Role of recycling in JET trace tritium transport experiments

JET experiments with transient puffed injection of low tritium levels were carried out with the goal of using measured time- and space-dependent neutron emission to determine the scaling of particle transport in the core for ITB, ELMy H-mode and hybrid scenarios. Several attempts to find core τp scaling based on the same dataset have reached contradictory conclusions: core particle diffusivity inversely proportional to density, scaling proportional to density, and scaling independent of collisionality (density independence). The analyses have also varied the assumed boundary conditions. In all three cases, ELM-averaged transport was analyzed. To see what can be learned about resolution of such differences through possible systematic SOL effects on particle transport scaling, using the available edge/SOL data for these core particle transport experiments, we analyze representative cases from the JET trace T experimental database. The pulses considered cover a range in average core electron density, with approximately constant tritium puffing rates. The study is motivated by the realization that, although these experiments were not designed for such a purpose, similar future 'trace T' experiments could better characterize the SOL, and thus its influence of on particle transport in the edge/pedestal region (0.8 < ρ < 1) since important elements of the tritium pathway are measured: the external T2 gas puff rate (Γpuffmax), and, from the neutron analysis, the influx to the plasma core region at ρ = 0.8 (ΓT+in,max) and the efflux from the core (ΓT+out,max). Systematic effects on edge/pedestal particle transport scaling in puffing experiments are found to depend on the status of saturation in the near-inlet region, on the details of ELM-induced particle expulsion from both core and SOL, and on the degree of near-wall T enrichment.