27th PSI

O20 He and D retention in bulk-W lamellae from the JET divertor measured by Laser-Induced Desorption with melting

21 May 2026, 08:40

20m

Oral C. Plasma Fueling, Particle Exhaust and Control, Tritium Retention Oral

Speaker

Dr Miroslaw Zlobinski (Forschungszentrum Jülich GmbH)

Description

In the final divertor setup of JET, the divertor bottom consisted of 4 toroidal rings of thin, separated bulk-W lamellae called “tile 5” or load-bearing septum replacement plates (LBSRP). [1]
Three of the lamellae studied here had been exposed during JET plasma operation only during the ITER-like wall campaign 1 (ILW1) and another three lamellae in addition during the ITER-like wall campaign 3 (ILW1 + ILW3). The divertor plasma time in the ILW1 campaign was 13 h and in the ILW3 campaign 18.5 h. The base temperature of the LBSRP was 70° C, while up to 1200° C was reached during plasma operation. They were only cooled inertially.
All 6 W lamellae were then cut in small samples and analysed ex situ by Laser-Induced Desorption (LID) with melting of 2 mm diameter spots using a 3 ms Nd:YAG laser pulse of 64 J in FREDIS [2]. The laser melts about 20 µm of the surface, the retained gases desorb and are quantified by Quadrupole Mass Spectrometry (QMS). FREDIS is equipped with a high-resolution QMS, that can separate the mass peaks of He and D₂ and measures masses 1 to 6 amu/e, while an overview QMS measures masses 1-50 amu/e with low resolution. The high-resolution QMS was especially useful in this case as the He and D₂ peaks at 4 amu/e were similarly high and thus distinct from each other.

LID-QMS measurements with melting were performed in 27 positions on the top surface of the lamellae, which does not show colour differences, while the side surfaces show colourful deposits. On the side surfaces of the lamellae 18 radial scans with 3 positions each were performed in a distance of 2, 5 and 7 mm from the top surface. In some positions the retention is about one order of magnitude larger on the side surface than on the top surface but mostly has a rapid decrease in the radial direction away from the plasma.

Afterwards some of the samples were cut through the centre of the laser spots, polished and etched. These metallographic cross-sections show the melting depth indicated by vertical grain boundaries, the heat affected depth, cracks at some laser spots and bubbles at the bottom of the molten volume.

[1] Ph. Mertens et al., Fus. Eng. Des. 84 (2009) 1289-1293,
doi:10.1016/j.fusengdes.2008.11.055
[2] M. Zlobinski et al., Fus. Eng. Des. 146 (2019) 1176-1180,
doi:10.1016/j.fusengdes.2019.02.035