M. Wirtz

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.

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.

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.

The effect of high-flux H plasma exposure with simultaneous transient heat loads on tungsten surface damage and power handling

The performance of the full-W ITER divertor may be significantly affected by the interplay between steady-state plasma exposure and transient events. To address this issue, the effect of a high-flux H plasma on the thermal shock response of W to ELM-like transients has been investigated. Transient heating of W targets is performed by means of a high-power Nd:YAG laser with simultaneous exposure to H plasma in the linear device Magnum-PSI. The effects of simultaneous exposure to laser and plasma have been compared to those sequentially and to laser only. Transient melting is found to be aggravated during plasma exposure and to occur at lower heat flux parameters. Roughness and grain growth are observed to be driven by peak temperature, rather than by the loading conditions. The temperature evolution of the W surface under a series of transients is recorded by fast infrared thermography. By accounting for changes in the reflectivity at the damaged surface as measured by ellipsometry, a reduction in power handling capabilities of the laser/plasma affected W is concluded. The evidence of reduced power handling of the W surface under conditions as described here is of great concern with respect to the durability of W PFCs for application in fusion devices.

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.

Thermal Shock Tests to Qualify Different Tungsten Grades as Plasma Facing Material

The electron beam device JUDITH 1 was used to establish a testing procedure for the qualification of tungsten as plasma facing material. Absorbed power densities of 0.19 and 0.38 GW m−2 for an edge localized mode-like pulse duration of 1 ms were chosen. Furthermore, base temperatures of room temperature, 400 °C and 1000 °C allow investigating the thermal shock performance in the brittle, ductile and high temperature regime. Finally, applying 100 pulses under all mentioned conditions helps qualifying the general damage behaviour while with 1000 pulses for the higher power density the influence of thermal fatigue is addressed. The investigated reference material is a tungsten product produced according to the ITER material specifications. The obtained results provide a general overview of the damage behaviour with quantified damage characteristics and thresholds. In particular, it is shown that the damage strongly depends on the microstructure and related thermo-mechanical properties.

Investigation of the Impact on Tungsten of Transient Heat Loads Induced by Laser Irradiation, Electron Beams and Plasma Guns

This contribution gives an overview of different simulation methods for transient events and damages induced in tungsten. The investigations were focussed on the resulting crack networks and special attention was paid to crack distance, width and depth. The results indicate that the different techniques show, in general, similar damage behaviours and the same damage thresholds.

Surface damage and Structure Evolution of Recrystallized Tungsten Exposed to ELM Like Transient Loads

Surface damage and structure evolution of the full tungsten ITER divertor under transient heat loads is a key concern for component lifetime and plasma operations. Recrystallization caused by transients and steady-state heat loads can lead to degradation of the material properties and is therefore one of the most serious issues for tungsten armor. In order to investigate the thermal response of the recrystallized tungsten under edge localized mode-like transient thermal loads, fully recrystallized tungsten samples with different average grain sizes are exposed to cyclic thermal shocks in the electron beam facility JUDITH 1. The results indicate that not only does the microstructure change due to recrystallization, but that the surface residual stress induced by mechanical polishing strongly influences the surface cracking behavior. The stress-free surface prepared by electro-polishing is shown to be more resistant to cracking than the mechanically polished one. The resulting surface roughness depends largely on the loading conditions instead of the recrystallized-grain size. As the base temperature increases from room temperature to 400 °C, surface roughening mainly due to the shear bands in each grain becomes more pronounced, and sub-grains (up to 3 μm) are simultaneously formed in the sub-surface. The directions of the shear bands exhibit strong grain-orientation dependence, and they are generally aligned with the traces of {1 1 2} twin habit planes. The results suggest that twinning deformation and dynamic recrystallization represent the predominant mechanism for surface roughening and related microstructure evolution.

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.

Tungsten damage and melt losses under plasma accelerator exposure with ITER ELM relevant conditions

Experimental simulations of ITER edge-localized mode have been performed with a Kh-50 quasi-stationary plasma accelerator. Heat loads exceeded the tungsten melting threshold. Droplet splashing and the solid dust ejection were observed. Droplets were emitted during the plasma exposure and dust generation dominated after the end of a plasma pulse, at the time of the material cooling. A decrease in the droplet velocity at an increase in the surface heat load was observed. It could be attributed to growing sizes of the droplets at higher energy loads. After one hundred pulses were performed at loads below the tungsten melting and above the cracking threshold, some porosity appeared. Such defects achieved dimensions of several tens of ?m. A heat load amounting to half the cracking threshold (0.3?MJ?m?2) could lead to the appearance of fatigue cracks after 100 plasma pulses. In this case the appearance of cracks could be caused by the accumulation and redistribution of linear defects (dislocations) in the affected tungsten layer.