# Experimental demonstration of the correction of coupled transverse dynamics aberration in an rf photoinjector

## TL;DR

This paper experimentally demonstrates the correction of coupled transverse dynamics aberration in an rf photoinjector, leading to preservation of low emittance in electron beams, aligning well with theoretical and simulation predictions.

## Contribution

It provides the first experimental validation of a quadrupole corrector for coupled transverse aberration in an rf photoinjector, improving beam quality.

## Key findings

- Successful correction of aberration reduces emittance growth.
- Experimental results match theoretical and simulation predictions.
- Low charge beam emittance preserved at thermal value.

## Abstract

The production of electron bunches with low transverse emittance approaches the thermal emittance of the photocathode as various aberrations are corrected. Recently, the coupled transverse dynamics aberration was theoretically identified as a significant source of emittance growth and a corrector magnet was proposed for its elimination [D.H. Dowell, F. Zhou, and J. Schmerge, PRAB 21, 010101 (2018)]. This aberration arises when the beam acquires an asymmetric distribution that is then rotated with respect to the transverse reference axis thus introducing a correlation in the vertical and horizontal planes. The asymmetry is introduced by a weak quadrupole field in the rf gun or emittance compensation solenoid and the rotation is caused by the solenoid. This Letter presents an experimental study of the coupled transverse dynamics aberration in an rf photoinjector and demonstrates its elimination by a quadrupole corrector consisting of a normal and a skew quadrupole. The experimental results agree well with theoretical predictions and numerical simulations. The study also demonstrates the emittance of a low charge beam can be preserved during transportation at its thermal value, which was 1.05 mm mrad/mm, for the cesium telluride photocathode and 248 nm UV laser used.