Reviewed design of the high heat flux panels for the AUG and W7-X neutral beam calorimeter

Each of the neutral beam injectors in the experimental devices ASDEX Upgrade and Wendelstein 7-X can be equipped with up to four positive ion sources with an injected neutral beam power of 2.5 MW per source. For the conditioning of the system, a movable calorimeter can be placed in the path of the neutral beam to dump the heat load. The main heat load on the calorimeter is absorbed by a set of calorimeter panels (CP). Its design, originally from 1988, specified the lifetime of the CPs to 25,000 heating cycles and ruled out any plastic deformation due to thermal cyclic loading. This was predicted with the tools available at the time but years of operation show that plastic deformation of the CPs already occurs at the very beginning of the calorimeter full power operation. After some years mechanical fatigue has led a few times to water leaks and consequently to NBI system shutdowns for repair. For this reason, careful inspection of the CPs is performed frequently and deformed CPs are exchanged for new ones. This paper presents the analysis of the causes that lead to the failure of the old CPs and proposes an optimized design for new CPs to be manufactured in the near future to substitute the old ones. The new design aims at minimizing mechanical fatigue from thermal cycling by reducing maximum surface temperature and thermomechanical stresses and improving material properties.