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Simulation of the mitigation efficiency of the RE beam impact on the ITER first wall by special sacrificial W diaphragm has been performed using ENDEP, MEMOS and TOKES codes. According to the technique proposed, the RE beam should deposit its energy onto the special sacrificial diaphragm consisting of separate tungsten ‘nails’. The ‘nails’ is partly evaporated and then the beam power deposited into the vaporized and ionized W plasma shield. The wall heat load mitigation is due to the RE beam energy deposition into the W plasma and its re-radiation onto larger wall area. Calculated tolerable beam energy should not exceed 90 MJ to ensure absence of melting for 10 ms RE beam deposition. [ABSTRACT FROM AUTHOR]

Abstract: Brittle destruction of tungsten armour under action of edge localised modes of plasma instabilities (ELMs) in ITER is an important issue determining the lifetime of the divertor. Besides, cracking of the armour produces tungsten dust with characteristic size of 1–10μm flying from the armour surface with velocities up to 10m/s. Influx of the tungsten dust into the ITER confinement decreases the temperature of the plasma, reduces the thermonuclear gain and even may run the confinement into disruption. This paper describes experiments in QSPA-Kh50 plasma gun and modeling, which has been performed for providing more insight into the physics of tungsten cracking under action of ELMs and for confirmation of the important result on stabilization of the crack development at the tungsten armour surface, predicted in our previous paper – the same authors, 2010. The threshold value of energy density deposition for start of tungsten cracking has been measured as 0.3MJ/m2 after 5–10 shots. From analytical considerations three times smaller threshold value has been predicted with increasing number of shots. [Copyright &y& Elsevier]

Abstract: Simulations of tungsten armour cracking under small ELM-like plasma heat load, which does not cause surface melting, have been performed using the PEGASUS-3D code. A dedicated series of experiments have been performed in the QSPA-Kh50 facility for measurements of the unknown tungsten thermophysical properties and for verification of the PEGASUS-3D simulation results. The simulations revealed that a cellular crack network with average mesh size Λ ∼0.5mm formed after first ELMs and the pattern does not change further. With increasing number of repetitive ELMs loads, the average crack width Δ(n) has a maximum value Δ m . The ratio of Δ m /Λ is equal to the tungsten thermal expansion at the maximum surface temperature. Δ(n) tends to this value exponentially. The number of ELMs n m needed for Δ stabilization depends on the ELMs energy density and time duration, n m ∼300 for the simulated ELMs of 0.45MJ/m2 and 0.25ms duration. The PEGASUS-3D code is prepared for simulations of tungsten armour damage under action of ELMs of various energy deposition and time duration. These parameters of ELMs depend on ITER regimes of operation and on how successful will be the efforts on ELMs mitigation. [ABSTRACT FROM AUTHOR]

Abstract: The behavior of a preheated at 650°C tungsten targets under repetitive ELM-like plasma pulses is studied in simulation experiments with the quasi-stationary plasma accelerator QSPA Kh-50. The targets have been exposed up to 350 pulses of the duration 0.25ms and the surface heat loads either 0.45MJ/m2 or 0.75MJ/m2, which is below and above the melting threshold, respectively. The development of surface morphology of the exposed targets as well as cracking and swelling at the surface is discussed. First comparisons of obtained experimental results with corresponding numerical simulations of the code PEGASUS-3D are presented. [Copyright &y& Elsevier]

Abstract: Tungsten in the form of macrobrush structure is foreseen as one of the two candidate materials for the ITER divertor and the dome. Melting and thus melt motion and melt splashing are expected to be main mechanisms of metallic target damage that determine the lifetime of ITER plasma facing components. Experiments carried out at the plasma gun facility QSPA-T for ELM-like heat loads demonstrated a significant erosion of frontal and lateral brush edges, which was confirmed by further numerical simulations. In the experiments and numerical simulations a threshold of brush edge melting was determined. In this paper most important mechanisms of melt splashing and melt bridge formation under ITER transient heat loads are analyzed. Approximate criteria for droplet ejection are obtained and the range of transient events without significant droplet injection is calculated. The critical radius of brush edges rounding which prevents the bridge formation at the macrobrush edges is determined. [Copyright &y& Elsevier]

Abstract: This paper is concerned with investigation of an erosion of the ITER-like divertor plasma facing components under plasma heat loads expected during the Type I ELMs in ITER. These experiments were carried out on plasma accelerator QSPA at the SRC RF TRINITI under EU/RF collaboration. Targets were exposed by series repeated plasma pulses with heat loads in a range of 0.5–1.5MJ/m2 and pulse duration 0.5ms. Erosion of CFC macrobrushes was determined mainly by sublimation of PAN-fibres that was less than 2.5μm per pulse. The CFC erosion was negligible at the energy density less than 0.5MJ/m2 and was increased to the average value 0.3μm per pulse at 1.5MJ/m2. The pure tungsten macrobrushes erosion was small in the energy range of 0.5–1.3MJ/m2. The sharp growth of tungsten erosion and the intense droplet ejection were observed at the energy density of 1.5MJ/m2. [Copyright &y& Elsevier]

Abstract: In recent tokamak simulation experiments with the QSPA Kh-50 facility several mechanisms of dust generation from tungsten surfaces under ITER ELM-like energy loads have been identified. Here cracking and melting are reported. The brittle destruction dominates after a few transient impacts when a network of major cracks forms on the surface. Bifurcation of major cracks results in ejection of dust particles with sizes up to ∼30μm. Dust generation occurs also after surface melting and following resolidification when fine crack networks along the grain boundaries develop. In this process the destruction is accompanied by bridge formation due to capillary tension across the fine cracks. Next impacts (even weak melt-free ones) can destroy those bridges, which produces considerable amounts of dust particles of nm-size dust. Surface modification after the repetitive plasma pulses also results in creation of nm-size dust. [Copyright &y& Elsevier]