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HTS current leads | China launches series production
Because they reduce the input power requirement for plant operation, high-temperature superconducting (HTS) current leads are one of the enabling technologies (together with superconducting magnets) for large-scale fusion power plants. First driven by the high-energy physics accelerator community, the development of high-current HTS leads is now being pushed by magnetic confinement fusion towards larger currents. At 68 kA, the ITER toroidal-field type HTS current leads will be the largest ever operated.
Contractors to the Chinese Domestic Agency have produce first-of-series pairs for each type of HTS current lead. Series manufacturing is now underway and this pair—destined for the magnet feeder for poloidal field coil #4—will be the first to reach ITER. The components are seen mounted in a ''cold termination box,'' which isolates the cryogenic components of the feeders from the environment.
HTS current leads are key components of the ITER magnet system, transferring the large currents from room-temperature power supplies to very low-temperature superconducting coils at a minimal heat load to the cryogenic system. Although HTS current leads represent an additional cost over conventional current leads, this additional cost is quickly amortized due to savings in cryoplant operation.
ITER's largest magnets—18 toroidal field coils, 6 central solenoid modules, 6 poloidal field coils, and 18 correction coils—will be supplied with 60 current leads, ranging from very large (68 kA for the toroidal-field type) to medium (10 kA for the correction-coil type), transferring up to 2.6 MA into and out of the cryogenic environment of the machine. Located at the far end of the magnet feeder relative to the machine (see diagram below) the current leads operate in much lower magnetic field than the magnet coils themselves.
The largest toroidal-field type of current lead is over 3 metres long and weighs 600 kgs.
The HTS current leads for the ITER Tokamak are procured by the Chinese Domestic Agency through the Institute of Plasma Physics (ASIPP) in Hefei. The Procurement Arrangement signed between the ITER Organization and the Chinese Domestic Agency for magnet feeders laid out a multi-year plan to develop the designs and to qualify the HTS lead manufacturing technology in ASIPP and its sub-suppliers Juneng and Keye.
Magnet feeders at the bottom of the machine (and also the top, not shown here) will carry electrical power, cryogenic fluids and instrumentation cables in to the superconducting magnets. In this diagram we are able to look into the far end of the feeder—the cryogenic termination box—where the HTS superconducting leads are housed. Their role is to relay large amounts of current across the room-temperature/low-temperature barrier.
Following the development of critical manufacturing technologies through targeted trials in mockups, Chinese contractors recorded a string of qualification milestones¹:
The successful testing of a pair of correction coil 10 kA current lead prototypes in March 2015;
The successful testing of a pair of toroidal-field type 68 kA current lead prototypes in July 2015;
The successful testing of a pair of poloidal-field/central-solenoid type current leads in 2016;
The completion of a Manufacturing Readiness Review in August 2016 (marking the end of the qualification phase).
Series manufacturing is now underway, and the first-of-series for all three types of HTS lead have been completed (see gallery). The fact that manufacturing is proceeding strongly, with only a small number of non-conformities, is a tribute to the thorough qualification efforts as well as the Chinese manufacturers' high level of expertise.
It should also be noted that the Chinese Domestic Agency and the ITER Organization put a supervision framework into place allowing local inspectors to witness critical manufacturing steps. Erwu Niu of the ITER China office now manages at least two inspectors who are permanently stationed at the suppliers' sites in Hefei.
Thousands of documents have already been uploaded to the ITER Organization Manufacturing Database—from material certificates, to personnel certificates and test reports. Documents attesting to the components' performance during testing—for example the final factory acceptance cold test in near-to operational conditions under full current—can be fully verified through the database before the final ITER Organization hold point is released.
At ASIPP, lead engineers Quan Han and Qingxiang Ran are now turning their attention to ramping up the pace of production to meet the ITER schedule. A number of additional pieces of large-scale manufacturing equipment—such as another electron-beam welding machine, insulation curing autoclaves and a third cold test station—are being commissioned to handle the extra load.
This year and next, up to 20 current lead pairs will be manufactured in parallel in by Juneng and Keye for shipment to ITER.
¹The qualification of the HTS current leads in China is summarized in an ITER Technical Report (Reference: ITR-18-001). You can download it on this page.