1074 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

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1 1074 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 Test of a 10 ka HTS Current Lead for ITER Pierre Bauer, Yanfang Bi, Arnaud Devred, Kaizong Ding, Hansheng Feng, Chen-yu Gung, Xiongyi Huang, Chenglian Liu, Neil Mitchell, Qing Ni, Guang Shen, Yuntao Song, and Tingsi Zhou Abstract The ITER Organization (IO) and the Institute of Plasma Physics at the Chinese Academy of Sciences (ASIPP) are jointly developing the design of the 10 ka Current Leads (CL) using High Temperature Superconductors (HTS) for the Correction Coil (CC) Magnet System of the International Thermonuclear Experimental Reactor, ITER. The proposed design combines a conventional helium cooled heat exchanger operating between 65 K and 300 K with a HTS module covering the low temperature end using Bi-2223 tapes. The details of the design and test results obtained on a first prototype lead will be discussed in this paper. Index Terms Bi-2223, fusion tokamak, HTS current leads, superconducting magnets. I. INTRODUCTION T HE ITER device, currently in the early construction phase in Cadarache/France, uses strong magnets to confine and heat an inductive plasma in a large volume in order to demonstrate the feasibility of thermo-nuclear fusion for power generation. With a total of 60 large current leads, ITER is yet another major application for HTS current leads after the Large Hadron Collider (LHC) at CERN. A total of 42 high current leads are required for the main ITER magnets and 18 medium current leads for the correction coils. Most of the ITER leads are for pulsed operation. The HTS-CLs for ITER are of the binary type and consist of a conduction cooled HTS section between 4.5 K and 65 K and a conventional counter-flow heat exchanger (HEX) made from copper between 65 K and 300 K. Following the development of a demonstrator lead by the EU ITER partner in [1], the Institute of Plasma Physics at the Chinese Academy of Sciences (ASIPP) has developed trial CL prototypes for 68 ka and 52 ka as for the TF and PF coil feeders respectively [2], [3]. Now this series was completed with the fabrication and test of a 10 ka pre-prototype current lead for the ITER CC system (Fig. 1). Table I summarizes the most important specifications for the ITER CC feeder CL. It uses Bi-2223, Ag stabilized tapes, as Manuscript received August 02, 2010; accepted December 06, Date of publication January 20, 2011; date of current version May 27, This work was supported by the Institute of Plasma Physics of the Academia Sinica (ASIPP) and ITER. P. Bauer, A. Devred, C.-Y. Gung, and N. Mitchell are with the ITER Organization, CS St Paul Lez Durance Cedex, France ( Pierre. Bauer@iter.org). Y. Bi, K. Ding, H. Feng, X. Huang, C. Liu, Q. Ni, G. Shen, Y. Song, and T. Zhou are with ASIPP, Hefei, Anhui Province, China. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TASC Fig. 1. Pre-prototype of the current lead for the ITER CC magnet system (insulation cover not shown, total length 2.0 m excluding twinbox joint). HTS. The Ag matrix is alloyed with 5.3% Au to reduce its thermal conductivity. In the HEX cold 50 K helium gas flows from the bottom to exit the top at 300 K. Two different HEX designs were developed as part of the R&D effort at ASIPP. First, a so called foil-type HEX. Second, a so-called zig-zag HEX, similar to that used in the LHC current leads [4]. The foil-type HEX is made of stacked perforated Cu plates, which at the same time serve as heat exchange surface and current flow path. The zig-zag HEX has fins for the heat exchange with the cold He gas, machined into a large diameter Cu rod whose remaining central core transports the current. In this 10 ka CL R&D step only the lead with the foil-type HEX is of the ITER CC type. The lead with the zig-zag HEX ( EAST ) is used in the test only for the purpose of current return for the CC lead (and to test the HEX technology). Another important feature of the current lead design is the transition between the HTS section and the HEX. As was found during this R&D, efficient heat exchange in this part of the lead is important for controlling the temperature at the HTS top [3]. Finally the HTS to LTS splice and the twin box joint at the lower end of the lead are also key components. II. DESIGN OF THE CC PRE-PROTOTYPE CURRENT LEAD The main elements of the design of the 10 ka pre-prototype lead are presented in further detail below. A. HTS Module Design The HTS module includes the Cu/stainless steel shunt and the HTS stacks. The shunt consists of 50 mm Cu sections at the two extremities and a 310 mm long stainless steel section in the middle. The Cu sections are part of the current transfer path into and out of the HTS. With a top and bottom (middle) cross-section of 9.9(5.2), the stainless steel center module limits the heat conduction while providing enough thermal mass to protect from burnout. The 410 mm long Sumitomo Bi-2223 HTS tapes are soldered into stacks (4 tapes/stack) and then soldered into 18 grooves running along the brazed shunt assembly (Fig. 2). The grooves are machined into the shunt assembly after brazing /$ IEEE

2 BAUER et al.: TEST OF A 10 ka HTS CURRENT LEAD FOR ITER 1075 TABLE I SPECIFICATION FOR THE CC CURRENT LEADS Fig. 3. Foil type HEX section and room temp termination of the 10 ka preprototype lead for the ITER CC magnet system. TABLE III RESISTIVE HEX PARAMETERS (FOIL TYPE) 3 Burn-Out Time is Defined as Time Needed to Reach the Hot Spot Temperature at Full Current During a Quench 33 LOFA Time is the Time Needed to Reach Max HEX Voltage After a Loss of Flow Accident in HEX Cooling (HTS Should not Quench During LOFA Time) 3 Assuming Uniform Current Distribution flows through a 136 mm long section with radial fins machined by wire EDM before entering the HEX. Fig. 2. HTS shunt section of the CC pre-prototype lead. TABLE II HTS PERFORMANCE IN ITER ENVIRONMENT 3 Assumes 130 A/Tape at 77 K, Self Field (Measured Average Performance of Tapes) and T Perpendicular Field (incl. ext. Field as Specified in Table I) Note that at the top end the shunt is brazed with the transition heat exchanger, which is described next. Table II summarizes the calculated electromagnetic performance of this HTS section in the CC coil feeder environment (note that the external magnetic field is not applied in the test). The calculated shunt heat load is 1.4 W (0.4 W from SS shunt, 1.0 W through matrix and solder of stacks). The critical current and heat load are well within the IO specifications. B. Transition Heat Exchanger Design Effective heat exchange between the lead and the incoming 50 K GHe is important for the control of the temperature of the HTS section. Here the transition heat exchanger is machined from Cu, and brazed to the shunt at the bottom and e-beam welded to the HEX at the top. The helium is injected into a central SS pipe from above, collected in a cavity at the bottom and then C. Resistive Heat Exchanger Design Two different HEX designs were developed as part of this R&D. The first is a so called foil-type heat exchanger. The second is a so-called zig-zag heat exchanger, similar to that used in the 68 ka trial lead [2]. The foil type HEX used in the first lead is made of stacked, perforated, thin Cu foils, which at the same time serve as heat exchange surface and current flow path. The foils are assembled on centering pins and compressed into a steel box, which is welded closed under the press. Thin spacers are placed under the two long edges of the foils to keep the foils spaced apart for the GHe flow. The most critical technological step is the welding of the foil stack to the Cu end-shoes. Electron-beam-welding has been successfully applied to produce these 50 mm deep welds, as indicated in Fig. 3. Table III lists the main geometrical parameters of the foil type HEX design. The zig-zag heat exchanger was used in the return current lead, not otherwise designed to the IO specification. It uses a scaled down version of the design already applied in the trial 68 ka lead [2], with the only exception that the wire-wrap was replaced by a tightly fitting cover as a more reliable way to prevent He gas by-passing the HEX. Unlike the HTS section, which is designed for 10 ka, the HEX is designed for 8 ka, the estimated average current for the reference CC operating scenario. The calculated and optimized HEX nominal performance (at 8 ka) parameters are: 79 mv voltage drop and 0.63 kw Joule heating, requiring a mass-flow of 0.49 g/s of 50 K GHe. At 10 ka the flow rate required would increase to 0.68 g/s. This calculation assumes that the heat transfer parameter approximates 1300 W/K/m in the 50 K transition area and lies in the range 900 to 2300 W/K/m for the

3 1076 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 Fig. 4. Twin box joint (box cover removed). Total length of joint box 0.5 m. rest of the HEX in laminar flow. The calculated pressure drop is 0.02 MPa, well below the specified limit. D. Room Temperature Termination Design In the 10 ka pre-prototype lead the room temperature termination is separate from the heat exchanger. After machining and brazing of the steel ring (attachment for the HEX cover), the termination is electron beam welded to the HEX. The peak current density in the termination is 3.6. An active water cooling circuit is integrated into the termination as well as two 150 W heaters to prevent freezing in case the water stops flowing. E. LTS Module and Joint Design The design of the low temperature module is very critical to reduce the total joint resistance at the cold end. Note that this joint is actually composed of two joints, one at the lower end of the shunt where the HTS stacks pass the current to LTS wires and in the busbar joint shown in Fig. 4. The design adopted for the former is a Cu cone block soldered into the bottom Cu end of the shunt, with the LTS wires soldered into the interface between the two. The lower Cu cone is cooled from the inside with 4.5 K SHe. The latter is a twin-box type joint as used for all joints inside the ITER magnet systems. The twin box joint is described elsewhere [5]. F. Insulation and Instrumentation The CL for ITER uses an external conduit concept for the HV insulation. The naked current lead is inserted and welded to the duct at top and bottom. This concept allows testing of the insulation of the external duct independently of the CL. The insulation conduit also works as outer wall for the CL thermal insulation vacuum jacket. The flange used to affix the CL to the cryostat is mounted on the outside of the insulation duct, partially embedded in the insulation. The flange is therefore not at high voltage. Only a section of the insulation cover was procured and tested at this point. A full system test at HV is still outstanding. The pre-prototype lead is equipped with the following sensors: -1- voltage taps to measure the voltage drop over the HTS stacks for critical current measurements and quench detection -2- temperature sensors placed on the HTS stacks to measure the temperature during normal operation as well as the Loss Of Flow Accident (LOFA) and burnout tests -3- flow controller, pressure sensors and temperature sensors at inlet and outlet to the HEX (50 K 300 K) GHe circuit to measure the HEX operational performance (pressure drop, etc), -4- temperature sensors at top, middle and bottom of the HEX to measure the temperature profile -5- voltage taps over the HEX and -6- voltage Fig. 5. Current sharing temperature 10 ka lead. taps and temperature sensors to measure the different joint resistances (twin box joint, HTS to LTS and HTS to HEX joints as well as of the room temperature termination). III. TEST RESULTS The test program consists of the following: 1) Controlled cooling down, heat load at 4.5 K end 2) Standby (zero-current) tests at 65 K, 80 K and 100 K 3) Steady state tests at 7 ka, 8 ka, 9 ka and 10 ka 4) LOFA in 50 K GHe circuit and burnout at 10 ka 5) Current sharing temperature test at 7 ka, 8 ka and 9 ka 6) Joint resistances (HEX/LTS, HTS/LTS, twinbox) 7) Current waveform for CC coils A. HTS Temperature Margin The HTS temperature margin (Fig. 5) is measured by varying the HTS warm end temperature by interrupting the flow of the GHe into the HEX. The current sharing temperature reduces linearly with increasing current, reaching 85 K at 10 ka, well above specification. Note that because of the absence of external magnetic field in the test, an additional 3 K needs to be subtracted from the found value to account for it (and in order to compare the number to that in Table II). B. HEX Performance The internal temperature sensors mounted inside the HEX allows measurement of the HEX temperature profile. The optimal temperature profile is characterized by a zero temperature gradient at the top end and 65 K at the HTS top end in the steady state. Since the CC lead HEX is optimized for 8 ka the optimal temperature profile appears at 8 ka. At 9, 10 or 7 ka the profiles clearly depart from ideal (Fig. 6). For 10 ka a near optimal HEX temperature profile is observed if allowing the HTS top to cool to 58 K, less than the specified 65 K. Note that the larger at the HTS-HEX joint in this case (more in Section III-C) results in a positive slope boundary condition at the top of the HTS and a noticeable increase in the gradient at the lower HEX section. If plotting the mass flow data as specific mass-flow rate (per ka) against the current, a minimum appears, indicating the current for which the design is optimized. As expected the minimum is at 8 ka for the foil type HEX and at 9 ka for the zig-zag HEX. The minimum is lower for the foil-type HEX,

4 BAUER et al.: TEST OF A 10 ka HTS CURRENT LEAD FOR ITER 1077 Fig. 6. Axial temperature profile in CC and return ( EAST ) lead. Fig. 8. Voltage-current characteristic of both 10 ka lead twin box joints. Fig. 9. LOFA and burnout test. Fig. 7. HEX GHe flow vs operating current in CC and return ( EAST ) lead. but the difference might well be within the measurement uncertainty. At 10 ka, however, the specific mass flow rate in the foil type HEX is higher than in the zig-zag type (EAST) (Fig. 7). This is not in contradiction with the above, but simply the result of the fact that the optimization current for the zig-zag lead is 9 ka, so that the over-cooling required is less important than in the foil type lead which is optimized for 8 ka. C. Joint Resistances The measured joint resistances of HTS-HEX and HTS-LTS are of similar magnitudes,. Unfortunately the HTS-LTS joint should have a much lower value (Table I). A possible explanation is the use of a low quality solder. Also the twinbox joint of the CC lead with 7.5 is 50% above the specified value. The current dependence indicates that the EAST twinbox joint hits critical current at 7.5 ka, while the CC s critical current appears to be lower (see question mark in Fig. 8). There are questions left unanswered. One possible explanation is that the twin-box joint for the CC lead was not compressed sufficiently during the welding of the box. D. LOFA and Burnout Times The LOFA experiment in the CC pre-prototype lead is conducted starting from a 10 ka steady state condition with temperature at the top end of the HTS of 58 K. If neglecting the time needed for the rise of temperature to the nominal 65 K, and ending the experiment when the HTS voltage has reached Fig. 10. Top HTS temperature (measured, calculated) during a CC pulsed scenario. 5 mv, the LOFA time becomes 438 s, as indicated in Fig. 9. This is much longer than required by specification. The burnout time can also be obtained from the set of data in Fig. 9 and it is 31 s, almost double the specified value. E. ITER CC Operating Cycle During a special test the pre-prototype lead is pulsed with a 12 min version of the CC reference power cycle (the power supply does not allow negative current excursions, so these are reversed). To simulate realistic operating conditions the mass flow rate in the HEX is kept constant at the optimal rate for 8 ka steady state operation. As shown in Fig. 10 the temperature excursion in the top of the HTS is less than 3.5 K (in line with the simulation). This result is obtained after pulsing with several cycles (12 min apart). This indicates that the choice of optimization current for the HEX is reasonable.

5 1078 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 F. Other Measurements In zero current operation a mass-flow rate of 0.11 g/s is found to result in a steady state 100 K at top of the HTS. Direct water cooling in the RT termination is found to be a very effective way to impose a fixed temperature boundary at the top of the lead in all cases, with or without current. The shunt to HEX transition heat exchanger works well in ensuring a fast thermalization at the HTS top. Due to instrumentation issues the 4.5 K heat load and the HEX pressure drop could not be measured in this test-campaign. IV. SUMMARY A 10 ka pre-prototype current lead for the ITER CC coil feeders was successfully designed, built and tested at ASIPP/ China. Series production of the HTS-CL for ITER shall commence soon. ACKNOWLEDGMENT The authors thank Prof. Yifeng Yang (University of Southampton) for this project is kindly acknowledged. REFERENCES [1] R. Heller et al., 70 ka high temperature superconductor current lead operation at 80 K, IEEE Trans. Appl. SC., vol. 16, no. 2, pp , June [2] Y. Bi et al., R&D towards HTS current leads for ITER, IEEE Trans. Appl. SC, vol. 19, no. 3, pp , June [3] Y. Bi et al., Test results of 52/68 ka trial HTS current leads for ITER, IEEE Trans. Appl. SC, vol. 20, no. 3, pp , June [4] A. Ballarino, Physica C, vol. 468, pp , [5] X. Huang et al., Design and test of joints for the ITER coil feeders, IEEE Trans. Appl. SC, vol. 20, no. 3, pp , June 2010.