## 2 Development of the CHSN01 Jacket  
The objective of China's compact fusion device is to demonstrate fusion energy production with a fusion power of 40-200 MW and a fusion power gain (Q) exceeding 1. To accomplish this, the central solenoid (CS) must provide more than 50 volt-seconds to induce and sustain a plasma current of 4-7 MA. The CS magnet, which should generate a high magnetic field, is composed of six vertically stacked modules, each containing an inner HTS coil and an outer low-temperature superconducting (LTS) coil. The schematic diagram and the key parameters of the CS magnet are presented in Figure 1 and Table 1. The HTS coil is wound by a rare-earth barium copper oxide (REBCO) cable-in-conduit conductor (CICC) [30], which comprises six conductor on round core (CORC) sub-cables helically wound around a copper tube. With a maximum magnetic field strength reaching 20 T and electromagnetic stress on the CS magnet jacket up to 1300 MPa, employing high-strength stainless steel as the jacket material becomes imperative. Developed in China and based on the N50 nitrogen-controlled austenitic stainless steel material, modified N50 (CHSN01, formerly known as N50H) [31; 32] exhibits exceptional low-temperature mechanical properties by the approaches compared with the standard UNS S20910 (N50) as follows:  
1. The C content in CHSN01 is less than 0.01, while the C content in UNS S20910 is less than 0.06. The presence of carbon elements in steel will lead to the precipitation of M23C6 carbides in the matrix during the long aging process, which seriously deteriorates the low-temperature plastic toughness and corrosion resistance of steel; therefore, the ultra-low carbon content is controlled in CHSN01;
2. Ni, as one of the most important alloying elements in austenite formation, can enlarge the austenite phase region and inhibit the formation of high-temperature \(\delta\) ferrite. In addition, Ni can improve the low-temperature performance of austenitic nickel-chromium stainless steel, and the low-temperature toughness is significantly improved with the increase in nickel. Overall, the Ni content (14.5-15.5) in CHSN01 is slightly higher than the UNS S20910 specification of 11.5-13.5; \begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **HTS** & **LTS** \\ \hline I\({}_{\text{op}}\) (kA) & 46.5 & 46.5 \\ B\({}_{\text{max}}\) (T) & 18.8 & 12.4 \\ Radial turns & 5 & 10 \\ Axial turns & 30 & 30 \\ Inner radius (mm) & 447.5 & 705.5 \\ Outer radius (mm) & 685.5 & 984 \\ Axial spaces between modules (mm) & 56 & 28 \\ CICC length (m) & 534 & 1592 \\ Jacket dimensions (mm) & 40 \(\times\) 40 & 41.6 \(\times\) 23 \\ Cable dimensions (mm) & \(\Phi\)31.4 & 33 \(\times\) 14.4 \\ Turn/ground insulation (mm) & 1/8 & 1/8 \\ SC strand/tape & REBCO & High J\({}_{\text{c}}\) Nb\({}_{\text{3}}\)Sn \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The key parameters of the CS magnet. Figure 1: The schematic diagram of the CS magnet. In addition, the O content in CHSN01 is about 20 ppm, which is not mentioned in the UNS S20910. If the oxygen content is too high, oxygen will exist in the form of various inclusions, which will seriously reduce the plastic toughness, fatigue properties, and cold-hot processing properties of steel. With a yield strength of over 1500 MPa at 4.2 K, which meets the stress requirements, CHSN01 is utilized as the CS jacket material. An FEI Quanta 600 (Thermo Fisher Scientific Inc., Waltham, MA, USA) scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) was used for the observations. As shown in Figure 2, the SEM-EDS elemental mapping results indicate that the spatial distribution of each CHSN01 element is homogeneous. Table 2 lists the chemical composition of CHSN01 after at least five measurements. Figure 3 illustrates the cross sections of the REBCO jacket and the Nb\({}_{3}\)Sn jacket; although they possess different geometries, their thickness remains uniform at 4.3 mm.