Alignment principles for ITER in-vessel diagnostic mirrors

Mirrors inside the ITER torus vacuum are used in many of the ITER port based diagnostic systems. As example, the core Charge Exchange Recombination Spectroscopy (core CXRS) concept is based on a mirror labyrinth of 7 consecutive in-vessel mirrors. For this system, ex-vessel alignment of up to three mirrors might be needed. Because of the unique and demanding environment in the ITER diagnostic port plugs, especially in the first meter from the plasma, existing designs of adjustable mirror holders cannot be used without modification. The wish to allow re-alignment of the mirrors during maintenance of the diagnostic system, if possible by remote-handling only, adds to the complexity. With many of the ITER diagnostics being in the stage of a conceptual design and advancing towards a more detailed design, the mechanical design of alignable mirrors themselves is starting. This paper discusses mirror alignment principles for use by ITER diagnostics. The core CXRS system is used as example throughout the paper. Loads and requirements for in-vessel diagnostic mirrors are discussed. 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.