Yanchaojuneng Completes VPI of Stainless Steel-Jacketed Superconducting Prototype Coil for Stellarator

Recently, Yanchaojuneng (Shanghai) Technology Co., Ltd. completed vacuum pressure impregnation (VPI) of a next-generation stellarator stainless steel-jacketed three-dimensional shaped superconducting prototype coil. Previously, the prototype coil had completed two core processes: three-dimensional precision winding and ground insulation wrapping. The completion of VPI signifies that the company has established an autonomous process chain from coil winding and forming, insulation wrapping, to integrated insulation curing.

The stellarator is an important device type for steady-state operation in the field of controlled nuclear fusion, capable of reducing the risk of major plasma disruptions. Three-dimensional shaped superconducting coils are the core components for building the magnetic field that confines plasma in a stellarator. Public information shows that Germany's W7-X stellarator uses aluminum alloy jackets, and Japan's LHD device uses copper jackets. By comparison, high-strength stainless steel jackets offer advantages in cryogenic yield strength, cryogenic toughness, and fatigue resistance, with cryogenic yield strength exceeding 700 MPa, which is conducive to improving the load-bearing capacity and long-term service stability of magnets under strong electromagnetic loads.

Stainless steel-jacketed conductors have a narrow plastic forming window and pronounced springback effects. Combined with the multi-layer, multi-turn, and complex spatial configuration of stellarator prototype coils, their manufacturing and insulation curing are highly challenging. Yanchaojuneng, relying on China's first dedicated production line for stellarator three-dimensional shaped superconducting magnets, which was put into operation in early 2026, continues to advance engineering validation of related processes.

Figure 1 Coil after ground insulation wrapping (6*12 turns)

The prototype coil that completed VPI validation this time is a 6-layer, 72-turn three-dimensional shaped winding with continuously varying curvature and a non-planar complex spatial configuration. Compared with the double-pancake coil previously validated, this prototype coil has significantly more turns and layers, and the stacking of multiple winding layers leads to more prominent internal stress accumulation. Stainless steel jackets differ from traditional aluminum and copper jackets in stiffness and cryogenic thermal expansion coefficient, further increasing the difficulty of VPI process control.

Vacuum pressure impregnation is a core process in superconducting magnet insulation manufacturing, typically including vacuum degassing and drying, high-pressure resin impregnation, and stepped temperature-controlled curing, with the goal of forming an integrated insulation structure free of bubbles, voids, and cracks. For large-sized, multi-layer three-dimensional shaped prototype coils, issues such as uneven resin filling, interlayer interface bonding failure, and coil dimensional accuracy deviation caused by stress release during the curing stage are prone to occur, potentially affecting the previously achieved millimeter-level forming accuracy.

Focusing on the structural characteristics of this prototype coil, the Yanchaojuneng R&D team iteratively optimized the VPI scheme, conducted multiple sets of process experiments, adjusted the degassing process, impregnation pressure, and staged temperature-controlled curing curves, and resolved issues such as resin filling blind zones inside complex curved windings, interlayer debonding, and residual bubbles.

After VPI completion, the team obtained a complete three-dimensional model of the coil through 3D scanning and conducted assembly simulation verification with the theoretical model of the coil housing. The results showed that the coil exhibited no significant deformation after VPI treatment, and the three-dimensional overall dimensional error remained stable at the millimeter level; there was no spatial interference between the coil and the coil housing model, meeting the requirements for housing assembly. Test results also showed that the resin filling inside the coil was dense, the insulation layer had no bubbles, no voids, and no cracks, and the jacket conductor and insulation interface were tightly bonded; the ground insulation and interlayer insulation performance indicators all met the requirements, making it suitable for service environments with combined extremely low temperatures, strong magnetic fields, and large electromagnetic stresses.

Yanchaojuneng stated that the completion of VPI for this prototype coil indicates that it has mastered the complete set of processes for high-strength stainless steel-jacketed superconducting magnets, from conductor design, high-precision bending—twisting composite three-dimensional forming, multi-point fixed-shape ground insulation wrapping, to vacuum pressure impregnation integrated insulation curing. Related technologies have obtained multiple national patent authorizations.

Next, Yanchaojuneng will advance key processes such as nano-ohm-level superconducting joint assembly and coil housing insertion, complete prototype coil integration, and conduct full-scale superconducting comprehensive performance testing under extremely low temperature and strong magnetic field conditions to assess coil electromagnetic performance, insulation reliability, and structural load-bearing capacity. The company will also continue to improve the stainless steel-jacketed superconducting magnet process system and promote the engineering application of related technologies in stellarator devices.