Latest results from the JET tokamak, with beryllium as the fist wall material facing the hot plasma, have shown substantial improvements in plasma purity and corresponding reductions in plasma dilution. This has allowed a fusion product (nDT,Ti) of 8-9~ 10" me3 s keV to be reached (within a factor of 8 of that required in a fusion reactor), albeit only rransientl~. Even so, at high heating powers, an influx of impurities still limits the achievement of better performance and steady state operation.
A New Phase for JET is planned in which an axi-symmetric pumped divertor configuration will be used to address the problems of impurity control, plasma fuelling and helium ash exhaust in operating conditions close to those of a Next-Step tokamak with a stationary plasma of thermonuclear grade. The New Phase should demonstrate a concept of impurity control; determine the size and geometry needed to realise this concept in a Next-Step tokamak; allow a choice of suitable plasma facing components; and demonstrate the operational domain for such a device. With an efficient ax&symmetric pumped divertor, ignition should occur in a tokamak reactor of about 2 to 3 times the size of JET.
It seems prudent to envisage international collaboration on a Next Step Programme, which could comprise several complementary facilities, each optimised with respect to specific clear objectives. There could be two Next Step tokamaks, and a Materials Test Facility. Such a programme would allow division of effort and sharing of risk across the various scientific and technical problems, permit cross comparison and ensure continuity of results. A single Next Step device (such as the ITER Project as currently conceived) has higher scientific, technical and management risks and does not provide such comprehensive information, particularly in the areas of ignition, reactor performance and blanket testing. Further details of these facilities, expected costs and timescales are discussed.