TArget Reflectivity Measurement System for large objects and hazardous environments (TARMS)
Description
ASE Optics Europe and F4E have jointly developed TARMS [1], an optical prototype for measuring the backscattered reflectivity of large objects and hazardous environments for application in the aerospace, energy, industrial and automotive sectors.
This technology has been created to characterise the reflectivity of components facing the ITER (International Thermonuclear Experimental Reactor) plasma and offers the possibility of measuring the reflectivity of extremely large objects, with 100dB of dynamic range at a wavelength of 800nm and 86dB of dynamic range at 1550nm.
Challenges
The ultimate goal of TARMS is to measure the reflectivity of components that cannot be analysed with traditional bidirectional measurement instruments (BRDF), which is essential for the development of tools for the inspection of the ITER reaction chamber, and which have a high dynamic range.
This system will allow in situ characterisation of the reflectivity of ITER nuclear fusion reactor components whose size, radioactivity, or toxicity are unsuitable for insertion into traditional BRDF measurement instrumentation.
How do we solve it
TARMS technology is based on a bistatic probe with lock-in detection and a neutral density filter wheel to attenuate the strongest returns, mounted on a scanning system designed to rotate around a 100 mm circle and cover all angles of incidence, using step-and-hold measurement technology.
This is intended to minimise the effects of unwanted scattering and stray light without involving complex movements of heavy components and control of backscattered reflectivity and/or control of beryllium contamination.
The acquired reflectivity data are fed into a simulator of the ITER vacuum vessel geometry to assess the coverage and verify compliance with the relevant requirements.
Results
TARMS will allow data to be acquired after the completion of the entire component construction process and thus of all modifications to the surface reflectivity.
Knowledge of the reflectivity of the surfaces to be inspected is therefore key to selecting the most appropriate technology to perform a dimensional metrology measurement, as well as to estimate the measurement coverage.
The current TARMS performance shows the characterisation in one hour of 2000 data points taken at uniform solid angles every 2°, over an eighth of a sphere, in a range from normal to 70°.
The acquired data can be used to generate predictive models of the capability of light-based measurement systems for specific surface finish components, such as IVVS.
References
[1] Thomas Siegel et al, “High-dynamic-range instrument for characterisation of the angle-dependent reflectivity of ITER plasma-facing components,” Proc. SPIE 11485, Reflection, Scattering, and Diffraction from Surfaces VII, 114850O (20 August 2020); https://doi.org/10.1117/12.2567372
