Alignment Principles for ITER In-Vessel Diagnostic Mirrors

Optical mirrors inside the ITER torus vacuum are required by many of the ITER port-based diagnostic systems. As an example, the core charge exchange recombination spectroscopy (core CXRS) concept is based on a mirror labyrinth of seven consecutive in-vessel (exposed to the torus vacuum) mirrors. For this system, alignment of up to three mirrors during assembly might be needed. Because of the unique and demanding environment in the ITER diagnostic port plugs, particularly in the first meter from the plasma, existing designs of adjustable mirror holders cannot be used without modification. The wish to allow the realignment of the mirrors during maintenance of the diagnostic systems, if possible by remote handling only, adds to the complexity. This paper discusses mirror alignment principles in the light of the requirements by ITER diagnostics. The core CXRS system is used as example throughout this paper. Loads and requirements for in-vessel diagnostic mirrors are looked into. Three basic groups of alignment principles are assessed for applicability in close proximity to the ITER plasma. The strength and weaknesses of the different alignment principles are discussed, and a conclusion about their general applicability in ITER is given.