Helium enrichment and tritium burn efficiency in simulations of divertor plasmas*

The connection between projected fusion reactor performance and helium enrichment in the edge plasma is explored using the plasma edge code SOLPS-ITER. Recently, it was proposed that the tritium usage in steady state, equilibrated fusion reactors could be characterized by a generic, dimensionless figure of merit: the “Tritium Burn Efficiency”, or “TBE” [1] which connects the reactor performance with the permitted helium gas fraction in the divertor. This study addresses the applicability of TBE for characterizing fusion devices through evaluation of helium transport and enrichment in the divertor plasma, in both existing experiments and for next-step devices. Existing impurity enrichment studies using injected helium performed on the DIII-D tokamak with induced scrape-off layer flows at various puffing/pumping speeds are modeled using SOLPS-ITER, matching reported experimental conditions [2] to assess the validity of the helium physics models in the code. A helium enrichment value defined as f_He,exh/f_He,core of 1.0 is achieved in the simulation, in good agreement with the reported value of 1.1 in the experiment. This simulation acts as a validated baseline for further physics studies on divertor helium enrichment in the DIII-D tokamak as an injected impurity and the next-generation fusion device SPARC as fusion-born alpha particles. 

[1] D.G. Whyte et al 2023 Nucl. Fusion 63 126019

[2] M.R. Wade et al 1998 Nucl. Fusion 38 1839