Experimental Results from Early Non-Planar NI-HTS Magnet Prototypes for the Columbia Stellarator eXperiment (CSX)

TL;DR

The paper presents a staged prototype program (P1–P3) to develop non-planar HTS magnets for the CSX stellarator, targeting an on-axis field of $0.5T$ while addressing ReBCO tape strain sensitivity. It introduces a winding approach on 3D-printed aluminum frames with a gimballed constant-tension mechanism and solder potting to enable low-resistance lap joints and passive quench mitigation. Experimental results show P1 delivering $0.55mT$ on-axis at $10A$ in LN2 and P2 reaching $4.5mT$ at $110A$ at $20K$ with $R≈1.67μΩ$, plus quenching near $120A$ and an inductance around $1.04mH$ (Grover estimate $1.53mH$); joint tests at $77K$ achieve $R≈380nΩ$ for 30 mm joints. P3 targets higher fields with 200 turns and concave geometry to approach $0.5T$ on-axis and ~1.5T on-coil, enabling full-scale CSX coil fabrication; the work de-risks manufacturing, cooling interfaces, quench management, and diagnostics, and lays the groundwork for higher-field tests, conformal cryostats, co-winding HTS layers, and tighter tolerances to meet quasi-symmetry requirements.

Abstract

The Columbia Stellarator eXperiment (CSX) is an upgrade of the Columbia Non-neutral Torus (CNT) that aims to demonstrate a university-scale, quasi-axisymmetric stellarator using high-temperature superconducting (HTS) technology at an on-axis magnetic field target of 0.5 T. Due to the strain sensitivity of ReBCO (Rare-earth Barium Copper Oxides), adapting it to non-planar stellarator geometries requires new winding, structural, and cooling strategies. We report on the results of a staged prototype program (P1, P2, P3) employing 3D-printed, sectional aluminum coil frames with winding channels, gimballed constant-tension winding mechanics, and solder potting for radial current redistribution and passive quench mitigation. The first prototype, P1 (planar elliptical, double-pancake) tested additive manufacture, sectional joining and baseline winding, achieving predicted fields at 77 K. P2 (non-planar, higher strain) was wound to 42 turns, energized at 30-40 K to produce expected magnetic fields, and studied thermal gradients and resistance at up to 4.5 kAt. Design evolution in P3 introduces concave geometry with dual double-pancakes, 200 turns, and approaches the 70 kAt target at 20 K. In parallel, sub-microhm lap joints have been developed. Together, these results de-risk manufacturing, cooling interfaces, quench management, and diagnostics, paving the way for full-size non-planar HTS stellarator coils for CSX.

Figures (9)

• Figure 1: Prototype 2 (P2) HTS magnet installed inside the cryogenic test-stand prior to cooldown. The photograph shows voltage taps and silicon diodes attached for diagnostics. The HTS leads are interfaced to copper current terminals at the top of the assembly, which connect to HTS-110 G10 leads.
• Figure 2: 77 K tests of elliptical prototype magnet P1, shown immersed in a liquid nitrogen bath during characterization. The coil consists of 22 turns of HTS tape, with copper terminations clamped at either end and leads attached for resistance measurements.
• Figure 3: CAD rendering (top) and photograph (bottom) of the P3 prototype non-planar HTS magnet mounted in a custom gimbal winding system. A central ball joint allows the winding angle to vary dynamically as the coil is rotated, enabling accurate placement of HTS in the 3D-printed channels.
• Figure 4: CAD rendering (left) and photograph (right) of the cryogenic test-stand with P2 installed. The modular design allows for rapid installation and removal of prototype magnets for testing. Feedthroughs on the top flange accommodate high-current leads, vacuum sensors, and other diagnostics. The rendering omits the radiation heat shield and multilayer insulation (MLI).
• Figure 5: Resistance characterization of a $30$ mm HTS lap joint measured at $77$ K. The slope of the V–I curve indicates a joint resistance of approximately $380$ n$\Omega$, with good linearity across the $0–10$ A range