Beryllium melting and erosion on the upper dump plates in JET during three ITER-like wall campaigns

Abstract

Data on erosion and melting of beryllium upper limiter tiles, so-called dump plates (DP), are presented for all three campaigns in the JET tokamak with the ITER-like wall. Highresolution images of the upper wall of JET, show clear signs of flash melting on the ridge of the roof-shaped tiles. The melt layers move in the poloidal direction from the inboard to the outboard tile ending on the last DP tile with an upward going waterfall-like melt structure. Melting was caused mainly by unmitigated plasma disruptions. During three ILW campaigns around 15% of all 12376 plasma pulses were catalogued as disruptions. Thermocouple data from the upper dump plates tiles showed a reduction in energy delivered by disruptions with fewer extreme events in the third campaign, ILW-3, in comparison to ILW-1 and ILW-2. The total Be erosion assessed via precision weighing of tiles retrieved from JET during shutdowns indicated the increasing mass loss across campaigns of up to 0.6 g from a single tile. The mass of splashed melted Be on the upper walls was also estimated using the high-resolution images of wall components taken after each campaign. The results agree with the total material loss estimated by tile weighing (~130 g). Morphological and structural analysis performed on Be melt layers revealed a multilayer structure of re-solidified material composed mainly of Be and BeO with some heavy metal impurities Ni, Fe, W. IBA analysis performed across the affected tile ridge in both poloidal and toroidal direction revealed a low D concentration, in the range 1-4 x 10 17 D atoms/cm 2 .

Micro-analyses of dust particles generated in the JET tokamak with the ITER-like wall

“…As a result, the conversion rate of the eroded material to dust is very low. This is supported by the fact that splashed metal droplets stick firmly to surfaces where they were originally deposited [29,36].…”

“…Low retention in metal particles, as shown above, together with a very small amount of particles retrieved after each of the three ILW campaigns (~1 g per campaign) [5,6,9] and a small fraction Be-rich particles in the loose matter, below 5 % [9] indicate that the operation with metal PFC significantly reduces risks related to dust: generation rate and fuel retention. Only small amount of loose Be dust is found despite melt erosion and splashing of Be from limiters [29]. As a result, the conversion rate of the eroded material to dust is very low.…”

“…A small Be droplet (approx. 300 µm in diameter) released during limiter melt events (see for instance [29]) in Figure 2 is partly coated by a deposited layer of W eroded in the divertor. The X-ray spectra recorded under the same conditions (accelerating voltage 10 keV, emission 1,6 µA, integration time 30 s) in Points 1 and 2 differ significantly.…”

Fuel inventory and material migration of JET main chamber plasma facing components compared over three operational periods

“…For the dump plate tiles DP2 and DP4, Figure 4 (a) and (b), the D concentrations on the surface exposed to ion current (-45 -0 mm) are lower than on the surface shadowed from the ion current (0 -45 mm) where a small amount of deposition occurs. DP8 (results in [20]) was subject to strong interaction with plasma during vertical displacement events (VDEs) (most of which occurred during resulting in melting. Despite the periodic heating above the melting temperature the fuel retention across DP8 (0.2 -0.4 × 10 18 atoms/cm 2 [20]) is higher than the high heat flux regions on the limiter tiles and is similar to the levels to DP2 and DP4.…”

“…DP8 (results in [20]) was subject to strong interaction with plasma during vertical displacement events (VDEs) (most of which occurred during resulting in melting. Despite the periodic heating above the melting temperature the fuel retention across DP8 (0.2 -0.4 × 10 18 atoms/cm 2 [20]) is higher than the high heat flux regions on the limiter tiles and is similar to the levels to DP2 and DP4. Lower D values are seen for ILW2 on DP4 after H plasmas, Figure 4(a).…”