Design and Evaluation of a Retroreflector System for Primary Mirror Health Diagnostics in ITER Optical Systems

Abstract

The integrity of optical components in the ITER tokamak is critical for accurate plasma diagnostics, yet prolonged exposure to high-energy plasma environments leads to contamination and degradation of reflective surfaces. This thesis presents the design, implementation, and validation of an in-situ health monitoring system for the primary diagnostic mirror, utilizing a 1D retroreflector mounted on the shutter to assess its condition. The proposed system enables back-illumination from the image plane, allowing for the analysis of mirror reflectivity, contamination levels, and overall surface quality.Through extensive optical modeling in Zemax and FRED, this work demonstrates that the retroreflector provides a viable and effective method for monitoring mirror health by analyzing the return signal from the reflector. The study establishes a quantitative framework for evaluating mirror performance, showing that quality and contamination can be directly correlated with scattering effects. By leveraging Bidirectional Scattering Distribution Function (BSDF) models and Total Integrated Scatter (TIS) analysis, this research quantifies how mirror degradation leads to increased optical noise, signal attenuation, and cross-talk between image channels. Additionally, this thesis identifies ghost imaging and stray light as significant sources of optical contamination, with simulations revealing that up to 69% of power can be lost due to unintended reflections. These findings emphasize the need for careful system design, optimized anti-reflective coatings, and advanced scattering control strategies to maintain high signal integrity. The results further demonstrate that the retroreflector method is highly sensitive to scattering variations, amplifying the observed scattering effects and enabling early detection of mirror degradation. By comparing beam-based and retroreflector-based measurements, this study confirms that scattering effects can be quantified and used as a diagnostic metric for assessing long-term mirror health. Ultimately, this work proves that a retroreflector-based system provides a practical, scalable, and non-invasive solution for monitoring primary mirror health in high-exposure plasma environments. The findings contribute to the development of an optimized calibration and monitoring strategy, ensuring that IT-ER’s optical diagnostics maintain long-term accuracy and reliability despite the challenges posed by plasma-induced contamination and degradation.