Preliminary Design of Insulation System for Superconducting Conductor Testing Facility

The Superconducting Conductor Testing Facility, which is developed to evaluate the reliability of engineering technology and safe operation in fusion reactor operation environment, is under engineering design by ASIPP. In case of an emergency shut-down like in succession of a quench, the voltage across the coil may rise to about 2.5 kV. The facility has to be reliably insulated. In this study, we present the preliminary design of the insulation system of the Superconducting Conductor Testing Facility, which includes turn and ground insulation. Since the magnet of the facility adopts the manufacturing process of “Wind&React”, the turn insulation material should withstand a heat treatment with temperature up to 650 °C. The insulation system is composed of S-glass fiber reinforced tape and vacuum-pressure impregnated in a DGEBF epoxy system. The mechanical properties are being subjected to investigations with respect to the design requirements and operating conditions. The test results are analyzed to verify the feasibility of the insulation system design.


Introduction
The Comprehensive Research Facility for Fusion Technology (CRAFT) in China is a national large-scale scientific and technological facility [1,2]. The superconducting conductor test facility is one of the important components of CRAFT, which is used to evaluate the performance of future superconducting components under high magnetic field and large size. The structure diagram of superconducting conductor test facility is shown in Fig. 1. As a key component of superconducting conductor test facility, the magnet system can provide 15 T background magnetic field for the test sample [3,4]. The magnet system consists of two split coils, each of which is composed of three concentric pairs of solenoid coil, including high field coil (HFC), medium field coil (MFC) and low field coil (LFC). The names of the 3 coils are based on their magnetic field respectively. The structure of coils is Nb 3 Sn cable-inconduit conductor (CICC). The HFC and MFC are layer winding, and the LFC is pancake winding.
The insulation system of the superconducting magnet plays an important role in the safe operation of the magnets. The insulation failure will lead to arc discharge, which will damage the magnet device and cause irreversible performance impairing. The insulating structure requires high mechanical strength to ensure that it can withstand the impact of multiple cold and heat cycles, as well as strong electrical strength to withstand the impact of high voltage [5,6].
According to the operation parameters of the magnet system of Superconducting Conductor Testing Facility, the insulation system was designed. The insulation system consists of S-glass fiber and Kapton tape. The epoxy resin and glass fiber tape are combined by vacuum pressure impregnation to form an insulation structure. The treatment of the insulating material was completed, and the mechanical test samples were prepared. The mechanical property tests were carried out to verify the feasibility of the design.

Design of Insulation System
Magnet System The magnet system consists 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 MFC1 and the MFC2. The MFC1/MFC2 is divided into 5 parts to ensure that each part has the same length of flow channel and prevent uneven flow when the magnet is cooled, resulting in insufficient temperature margin at some positions of the magnet.
The HFC and MFC are connected in series and powered by the same power supply, the operating current of which is set at 8.5 kA. The LFC is powered by another power supply alone, with the operating current set at 14 kA. The total stored energy of the magnet system of the test facility is about 560 MJ. The main parameters of the magnet system are shown in Table 1.

Design Requirements
The dielectric strength is a key requirement for the design of the electrical insulation systems. When quench occurs, the maximum induced voltage of the HFC and MFC terminal is 680 V, and the maximum induced voltage of the LFC terminal is 2520 V. The neutral point of the magnet is grounded to reduce the voltage to ground. Thus, 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 9 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 developted 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 a porous medium with elasticity modulus of 4 GPa. For HFC and MFC, the layer insulation   7) Connecting wire between medium field coils. (8) Connecting wire between low field coils is used to compensate the winding tolerances and the thickness of layer insulation 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 needs to be larger than 100.2 MPa, and the tensile stress in vertical direction should be larger than 40 MPa.

Insulation System Design
The insulation system of magnet includes turn insulation and ground insulation. The insulation system structure is represented in Fig. 4. The blue part is glass fiber and the red part is Kapton tape. The turn insulation of HFC and MFC needs to be wrapped first, and then the coil is heattreated, so the turn insulation is composed of four layers of temperature-resistant and high-strength glass fibers. The glass fibers are wound on the conductor in a half-lapped manner. 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 3.8 mm under compression.
The LFC winding consists of seven QPs and one DP. Overlapping joints close to the outer surface of LFC are   The designed turn insulation of LFC. c The designed ground insulation of QP and DP. d The designed ground insulation for magnet system 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 0.8 mm. 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 was wrapped, the vacuum pressure impregnation (VPI) technique is adopted. The vacuum environment is used to remove the moisture and volatile matter in the insulation material, reduce the gas in the gap between insulation layers. This avoids the formation of gaps during the impregnation process, and reduces the impregnation resistance, thereby increasing the resin permeability of the coil as a whole [7,8].

Mechanical Properties Test
During cooling and operation, the composite insulation structure should endure complex interlayer shear and tensile stresses. Therefore, it is necessary to conduct a mechanical performance test of the designed insulation system, which can validate the feasibility of insulation design and provide practical information for optimizing the insulation. The HFC and MFC adopts the manufacturing process of ''Wind&React'', therefor, its turn insulation material needs to endure a heat treatment of 650°C, so there is no Kapton in the turn insulation [11]. This structure is different from the commonly used one for superconducting magnets. Therefore, the mechanical performance test of the turn insulation of HFC and MFC was first carried out.

Sample Preparation
Organic compound is used as a sizing agent to coat on to the surface of glass fiber, providing anti-static festure and ensuring the mechanical integrity of the glass fiber fabric. During the heat treatment of Nb 3 Sn superconducting magnets, these organic compounds will carbonize under high temperature, reducing the insulating properties of superconducting magnets. Therefore, the organic compounds on the surface of glass fiber tape must be removed before heat treatment [9][10][11][12]. The experiments on the decarburization process of the high-strength glass fiber were carried out. The carbon removal process parameters of the glass fiber tape were determined, that is, with oxygen conditioning, the carbon removal temperature was 350°C, and the carbon removal process takes 5-7 h, during which 95% of the carbon content in the glass fiber tape is effectively removed. The SEM images of the glass fiber tape under different treatment processes are shown in Fig. 5. It is found that the conductivity of the carbonized sample of the glass fiber cloth is better than that of the original sample and the decarburized sample, and the surface C content of the carbonized and decarburized glass fiber cloth is significantly reduced.
The turn insulation studied is made of glass fibber wound on stainless steel plate instead of rectangular sheath to facilitate sample sampling. The first layer uses polytetrafluoroethylene (PTFE) tape to separate the insulation layer from the SS plate. The maximum heat treatment temperature is 650°C. Since PTFE decomposes at 650°C, the glass fiber, whose sizing is removed from glass before use, was heat treated before wrapping, as shown in Fig. 6.The parameters of heat treatment are consistent with the ones used for the actual coil, and then the insulation wrap was performed. The wrapped and fixed samples were placed into custom containers for VPI. As shown in Fig. 7, the process of VPI include tightness test, de-gassing for component, resin injection, gelling and curing. After curing, the plate was used to section test samples. The samples are cut by numerically controlled machine.

Test Procedure
Due to the anisotropic properties of the insulator, the sampling of the mechanical tensile properties test samples consists in two directions including parallel (0°) and perpendicular (90°) to the winding direction of the glass fibers [13,14]. The Ultimate Tensile Strength (UTS) were evaluated at 77 K and the test speed was about 2 mm per minutes. The tensile test is designed and made according to ASTM D638 standard, and the sample shape and size of the tensile test are shown in Fig. 8. The design thickness of the turn insulation structure is 0.8 mm. In order to prevent the sample from being damaged during the sampling process due to the thinness of the sample, four layers of inter-turn insulation were wrapped when the insulation was wrapped. The thickness of the turn insulation sample is 3.2 mm.
The interlaminar shear stress at 77 K was measured by three-point bending test (short beam bending method) [10]. The samples were processed and manufactured according The glass fiber without decarburization after 650°C heat treatment. c The glass fiber with decarburization after 650°C heat treatment to ASTM D2344 standard. The test speed is about 1 mm/ min. Before the test, the sample is immersed in liquid nitrogen for precooling. Both the clamp and the sample were immersed in liquid nitrogen to obtain the shear properties of the insulating sample at 77 K.

Test Result
There are 6 samples in 0°and 90°directions respectively, with 3 being tested at 77 K. The UTS of the turn insulation sample detected under 77 K are summarized in Table 2.
From the tensile performance test results, it can be easily obtained that UTS could meet the design requirements of higher than 100 MPa in the parallel direction and 40 MPa in vertical direction at 77 K. And at the same test temperature, the UTS in the 0°direction of the insulation system is much higher than the UTS in the 90°direction, it could be inferred from the fracture surface. For the parallel direction, the fracture did not occur until S-glass fiber and epoxy resin were broken, demonstrating strong strengthening effect of S-glass. But for the 90°direction, although 50% overlap is used, the epoxy resin plays a major role in the fracture process, resulting in a greatly reduced reinforcement strength.
The interlaminar shear strength is used to measure the ability of the re-cored material to withstand the shear stress, and it reflects the interface performance between the fiber material and the resin. The test equipment of MTS-SANS CMT5105 was used to maintain axial load as much as possible to obtain pure shear stress. The inter turn insulation shear stress measured at 77 K is summarized in Table 3. Similar to the trend of tensile performance, the shear stress in the 0°direction is higher than that in the 90°d irection. The result show that the shear stress satisfies the design requirements of shear strength higher than 37.8 MPa at 77 K.

Conclusion
The magnet system of Superconducting Conductor Testing Facility consists of three concentric split pairs of Nb 3 Sn coil. The insulation of magnet system, which is composed of S-glass fiber reinforced tape and vacuum-pressure impregnated in a DGEBF epoxy system, has been designed and tested for mechanical performance. The tensile performance and inter-laminar shear properties of the turn insulation of HFC and MFC at 77 K were investigated first, and the results showed that it can meet the design    requirements, suggesting that the designed structure and manufacture process is applicable. Furthermore, more R&D activities of the insulation system need be conducted, including electrical testing of insulation systems and insulation performance testing of mock up coil, etc.