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

Abstract

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.

Observation of the tungsten surface damage under ITER-relevant transient heat loads during and after electron beam pulse

“…It will undergo impact of the intense continuous and pulsed heat loads [1]. Different mechanisms of the surface damage can occur at such severe conditions [2,3]. Cracking, melting, dust generation may affect plasma confinement.…”

Synergistic effects of ELMs and steady state H and H/He irradiation on tungsten

“…Nevertheless, many of the exposures affect tungsten and can cause unacceptable material damage. Different experimental set-ups, such as linear plasma devices, laser-, particle-, or electron-beam facilities have been used to investigate the plasma-material interactions of different tungsten-grades to a broad range of loading conditions [1][2][3][4]. These experiments made it possible to determine material behaviour, damage mechanisms, and threshold-values, among others.…”

Comparison of tungsten modification after irradiation at different facilities for PSI studies

“…Moreover, in previous experiments, irradiation at the GOL-3 facility was compared with irradiation using the electron beam facility JUDITH-1 and the Nd:YAG laser [4], [7]. The different techniques show, in general, similar damage behaviour and the same damage thresholds [8], [11].…”

“…Under loads of about 0.5 MJ/m 2 , the tungsten surface did not melt, but after multiple plasma impacts crack patterns are formed (found after irradiation by using Nd:YAG laser [4], [5], quasi-stationary plasma accelerators QSPA Kh-50 [6], linear plasma machines PSI-2 [7], Nd:YAG laser, JUDITH-1 and GOL-3 [8], [9]). …”

“…Bubbles and three different crack networks with typical cell sizes of 1000, 10 and 0.3 µm were detected after irradiation of the tungsten surface by different methods (after irradiation at the Nd:YAG laser [4], QSPA Kh-50 and GOL-3 [6], [10], PSI-2 [7], Nd:YAG laser, JUDITH-1 and GOL-3 [2], [8] [11]. The typical erosion depth is less than 20 μm [2], [10].…”