EFDA-JET-CP(02)02/17

Helium Ash Simulation Studies with Divertor Helium Pumping in JET Internal Transport Barrier Discharges

Improved core confinement by the formation and sustainment of an Internal Transport Barrier (ITB) is seen as a possible route to steady state tokamak operation because of the potential of this regime for full non-inductive current drive. Such a regime must have a sufficiently fast chain of He transport from the core to the edge, and from the edge to the divertor, such that He can be pumped effectively. If one of the links of this chain is not fast enough, He ash will accumulate and the fusion reactions will self-extinguish. This can be quantified by measuring the ratio of the He retention time to the energy confinement time [1]. If small levels of additional impurities are present, the requirement is for tHe* / tE < 10 to obtain steady state burn conditions. With additional impurities the requirement becomes more strict, e.g. tHe* / tE < 5 with the upper limit of five corresponding to e.g. carbon concentrations of 3%. The relationship between He retention time and He replacement time,tHe*= tHe/(1-Reff), defines Reff i.e. the global recycling coefficient. We note that Reff is not the same as the edge recycling coefficient since central source and edge source are not characterised by the same replacement time. Nevertheless, without He pumping Reff is close to unity and the He retention time becomes very large. The potential for pumping under these conditions is expressed by the He enrichment factor, i.e. the ratio of the partial pressures of He and D2 in the sub divertor region (at the pump throat) to the ratio of He2+ to D+ in the plasma core, h = (pHe / 2pD2) / (nHe / nD) which needs to be larger than 0.2 for stationary operation of ITER [2].
Name Size  
EFDC020217 758.19 Kb