G. Pintsuk

Performance of different tungsten grades under transient thermal loads

Plasma facing components in future thermonuclear fusion devices will be subjected to intense transient thermal loads due to type I edge localized modes (ELMs), plasma disruptions, etc. To exclude irreversible damage to the divertor targets, local energy deposition must remain below the damage threshold for the selected wall materials. For monolithic tungsten (pure tungsten and tungsten alloys) power densities above ?0.3?GW?m?2 with 1?ms duration result in the formation of a dense crack network. Thin tungsten coatings for the so-called ITER-like wall in JET, which have been deposited on a two-directional carbon?fibre composite (CFC) material, are even less resistant to thermal shock damage; here the threshold values are by a factor of 2 lower. First ELM-simulation experiments with high cycle numbers up to 104 cycles on actively cooled bulk tungsten targets do not reveal any cracks for absorbed power densities up to 0.2?GW?m?2 and ELM-durations in the sub-millisecond range (0.8?ms); at somewhat higher power densities (0.27?GW?m?2, ?t = 0.5?ms) cracks have been detected for 106 cycles.

Development of W/Cu—functionally graded materials☆

Abstract Plasma facing components (PFCs) consist of a plasma facing and a heat sink material. These have to fulfil different functions that require different material properties, for example the coefficient of thermal expansion (CTE) of tungsten and copper. Joining of these materials (e.g. by brazing or HIPing) results in the formation of thermal-induced stresses at the interface. Functionally graded materials (FGMs), used as an interlayer, reduce these thermally induced stresses. Two different methods, laser sintering and plasma spraying, have been investigated as a means to produce W/Cu FGMs to be used in PFCs of next step confinement experiments. In addition to mixtures of tungsten and copper powders, 40 wt.% Cu-coated W powder was used to produce W/Cu composites with a content of either 25 or 60 vol.% Cu. The composite microstructure has been analyzed according to Cu content, particle distribution and layer structure. The difference in the behavior of powder mixtures and coated powder is outlined. A comparison of plasma sprayed to commercially produced Cu-infiltrated W samples is made and the results of thermomechanical and thermophysical testing are discussed with respect to different microstructures.

Evolution of tungsten degradation under combined high cycle edge-localized mode and steady-state heat loads

Combined thermal shock and steady-state heat loads (SSHLs) can have an impact on divertor materials and are therefore important for lifetime estimations and evaluations of operational thresholds of divertor components in future fusion devices such as ITER. This paper discusses the results of tests performed in the electron beam facility JUDITH 2 (Forschungszentrum Julich, Germany) on actively cooled tungsten specimens, loaded with edge-localized mode-like thermal shocks (pulse duration 0.48 ms, power densities 0.14–0.55 GW m−2, frequency 25 Hz and up to 1000 000 pulses) either with or without an additional SSHL of 10 MW m−2. The material showed no damage at 0.14 GW m−2 (independent of the SSHL) for up to 250 000 pulses. At a power density of 0.27 GW m−2 (without SSHL), surface roughening occurred at 100 000 pulses, developing into a crack network at 1000 000 pulses. In general, the additional SSHL resulted in an earlier (in terms of pulse number) and more severe material degradation.

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.

The processing of vacuum plasma-sprayed tungsten-copper composite coatings for high heat flux components

Abstract This study investigates the effect of different vacuum plasma spraying conditions on the deposition of composite tungsten–copper coatings. Three different powder conditions were used: copper coated tungsten powder (d50=55 μm W+a 8-μm thick layer of Cu), a pre-mixed powder of tungsten (75 wt.%, d50=5.5 μm) and copper (25 wt.%, d50=35 μm) and the same initial powders of the pre-mixed powder, but not mixed until injected separately to the plasma plume. Additionally, the influence of transferred arc cleaning was studied in all three cases.

Materials for DEMO and reactor applications-boundary conditions and new concepts

DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.

An overview of the comprehensive First Mirror Test in JET with ITER-like wall

The First Mirror Test in Joint European Torus (JET) with the International Thermonuclear Experimental Reactor-like wall was performed with polycrystalline molybdenum mirrors. Two major types of exp ...

Development and testing of a bulk tungsten tile for the JET divertor

The ITER-like wall project has been launched to design, manufacture and test all the necessary components in view of their installation in a dedicated shutdown 2008?09. One of the RD (ii) W lamellae design, which is a mechanical assembly of several stacks of W blades. Both concepts were realized as test tiles and tested under high heat flux loading. The lamellae test tile showed a high performance fulfilling the requirements. Finally, the lamellae concept was chosen to be the primary solution and comprehensive numerical analyses were performed for the further optimization of the concept.

Performance of yttrium doped tungsten under 'edge localized mode'-like loading conditions

Spark plasma sintered tungsten grades, with an yttrium content varying between 0.25 and 1 wt%, were characterized and exposed to transient thermal loads. The samples were cyclic tested at room temperature applying 1 ms long heat pulses using a Nd:YAG laser beam and the electron beam facility JUDITH 1. The absorbed power density of these pulses varied between 0.37 and 1.14 GW m−2. The material modifications were analysed with scanning electron microscopy, optical microscopy and laser profilometry. Comparison showed an improvement of the thermal shock resistance with increasing yttrium content. Additionally, three samples were tested at an elevated base temperature at 400 °C. The two materials with highest yttrium content cracked, indicating still brittle behaviour at the elevated base temperature when adding yttrium.

Melt-layer ejection and material changes of three different tungsten materials under high heat-flux conditions in the tokamak edge plasma of TEXTOR

The behaviour of tungsten (W) plasma-facing components (PFCs) has been investigated in the plasma edge of the TEXTOR tokamak to study melt-layer ejection, macroscopic tungsten erosion from the melt layer as well as the changes of material properties such as grain-size and abundance of voids or bubbles. The parallel heat flux at the radial position of the exposed tungsten tile in the plasma ranges around q|| ~ 45 MW m−2 causing samples to be exposed at an impact angle of 35° to 20–30 MW m−2. Locally the temperature reached up to 6000 K, high levels of evaporation and boiling are causing significant erosion in the form of continuous fine spray or droplet ejection. The amount of fine-spray tungsten emission depends strongly on the material properties: in the case of the tungsten–tantalum alloy the effect of spraying and droplet emission is significantly higher at even low temperatures when compared with regular tungsten or even ultra-high purity tungsten which shows almost no spraying at all. Differences in the material composition, grain structure and size may be related to the different evolution of macroscopic erosion. In addition the re-solidified material is studied and strong differences in terms of re-crystallized grain size and evolution of the grain structure and grain orientation are observed. The build up of large voids has been observed.