First plasma exposure of a pre-damaged ITER-like plasma-facing unit in the WEST tokamak: procedure for the PFU preparation and lessons learned

Abstract

The evaluation of the impact of plasma-facing components (PFCs) damage on subsequent plasma operation is an important issue for ITER. During the first phase of operation of WEST, a few ITER like divertor plasma-facing units (PFUs) have been installed on the lower divertor. One PFU was pre-damaged under electron beam gun thermal loading, before its installation in WEST, and the subsequent evolution of the damage was studied after the WEST plasma exposure. This paper presents the procedure followed to get the pre-damaged PFU. It consists in the characterization of the response of tungsten samples representative of WEST PFU under high heat flux (HHF) loading, the selection of damage (namely small cracks, crack network, crack network and W melt droplets). Finally, according to the WEST plasma loading conditions, the blocks with damage within the PFU and the position of the pre-damaged PFU on the WEST lower divertor are attributed. The first results obtained after an initial plasma exposure in WEST lead to assess, as expected with regard to the heat loading conditions, that no major surface aspect modification was found. This result emphasized the possibility to implement as pre-damage some small local droplets of melted tungsten in a high heat loaded zone for a future WEST experimental campaign.

Overview of the emissivity measurements performed in WEST: in situ and post-mortem observations

“…The emissivity and cumulated conducted energy before and after the pulse sequence (last pulse) are plotted in black and red, respectively. The conducted energies for these pulses are: (a) 230 MJ for 12 similar pulses at R = 2.27 m, (b) 720 MJ for 24 pulses at R = 2.25 m, (c) 112 MJ for eight pulses at R = 2.29 m, and (d) 2197 MJ for 106 pulses at R = 2.235 m. These different periods correspond to experiments performed in the EU framework during C4: (a) PFU shaping experiment [28], (b) predamaged PFU experiment [29], (c) sustained melting experiment [30] and (d) the helium campaign [31]. The experiments (a)-(c) were performed with deuterium fueling and about 4 MW of LHCD heating.…”

Long-term plasma exposure of ITER-like W/Cu monoblocks with pre-damaged surfaces in EAST experiments

“…According to high heat load test experiments conducted using laser, electron or ion beams, the extremely high transient heat flux up to thousands of MW m −2 is expected to cause the shallow surface damages, like roughing, cracking, melting even boiling of W/Cu monoblocks [4][5][6]. WEST installed monoblocks with pre-damage (induced by electron beams) and subjected them to steady-state plasma heat flow for a cumulative maximum of 1400 s. It was confirmed that the pre-damaged monoblocks remained stable and continued to operate under steady-state heat flow [7]. However, the impact of severer damages, such as edge-melting or boiling, on longterm plasma operation is one of the main concerns for ITER operating with W divertor.…”

“…If the hill structure increase, the temperature rise is expected to be more severe, which will significantly decrease the threshold parallel heat flux for melting. To identify this theoretical effect, a special experiment should be designed and carried out in tokamak, such as WEST with high spatial resolution (∼0.1 mm) camera [7] which will be able to conduct such kind of dedicated tests. In addition, the brittle destruction in the resolidified layer may generate cracks as shown in figure 7 and thus produce submicron-sized dust particles.…”