Description
Understanding and controlling power exhaust is crucial for the success of SPARC and other future tokamaks. This work presents a comprehensive review of power exhaust studies conducted across European tokamaks. The analysis integrates data from both carbon-based and metallic-wall regimes to characterize divertor heat fluxes. We detail the foundational research that established the empirical multi-machine scaling law for the inter-ELM scrape-off layer (SOL) power decay length. This parameter is needed to predict the steady-state heat flux to the divertor, which in turn determines the level of power dissipation needed to protect the divertor targets from excessive damage or erosion.
We then review the experimental efforts from JET, ASDEX Upgrade, and MAST to characterize transient heat loads caused by Edge Localized Modes (ELMs). A relationship between the ELM energy fluence and the upstream pedestal-top pressure was identified and later validated on several other tokamaks, including COMPASS, allowing for the prediction of transient heat fluxes in next-step devices. Using these scalings, we show that SPARC and ITER will have similar steady-state and ELM-transient heat fluxes.
To link these exhaust challenges to operational constraints, we have developed the Separatrix Operational Space (SepOS) framework. This framework describes operational domains such as LH transition and density limit in terms of separatrix parameters which are key for power dissipation and detachment access. In addition, it has been shown to correlate with the transition between large “type-I” ELMs and the small-ELM QCE/EDA regime. This allows us to identify conditions where both the steady-state and transient heat fluxes can be tolerated in high-performance regimes on future tokamaks.
However, the studies on SepOS do not yet include impurity seeding cases. Furthermore, SOLPS modeling for ITER has shown that the highest achievable separatrix density is limited by the presence of impurities in the divertor region. The application of this methodology to SPARC yields similar results, identifying a corresponding reduction in the available separatrix operational space. Thus, the presentation concludes with both a discussion on these remaining gaps in our understanding, as well as a brief outlook to the boundary digital twin for SPARC, which incorporates the extensive heritage and data from European tokamaks.