Self-radiometric calibration solution for tokamak's optical diagnostics

The radiometric calibration of optical diagnostics used in fusion reactors is critical for monitoring and studying reactions.

Optical systems installed in particularly harsh environments will likely see their optical throughout degrades over time due to optics damages. A shutter and a cleaning system can be implemented to protect optics but won’t fully prevent throughout losses. Consequently, the transmission of the system needs periodic recalibration.

It’s not possible to permanently install a calibrated light source in front of the system, so one solution is to place a retroreflector behind the shutter to do the calibration remotely. Various retroreflector-based solutions have been proposed, but since they rely on knowing the reflectivity on the material, they are not immune against environmental aggressions. Therefore, the calibration will drift over time.

Many variants of design have been proposed in order to calibrate the retroreflector itself but without guaranteeing the radiometric calibration.

We propose an innovative solution that offers true radiometric calibration based on dual-shape retroreflector. It combines two types of mirrors with different effects. The first returns the light like a corner cube after three reflections while the seconds induce a diffusing effect with a single reflection. The difference in reflection number associated with a different optical conjugation allows the measurement of the intrinsic reflectivity of the material of the retroreflector as well as the transmission of the optical system. Such reflector has been implemented in the design of the CXRS Core diagnostic for F4E/ITER. It has been demonstrated experimentally by Bertin that both signals can be separated and detected with enough SNR to get a self-calibration of the reflector and thus a true calibration of the transmission optical path