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Thermal Shock Tests at High Numbers of Pulses

Type I Edge localised modes (ELMs) are transient events at frequencies > 1 Hz that repeatedly load the plasma facing components (PFCs) of a fusion device with high heat power densities (GW/m²) for a short time span (< 1 ms). Under these conditions plasma facing materials (PFMs) will be subject to different degradation processes like (depending on material and conditions) erosion, particle release, surface roughening, recrystallisation, grain growth, cracking, melting, melt layer removal, etc. These degradation processes depend on the number of loading cycles. As many experiments are conducted at low numbers of pulses (1 – 104) it is of special interest to study the impact of a high number (> 104 pulses) of ELM-like heat loads on PFMs as well as the evolution of degradation in the course of pulses. Combination of ELM loading with additional steady state heat loading is used to simulate conditions that come close to ITER-like heat loads.
These experiments are performed with the electron beam facility JUDITH 2 which provides the necessary power density, a high performance active cooling circuit and the flexibility to allow such fast events.

Test specimenFigure 1: shows a typical test specimen with tungsten tiles brazed to a copper heat sink which can be actively cooled

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The energy density per load is shown in figure 2, the resulting damage in figure 3.

Energiedichte pro BelastungspulsAbbildung 2: Die Energiedichte für eine ELM-artige Belastung mit 0,27 GW/m² und einer Pulsdauer von 0,5 ms. Die Probenform wird durch das schwarze Quadrat (unten) dargestellt

Image of a tileFigure 3: Image of a tile loaded with 1,000,000 pulses of 0.27 GW/m² and 0.5 ms pulse duration at 25 Hz repetition rate. The loaded surface is cracked and roughened.

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Testing procedures are also simulated with the finite element simulation program ANSYS. An animation of the results of a thermal analysis shows the temperature development for the experiment in slow motion (~100x slower than real time, figure 4). The first tile is loaded according to the above mentioned parameters, tile 2 and 3 are additionally loaded with a steady state heat flux of 5 MW/m² and 10 MW/m² respectively. The different base temperatures and overall heat flux can lead to different material behaviour.

animation of the results of a thermal analysisFigure 4: Simulation of the surface temperature development during one shot of 0.27 GW/m² and 0.5 ms with different additional steady state heat loads (0, 5 and 10 MW/m²) in slow motion (100x slower than real time).