I. Steudel

Investigation of the impact of transient heat loads applied by laser irradiation on ITER-grade tungsten

Cracking thresholds and crack patterns in tungsten targets after repetitive ITER-like edge localized mode (ELM) pulses have been studied in recent simulation experiments by laser irradiation. The tungsten specimens were tested under selected conditions to quantify the thermal shock response. A Nd:YAG laser capable of delivering up to 32 J of energy per pulse with a duration of 1 ms at the fundamental wavelength λ = 1064 nm has been used to irradiate ITER-grade tungsten samples with repetitive heat loads. The laser exposures were performed for targets at room temperature (RT) as well as for targets preheated to 400 °C to measure the effects of the ELM-like loading conditions on the formation and development of cracks. The magnitude of the heat loads was 0.19, 0.38, 0.76 and 0.90 MJ m−2 (below the melting threshold) with a pulse duration of 1 ms. The tungsten surface was analysed after 100 and 1000 laser pulses to investigate the influence of material modification by plasma exposures on the cracking threshold. The observed damage threshold for ITER-grade W lies between 0.38 and 0.76 GW m−2. Continued cycling up to 1000 pulses at RT results in enhanced erosion of crack edges and crack edge melting. At the base temperature of 400 °C, the formation of cracks is suppressed.

Impact of combined hydrogen plasma and transient heat loads on the performance of tungsten as plasma facing material

Experiments were performed in three different facilities in order to investigate the impact of combined steady state deuterium plasma exposure and ELM-like thermal shock events on the performance of ultra high purity tungsten. The electron beam facility JUDITH 1 was used to simulate pure thermal loads. In addition the linear plasma devices PSI-2 and Pilot-PSI have been used for successive as well as simultaneous exposure where the transient heat loads were applied by a high energy laser and the pulsed plasma operation, respectively. The results show that the damage behaviour strongly depends on the loading conditions and the sequence of the particle and heat flux exposure. This is due to hydrogen embrittlement and/or a higher defect concentration in the tungsten near surface region due to supersaturation of hydrogen. The different results in terms of damage formation from both linear plasma devices indicate that also the plasma parameters such as particle energy, flux and fluence, plasma impurities and the pulse shape have a strong influence on the damage performance. In addition, the different loading methods such as the scanning with the electron beam in contrast to the homogeneous exposure by the laser leads to an faster increase of the surface roughness due to plastic deformation.

Sequential and simultaneous thermal and particle exposure of tungsten

The broad array of expected loading conditions in a fusion reactor such as ITER necessitates high requirements on the plasma facing materials (PFMs). Tungsten, the PFM for the divertor region, the most affected part of the in-vessel components, must thus sustain severe, distinct exposure conditions. Accordingly, comprehensive experiments investigating sequential and simultaneous thermal and particle loads were performed on double forged pure tungsten, not only to investigate whether the thermal and particle loads cause damage but also if the sequence of exposure maintains an influence. The exposed specimens showed various kinds of damage such as roughening, blistering, and cracking at a base temperature where tungsten could be ductile enough to compensate the induced stresses exclusively by plastic deformation (Pintsuk et al 2011 J. Nucl. Mater. 417 481–6). It was found out that hydrogen has an adverse effect on the material performance and the loading sequence on the surface modification.

Impact on the deuterium retention of simultaneous exposure of tungsten to a steady state plasma and transient heat cycling loads

The impact on the deuterium retention of simultaneous exposure of tungsten to a steady-state plasma and transient cyclic heat loads has been studied in the linear PSI-2 facility with the main objective of qualifying tungsten (W) as plasma-facing material. The transient heat loads were applied by a high-energy laser, a Nd:YAG laser (λ = 1064 nm) with an energy per pulse of up to 32 J and a duration of 1 ms. A pronounced increase in the D retention by a factor of 13 has been observed during the simultaneous transient heat loads and plasma exposure. These data indicate that the hydrogen clustering is enhanced by the thermal shock exposures, as seen on the increased blister size due to mobilization and thermal production of defects during transients. In addition, the significant increase of the D retention during the simultaneous loads could be explained by an increased diffusion of D atoms into the W material due to strong temperature gradients during the laser pulse exposure and to an increased mobility of D atoms along the shock-induced cracks. Only 24% of the retained deuterium is located inside the near-surface layer (d<4 μm). Enhanced blister formation has been observed under combined loading conditions at power densities close to the threshold for damaging. Blisters are not mainly responsible for the pronounced increase of the D retention.

Performance of plasma facing materials under thermal and plasma exposure

A relatively clean, safe, and promising solution to cover the globally increasing energy demand but also to avoid energy supply problems could be nuclear fusion. In recent decades, this ambitious project, to make energy generation via fusion possible, demanded a lot of work and the construction of the experimental fusion reactor ITER (latin for ̋the way) in Cadarache, South France, plays an important role to the next big step forward. ITER, a large scale experiment, will demonstrate the scientific and technological feasibility of nuclear fusion and should test all key technologies that are necessary for the next steps, which will be a demonstration power plant (DEMO) and finally a commercial fusion power plant. Furthermore, potential plasma facing materials (PFMs) for in-vessel components have to sustain heat fluxes, neutronic volumetric heating and neutron activation, electromagnetic loads, and environment and safety requirements just to list the most significant ones. At this time beryllium and tungsten are the PFMs in ITER, for DEMO it could be ferritic martensitic steel in addition to tungsten. In this context, this work examines two materials, tungsten und stainless steel, from the material scientific point of view under ITER and DEMO relevant heat and particle fluxes. Building on the results of former works, pure tungsten was exposed in the linear plasma device PSI-2 to sequential and simultaneous transient thermal loads with absorbed power densities up to 0.76 GW/m2 and pure deuterium plasma and deuterium plasma with 6 % helium content, respectively. Furthermore, base temperatures of 400 °C and 730 °C were used and the pulse number was limited to a maximum of 1000 to cover a wide range of loading conditions within the available machine time. The results of this campaign identified that the microstructure, the order of exposure as well as the loading parameters have a substantial impact on the surface modification and damage behaviour and furthermore, that deuterium and helium exacerbates the material performance considerably. In addition, high pulse number tests (≤ 100, 000 pulses) with deuterium plasma background and an absorbed power density of 0.38 GW/m2 were executed to quantify fatigue effects. These experiments led not only to tremendous plastic deformations, microstructural changes like subgrain formation and recrystallisation, but also to the formation of nanostructures and helium induced bubbles below the sample surface. In matters of ITER, where more than 10 transient events are expected, these results indicate severe disturbances of the operation as well as detractions of plasma facing components (PFCs).