Deriving the size and shape of the ALBA electron beam with optical synchrotron radiation interferometry using aperture masks: technical choices

TL;DR

This paper demonstrates a method using optical synchrotron radiation interferometry with aperture masks to accurately determine the size and shape of the ALBA electron beam, emphasizing the importance of phase stability and self-calibration.

Contribution

It introduces a self-calibration process for aperture masking interferometry to measure electron beam properties at optical wavelengths, addressing phase fluctuations and illumination issues.

Findings

Closure phases are stable and near zero, indicating reliable phase measurements.

Visibility measurements have an estimated accuracy of about 1% per frame.

Optimal procedures with the 5-hole mask improve measurement reliability.

Abstract

We explore non-redundant aperture masking to derive the size and shape of the ALBA synchrotron light source at optical wavelengths using synchrotron radiation interferometry. We show that non-redundant masks are required due to phase fluctuations arising within the experimental set-up. We also show, using closure phase, that the phase fluctuations are factorizable into element-based errors. We employ multiple masks, including 2, 3, 5, and 6 hole configurations. We develop a process for self-calibration of the element-based amplitudes (square root of flux through the aperture), which corrects for non-uniform illumination over the mask, in order to derive visibility coherences and phases, from which the source size and shape can be derived. We explore the optimal procedures to obtain the most reliable results with the 5-hole mask, based on the temporal scatter in measured coherences and closure phases. We find that the closure phases are very stable, and close to zero (within $2^o$). Through uv-modeling, we consider the noise properties of the experiment and conclude that our visibility measurements per frame are likely accurate to an rms scatter of $\sim 1\%$.