A. Magnet system
The magnet system consits of 3 split pairs of the solenoid coil, as shown in Fig. 2. In order to ensure that the temperature between the windings meets the requirements during the heat treatment of the coil, the HFC1 is divided into two coils, and the same is true for the HFC2, the MHC1 and the MFC2.
The HFC and MFC are connected in series and powered by the same power supply, the operating current was set at 8.5 kA. The LFC is powered by antoher power supply alone, and the operating current is set at 14 kA. The total stored energy of the magnet system is about 560MJ. The main parameters of the magnet are shown in Table 1.
Fig. 2. Cross section of magnet system. (1) Left high field coil (HFC-1). (2) Right high field coil (HFC-2). (3) Left medium field coil (MFC-1). (4) Right medium field coil (MFC-2). (5) Left low field coil (LFC-1). (6) Right low field coil (LFC-2). (7) Connecting wire between medium field coils. (8) Connecting wire between low field coils.
TABLE I
The main parameters of magnet
Item
|
HFC1/HFC2
|
MFC1/MFC2
|
LFC1/LFC2
|
Conductor size (mm×mm)
|
30.8×14
|
24.6×14.8
|
33.5×15.2
|
Radial turn
|
16
|
3
|
2
|
2
|
2
|
3
|
31
|
Axial turn
|
15
|
28
|
26
|
25
|
24
|
22
|
32
|
Total turn
|
240
|
300
|
992
|
Inner radius (mm)
|
322
|
621.6
|
673.8
|
708.6
|
754.4
|
789.2
|
878.1
|
Outer radius (mm)
|
596
|
671.2
|
706.0
|
751.8
|
786.6
|
838.8
|
1421.3
|
Conductor length (m)
|
691.6
|
1363
|
7136
|
Maximum magnetic field (T)
|
15.7
|
12.8
|
10.6
|
Current (kA)
|
8.5
|
8.5
|
14
|
B. Design requirements
For design of electrical insulation systems, the focus has been on dielectric strength. When quench occurs, the maximum inducted voltage of the HFC and MFC terminal is 680 V, and the maximum inducted voltage of the LFC terminal is 2520 V. The neutral point of the magnet is grounded to reduce the voltage to ground. So the maximum voltage to ground of the HFC and MFC is 340 V, and the maximum voltage to ground of the LFC is 1260 V. The fabricated coil insulation sections must be able to withstand test voltages of (2 x maximum operating voltage) + 1 kV. Thus, the withstand acceptance test voltage of HFC and MFC is 1700 V, and the withstand acceptance test voltage of LFC is 3500 V.
In order to obtain the mechanical properties of the insulation, the two-dimensional finite element method was used to analyze the mechanical properties of the magnet. The applied loads include Pre-compression load (P), thermal load (T) and Electromagnetic load (E). A two-dimensional model with axisymmetric and mid-plane symmetry is used to calculate the stress levels of the conductor and the insulator in the pre-tightened state (P), the cooling state (P+T), and the operating state (P+T+E). In this model, the superconducting cable with voids can be seen as porous medium with elasticity modulus of 4 GPa. In order to compensate the winding tolerances, the layer insulation for HFC and MFC is set as 1 mm. As for LFC, each pancake of the quad-pancakes (QP) and the double pancake (DP) is with 1 mm insulation, and 1.5 mm among QPs. The material properties used for the analysis are retrieved from the ITER database[8]. After analysis and calculation, for the turn insulation part, the peak shear stress of the HFC is 34.1 MPa, the peak shear stress of the MFC is 37.8 Mpa, and the peak shear stress of the LFC is 35.5 MPa. The peak shear stress is distributed at the rounded corners of the conductor, as shown in Fig. 3. The tensile stress in 0° direction (the same as the winding direction of glass ribbon) is 50.1 MPa, and the tensile stress in 90° direction (which is perpendicular to the winding direction of the glass ribbon) is 20 MPa. The safety factor for insulation design is 2 , that is, the tensile stress in the parallel direction need to be lager than 100.2 MPa, and the tensile stress in vertical direction should to be larger than 40 MPa.
C. Insulation system design
The insulation system of magnet include turn insulation and ground insulation. The insulation system structure is represented in Fig. 4, the blue part is glass fiber and red part is Kapton tape. The turn insulation of HFC and MFC needs to be wrapped first, and then the coil is heat-treated, so the turn insulation is composed of 4 layers of temperature-resistant and high-strength glass fibers. The glass fibers are wound on each turn of the bending conductor in turn, and the coil is surrounded by 50% overlap method. The thickness of turn insulation is 0.8 mm. The ground insulation layer is composed of five layers of glass fiber and four layers of glass Kapton composite tape, with a thickness of 3.8mm under compression. The widths of glass fiber and Kapton used are 30mm and 25mm respectively. The minimum tracking length is 20 mm (10 mm/GK layer) for such two layer Glass-Kapton overlapping, so the ground insulation has enough tracking length.
The LFC winding consists seven QPs and one DP. Overlapping joints close to the outer surface of LFC are used to connect QPs and DP. The turn insulation is composed of two layers of glass fiber and one layer of Glass-Kapton composite tape. The total thickness of the 3 layers is 0.8mm.The ground insulation of QP and DP is composed of four layers of glass fiber and one layer of Glass-Kapton composite tape half-stacked. The total thickness of this 5 layers is 2 mm. The ground insulation of LFC, HFC and MFC are the same.
After the conductor were wrapped, the vacuum pressure impregnation (VPI) technique is adopted. The purpose of VPI is transfering the resin into the pre-wound fiber-glass and Kapton for curing. The advantage of this technique is that, there are no defects as gas pocket during the treatment process, in order to assure the mechanical and electrical performance of the manufactured insulation system.