Development and Techniques of High Current Leads for HTS Device Applications

Development and Techniques of High Current Leads for HTS Device ApplicationsXiaoyuan Chen, Jianxun JinCenter of Applied Superconductivity and Electrical Eng. University of Electronic Science and Technology of ChinaChengdu, Chinajxjin@, cxy_yjs@Jie Liu, Hongtao Ren, Tianfa Xiang Jiangsu New Hengji Industrial and Trading Co.Nanjing, Chinaliujie19830911@Abstract—Various current leads (CLs) have become suitable for practical high temperature superconducting (HTS) device applications. This paper made a comprehensive summary on CLs. A practical HTS CL has also been made using Bi-2223 tapes to optimize HTS design for CLs’ applications.Keywords-HTS; current lead; heat leakage; AC lossI.I NTRODUCTIONIn the current high temperature superconducting (HTS) applications, e.g., HTS transformer, HTS fault current limiter, HTS motor, etc, various current leads (CLs) are required to electrically connect and thermally insulate the low-temperature superconducting components inside the cryostat with the room-temperature (RT) components. The copper CLs were commonly used and researchers had made several optimization works: J. M. Lock developed some optimization methods for copper CLs based on the ideal heat exchange condition and Wiedemann-Franz law [1], J. R. Hull, K. Maehata, R. Wesche, et al furthered the optimization works for the theoretic CL [2-8]. Following by the appearance of HTS materials, F. J. Mumford introduced HTS materials into the cold segment in copper CL to form a so-called binary CL. The results illustrated that the thermal leakage of the binary CL was only one-sixth of that in the similar copper CL [9]. After that, B. Dorri, J. L. Wu, P. Hanzlka, et al carried out various theoretic and experimental optimization works [10-14]. In 1996, S. Yamaguchi proposed a new-type Peltier CL (PCL) made by thermoelements instead of HTS and copper materials [15].Recently various CLs including unitary CLs (e.g., copper CLs), binary CLs and PCLs have been progressed actively in practical HTS applications. This paper mainly introduces the basic classifications and relevant characteristics, theoretic analysis on the heat leakage, and development status of various CLs. Studies on a HTS CL using Bi-2223 tapes have also been presented to optimize and verify HTS for CLs’ applications.II.C LASSIFICATIONS AND C HARACTERISTICS The main classifications of various CLs can be summarized as follows: according to different structural materials, CLs can be classified as unitary CLs, binary CLs and Peltier CLs; according to different constitutional structures, CLs can be classified as single-channel CLs, multi-channel CLs, double-spiral CLs, disk-fin CLs, variable-sectional CLs, etc.; according to different cooling methods, CLs can be classified as conduction-cooled CLs and vapor / gas-cooled CLs; according to different cooling temperatures, CLs can be classified as ultra-low-temperature CLs from 4.2 K to 20 K, low-temperature CLs from 21 K to 62 K, LN2-temperature CLs from 63 K to 77 K and room-temperature CLs from 77 K to RT; according to different levels of the operating current, CLs can be classified as large-scale CLs at kilo-amperes’ level, small-scale CLs from several amperes to hundreds of amperes and weak CLs below 1 A.Conventional copper or brass CLs can be used to transport the current from RT components to low-temperature components inside the cryostat. However HTS materials are recommended for the cold part of the CLs inside the cryostat because HTS CLs generate no resistive loss and their thermal conductivity is relatively small.Recently, HTS materials which commonly used to develop CLs include Bi-2223 tapes, Bi-2212 bulks, Bi-2223 bulks, Y-123 bulks (rods, tubes, bars), etc. HTS tapes sheathed with pure Ag have several advantages, e.g., high J c characteristics due to the increase of the crystal orientation in the core superconductor, flexibility in use and an increase in thermal stability, etc. The high κAg value however causes a serious problem of heat intrusion into the cryostat. Some practical methods for reducing the thermal conductivity of the sheathed HTS tapes are: i) reduce the cross-section area of the sheath, i.e., increase the filling factor of tape; ii) reduce the thermal conductivity of the sheathed materials. Compared to the CLs made from Ag sheathed Bi-2223/Ag tapes, advantages of the bulk Bi-2223 CLs are: i) low thermal conductivity [κBi-2223 ~ 0.2 << κAg ~ 1.3 × 104 (W/mK, 4 K) and κBi-2223 ~ 3 << κAg ~ 4 × 102 (W/mK, 300 K)]; ii) single lead handling large current (~ 103 A/cm2); iii) auto quench protection; iv) low manufacturing cost. However the disadvantages identified are lower in-field capability and higher AC loss [16-18].III.T HERMAL A NALYSIS ON V ARIOUS C URRENT L EADSThe heating transfer phenomenon includes three basic types of thermal conduction, thermal convection and thermal radiation. The thermal radiation in the CLs is relatively weak comparing to the thermal conduction and thermal convection and can be neglected in practical analysis and calculation. TheProceedings of 2009 IEEE International Conference onApplied Superconductivity and Electromagnetic DevicesChengdu, China, September 25-27, 2009ID1022thermal leakage of the CLs in the HTS systems mainly includes conductive thermal leakage own to the temperature difference between warm part and cold part, and Joule heat own to the self-resistance of the CLs.The common CLs in different HTS system have different types of unitary CLs, binary CLs and Peltier CLs: unitary CLs are formed by copper or other conventional materials; binary CLs are formed by copper materials in the warm part and HTS materials in the cold part; Peltier CLs are formed by the thermoelectric element (TE) inserted into a pure copper lead at warm part, with or without HTS conductors below 77 K. The above three CLs can be modeled and optimized as follows: A. Unitary Current LeadThe unitary CL is modeled to be a rectangular bar, as shown in Fig. 1. Assume that the initial temperatures in cool end and warm end are given by T b = T L and T a = T H , the temperature in the middle part equably increases along the CL, and the CL is operated in the adiabatic vacuum condition, then the thermal conduction equation and thermal balance equation for a steady unit dx along the CL can be expressed by [1,4,7]dxdT AT K Q )(= (1)AT I dx dQ )(2ρ= (2)where Q is the thermal flow along the CL, K (T ) is the coefficient of the thermal conduction of the CL, A is the effective cross-sectional area of the CL, I is the operating current of the CL, ȡ(T ) is the electrical resistivity of the CL.Figure 1. A model of the unitity CLs.The finite element method (FEM) is applied to simplify the above equations for the thermodynamic analysis. Assume that all the elements along the CL have fixed temperature value, thermal characteristics and physical size, then the differential equation of thermal transfer can be expressed by [7]0)()(222=+i i ii ii A T I dx T d A T K ρ (3) where the variables with the suffix i are the correspondingfactors in a certain element i . The solution of (3) is)0(|)()()()(0211112i i x ii i i i i i T x dx dT x x A x A T I x T i ++−==ρ (4)So the conductive heat and total temperature distribution alongthe CL can be obtained by referring to (6) and relevant boundary conditions.B. Binary Current LeadThe binary CL is modeled to be a rectangular bar formed by two parts, i.e., HTS part AB (cold end) and copper part BC (warm end), as shown in Fig. 2. Assume that the HTS part has the length of L AB and initial temperature of T AB , copper part has the length of L BC and initial temperature of T BC , thetemperature in the joint between two parts is T b , the temperature in the middle part equably increases along axis-direction, and the whole CL is operated in the adiabatic vacuum condition, then the steady thermodynamic conduction balance equations can be expressed by [12-14]0)())((2=+AT I dx dT A T K dx d ρ (5) 0))((=dx dTA T K dxd (6)Figure 2. A model of the binary CLs.Divide the CL model into M + N elements, i.e., HTS part accounts for M elements, copper part accounts for N elements. Assume that all the elements along the CL have fixed temperature value, thermal characteristics and physical size, then the differential equation of thermal transfer can be expressed by)(22=jj jj dx T d A T K (7)where the variables with the suffix j are the corresponding factors in a certain element j (1 j M ). The solution of (7) is)0(|)(0j j x jj j j T x dx dT x T j +== (8)So the conductive heat and total temperature distribution along the HTS part can be obtained by referring to (8) and relevant boundary conditions.The conductive heat and total temperature distribution along the copper part is similar to that along the unitary CL and can be calculated by referring to (6) [7].C. Peltier Current LeadsMost thermoelements are the alloys of bismuth (Bi), tellurium (Te), selenium (Se), and antimony (Sb), heavily doped to create excess (n-type) or deficiency (p-type) of electrons. In 1996, S. Yamaguchi observed that the thermoelements can be used to pump heat out of cryostats for superconducting magnets, i.e., PCLs [15].Assume that the initial temperatures in cool end and warm end of a thermoelement are given by T b = T L and T a = T H , as shown in Fig. 3, then the heat consumed at the cold part is [19])(211221T T K R I I T Q −−−=α (9)where I is the operating current through the thermoelement, three parameters (a , R , K ) characterize the thermoelement: a is the Seebeck coefficient, R is the electric resistance, and K is the thermal conductance.Figure 3. A model of the Peltier CLs.To calculate and minimize the thermal leakage, the Sato’s code [19] was used based on the Okumura’s model0)(2=+−AI dx x dT fmC dx dQ p η (10) where T (x ) is the temperature in the section of the coordinate of the CL, A is the cross-sectional area of the CL, f is a parameter describing the exchange ratio of the coolant gas, m is mass flow rate of the coolant gas, C p is the specific heat of the coolant gas under constant pressure, I is the operating current. Q is the heat flow and can be expressed by [20]I x T dxx dT AQ )()(ακ+−= (11) where ț is thermal conductivity, Į is the Seebeck coefficient. Various theoretic analysis and experimental results verify that the thermoelements are more effective near RT segments of CLs (e.g. 200 K - 300 K), whereas the HTS and copper materials are used as the low-temperature (e.g., 4 K - 77 K) and middle-temperature (e.g., 77 K - 200 K) segments [19-21].IV. D EVELOPMENT AND A PPLICATIONSMany research groups in the world, mainly including USA, Japan, Europe, etc., began to study the feasibility and practicality of various CLs for practical HTS applications. The development and relevant applications of the CLs have been progressed actively over past years, as shown in Table I. The European Organization for Nuclear Research (CERN) has carried out a Large Hadron Collider (LHC) CL project [22-26]. About 3300 CLs with the rating current values from 60 A to 13 kA were developed to transfer altogether more than 3 MA for the LHC, mainly including: 64 CLs with the rating current of 13 kA for the main quadrupole magnets, 310 CLs ranging from 3.9 kA to 6.9 kA for the quadrupole and dipole magnets of the insertion regions, 750 CLs with the rating current of 600 A for the corrector magnets, 504 corrector dipole CLs with the rating current of 120 A, and 1504 corrector dipole CLs with the rating current of 60 A. In addition, a pair of 20 kA CLs for the Compact Muon Solenoid (CMS) has been developed by CEA Saclay in Italy, with an aim to form a general-purpose proton-proton detector designed to run at the highest luminosity at the LHC [27]. As the collaboration program between CERN and Japan for LHC, a pair of 8 kA CLs for a half of the high gradient quadrupole magnets for the LHC low-beta insertions has been developed at High Energy Accelerator Research Organization (KEK) [28].In the frame of the European Fusion Technology Programme, the Forschungszentrum Karlsruhe (FZK), Germany, and the Centre de Richerches en Physique des Plasmas (CRPP), Switzerland, have started a development program of a 60 kA HTS binary CL for the International Thermonuclear Experimental Reactor (ITER) Toroidal Field (TF) coils [29]: i) design and test of different materials and concepts in 1 kA modules for selection at CRPP [30]; ii) designand test of a 10 kA HTS CL using the selected material at CRPP [31,32]; iii) design and test of a 20 kA HTS CL for the Large Helical Device (LHD) International Mutual Experiment (LIME) project in a joint collaboration between the FZK andCRPP [33]; iv) design and test of a 60 kA HTS CL at JAERI in collaboration with Fuji Electric and Sumitomo [34].FZK and CRPP are responsible for a development program of a 70 kA current lead for the TF Coil system of ITER-FEAT. 60 CLs with the total current of more than 2.5 MA were needed. EFDA CSU Garching had launched a development program of a 70 kA HTS CL demonstrator for the ITER TF coils [35]. The FZK and CRPP developed the 70 kA CL at 50 K, as shown in Fig. 4 [36]. The 70 kA CL was also successfully tested in the TOSKA facility of FZK at 80 - 85 K in 2004 [37]. In addition, based on the research results of the 30 kA CLs for the POLO model coil tests [38], the LCT coil 1.8 K tests [39], the W7-X conductor tests [40], and the W7-XDEMO coil tests [41], a pair of 80 kA CLs were developed for ITER Toroidal Field Model Coil (TFMC) in the TOSKAfacility of FZK in 2001 [42,43].Figure 4. A 70 kA CL for the ITER TF coils [36].In Japan, the development of various CLs is mainly summarized as follows: a pair of 1 kA CLs using Bi-2223 tubes [44,45] and a pair of 1 kA CLs using Bi-2223 bars [46] were developed for low temperature superconducting (LTS) magnet systems; a 2 kA CL using Bi-2223 tapes was developed for AC applications [47]; a 1 kA CL using Bi-2212 cylindrical bulk was developed for a l kWh/l MW module-type SMES system [48,49]; a 1 kA low heat leak CL using YBCO tapes was developed for HTS magnet [50]; a CL with kA capacity and small AC loss was developed for superconducting fault current limiter (SFCL) cooled with liquid helium (LHe) [51,52]; 10 CLs using YBCO bulks were developed and investigated the transport properties in liquid nitrogen (LN 2) at an applied magnetic field of 0.5 T in 2003 [53].The National Institute for Fusion Science (NIFS), in collaboration with universities and laboratories in Japan, FZKand the Max-Planck Institut für Plasma Physik (IPP) in Germany carried out a CL development for large-scale superconducting coils. A 20 kA YBCO CL from 4.4 K to 1.8 K was developed for the superfluid helium (He II) cooled superconducting coils, as shown in Fig. 5 [54]. A pair of CLs with the short-time rated current of 30 kA and continuous rated current of 12 kA and a pair of CLs with the short-time ratedcurrent of 100 kA and continuous rated current of 40 kA were developed to measure the critical current for LHD [55,56]. In USA, the development of various CLs is mainly summarized as follows: a pair of 1.5 kA binary CLs wasdeveloped at UW-Madison for ȝSMES applications [57,58]; a pair of 10 kA CLs was developed for the incorporating HTS sections in the N HMFL 45 T Hybrid Magnet System [59]; a pair of 6 kA CLs was developed for LHe-cooled magnets [60];a pair of 100 kA DC CLs was developed for the Fermilab’s transmission line magnet [61]; a semi-retractable 600 A binary CL was conceptually designed for a 21 T Fourier Transform Ion Cyclotron Resonance (FT-ICR) superconducting magnet system in 2006 [62].In Korea, the development of various CLs is mainly summarized as follows: a pair of 2.1 kA-class CLs [63] and a pair of 1 kA CLs [64] were developed for 0.7 MJ SMES and 3 MJ SMES, respectively; a pair of 667 A CLs was developed for resistive superconducting fault current limiter [65,66]; a pair of 40 kA CLs for the toroidal field (TF) magnet and 11 pairs of 20 - 30 kA CLs for the poloidal field (PF) magnet were developed in the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting magnet system [67].In China, the development of various CLs is mainly summarized as follows: 12 pairs of 15 kA CLs [68] and a pair of 16 kA CLs [69,70] using Bi-2223 tapes were developed for the experimental advanced superconducting tokamak (EAST) in Chinese Academy of Sciences; 6 pairs of CLs with four different rating current values were developed for superconducting quadruple magnets (SCQ) and a pair of 4 kA CLs was developed for the superconducting solenoid magnet (SSM) in the development of Beijing Electron-Position Collider Upgrade (BEPC II) [71]; a pair of CLs using Bi-2223 tapes were developed for a 0.5 MJ SMES magnet in Tsinghua University [72]; a pair of 400 A CLs using Bi-2223 tapes were developed for 300 kVA HTS traction transformer in Huazhong University of Science & Technology [73].TABLE I. T HE D EVELOPMENT AND R ELEVANT A PPLICATIONS OF V ARIOUS CL SResearchgroups Number CurrentcapacityComponent materials Cooling conditionsApplications Warm part Cold part Warm part Cold partCERN, Italy, Japan64 13 kA Copper Bi-2223 tapes 50 K GHe 20 K GHe LHC [22-26] 310 3.9 - 6 kA Copper Bi-2223 tapes 50 K GHe 20 K GHe LHC [22-26] 750 600 A Copper Bi-2223 tapes 50 K GHe 20 K GHe LHC [22-26] 504 120 A Copper 50 K GHe 20 K GHe LHC [22-26] 1504 60 A Copper 50 K GHe 20 K GHe LHC [22-26] 2 20 kA Copper GHe (4.5 K - RT) LHC 27] 2 8 kA Copper Bi-2223 tapes 50 K GHe 4.4 K GHe LHC [28]FZK, CRPP, Switzerland 2 10 / 14.5 kA Copper Bi-2223 tapes 60 K GHe (70 K - RT) 4.5 K GHe (4.5 - 70 K) LHD [31,32] 1 20 kA Copper Bi-2223 tapes 60 K GHe (70 K - RT) 4.5 K GHe (4.5 - 70 K) LHD [33] 40 60 kA Copper Bi-2223 tapes 35 K GHe (50 K - RT) 4.5 K GHe (4.5 - 50 K) ITER [34] 2 70 kA Copper Bi-2223 tapes 50 K GHe (65 K - RT) 4.5 K GHe (4.5 - 65 K) ITER [35,36] 2 80 kA SF-Copper 4.5 K GHe ITER [42,43]Japan 2 > 1 kA Bi-2223 tubes Cryocooler (11 - 60 K) LTS magnet [44,45] 2 1 kA Copper Bi-2223 bars GHe (4 - 5 K) LTS magnet [46] 1 2kA Copper Bi-2223tapes GN2 (77 K - RT) Cryocooler AC applications [47] 2 1 kA Copper Bi-2212 bulk GHe (77 K - RT) GHe (4.5 - 77 K) SMES [48,49]1 1 kA Copper alloy YBCO tapes Cryocooler (77 K - RT) Cryocooler (20 - 70 K) HTS magnet [50] 1 1 kA Copper Bi-2222 bulks GHe (56 K - RT) GHe (4.2 - 56 K) SFCL [51,52] 10 > 2.5 kA Copper YBCO bulk LN2 (77 K) Experiment [53]1 20 kA Copper YBCO bulk LHe (4.4 - 1.8 K) LHD [54]2 30 kA Copper GHe (4.4 - 60 K) LHD [55]2 100 kA Copper GHe (4.4 - 60 K) LHD [56]USA 2 1.5 kA Copper YBCO rods Cryocooler (60 K - RT) Cryocooler (60 K) SMES [57,58]2 10 kA Copper Bi-2223 tapes GN2(77 K - RT) LN2(77 K - RT) Hybrid magnet [59] 2 6 kA Copper Bi-2223 tapes GHe(77 K - RT) LHe (4.4 - 77 K) LHe-cooled magnet [60] 2 100 kA Copper rods Copper rods 4.4 K GHe HTS magnet [61]1 600 A Copper Bi-2223 tapes Cryocooler (52 K - RT) Cryocooler (52 K) FT-ICR [62]Korea 2 1.8 / 2.1 kA Copper Bi-2223 tapes GN2(77 K - RT) LH2(4.2 - 77 K) SMES [63]2 1 kA brass Bi-2223 tapes Cryocooler (60 K - RT) Cryocooler (4.2 - 60 K) SMES [64]2 667A Brass GN2(77 - 298K) SFCL [65,66]2 200 A Brass GHe (4.2K - RT) KSTAR magnet [67]China 24 15 kA Copper Bi-2223 tapes GN2 (78 K - RT) GHe (4 - 78 K) EAST [68]2 16 kA Copper Bi-2223 tapes GN2 (78 K - RT) GHe (4 - 78 K) EAST [69,70] 2 1.6kA Copper GN2 (77 K - RT) SCQ [71]4 630A Copper GN2 (77 K - RT) SCQ [71]1 150A Copper GN2 (77 K - RT) SCQ [71]5 75A Copper GN2 (77 K - RT) SCQ [71]2 4kA Copper GN2 (77 K - RT) SSM [71]2 350 A Copper Bi-2223 bars GN2 (77 K - RT) SMES [72]2 400 A Copper Bi-2223 tapes LN2 (67 K - RT) Transformer [73]France 1 1 kA Copper YBCO tubes GHe (77 K - RT) GHe (4.2 - 77 K) LHe application [74] Demark 1 2kA Copper GN2(77 K - RT) HTS cable [75] Italy 2 1.1 kA Copper Bi-2223 tapes Cryocooler (65 K - RT) GHe (4.2 - 65 K) SMES [76] Switzerland 6 20.5 kA Copper bars GHe (4.5 - 5 K) ATLAS magnet [77] UK 2 300 Copper, brass Bi-2223 tapes Cryocooler (50 K - RT) GHe (4.2 - 50 K) LTS magnet [78] Finland 2 1 kA Copper Bi-2223 tapes Cryocooler (77 K - RT) GHe (20 - 77 K) HTS magnet [79]Figure 5. A 20 kA HTS CL and a 30 kA Cu CL for the LHD [54].The development of various CLs in some other research groups around the word is mainly summarized as follows: a 1 kA CL using YBCO tubes was developed for LHe temperature applications in France [74]; a 2 kA CL using copper tapes was developed for HTS cable applications in Demark [75]; a pair of 1.1 kA CLs using Bi-2223 tapes was developed for a 1 MJ / 1 MW SMES system in Italy [76]; three pairs of 20.5 kA CLs were developed for the superconducting barrel toroid of the ATLAS toroid magnets in Switzerland [77]; a pair of 300 kA CLs using Bi-2223 tapes was developed for Oxford Instruments’ Cryofree and LHe-cooled magnets in UK [78]; a pair of 1 kA CLs using Bi-2223 tapes was developed for superconducting magnet systems in Finland [79].V.A HTS C URRENT L EAD D ESIGN AND A NALYSISA HTS CL using YBCO tapes has been, as shown in Fig. 6. The framework of the developed CL is an octahedron made of plastics, with eight grooves (5 mm depth) locating in its eight sides. The HTS YBCO wires with the critical current of 120 A (77 K, 0 T) are placed into the grooves to form a whole CL inthe work.Figure 6. The developed CL by UESTC.A simulation model of the designed CL is built by ANSYS, as shown in Fig. 7. Assume that each groove has a HTS wire to reach the total current of 1 kA, the magnetic field distribution and flux densities [B , B x (//c), B y (ŏc)] along the surface of arbitrary HTS wire are shown in Fig. 8 and Fig. 9. The maximum parallel flux density and maximum perpendicularflux density are 0.015 T and 0.0157 T.Figure 7. The simulation model of the developed CL.The practical critical current of the HTS wires is influencedby the external magnetic field own to the anisotropic property.The external magnetic filed for each HTS wire is generated by the certain wire itself and other seven HTS wires. Assume that a certain HTS wire is with no current and other seven HTS wires are operated with the current density of 12 kA/cm 2, the magnetic field distribution and flux densities along the surface of the HTS wire with no current are shown in Fig. 10 and Fig. 11. The maximum parallel flux density and maximumperpendicular flux density are -0.007 T and -0.00066 T.Figure 8. The magnetic field distribution of the 1 kA CL.Wire width [ mm ]M a g n e t i c d e n s i t y [ T ]Figure 9. The flux densities along the surface of arbitrary wire in 1 kA CL.Figure 10. The magnetic field distribution of the 7-wires CL.B x [ T ]Wire width [ mm ]B y [ T ]Figure 11. The flux densities along the surface of a certain wire in 7-wires CL.Assume that the stack number of the HTS wires in each groove is 2 and the total current of the CL is 2 kA, the flux densities [B , B x (//c), B y (ŏc)] along the surface of arbitrary HTS wire is shown in Fig. 12. The maximum parallel flux density and maximum perpendicular flux density are 0.0272 T and 0.02327 T.Wire width [ mm ]M a g n e t i c d e n s i t y [ T ]Figure 12. The flux densities along the surface of arbitrary wire in 2 kA CL.The 77 K critical line (critical current I c vs. B x ) [80] of the HTS Bi-2223 wires in the work, the load lines of 1 kA CL and 2 kA CL are shown in Fig. 13. The allowable operating current values are about 87.3 A and 75.2 A for arbitrary wire in 1 kA and 2 kA CLs at 77 K.B x [ T ]I o [A ]Figure 13. The 77 K critical line of the HTS wires vs. the load lines in 1 k A and 2 kA CLs (a : load line of 1 kA CL; b : load line of 2 kA CL).Based on the above analysis, the practical critical current seems to be feebly affected by self-field. Several relevant experimental tests have been carried out to study and verify the I - U characteristics of the developed CL. The test results of the developed CL with one HTS wire and two HTS wires in the groove(s) are shown in Fig. 14. The practical critical current of the developed CL with one HTS wire is about 92.3 A (reference standard of quench: 1 ȝV/cm). The voltage across the HTS wire(s) seems to be approximately linear increase along with the total carried current when the quench occurs. The fitted I -U curve of the developed CL with one HTS wire in the groove is shown in Fig. 15. Assume that the current is evenly distributed in eight HTS wires of the 1 kA CL, then the fitted critical current of the 1 kA CL with eight HTS wires is about 740 A. However the practical critical current of the developed CL with eight HTS wires should be smaller than the fitted value own to the magnetic influence among different HTS wires. So the practical operating condition with lower temperature (< 77 K) should be applied to obtain 1 kA or 2 kA operating current in the developed CL.U [ V ]I [ A ]Figure 14. The experimental results of the developed CL (ż: I -U curve of the developed CL with one HTS wire in the groove; Ƒ: I -U curve of the developed CL with two HTS wire in the grooves).U [ V ]I [ A ]Figure 15. The fitted I -U curves of the developed CL with one HTS wire in the groove.VI. C ONCLUSIONSVarious CLs with different materials, structures, capacities have been developed for practical HTS applications in recent years. Comprehensive summary has been made and verified with practical prototype and analysis. For practical HTS application in building high CLs, more optimization works to increase current density and reduce the thermal leakage of various CLs are necessary for large-scale HTS applications.R EFERENCES[1] J. M. Lock, “Optimization of current leads into a cryostat,” Cryogenics,vol. 9, no. 6, pp. 438-442, 1969.[2] K. Maehata, A. Katase, K. Ishibashi, M. Wake, and M. 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