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Description
Progress in mapping the tokamak operational space to the separatrix plasma parameters via the separatrix operating space framework (SepOS [1]) has enabled cross-machine comparisons of small/no-ELM regime access (e.g., QCE [2] or EDA H-mode [3]). In this work we extrapolate the SepOS projections to SPARC and ITER, and, crucially, introduce detachment access criteria, thus formulating the combined power exhaust constrained SepOS (i.e., PE-SepOS) to evaluate integrated power exhaust solutions in next-step devices.
Through the interpretation of multi-device (ITER, JET and SPARC) SOLPS-ITER datasets and foundational work demonstrated on JET [4,5], we develop an as simple as possible description of the SOL net power and momentum losses in dissipative regimes to link the main power exhaust quantities (qdep,t, Te,t, cz) with the SepOS parameters (ne,sep, Te,sep, Zeff, αt). Leveraging normalizations for machine parameters (R, a, Bt, Ip) through the SepOS LH transition boundary minimum, while taking advantage of self-similarities in SOL transport physics, we demonstrate the utility of a normalized PE-SepOS framework in identifying accessible operational points for given exhaustible Psep requirements. In applying the PE-SepOS framework to project the SPARC and ITER operational space, we find inherent trade-offs, namely: i) accessing high impurity radiation scenarios leads to pronounced reductions in ne,sep (e.g., 50% ne,sep reductions at cNe=2%) as a consequence of power starvation, and ii) a compromise between high radiative fraction and high density/neutral pressure is required for access to small/no-ELM regimes at sufficiently high density, high αt conditions, with the divertor dissipative regime transitioning to pronounced detachment.
We further show that by establishing robust correlations between actuators ΓD2, ΓNe, Psep and physics parameters Te,t and cz, the PE-SepOS framework can be used to inform experimental strategies in scaling up from low heating power to reactor-scale values of Psep/R. Taking advantage of universal trends enabling projections of density and impurity seeding scans, the PE-SepOS thus provides a framework for mapping out the edge plasma operational space in a scalable manner.
[1] Eich T. et al 2025, NME 42 101896
[2] Faitsch M. et al 2023, NF 63 076013
[3] Miller M.A. et al 2014, NF 65 052002
[4] Lomanowski, B. et al 2022, NF 62 066030
[5] Lomanowski, B. et al 2023, NME 35 101425
The work was supported in part by the US DOE Office of Fusion Energy Sciences under contract DE-AC05-00OR22725 by the Innovation Network for Fusion Energy (INFUSE), and by Commonwealth Fusion Systems.