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

TL;DR

This paper introduces the separatrix operational space (SepOS) as a tool to identify ELM-free regimes in fusion experiments, validating it with ASDEX Upgrade data and applying it to predict operational points for the SPARC device.

Contribution

The paper develops a normalized SepOS framework that can be used across different fusion devices and demonstrates its predictive capability for next-step experiments like SPARC.

Findings

SepOS accurately describes H-mode boundary variations with plasma current and magnetic field.

Normalized SepOS boundaries are nearly identical across multiple machines.

SepOS predictions closely match multi-machine scalings and identify viable operational points for SPARC.

Abstract

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 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 QCE operational point for SPARC, at a separatrix density of 4e20/m3, a separatrix temperature of 156eV and an alpha-t of 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.