H-mode access projections for high field tokamaks under various scaling assumptions

Power requirements for H-mode on the SPARC tokamak are assessed using a range of empirical scalings for the L-H power threshold P_L-H. The most well studied SPARC scenario is the Primary Reference Discharge (PRD), a DT plasma with B_T=12.2T, which has a reasonable window for H-mode access [1] based on transport simulations at varied density and input power, and on the commonly used 2008 ITPA scaling for P_L-H. Recently, new empirical scalings have been derived from a L-H database comprising multiple metal wall devices, with P_L-H generally having a stronger dependence on density and a weaker dependence on field, relative to the 2008 scaling. In SPARC, and also in ARC-class fusion devices, the new scaling introduces modest changes in L-H power requirements, with lower P_L-H projected at sufficiently low Greenwald fraction. Since SPARC will reside at the extremes of legacy devices’ field and density values, the uncertainties in projection from extrapolation become important to address and, eventually, to resolve through actual power threshold measurements on the new device. Meanwhile Alcator C-Mod L-H data are used to validate P_L-H scalings, for more confident projection to SPARC and ARC.

[1] J.W. Hughes et al., Nucl. Fusion 65 (2025) 052001.