Prototyping and Test of the "Canis" HTS Planar Coil Array for Stellarator Field Shaping

TL;DR

The paper presents the design, manufacture, and cryogenic testing of the Canis 3x3 HTS planar shaping coil array as a proof-of-concept for a planar coil stellarator. It demonstrates closed-loop control of nine HTS coils at $T\approx 20\ \mathrm{K}$, achieving field shapes projected onto a plane 25 cm from the array with RMS error $E_{RMS}\lesssim 1\%$ relative to predictions, and validates the feasibility of scalable, demountable shaping coil units. Key insights include the validated mutual-inductance interactions, robust manufacturing across multiple HTS suppliers, and a comprehensive cryogenic/test infrastructure (cryostat, in-vessel structure, cooling, instrumentation, and ATLAS scanning). The results support the viability of HTS planar shaping coils for confining stellarator plasmas and inform the path toward larger, more integrated FSUs. The work has practical impact by reducing coil complexity and enabling modular maintenance for future fusion devices. $E_{RMS}$ values for EOS shapes reached sub-percent accuracy, underscoring the approach’s potential for reliable stellarator field shaping.

Abstract

Thea Energy, Inc. is currently developing the "Eos" planar coil stellarator, the Company's first integrated fusion system capable of forming optimized stellarator magnetic fields without complex and costly modular coils. To demonstrate the field shaping capability required to enable Eos, Thea Energy designed, constructed, and tested the "Canis" 3x3 array of high-temperature superconductor (HTS) planar shaping coils after successfully demonstrating a single shaping coil prototype. Through the Canis 3x3 magnet array program, Thea Energy manufactured nine HTS shaping coils and developed the cryogenic test and measurement infrastructure necessary to validate the array's performance. Thea Energy operated the array at 20 K, generating several stellarator-relevant magnetic field shapes and demonstrating closed loop field control of the superconducting magnets to within 1% of predicted field, a margin of error acceptable for operation of an integrated stellarator. The Canis magnet array test campaign provides a proof of concept for HTS planar shaping coils as a viable approach to confining stellarator plasmas.

Figures (20)

• Figure 1: A CAD rendering of the Canis 3x3 magnet array, showing plumbing and current leads.
• Figure 2: Double pancake (DP) performance trends over serial number as measured in liquid nitrogen testing at 77 K, for DPs manufactured from three different suppliers. Shown are (a) radial resistance, (b) equivalent series resistance (ESR) and (c) field strength normalized by power supply current. Field is measured by a Lake Shore HGCA-3020 cryogenic Hall effect sensor, located along the central axis of the coil 36.3 mm from the coil midplane.
• Figure 3: Winding pack performance trends over serial number as measured in liquid nitrogen testing at 77 K. Shown are (a) radial resistance, (b) equivalent series resistance (ESR) and (c) field strength normalized by power supply current. Field is measured by a Lake Shore HGCA-3020 cryogenic Hall effect sensor, located along the central axis of the coil 36.3 mm from the coil midplane.
• Figure 4: Canis 3x3 magnet array layout, showing mounting pitch, location indexing, and WP allocation. Also shown is the location of each Hall effect sensor.
• Figure 5: CAD rendering of in-vessel structure, including magnet array structural plate and gravity supports.