Proposal for halo current diagnostic system for JET

One of the critical issues for the International Thermonuclear Experimental Reactor is a deep understanding of the origin, distribution, and scaling of halo currents flowing through plasma and vacuum vessel during vertical displacement events. The Joint European Torus (JET) can play a key role in this study, provided that new and more detailed information becomes available. This fact motivated the design of a set of sensors specifically aimed at improving the diagnostic capability of halo currents at JET. This new system is described in the article. It consists mostly of Rogowski coils and of a small number of toroidal field pickup coils. The former will measure directly the current flowing through the tiles of the first wall. The latter will allow the estimation of the total poloidal halo. The system is a compromise between the maximization of the spatial resolution in both toroidal and poloidal direction and the JET capability of adding new in-vessel probes, compatibly with existing feedthroughs needed to bring the signals outside the vessel. The main issues of the system are briefly reported, then the modifications of the in-vessel components, necessary to house the probes, are described together with the technological solutions adopted to improve the measurement quality. Finally, results of the electromechanical analyses performed to assess the reliability of the modified tiles are presented, too.