Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET Campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma facing materials. The large reduction of the carbon content (more than a factor ten) led to a much lower Zeff (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that fuel retention rates with the new wall are in line with the ITER needs. The re-establishment of high-confinement scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type I ELMy H-mode regimes with H98,y2 close to 1 and βN∼1.6 have been achieved in high triangularity plasmas. The ELM frequency is the main factor for the control of the metallic impurities accumulation. Pedestal temperatures tend to be lower with the new wall, leading to somewhat reduced confinement, but nitrogen seeding restores high pedestal temperatures and high confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on plasma-facing components due to lower radiation, made the routine use of massive gas injection for disruption mitigation essential.
ASJC Scopus subject areas
- Nuclear and High Energy Physics
- Condensed Matter Physics