The key outcomes in the field of 9 T NbTi liquid helium-free superconducting magnet technology in 2024 have been summarized. This 9 T NbTi magnet utilizes an independently developed conduction cooling technique for temperature reduction. It features self-developed conduction cooling current leads that offer low heat leakage and high strength. The magnet was successfully magnetized to its operational field in September 2024.

Development of Conduction Cooling Technology

On one hand, we achieved significant optimization of low-temperature system component materials, configurations, and connections through theoretical analysis and numerical calculations, reducing the conduction-cooled low-temperature system thermal shield temperature from 70 K to 30 K and lowering the second-stage temperature from 10 K to below 3 K, greatly enhancing the magnet's temperature margin.

On the other hand, we independently developed NbTi magnet conduction cooling technology and continually optimized it based on experimental results, ultimately forming an efficient construction plan for the conduction cooling path of NbTi magnets. Under this plan, the temperature difference between the cryocooler’s second-stage cold head and the magnet in the magnet’s no-load state is less than 0.2 K, and during stable operation under full load, the temperature difference remains below 0.4 K.

Fig. 1 Model of the 9 T NbTi Liquid Helium-Free Superconducting Magnet Cryogenic System.

Development of Conduction Cooling Current Lead Technology

NbTi liquid helium-free superconducting magnets are typically powered by commonly used high-temperature superconducting dual current leads. However, conventional YBCO current leads often face issues such as significant heat leakage and low strength; the former makes it difficult to lower the magnet's temperature, while the latter can cause the current leads to burn out in strong magnetic fields. To address these problems, we have independently designed a conduction cooling YBCO dual current lead that offers the advantages of low heat leakage and high strength.

The first stage of the current lead utilizes an optimal design for conduction cooling, employing brass material to balance the requirements of lead strength and thermal load, with a single lead exhibiting heat leakage of less than 5 W. The second stage employs YBCO tape for conduction. This YBCO current lead has a heat leakage of less than 25 mW per lead and its high-strength design can support the electromagnetic forces under a 9 T magnetic field for extended operation.

Fig. 2 Magnet Excitation Curve

9T NbTi Liquid Helium-free Superconducting Magnet

We have successfully applied our independently developed conduction cooling technology and conduction cooling current lead technology to the 9 T NbTi liquid helium-free superconducting magnet, which has now been successfully excited to the field. This magnet weighs less than 80 kg, has a cold bore diameter of 100 mm, and achieves a field uniformity of ±0.1% at a 1 cm sphere, with a central magnetic field strength of up to 9.2 T. Moreover, the overall temperature of the magnet in stable operation at 9.2 T remains below 3.3 K, providing a significant temperature margin.

Fig. 3 9 T NbTi Liquid Helium-free Superconducting Magnet