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Diversification of the Position Sensing Instrumentation for the JET Neutral Beam Calorimeters

The Joint European Torus neutral beam injection system incorporates a copper panel calorimeter into each beamline, instrumented with thermocouples to provide diagnosis of the beam shape and alignment. The panels are rotated out of the beam path to allow the beam to enter the torus; they are inertially cooled and can only sustain full beam power for a fraction of a second, hence must be fully withdrawn during plasma operation. Calorimeter position is monitored with in-vacuum microswitches close to the limits of travel, but these have proved unreliable in the past. Along with various other weaknesses in the mechanism, this has led to periods of operation where the bottom of the panel has unknowingly scraped the edge of the beam, and in 2013, this resulted in melting of the edge of one panel and a large water leak. A procedure has been implemented to check the calorimeter position and thus avoid a repeat of the melting incident; however, in 2015, an independent review panel examined neutral beam injector reliability and recommended that a diversity of methods should be used to detect the positions of the calorimeters. This paper summarizes the methods considered and details the option selected for installation during the 2017 shutdown.