Combined Steady State and High Cycle Transient Heat Load Simulation with the Electron Beam Facility JUDITH 2

The increasing world energy needs lead to strong efforts in today's energy R&D trying to open up new energy resources. One possible option to access energy in large scale power plants is to use the process of nuclear fusion to generate heat and, from that, electricity with conventional steam turbine technology. However, the realisation is technologically and scientifically very challenging. The heat fluxes that load the inner walls of a fusion device, especially the most severely loaded part, the divertor, are one of the issues currently being under investigation. A distinction is made between steady state heat loads (SSHLs) that are continuously active during operation and transient heat loads (THLs) that are superimposed short-time events. The potentially most harmful THLs during normal operation are type I Edge Localised Modes (ELMs). They are estimated to have a power density of 1 - 10 GW/m² for 0.2 - 0.5 ms duration in the upcoming next step fusion experiment ITER. Because of high pulse repetition frequency more than 1,000,000 ELM events are expected during the foreseen lifetime of divertor components. However, only data regarding behaviour of materials for a low number of pulses (typically 100 - 1000) exists. This work describes the development of a procedure to simulate THLs at high repetition frequency using an electron beam facility and the tests done on tungsten and carbon-based (carbon fibre composite, CFC) plasma facing materials. The developed procedure uses a pulse frequency of 25 Hz, hence actively cooled components are necessary and were designed. A novel electron beam guidance procedure, called circular loading method, was a result of the developmental process. It was used for all later tests because it provides a stabilisation of the applied power density against test parameter fluctuations (e.g. vacuum quality). The electron beam guidance is flexible enough to provide a SSHL pattern during the interpulse time (between two successive THLs) additionally to the THL pulses. This allowed to influence the base temperature of the sample surface. The material tests were done with pulse numbers of 100 - 1,000,000 and absorbed power densities of up to 0.55 GW/m² and 0.68 GW/m² per pulse for tungsten and CFC materials respectively. The surface base temperature was predicted by finite element analyses and monitored by pyrometer measurements. Damage thresholds of the investigated tungsten and CFC were found to be < 0.27 GW/m² and < 0.68 GW/m² respectively. Below these power densities no damage/degradation was found for pulse numbers up to 1,000,000 (tungsten) or 100,000 (CFC). Tungsten showed long term fatigue, which did not occur in CFC. Although it was expected that tungsten would be more resistant at higher base temperatures due to higher ductility, it was found to show earlier degradation at higher temperatures. It is proposed that an increased ductility leads to stronger fatigue damage