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What sets SPARC apart from ITER, JET, and other previous fusion tokamaks will be its use of a new type of high-temperature superconductor (HTS), yttrium barium copper oxide (YBCO). Current-carrying tapes made from YBCO remain superconducting at considerably higher magnetic fields than is possible with older superconductors. This is valuable because higher magnetic fields improve the thermal insulation of the plasma and thus allow for considerable improvement over the performance of previous tokamaks.
YBCO superconductors have existed for a number of years, but they have only recently become commercially available in the quantity and quality required for fusion devices. By using this new superconductor to develop high-field magnets – capable of producing fields of 12 T at the centre of the plasma, compared to 5 T in ITER – CFS and MIT hope to drastically accelerate the timetable to fusion energy and achieve net energy gain in a device that is roughly 2% the size of ITER. To make a comparison, we believe that YBCO superconductors will be an enabling technology for fusion in the same way that lightweight internal combustion engines were an enabling technology for powered flight.
The prototype HTS coil will demonstrate the operation of the magnet system and test the integrated cryogenic coolant system used to cool the superconductors to 20 K – the operating temperature required to generate the high magnetic fields in SPARC. Maintaining this temperature in SPARC will be challenging, since the compact nature of SPARC leads to higher power density and thus high heat fluxes generated by the fusion reactions in the core of the machine. Additionally, the properties of the mechanical structure around the conductor in the magnet depend strongly on the temperature of the materials, and therefore this structure must be kept cold in order to withstand the stresses generated in the magnet. The model HTS coil will therefore experimentally validate the heat removal capabilities in HTS magnet systems, and the lessons we learn from it will be critical in moving the SPARC project forward.