Speaker
Description
Following the third deuterium-tritium campaign, DTE3, and the end of JET operations in 2023, a selection of plasma-facing components was removed from JET vessel which included the modules of bulk tungsten divertor tile, so called “tile 5” or LBSRP. During JET operations, these bulk tungsten tiles were exposed to deuterium, tritium and helium plasmas. After the end of plasma operations and prior to removal they also were targeted by laser-induced breakdown spectroscopy (LIBS) measurements. Following their removal from JET vessel, divertor tiles were disassembled into their constituent lamellae, and these lamellae were used for the microscopy studies presented in this contribution. Investigated lamellae were extracted from different sections of tile 5. Within this tile, lamellae are arranged in four stacks, denoted A to D, whereby lamellae are arranged toroidally within a stack, and stacks are arranged poloidally, with stack A at the inboard and D at the outboard side.
Scanning electron microscopy (SEM) was used for imaging of plasma-exposed surfaces. Surface imaging was complemented with focused ion beam (FIB) cross-sectioning, whereby the cross-sections of the plasma exposed surfaces were prepared and imaged using SEM. This allowed the studies of morphology and microstructure in the under-surface volumes of tungsten lamellae.
The main focus of the study was on the effects of exposure of tungsten to helium plasma and to high heat flux. During helium plasma operations the outer strike point was located on stack B. Following helium operations, including deuterium-tritium campaigns, the outer strike point was at all times located on stacks C and D, such that the area of strongest interaction with helium plasma was not further disturbed. The lamellae studied here come from stacks A (private flux region), B (where the highest helium ion flux and surface temperature were present) and D (outer SOL and location of the outer strike point during deuterium-tritium operations, where the highest heat flux during high power operations was present). Surface effects of helium plasma exposure in JET environment, as well as microstructural changes, such as grain growth or recrystallization, induced in tungsten by high heat flux, are presented and discussed.
Additional area of study involved surface and sub-surface imaging of the craters produced by LIBS in the tungsten surfaces. Microstructural effects of laser-surface interaction are presented as well.