Sensitivity of optical system positioning errors applied to the Wendelstein 7-X Phase Contrast Imaging diagnostic

The Phase Contrast Imaging (PCI) diagnostic uses a CO₂ laser to measure line-integrated electron density fluctuations and images poloidally resolved fluctuations from coherent and turbulent fluctuations [1]. The PCI system on the Wendelstein 7-X stellarator [2] measures frequencies in the range of about 2 kHz to 1 MHz under standard digitizing speeds and can detect wavenumbers in the range of about k_⊥r_s ~ 0.1 – 10 in a typical operating configuration. The optical pathway from source to detector includes more than 50 optical surfaces on the beam path. The design analysis has, until now, not considered the sensitivity of the optical performance to error in position and orientation of the components. This work presents first steps at providing a full system sensitivity analysis that considers simultaneous variations in all optical components for the W7-X PCI diagnostic. The main goals of this work are to determine a general system uncertainty, the optical components that contribute the largest errors, and critical observation points where errors can be best compensated.

[1] Z. Huang et al., JINST 16, P01014 (2021).

[2] T. Klinger et al., Nuc. Fus.59, 112004 (2019).