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In vessel Mirnov coils are an essential diagnostic in present day tokamaks. Their use in ITER and future Fusion reactors presents some disadvantages linked to the high radiation environment. Furthermore large Electro Magnetic forces can be experienced by the coil, due to the pulsed operation of the tokamak device [1], and disruptions [2].
Since the operation with the ITER-like wall, JET has experienced severe faults in the high-bandwidth Ti wire coils. Surface discoloration of the wire pointed to the formation of chemical surface layers that can produce hardening of Ti, leading to increased mechanical failures. Disintegration of 0.5mm thick mica plates covering slots in the coils casing was also found.
The tensile strength of the failed wire has been tested, comparing it to new samples of the same composition and manufacturer, and hardening has been observed. Scanning Electron Microscope investigations have been carried out on the rupture surface, which is flat but rough, with no wire deformation, compatible with fatigue failure. Modelling of EM loads showed that the forces, combined with fatigue on hardened Ti are enough to cause failure on sharp wire bends.
During 2016-17 new coils have been designed and installed. These can be replaced using remote handling, and they use Cu alloy wire, in order to reduce chemical interaction with in-vessel gases and increase heath conduction to the ceramic former.
The presented work includes the failure analysis and modelling, motivating the design differences between old and new coils. The latter will provide valuable information on the long term effects of EM loads during disruptions, as well as chemical degradation processes that will be encountered for ITER HF coils, which are characterized by the same materials.
1) Vayakis, George, et al. Journal of Nuclear Materials 417.1 (2011): 780-786.
2) Gerasimov S.N. et al Nucl. Fusion 55 (2015) 113006
