The separatrix operational space of next-step fusion experiments: From ASDEX Upgrade data to SPARC scenarios

Fusion power plants require ELM-free, detached operation to prevent divertor damage and erosion. The separatrix operational space (SepOS) is proposed as a tool for identifying access to the type-I ELM-free quasi-continuous exhaust regime. In this work, we recast the SepOS framework using simple parameters and present dedicated ASDEX Upgrade discharges to demonstrate how to interpret its results. Analyzing an extended ASDEX Upgrade database consisting of 6688 individual measurements, we show that SepOS accurately describes how the H-mode boundary varies with plasma current and magnetic field strength. We then introduce a normalized SepOS framework and LH minimum scaling and show that normalized H-Mode boundaries across multiple machines are nearly identical, suggesting that the normalized SepOS can be used to translate results between different machines. The LH minimum density predicted by SepOS is found to closely match an experimentally determined multi-machine scaling, which provides a further indirect validation of SepOS across multiple devices. Finally, we demonstrate how SepOS can be used predictively, identifying a viable type-I ELM free Quasi-Continuous-Exhaust (QCE) operational point for SPARC, at n e , s e p = 4 × 1 0 20 m − 3 , T e , s e p = 156 e V and α t = 0 . 7 — a value solidly within the QCE operational space on ASDEX Upgrade. This demonstrates how SepOS provides a concise, intuitive method for scoping ELM-free operation on next-step devices. • Validation and Application of SepOS Framework: The separatrix operational space (SepOS) framework was validated for an extended database of 6688 individual measurements from ASDEX Upgrade, showing that it accurately predicts the operational state (L-mode, H-mode, or density-limit disruption) under varying plasma current and magnetic field strength conditions. This validation supports its application for future fusion devices. • Predictive Capability for Next-Step Devices: The SepOS framework identified a viable Quasi-Continuous Exhaust (QCE) operational point for SPARC, highlighting its potential for predicting ELM-free operation in next-step fusion devices. As well as avoiding type-I ELMs, the higher separatrix density and reduced separatrix temperature offer more benign steady-state heat exhaust conditions. • Normalization and Multi-Machine Applicability: Normalizing the SepOS results to the LH minimum density and temperature demonstrated nearly identical operational spaces across different machines, suggesting that the framework can translate results between devices. This finding indicates the framework’s broader applicability, including for future high-performance devices like SPARC, ARC, ITER and DEMO.