Wall conditioning of WEST after installation of the ITER grade, actively cooled lower divertor

Future fusion reactors like ITER and DEMO will have all-tungsten (W) walls and perform long pulses. These features will make wall conditioning more challenging than in most of the existing devices. The W Environment in Steady-state Tokamak (WEST) is the only long pulse (~100 s) fusion device with W plasma-facing components in the EU. WEST is a unique test bed to study radiation losses mitigation and plasma density control via reactor relevant wall conditioning. The phase II of WEST operations has begun in 2022, after the installation of a new lower divertor, now entirely constituted by actively cooled, ITER grade, W monoblocks. After pump down, we baked WEST between 90 °C and 170 °C for  about 2 weeks. After 82.5 h at 90 °C and 33 h at 170 °C, vacuum conditions were stable with a vessel pressure of 6x10^-5 Pa and mass spectra dominated by H₂ molecules. We then performed glow discharge cleaning (GDC) and boronization (GDB) at 170 °C, for the first time at such high temperature in WEST. Three sessions of D₂ GDC, for a total of ~40 h, allowed for a further ~10 times reduction of the H₂O signal in mass spectra. Then we carried out GDB for ~5 h using a 15%-85% B₂D₆-He mix, injecting a total boron mass of ~12 g. Once back to 70 °C, the vessel pressure was 5.5x10^-6 Pa, lower than ever in WEST phase I. Plasma restart was seamless with ~30 s already cumulated over the very first 5 pulses, proving that the procedure described above was successful in conditioning a freshly installed ITER grade, actively cooled lower divertor.