JET with a pumped divertor: design, construction, commissioning and first operation

Eleven years of JET operations have brought studies of tokamak plasmas up to reactor-relevant performance. This has resolved some key issues for the design of a “next step” machine. However it has been clearly shown that active control of the influx of impurities is essential for a long-burn fusion reactor. Therefore, a single-null pumped divertor has been installed inside the JET vacuum vessel to study impurity control, and it is now operational. It consists of four poloidal coils (manufactured inside the vessel), carbon-fibre composite (CFC) target plates and a toroidal cryo-pump. The new magnetic configuration produces plasma shapes which required a complete redesign of the first wall, including r.f. antennae and limiters. New power supplies were introduced for the divertor coils and for plasma fast vertical position control. Additional cooling systems, a new digital plasma control, a comprehensive machine protection system and new diagnostics to measure plasma parameters in the divertor region were also installed. Well-controlled 4-MA discharges have already been achieved.