Non-planar, no-insulation ReBCO coils for an optimized tabletop stellarator

We report on design, construction and testing of non-planar, non-insulated coils made from ReBCO (Rare earth barium copper oxide) high temperature superconductors (HTS) for the EPOS (Electrons and Positrons in an Optimized Stellarator) experiment, which aims to confine a pair plasma in a tabletop stellarator with a major radius of 0.2 m.

Utilizing the higher operating temperatures and in-field performance of ReBCO, we aim to realize the 2 T EPOS magnet system at 20 K operating temperature without cryogens in a compact form factor with high current density.

Over the past 2 years, we have designed, built and tested several coil prototypes starting with simple low-field coils at 77 K in liquid nitrogen and culminating in high-field and non-planar coils at below 20 K operating temperature.

During our test campaign, we achieved low manufacturing tolerances of less than 0.1 mm, the desired magnetic field strength on the superconductor (>3 T) and remarkable resilience against quenches due to the no-insulation winding pack. We avoid damage to the superconductor by optimizing the winding pack orientation to minimize strain from torsion and bending which originates from the complicated, non-planar shape of modular stellarator coils. The coil tests informed our engineering design and the manufacturing process for the EPOS magnet system.

We discuss coil tests, the engineering design of EPOS and the current state of its assembly currently underway at the Max-Planck Institute for Plasma Physics in Garching, Germany.