CRYOGENIC TEMPERATURE. Wooho Chung, Bungsu Lim, Myungkyu Kim, Hyunki Park, Keeman Kim,. Yong Chu, Sangil Lee. Korea Basic Science Institute.
MECHANICAL AND THERMAL CHARACTERISTICS OF INSULATION MATERIALS FOR THE KSTAR MAGNET SYSTEM AT CRYOGENIC TEMPERATURE
Wooho Chung, Bungsu Lim, Myungkyu Kim, Hyunki Park, Keeman Kim, Yong Chu, Sangil Lee Korea Basic Science Institute Daejeon, 305-380, Korea
ABSTRACT The KSTAR(Korea Superconducting Tokamak Advanced Research) superconducting magnet is electrically insulated by the composite material of epoxy resin and glass fiber (2.5 kV/mm) and Kapton (8 kV/mm). The insulation composite material of epoxy resin and glass fiber is prepared using a VPI (Vacuum Pressure Impregnation) process. The superconducting magnet is under mechanical stress caused by the large temperature difference between the operation temperature of the magnet and room temperature. The large electro-magnetic force during the operation of the magnet is also exerted on the magnet. Therefore, the characteristics of the insulation material at cryogenic temperatures are very important and the tensile stress and thermal expansion coefficient for the insulation materials of the KSTAR superconducting magnet are measured. This paper presents results on mechanical properties of the insulation material for KSTAR magnets, such as density, ultimate tensile stress and thermal contraction between room temperature and cryogenic temperatures.
INTRODUCTION The KSTAR(Korea Superconducting Tokamak Advanced Research) device is a tokamak with a fully superconducting magnet system, , which enables an advanced quasisteady-state operation. The major radius of the tokamak is 1.8 m and the minor radius is 0.5 m with the elongation and triangularity of 2 and 0.8, respectively [1]. The KSTAR superconducting magnet system consists of 16 TF coils and 14 PF coils. The TF coil system provides a field of 3.5 T at the plasma center and the PF coil system is able to
CP711, Advances in Cryogenic Engineering: Transactions of the International Cryogenic Materials Conference - ICMC, Vol. 50, edited by U. Balachandran © 2004 American Institute of Physics 0-7354-0187-X/04/$22.00
297
3730mm Cryostat
Concrete Floor
FIGURE 1. KSTAR Tokamak Configuration
provide a flux swing of 17 V-sec. Both of the TF and PF coil systems use internally cooled superconductors. The overall tokamak configuration of the KSTAR is shown in FIGURE 1 and major parameters are summarized in TABLE I. [2] TABLE 1. Major Parameters of KSTAR Parameters
Baseline
Toroidal field, BT (T) Plasma current, Ip (MA)
3.5 2.0
Major radius, RQ (m)
1.8
Minor radius, a (m)
0.5
Elongation, KX
2.0
Upgrade
Triangularity, 8X
0.8
Poloidal divertor nulls
2
1&2
Pulse length (s)
20
300
Heating power (MW) Neutral beam
8.0
16.0
Ion cyclotron
6.0
6.0
Lower hybrid
1.5
3.0
Electron cyclotron
0.5
1.0
1.5 x 10
2.5 x 10
Peak DD neutron source rate(s )
298
Turn
Ground
: 0jl mm
: 7*6 mm
B
Scale 20.0 FIGURE 2. Schematic of PF Coil Insulation Structure
The procedures for coil fabrication are as follows: (i) CICC (Cable-In-Conduit Conductor) leak test; (ii) grit blasting; (iii) CICC winding; (iv) attachment of helium feedthroughs and joint terminations; (v) A15 reaction heat treatment for Nb3Sn superconducting magnets; (vi) insulation taping and ground wrapping; (vii) VPI; (viii) encasing; and (ix) test. A continuous winding scheme without internal joints is adopted to reduce the joint losses. Since PF6 and PF7 coils use NbTi CICC and the reaction heat treatment process is not required, the helium feed-throughs attachment and Kapton and S2glass insulation taping are carried out during the winding process. Each turn of the coil is individually separated and the CICC is insulated with Kapton and S2-glass tapes. The thicknesses of Kapton and S2-glass tapes are 0.05 mm and 0.178 mm respectively. FIGURE 2 shows the schematic of PF coil insulation structure. S2-glass roving is applied at the corner of the CICC to minimize the resin rich area. G10 pieces, which are shaped to fill the empty space of the layer transition area, are also inserted and the coil bundle is ground wrapped using S2-glass tape. The thickness of S2glass tape for the ground wrapping is 0.254 mm. The coil bundle is placed in a molding die and vacuum pressure impregnated. VANTICO GY282(100w%), HY918(82w%), and DY073-l(0.25w%) are used as the epoxy resin, hardener, and accelerator, respectively. The pre-mixed resin is warmed to 40 °C and injected to the molding die. The curing occurs at 80 °C for 12 hours and at 120 °C for 24 hours. Under the nominal operation condition of the KSTAR device, a large electromagnetic stress is generated in the CICCs and supporting structures. The different thermal expansions of the CICC winding pack, ground wrap and supporting structures also produce a tensile stress at cryogenic temperatures. In order to define the mechanical stability of the KSTAR superconducting magnet system, it is important to measure the mechanical and thermal characteristics of the insulation material of the glass fiber reinforced epoxy for KSTAR magnets. TENSILE BEHAVIOR
The insulation material for tensile stress and thermal expansion measurements is prepared using the same process used for the KSTAR superconducitng magnet VPI process. Before resin injection, the vacuum pressure is maintained below 2x10~2 torr. After resin injection, the VPI die is pressurized to 2.5 bar. The properties of the insulation materials used in this study are summarized in TABLE 2. Static tensile testing is carried out at both 300K and 77K with an MTS Alliance RT/100 Material Testing System, equipped with a liquid nitrogen cryostat (FIGURE 3). 299
The static ultimate tensile strength (UTS) is measured according to ASTM standard D638. The KSTAR insulation material of the glass fiber reinforced epoxy and a commercial G10 composite material were prepared for tensile strength experiments. For the experiments at cryogenic temperature, a special grip is used (FIGURE 4). The grip is able to hold the specimen at cryogenic temperatures. The fractured KSTAR insulation material and G10 are shown at FIGURE 5. TABLE 3 shows the UTS of the KSTAR insulation material and G10. The UTS value of G10 from this experiment is same as the value given by the supplier within the measurement error. Earlier test results of ITER Insulation materials were obtained from the literature [3] for comparison. The UTS of the KSTAR insulation material is higher than the commercial G10 and ITER Insulation materials. TABLE 2. Properties of the insulation material. Material
Density at 300K (g/cm3)
Fiber Content (w%)
KSTAR insulation material G10 (Made by Hankook Fiber)
1.713
57.59
1.866
56.44
TABLE 3. Tensile Stress of Test Materials
Material
UTS (MPa) at 300K
UTS (MPa) at 77K
KSTAR Insulation material (0°)
896
1035
G10 (45°) (Made by Hankook Fiber)
383 (327*)
420
ITER Insulation material test (0°) [3]
938
* The value given by the supplier
(a) Material Testing System
(b) Liquid Nitrogen Cryostat
FIGURE 3. MTS Alliance RT/100 Material Testing System, which is equipped with a liquid nitrogen cryostat 300
(a) Grip for cryogenic temperatures
(b) Specimen in Grips
FIGURE 4. Grip for cryogenic temperatures and the specimen.
FIGURE 5. Specimen after extension testing
THERMAL EXPANSION COEFFICIENT
The thermal expansion coefficient is measured between 300K and 4K using a Quantum Technology Model 6000 PPMS system. Two well-matched strain gauges (WKseries) are bonded to the specimen of the reference material, which is OFHC copper, and the specimen of the test material. Figure 6 shows a schematic of PPMS and the specimen installed on the puck. Thermal outputs (temperature-induced apparent strain) from two strain gauges are measured and the thermal expansion coefficient is calculated with the difference of the thermal outputs as defined by equation (1). In this equation, a is the thermal expansion coefficient and s is the thermal strain. The subscript S and R represent the test material and reference material, respectively [6]. \£TIO(GIS)
(i)
301
Temperature control
(a) PPMS diagram
(b) Thermal expansion coefficient test module FIGURE 6. PPMS & Thermal expansion coefficient test module
TABLE 4. Thermal expansion coefficients in the range 273-1 OK for KSTAR insulation material (0°) ___ Temperature [K]
273
150
77
60
50
40
30
20
15
10
Thermal expansion coefficient [10'6/K]
9.52
9.96
8.15
7.31
6.72
5.83
4.48
3.35
3.97
4.77
302
0
50
100
150
200
250
300
Temperature [K| FIGURE 7. Thermal Expansion Coefficient of the KSTAR Insulation material (0°) between 300K and 4K
0.10-1
——— Type 316 STAINLESS STEEL [7] —— KSTAR INSULATION MATERIAL . ............... .............. ............... .............. ............... ............... .............. ............. .............. .............. ............... .............. .
0.05-
..................................................................... . / .. .___._______.^«^__. "c" -0.05. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ^^..........:......................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..._._^p?.__. _..____. E .._ . . . . . . . . . . . . . ^-^ _X_ _..__..____ . .1 -0.20........ ^^. .....^S.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... —*^ . . . .................. ............ . . .... . 0.00-
-—
_n 4C
-0.300
50
100
150
200
250
300
Temperature [K|
FIGURE 8. Thermal expansion of KSTAR Insulation material (0°) and type 316 stainless steel between 300K and 4K
The thermal expansion coefficient of the KSTAR insulation material from 300K to 4K is shown in FIGURE 7 and TABLE 4. FIGURE 8 shows thermal expansion of the KSTAR insulation material and the magnet structure material, which is stainless steel 316LN [7]. The total thermal expansion from 273K to 4K for KSTAR insulation material and stainless steel 316LN are 0.23% and 0.265%, respectively. 303
CONCLUSION
We have investigated the tensile and thermal expansion behavior of the KSTAR insulation material. The KSTAR Insulation material, which is an S-2 glass fiber reinforced epoxy, has higher ultimate tensile stress than the designed value. The difference of thermal expansion between the KSTAR CICC and the KSTAR insulation material is relatively small and the thermal stress at cryogenic temperatures will not be significant. Since the filling material between the KSTAR TF coil and the KSTAR TF coil case will be similar to the KSTAR insulation material, the detachment between the TF coil and the TF coil case during cooling down should not occur. However, a detailed structural analysis is needed in the future for thermal stress calculation due to the thermal expansion mismatch. REFERENCES 1. 2. 3. 4. 5. 6. 7.
G.S. LEE, et al, Nuclear Fusion 41, 1515 (2001). B. Lim, et al., IEEE Tram. On Applied Superconductivity 12, 591 (2002). P. Rosenkranz, et al., Fusion Engineering and Design 58-59, 289 (2001). H. Fillunger, et al., Fusion Engineering and Design 58-59, 135 (2001). A. Devred, et al., Advances in cryogenic engineering 46, 143 (2000). VISHAY Measurements Group, Tech note, TN-513-1 Handbook on Materials for Superconducting Machinery, 8.1.5-8 (11/74).
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