The first quasi-axisymmetric stellarator ever built — a joint China-Japan experiment designed to test whether stellarators can confine particles as well as tokamaks while keeping the stellarator's disruption-free advantage.
CFQS — the Chinese First Quasi-axisymmetric Stellarator — is a mid-scale stellarator built through a collaboration between China’s Southwest Jiaotong University (SWJTU) and Japan’s National Institute for Fusion Science (NIFS). It holds a distinctive place in the world stellarator fleet: CFQS is the first device ever constructed with a quasi-axisymmetric (QA) magnetic configuration, making it a direct experimental test of one of the most promising theoretical ideas in stellarator optimization.[1]
Stellarators confine plasma using external magnetic coils alone, without the large internal plasma current that tokamaks require. This eliminates the disruption risk that plagues tokamaks — a significant advantage for a power plant. The historic drawback has been poor particle confinement: in a conventional stellarator, the three-dimensional magnetic geometry allows trapped particles to drift out of the plasma on relatively short timescales, leading to high neoclassical transport losses.
Quasi-axisymmetry offers a way around this problem. By shaping the magnetic field so that its strength varies in a pattern that mimics a tokamak’s axial symmetry — even though the physical geometry of the coils and plasma is fully three-dimensional — particle orbits can be confined just as effectively as in a tokamak. The concept was developed theoretically by Allen Boozer and Juergen Nuehrenberg in the 1980s and 1990s, but CFQS is the first hardware built specifically to test it.[2]
CFQS is a compact device with a major radius of approximately 1.0 m, an average minor radius of roughly 0.18 m, and a magnetic field on axis of 1.0 T. The plasma volume is modest — this is an experiment designed to validate physics principles, not to approach reactor conditions. The device uses a set of carefully optimized modular coils — 16 non-planar coils in total — whose shapes were computed to produce the quasi-axisymmetric field pattern within the plasma volume. The coil design draws on experience from Japan’s Large Helical Device program and from the theoretical optimization tools developed for Wendelstein 7-X.[3]
The CFQS project was formalized through a bilateral agreement between SWJTU and NIFS, with coil design and fabrication shared between the two institutions. Japanese expertise in heliotron and stellarator physics — accumulated over decades of operating the Compact Helical System (CHS) and the Large Helical Device (LHD) — complemented Chinese manufacturing capabilities and the host institution’s plasma diagnostics program. The collaboration represents one of the few active international partnerships in stellarator research outside the European Wendelstein program.[4]
The primary scientific mission of CFQS is to measure neoclassical transport in a quasi-axisymmetric configuration and compare it directly to theoretical predictions. If the confinement improvement predicted by QA optimization is confirmed experimentally, it would validate an entire class of next-generation stellarator designs — including proposed QA reactor concepts that could compete with tokamaks for a power-plant role. CFQS also plans to study MHD stability, plasma beta limits, and electron cyclotron heating scenarios in the QA geometry. The results will inform the design of larger quasi-symmetric stellarators and contribute to the broader question of whether the stellarator path to fusion energy is viable at reactor scale.[5]