Pumped divertors and limiters for tokamaks

Abstract

Extended pulse experiments and fusion reactors will require plasma exhaust, specifically helium exhaust, in order to achieve long, stable pulses or steady-state burning. In addition, impurity control is required to minimize the value of nτ E T i needed to achieve full plasma performance. It is shown that high density, lower temperature plasma burn conditions, such as are predicted for high field tokamaks, place less severe requirements on exhaust efficiency for helium. Modular and toroidal belt pumped limiters have demonstrated exhaust efficiencies of greater than 10 to 20% for hydrogen and deuterium in ohmic, neutral beam and ICRF discharges. Recent experiments in TEXTOR have explored the details of helium transport and exhaust in mixed deuterium-helium plasmas. To control impurity introduction, pumped divertors may be essential and major experiments are planned for the JET and DIII-D tokamaks. Pumped divertors can exhaust plasma and reduce the heat flux to target plates. Analysis indicates that friction can restrain impurities generated in the divertor chamber from escaping the divertor plasma channel if high density, low temperature conditions are achieved near the divertor plate. Going further in this direction, experimental simulations in the linear experiment, PISCES, of the gaseous divertor concept, suggested for the ITER reactor, have demonstrated greatly enhanced radial plasma losses (by comparison with Bohm losses) in the channel and a large, 100 fold reduction in the heat flux to the end (divertor) plate. The results are encouraging with regard to handling the high heat fluxes in reactors. The exhaust of helium may, however, be limited by diffusion rates in the core, especially in H-mode plasmas.