An accurate Rb density measurement method for a plasma wakefield accelerator experiment using a novel Rb reservoir

TL;DR

This paper presents a precise method for measuring rubidium vapor density using interferometry, crucial for plasma wakefield experiments like CERN's AWAKE, with detailed plans and preliminary results from a prototype cell.

Contribution

It introduces a novel Rb vapor density measurement technique with high accuracy, tailored for large-scale plasma wakefield experiments, and details the implementation plan and initial testing results.

Findings

Achieved approximately 1% relative accuracy in vapor density-length product measurement.

Developed a fiber-based Mach-Zehnder interferometer for in-situ density measurement.

Obtained preliminary results from an 8 cm Rb vapor cell prototype.

Abstract

A method to accurately measure the density of Rb vapor is described. We plan on using this method for the Advanced Wakefield (AWAKE)~\cite{bib:awake} project at CERN , which will be the world's first proton driven plasma wakefield experiment. The method is similar to the hook~\cite{bib:Hook} method and has been described in great detail in the work by W. Tendell Hill et. al.~\cite{bib:densitymeter}. In this method a cosine fit is applied to the interferogram to obtain a relative accuracy on the order of $1\%$ for the vapor density-length product. A single-mode, fiber-based, Mach-Zenhder interferometer will be built and used near the ends of the 10 meter-long AWAKE plasma source to be able to make accurate relative density measurement between these two locations. This can then be used to infer the vapor density gradient along the AWAKE plasma source and also change it to the value desired for the plasma wakefield experiment. Here we describe the plan in detail and show preliminary results obtained using a prototype 8 cm long novel Rb vapor cell.