown in Fig.2a. It shows the characteristic localised melting and splashing of RE damage. The impact of RE damage was measured using 3D profiling to generate a surface map, revealing beryllium material dispersion and accumulation arising during melting (Fig.2b). Figure 2 a) RE damaged tile as removed from JET; b) 3D profiling analysis; c) Tile height estimation in poloidal direction as compared with the non-exposed case Initial assessment of the profiling data revealed a change in material height between 1 to 2 mm as shown in Fig.2c; material which was either dispersed generating “valleys” or accumulated generating “hills”. Using the DINA-SMITER-MEMOS-U [5] together with GEANT4 toolkit, modelling of this damage was performed, revealing a good agreement with the post-mortem measurements [6]. Optical microscopy to assess the damage depth was performed after tile cutting, revealing evidence of material bridging gaps between adjacent castellations. No accumulation of molten material penetrating between the gaps was observed. Further analysis to evaluate morphological and structural changes and fuel retention in the damaged regions will be presented to provide a detailed assessment of RE damage on beryllium components. Using thermocouple data, infrared imaging the EFIT plasma equilibrium reconstruction, the exact damaging plasma pulse was identified. The research aims to provide a comprehensive understanding of the effects of unmitigated plasma disruptions on future fusion devices, providing benchmark data for modelling and scaling for the prediction of component damage in ITER. ACKNOWLEDGEMENTS This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion) and from the EPSRC [grant number EP/W006839/1]. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.   REFERENCES

• PITTS R. et al, Physics World 19: 20–26 (2006)
• JEPU I. et al, Nucl. Fusion 59 086009 (2019)
• SERGIENKO G. et al., Phys. Scr. T128 81–86 (2007)
• REUX C. et al Nucl. Fusion 55 093013 (2015)
• COBURN J. et al. Nucl. Mater. Energy, 28, 101016, (2021)
• CHEN L. et al, 5th Asia-Pacific Conference on Plasma Physics, 26.09-1.10.2021