Search

link to homepage

Institute of Energy and Climate Research (IEK)

Navigation and service


Thermal shocks

Materials of the first wall and divertor in next step fusion devices, e.g. ITER or DEMO, must withstand severe steady state and transient heat loads. Candidate materials for an application as plasma facing materials (PFMs) are beryllium (Be), carbon fibre composites (CFC) and tungsten (W). The conditions in a fusion device like ITER (figure 1) can be separated into normal and off-normal events. Normal events are steady state heat fluxes up to 20 MW/m² and transient edge localised modes (ELMs) with heat fluxes of ca. 1 GW/m² and pulse durations of 0.2 – 1 ms. Off-normal events are vertical displacement events (VDEs) and plasma disruptions and are very severe. No material known nowadays can withstand them during long time operation.

Thermal ShocksFigure1: Overview of the expected heat loads and their respective pulse durations of in ITER. The events are separated into ‘normal’ ones which appear during the normal operation of ITER and off-normal ones which are a result of irregularities of the plasma confinement

leeres Bild

The thermal loading conditions are simulated with the electron beam test facilities JUDITH 1 and JUDITH 2 (Juelich Divertor Test Facility in Hot Cells). All candidate materials are tested under different loading conditions varying power density, base temperature, pulse length, and pulse number.
The induced damages on the PFMs strongly depend on these conditions. Figure 2 gives an example for this dependence. The pictures show two tungsten samples which were exposed to the same loading conditions except the base temperature was different. The material shown on the right hand side was tested at room temperature and thermal shock crack networks were formed in the loaded area. The material shown on the left hand side was tested at 400 °C and there were no cracks formed at the surface. At this loading condition we just observed surface modification like roughening of the loaded surface which appears a bit darker than the unloaded surrounding area.

Tungsten sample exposed to 0.63 GW/m² with a pulse length of 1 ms for 100 pulses at room temperature.Figure 2 : Tungsten sample exposed to 0.63 GW/m² with a pulse length of 1 ms for 100 pulses at room temperature.

Tungsten exposed to the same conditions but with a higher base temperature of 400°C.Tungsten exposed to the same conditions but with a higher base temperature of 400°C.

In order to understand the underlying damage mechanisms the so called damage mapping of the thermal shock behaviour of future plasma facing materials can be performed in a very high power density range (up to 1.5 GW/m²) at base temperatures up to 1200 °C. The obtained results are compared with material parameters such as mechanical strength, thermal conductivity and microstructure. The comparison of different materials enable us to identify important parameters which influence the thermal shock response and will therefore help us to improve the candidate materials for an application in future fusion power plants.


Servicemeu

Homepage