Measurement of the emittance of accelerated electron bunches at the AWAKE experiment

TL;DR

This paper reports the first measurement of electron beam emittance at CERN's AWAKE experiment, validating measurement techniques and analyzing emittance growth due to plasma effects, with implications for future beam control.

Contribution

It introduces a proof-of-principle emittance measurement at AWAKE using existing spectrometers and compares three data analysis methods, highlighting emittance growth sources.

Findings

Measured emittance of ~0.5 mm·mrad, larger than injection beam.

Emittance growth attributed to plasma density ramps and beam size.

Instrument performance suitable for emittances down to 1e-4 mm·mrad with improvements.

Abstract

The vertical plane transverse emittance of accelerated electron bunches at the AWAKE experiment at CERN has been determined, using three different methods of data analysis. This is a proof-of-principle measurement using the existing AWAKE electron spectrometer to validate the measurement technique. Large values of the geometric emittance, compared to that of the injection beam, are observed ($\sim \SI{0.5}{\milli\metre\milli\radian}$ compared with $\sim \SI{0.08}{\milli\metre\milli\radian}$), which is in line with expectations of emittance growth arising from plasma density ramps and large injection beam bunch size. Future iterations of AWAKE are anticipated to operate in conditions where emittance growth is better controlled, and the effects of the imaging systems of the existing and future spectrometer designs on the ability to measure the emittance are discussed. Good performance of the instrument down to geometric emittances of approximately $\SI{1e-4}{\milli\metre\milli\radian}$ is required, which may be possible with improved electron optics and imaging.