The Core-Plasma CXRS Diagnostic for ITER: An Introduction to the Current Design

The development and design of the CXRS diagnostic for the core plasma of ITER is used as a pretext to elaborate on several of the main challenges of optical diagnostics on such a large fusion device. The idea was to confront the students of the 16th Ettore Majorana School on Diagnostics and Technology Developments with as many aspects of the design of optical diagnostics as possible. After an elementary review of the basics of charge-exchange, of typical spectra, of intended measurements and of the expected associated background, the DNB (diagnostic neutral beam) is briefly presented. The light collection and transport to the ex-vessel fibres and spectrometers constitutes the main part of this contribution: building on existing telescope and endoscope systems, the choice of a suitable optical layout for the core-plasma CXRS diagnostic at ITER is discussed. An account follows of the protective measures against the degradation of the first mirror, which will be exposed to high particle and heat fluxes: an appropriate duct, a shutter and, possibly, a cleaning discharge with “End-of-Cleaning Indicator”.