ITER solves problem with superconducting cables

Science : A potential construction delay for ITER, the international fusion reactor, has been resolved. Tests run last year had determined that the superconducting cables, which generate the magnetic fields holding the superheated plasma in place at the heart of the machine, were lasting only one-tenth as long as needed. Three individual strands less than a millimeter across are wound together to form a triplet, and 288 triplets are combined to form a cable. Japan, the country responsible for providing the cables, made its triplets from two strands of niobium–tin and one of copper. The niobium–tin carries the current, and the copper provides an alternate path if the niobium–tin suddenly loses its superconducting ability, a process called quenching. Because only two-thirds of the cable then bears the load in normal operation, the cable degrades faster than planned. When ITER researchers combined copper and niobium–tin into a single strand, however, they found that the cables lasted almost twice as long as previously. The cables have proven to be only one of a number of stumbling blocks along the way, however. Last year’s earthquake and tsunami in Japan had already caused ITER’s scheduled start to be pushed back to 2020. And financial trouble could be ahead because the US government has just cut its promised funds.