Impurities in JET and their control

The problems of impurities in the JET (Joint European Torus) and proposals for their control are described. The latest results from JET in which beryllium has been used as the first wall material facing the hot plasma are presented. Substantial improvements in plasma purity and corresponding reductions in plasma dilution have been observed. This has allowed a fusion product of 5*10/sup 20/ m/sup -3/-s-keV to be reached (which is within a factor of 10-15 of that required in a fusion reactor), albeit in a transient situation. Even so, at high heating powers, impurity influx limits the attainment of higher parameters and prevents reaching a steady-state situation. A new phase is proposed for the JET, with an axisymmetric pumped-divertor configuration to address the control of impurities, fueling and exhaust of helium ashes in operating conditions close to those of a next-step tokamak with a stationary plasma of thermonuclear grade. The latest proposal for the configuration is described. It should do the following: demonstrate a concept of impurity control, determine the size and geometry needed for this concept in the next-step, allow a choice of plasma-facing components for the next-step, and demonstrate the operational domain for the next-step. >