Residual Stress Monitoring for ITER Diagnostic Windows

This article presents the measurement of residual stress distribution in ITER (“The Way” in Latin) diagnostics windows using a circular polariscope. The method being used is based on the measurement of the change in the state of polarization of a circularly polarized light after transmission/reflection from the stressed window under test. The theory based on Mueller calculus for the experimental setup has been presented. The measurement technique is validated for transmission mode by comparing the stress values measured for borosilicate glass (BK7) sample disks under diametrical compression with the theoretical values obtained using linear isotropic theory of elasticity. The method is, then, applied to an undamaged and a damaged ITER trial window, both in transmission and reflection modes, to observe the stress patterns inside the glass material and on the glass to metal seal. The major observations are that the undamaged window shows higher stress in majority of the cross section with lower stress values at localized regions. The glass near the seal shows high values of stress all along the circumference in case of the undamaged window. The damaged window that has got detached from the metal seal shows much lower stress values in majority of the cross section of the window with higher stress values in some regions. The region where the seal seems to have got detached from the glass shows lower values of stress at the seal location, and also the stress values in the glass near those locations are low. In conclusion, by observing the stress patterns inside the window, this technique helps determine the health of vacuum seal, and hence can be used in taking preventive measures.