Power and particle exhaust for the ARC fusion power plant

To successfully show that fusion is an attractive energy source, the ARC $^{\scriptstyle \mathrm{TM}}$ fusion power plant will need to operate with a robust, integrated power and particle exhaust solution. To maximise ARC’s fusion power output while avoiding excessive erosion of the plasma-facing components, we will need to radiatively dissipate most of the power crossing the last-closed flux surface, injecting radiating impurities such as argon or neon to access divertor detachment. Divertor detachment will need to be integrated with a high-performance core plasma, and with efficient impurity pumping to prevent the accumulation of helium ash in the core. To access and control detachment in high-performance plasmas, we have designed ARC with up–down-symmetric divertors, with secondary X-points in long, tightly baffled outer legs. Using a core-edge modelling workflow, we predict that with this divertor design, ARC will access detachment with modest argon seeding in the divertor ( ${c_{Ar,div}}\sim {0.9\,\%}$ ), which should have minimal impact on the core ( ${\Delta Z_{\textit{eff},\textit{core}}}\lt {0.5}$ ) for reasonable argon enrichment ( ${c_{Ar,div}/c_{Ar,\textit{core}}}={6}$ ). Due to the high predicted divertor neutral pressure ( ${p_{\textit{div}}}\sim {20\,\mathrm{Pa}}$ ), we predict that ARC will sufficiently pump helium to limit ash accumulation in the core ( ${c_{\textit{He},\textit{core}}}\lt {2\,\%}$ ) for a helium enrichment of ${c_{\textit{He},\textit{div}}/c_{\textit{He},\textit{core}}}={0.4}$ . ARC’s divertor design is expected to increase the stability of a detachment front in the outer divertor leg, helping to prevent divertor reattachment during smaller heat-flux transients such as scrape-off-layer filaments associated with the quasi-continuous exhaust regime. However, this buffering will not be sufficient to prevent divertor reattachment during large type-I edge-localised modes (ELMs), and as such these will need to be avoided on ARC. Experiments on SPARC will be used to select an integrated scenario which avoids or mitigates type-I-ELMs while maintaining access to detachment, good core fusion performance and sufficient impurity exhaust. SPARC experiments will also be used to finalise ARC’s divertor design, by studying the impact of magnetic and first-wall geometry on detachment stability, impurity enrichment and neutral baffling under conditions similar to those expected for ARC. In conclusion, our analysis finds that ARC will have a viable power and particle exhaust solution which is compatible with high-power operations, and this solution will be validated in experiments on SPARC.