ITER-Relevant First Mirrors Unit With Integrated Mirror Cleaning System Utilizing Capacitive Coupled RF Discharge

Basic principles are discussed of a First Mirrors Unit (FMU) design for ITER optical diagnostics, equipped with an RF capacitive coupled discharge cleaning subsystem. A non-tunable RF pre-matcher device is shown to be desirable in the proximity to the discharge load to avoid excessive heating of an in-vessel RF feeder part. A possibility of satisfactory RF pre-matching with a simple non-tunable L-shaped circuit is demonstrated for the experimentally measured RF discharge impedance variation. The structure of an FMU RF power distribution circuit is suggested, consisting of an RF pre-matcher, an RF power meter, and, when required, an RF decoupler of a DC-grounded water-cooled mirror. Planar implementation of an RF pre-matching device is recognized as the most feasible and universal, as a result of analysis of various base element types. Three options of RF-decoupled mirror cooling manifolds are discussed, revealing an optimal selection of their electric and dimensional parameters. A structural implementation of a planar RF pre-matcher ceramic parts fastening is suggested, taking into account their integrity under inertial, neutron and thermal loads expected during ITER operation. Successful pilot tests of the RF pre-matcher performance and structural integrity demonstrate the promise of the described design approach.