Wall conditions in WEST during operations with a new ITER grade, actively cooled divertor
Résumé
Future fusion reactors like ITER and DEMO will have all-tungsten (W) walls and long pulses. These features will make wall conditioning more difficult than in most of the existing devices. The W Environment Steady-state Tokamak (WEST) is one of the few long pulse (364 s) fusion devices with actively cooled W plasma-facing components in the world. WEST is a unique test bed to study impurity migration and plasma density control via reactor relevant wall conditioning techniques. The phase II of WEST operations began in 2022, after the installation of a new lower divertor, now entirely equipped with actively cooled, ITER grade, W monoblocks. After pump down, we baked WEST between 90 ◦C and 170 ◦C for ~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. While at 170 ◦C, we performed ~40 h of D2 glow discharge cleaning (GDC) and ~5 h of glow discharge boronization (GDB), using a 15 %-85 % B2D6-He mix and a total boron mass of ~12 g. This was the very first GDB at such high temperature for WEST. The whole wall conditioning sequence led to a ~10 times reduction of the H2O signal as well as to a ~3 times reduction of the O2 signal, according to mass spectra. Once back to 70 ◦C, the vessel pressure was 5.5x10-6 Pa and plasma restart was seamless with ~30 s cumulated over the very first 5 pulses and an Ohmic radiated power fraction Frad = 0.6, showing successful conditioning of the new ITER grade divertor. The effect of the first, ‘hot’ GDB faded with a characteristic cumulative injected energy of 2.45 GJ and saturation towards Frad ~0.8. After 1.4 h and 7.5 GJ of cumulative plasma time and injected energy, we carried out a second GDB, this time at 70 ◦C. This ‘cold’ GDB initially led to a much lower Ohmic Frad = 0.3–0.4 but the effect lasted ~7 times less, with a characteristic cumulative injected energy of 0.37 GJ. At the end of the campaign, we cumulated ~3h and ~30 GJ through repetitive, minute long pulses without any boronization. Throughout this 4-weekslong experiment, Frad in the 4 MW heating phase evolved only marginally (from 0.5 to 0.55). This increase is mostly due to the build-up of re/co-deposited layers on both lower divertor targets.
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