[en] Sn filled capillary porous structures were exposed to high flux low temperature plasma conditions at the Pilot-PSI linear device. Enhanced erosion above that expected classically was investigated via spectroscopic observation of Sn"0 emission from the plasma in front of the target surface while the surface temperature was monitored by both thermography and pyrometry. An anomalous erosion flux was observed as temperature increases, with onset for this occurrence varying strongly between different ion species. The results appear incompatible with existing ‘adatom’ models for the anomalous erosion flux. Further targets were exposed in turn to increasing heat fluxes and the heat removed determined from cooling water calorimetry, which was then compared to a solid Mo reference target. At high powers the total energy of the cooling water is reduced, indicating a shielding of the surface from the plasma heat flux by the vapour cloud in front

[en] The tungsten (W) material in the high heat flux regions of the ITER divertor will be exposed to high fluxes of low-energy particles (e.g. H, D, T, He, Ne and/or N). Combined with long-pulse operations, this implies fluences well in excess of the highest values reached in today’s tokamak experiments. Shaping of the individual monoblock top surface and tilting of the vertical targets for leading-edge protection lead to an increased surface heat flux, and thus increased surface temperature and a reduced margin to remain below the temperature at which recrystallization and grain growth begin. Significant morphology changes are known to occur on W after exposure to high fluences of low-energy particles, be it H or He. An analysis of the formation conditions of these morphology changes is made in relation to the conditions expected at the vertical targets during different phases of operations. It is concluded that both H and He-related effects can occur in ITER. In particular, the case of He-induced nanostructure (also known as ‘fuzz’) is reviewed. Fuzz formation appears possible over a limited region of the outer vertical target, the inner target being generally a net Be deposition area. A simple analysis of the fuzz growth rate including the effect of edge-localized modes (ELMs) and the reduced thermal conductivity of fuzz shows that the fuzz thickness is likely to be limited by the occurrence of annealing during ELM-induced thermal excursions. Not only the morphology, but the material mechanical and thermal properties can be modified by plasma exposure. A review of the existing literature is made, but the existing data are insufficient to conclude quantitatively on the importance and extent of these effects for ITER. As a consequence of the high surface temperatures in ITER, W recrystallization is an important effect to consider, since it leads to a decrease in material strength. An approach is proposed here to develop an operational budget for the W material, i.e. the time the divertor material can be operated at a given temperature before a significant fraction of the material is recrystallized. In general, while it is clear that significant surface damage can occur during ITER operations, the tolerable level of damage in terms of plasma operations currently remains unknown. (paper)

[en] Accidental melting of metallic plasma-facing materials in future fusion devices poses serious issues regarding the material lifetime and power-handling capabilities as well as core plasma performances. The behaviour of aluminium (as a proxy for beryllium) and tungsten materials was investigated in the Pilot-PSI linear plasma device to study the melt-layer motion and droplet ejection under ITER-relevant plasma conditions. Heat fluxes of up to 50 MW m⁻² raised the surface temperature to values up to 5000 K. The melt-layer rotation was found to depend on the magnetic field (up to 1.6 T) strength and target potential and is attributed to J × B forces caused by radial currents in the plasma. The amount of droplets ejected from the molten surface depends on the material—more droplets ejected from aluminium than from tungsten—and the heat flux to the target. The average droplet velocity was determined to be around 60 m s⁻¹ for both materials with droplets being ejected mainly in the axial direction. Droplet ejection is only observed during helium discharges, no ejection can be observed with hydrogen plasmas despite similar heat fluxes. Bubble boiling appears to be the main mechanism contributing to the observed droplet ejection. (paper)

[en] Magnum-PSI is an advanced linear plasma device uniquely capable of producing plasma conditions similar to those expected in the divertor of ITER both steady-state and transients. The machine is designed both for fundamental studies of plasma–surface interactions under high heat and particle fluxes, and as a high-heat flux facility for the tests of plasma-facing components under realistic plasma conditions. To study the effects of transient heat loads on a plasma-facing surface, a novel pulsed plasma source system as well as a high power laser is available. In this article, we will describe the capabilities of Magnum-PSI for high-heat flux tests of plasma-facing materials

[en] We report on the generation of high transient heat and particle fluxes in a linear plasma device by pulsed operation of the plasma source. A capacitor bank is discharged into the source to transiently increase the discharge current up to 1.7 kA, allowing peak densities and temperature of 70x10²⁰ m^-3 and 6 eV corresponding to a surface power density of about 400 MW m^-2.

[en] The A ²Π-X ²Σ⁺ band (the Δv = 0 sequence) emission of beryllium deuteride (BeD) molecules has been observed both in Be-seeded deuterium plasma and in front of Be targets exposed to deuterium plasma. The particle interchange reaction, Be⁺ + D₂ → BeD + D⁺, is thought to be a dominant process for the BeD formation in the plasma. On the other hand, BeD observed in front of Be targets is a product of chemical sputtering of Be bombarded by deuterium plasma. The photon emission coefficient of the A-X band at λ ∼ 497.3-499.2 nm around the prominent Q branch is estimated to be ∼5 x 10^-14 m³ s^-1 for electron temperatures ∼>8 eV as deduced from particle balance between Be, Be⁺, and BeD in D₂ neutral pressure scans. The surface temperature dependence of chemical sputtering of Be released as BeD is investigated, and the sputtering yield of BeD is found to peak at ∼440 K. This peak temperature is consistent with the onset temperature of the decomposition of BeD₂, obtained from thermal desorption spectrometry of BeD₂ powder. Also, the chemical sputtering yield is observed to be decreased with increasing incident ion flux, similar to carbon. Vibrational (T_vib) and rotational (T_rot) temperatures of BeD molecules are evaluated by fitting a measured A-X band spectrum with a simulated spectrum. Spectra taken from BeD formed in the plasma are well fitted with a single pair of T_vib and T_rot, which increase linearly with electron density. However, the vibrational and rotational energy of sputtered BeD is found not to be consistent with a Boltzmann distribution.

[en] The prediction of tritium retention in ITER relies on the extrapolation from present data. An empirical equation was proposed in (De Temmerman G. et al 2008 Nucl. Fusion 48 075008) to account for the influence of the beryllium deposition rate, the substrate temperature and the average energy of the deuterium neutrals on the deuterium retention. However, the beryllium deposition rate observed in PISCES-B is much lower than that expected in the ITER divertor. On the other hand, the flux ratios of deuterium and beryllium are comparable in magnitude. Therefore, a revised scaling equation is proposed here to take into account the flux ratio of deuterium to beryllium arriving at the co-depositing surface and to match the validity range of the scaling equation with the ITER parameters. (letter)

[en] Full text: Edge Localized Modes (ELMs) in ITER will lead to material erosion, cracking, melting and vaporization. During an ELM, the divertor surfaces are submitted to both the steady state detached divertor plasma and the intense heat and particle fluxes during ELMs. Such a situation will lead to synergistic effects which might strongly affect the material damage threshold. In this contribution we describe a new experimental setup for ELM simulation experiments with relevant steady-state plasma conditions and transient heat/particle source. The Pilot-PSI linear device produces plasma parameters relevant to the study of plasma-surface interactions in the ITER divertor. In parallel to the DC power supply, the plasma source is also connected to a capacitor bank (5 kV, 8.4 mF, 100 kJ) which is discharged in the plasma source to create a transient increase of the discharge current. The pulse duration is about 1 ms. This allows the superimposition of a transient heat and particle pulse to the steady-state plasma. The plasma parameters are measured by Thomson scattering and the sample temperature during the pulse is monitored by a fast infrared camera. The plasma source can be operated in pulsed mode in a variety of gases (Ar, H, He, N) as well as with mixed gases. So far, discharge currents of up to 11.6 kA have been achieved, which represents a peak power in the plasma source of about 5 MW. The highest plasma density of 1.5 x 10²² m^-3 and temperature of 4.5 eV leads to a surface peak heat flux of 0.9 GW·m^-2. The temperature rise time is in the range 300 - 500 μs, comparable to that observed during Type-I ELMs in JET. With higher gas flow, it is anticipated that peak heat fluxes in the range 1 - 3 GW·m^-2 will be reached. Exposure of tungsten targets to hydrogen plasma with transient revealed an abrupt increase of the plasma density after the pulse at the location of the TS system (17 mm from the target). The influence of such heat pulses on the deuterium retention in tungsten will be described. In addition, the pulsed plasma source has been used to assess the effect of transient heat/particle pulses on the growth of helium-induced nanostructure and characterize the generated dust. (author)

[en] A systematic study of the influence of the deposition conditions on the deuterium retention in co-deposited tungsten layers formed both by magnetron sputtering and in the PISCES-B linear device has been carried out. Experimental parameters such as the tungsten deposition rate, the incident particle energy and the substrate temperature are shown to affect the level of deuterium retention in the layers. A decreased retention for increased substrate temperature and deposition rates, and an increased retention for increasing incident deuterium particle energy are observed. A scaling equation is proposed to describe the influence of the conditions during the co-deposition process (surface temperature, incident particle energy and deposition flux) on the deuterium retention. In addition, the desorption kinetics of deuterium has been studied by TDS. Two desorption stages at 473-573 K and at 1073 K have been observed.

[en] Given the planned extensive use of metallic mirrors in the optical diagnostics systems of ITER, the study of reflectivity changes induced by erosion and/or redeposition of impurities on the mirror surfaces is of primary importance for the reliability of the diagnostics signals. This contribution will demonstrate that the mirror material choice can exert a significant influence on the relative importance of erosion/redeposition affecting the mirror reflectivity. A dedicated manipulator has been designed to allow exposure of mirror samples in the divertor region of the Tokamak a Configuration Variable (TCV) tokamak. Mirrors from different materials have been exposed both during short experimental campaigns and boronisation procedures. Before and after exposures the mirrors were characterized with different surface analysis techniques (XPS, SIMS, SEM, EDX, ellipsometry). Under identical exposure conditions, the mirror material can strongly influence the deposit thickness found on the sample: the carbon layer thickness on a Si sample is much higher than on a Mo sample. These results have potentially important consequences for the first mirror material choice in ITER