Interferometer-based high-accuracy white light measurement of neutral rubidium density and gradient at AWAKE

TL;DR

This paper presents a high-precision, automated white light interferometry method for measuring rubidium vapor density and its gradient in the AWAKE experiment, enabling real-time plasma diagnostics with sub-percent accuracy.

Contribution

It introduces a novel interferometric technique using broadband laser light and Fourier analysis for precise, online measurement of rubidium vapor density and gradient in plasma sources.

Findings

Achieved density measurement uncertainty between 0.11% and 0.46%.

Demonstrated real-time density monitoring for plasma source control.

Enabled detection of linear density gradients at the percent level.

Abstract

The AWAKE experiment requires an automated online rubidium (Rb) plasma density and gradient diagnostic for densities between 1 and 10$\cdot$10$^{14}$ cm$^{-3}$. A linear density gradient along the plasma source at the percent level may be useful to improve the electron acceleration process. Because of full laser ionization of Rb vapor to Rb$^{+}$ within a radius of 1 mm, the plasma density equals the vapor density. We measure the Rb vapor densities at both ends of the source, with high precision using, white light interferometry. At either source end, broadband laser light passes a remotely controlled Mach-Zehnder interferometer built out of single mode fibers. The resulting interference signal, influenced by dispersion in the vicinity of the Rb D1 and D2 transitions, is dispersed in wavelength by a spectrograph. Fully automated Fourier-based signal conditioning and a fit algorithm yield the density with an uncertainty between the measurements at both ends of 0.11 to 0.46 $\%$ over the entire density range. These densities used to operate the plasma source are displayed live in the control room.