# Operation Margin of the ITER Central Solenoid During the Plasma Scenario
## 2 The ITER Central Solenoid and the Cryogenic Plant  
The CS system is composed of six coils [31; 32] and its feeders with the associated cryogenic cooling loop, as shown in Figure 1[33; 34].  
The CS assembly consists of a stack of six identical circular coils, called modules CS3L (bottom lower module) to CS3U (top upper module), compressed together by a mechanical structure and attached to the toroidal field coils (TF) through a supporting system and centring system; see Figure 1a. The assembly has a diameter of 4.3 m and a height of more than 16 m. Each module is composed of 20 double pancakes (40 pancakes in total) [34].  
The modules are cooled with supercritical He (SHe) at 4.3 K in parallel by their feeders, as shown in Figure 1b. This loop is closed and independent of any other supercritical loops from the hydraulic point of view [34].  
In the CS modules, three different conductor types are used: the conductors in the modules (see Figure 2a), the busbar lead extensions (EXT; see Figure 2b) and the main busbar conductors (MB; see Figure 2c). The conductors in the modules and the lead extensions are manufactured as round cables in a square stainless-steel jacket, while the main busbars are manufactured as round cables in a round stainless-steel jacket. The different strands and corresponding manufacturers used in the aforementioned conductors are reported in Table 1. The main busbars are manufactured with NbTi strands produced by the Western Superconducting Technologies Co., Ltd. (WST, Xi'an, China). The lead extensions (EXT) and the CS modules are produced with Nb\({}_{3}\)Sn strands manufactured by Jastec (JAS, Hyogo, Japan), Furukawa (FUR, Tokyo, Japan) and Kiswire Advanced Technology (KAT, Daejeon, Republic of Korea); they have two main differences:  
Figure 1: (**a**) CS modules [33; 34]; (**b**) CS feeder system [33; 34].  
1. The conductor realised with KAT strands has a slightly larger central spiral--8 mm \(\times\) 10 mm instead of 7 mm \(\times\) 9 mm--which implies different helium sections and void fractions;
2. The JAS and FUR strands are based on the bronze route process, whereas KAT is based on the internal tin route, and they exhibit slightly different performance (T\({}_{\text{cs}}\), AC loss characteristic, etc.).  
In the bronze route process, niobium (Nb) filaments are embedded inside the bronze matrix (Cu-Sn alloy) used as the tin source. Instead, in the internal tin route, pure tin (Sn) is used as a direct source, instead of a bronze alloy. During heat treatment, the tin in the bronze diffuses into the Nb filaments, forming Nb\({}_{3}\)Sn at the Nb-bronze interface. In the internal tin route, the higher tin content leads to faster and more complete Nb\({}_{3}\)Sn formation. Typically, this results in a higher critical current density (\(J_{c}\)) than in the bronze route. However, the mechanical properties can be weaker compared to those of bronze route wires.  
The CS modules, except for the two central ones (CS1U and CS1L), have independent power supplies [33; 34], used forplasma current initiation, positioning and shaping control. The two central modules of the CS are connected in series to a common power unit.  
\begin{table}
\begin{tabular}{c c c} \hline
**CS Module** & **Strand Type** & **Manufacturer** \\ \hline CS3U & Nb\({}_{3}\)Sn & Kiswire Advanced \\  & (internal tin) & Technology (KAT) \\ CS2U & Nb\({}_{3}\)Sn & Furukawa \\ CS1U & (bronze) & (FUR) \\ CS1U & Nb\({}_{3}\)Sn & Jastec \\ CS1L & (bronze) & (JAS) \\ CS2L & Nb\({}_{3}\)Sn & Kiswire Advanced \\  & (internal tin) & Technology (KAT) \\ CS3L & Nb\({}_{3}\)Sn & Jastec \\  & (bronze) & (JAS) \\ busbar & Nb\({}_{3}\)Sn & Jastec \\ lead extension (EXT) & (bronze) & (JAS) \\ main busbar & NbTi & Western Superconducting \\ (MB) & & Technologies (WST) \\ \hline \end{tabular}
\end{table}
Table 1: Conductor strand manufacturers.  
Figure 2: Conductor cross-section: (**a**) CS, (**b**) busbar lead extension and (**c**) MB.