Overestimation of thermal emittance in solenoid scans due to coupled transverse motion

TL;DR

This paper demonstrates that solenoid scan measurements of thermal emittance are overestimated due to coupled transverse motion caused by weak quadrupole fields, and proposes correction methods to improve accuracy.

Contribution

The study identifies the cause of overestimation in solenoid scans and introduces a correction technique using quadrupole magnets, validated through experiments and simulations.

Findings

Overestimation of thermal emittance by 35% observed experimentally.

Coupled transverse dynamics cause correlation between horizontal and vertical motion.

Quadrupole corrector improves measurement accuracy.

Abstract

The solenoid scan is a widely used method for the in-situ measurement of the thermal emittance in a photocathode gun. The popularity of this method is due to its simplicity and convenience since all rf photocathode guns are equipped with an emittance compensation solenoid. This paper shows that the solenoid scan measurement overestimates the thermal emittance in the ordinary measurement configuration due to a weak quadrupole field (present in either the rf gun or gun solenoid) followed by a rotation in the solenoid. This coupled transverse dynamics aberration introduces a correlation between the beam's horizontal and vertical motion leading to an increase in the measured 2D transverse emittance, thus the overestimation of the thermal emittance. This effect was systematically studied using both analytic expressions and numerical simulations. These studies were experimentally verified using an L-band 1.6-cell rf photocathode gun with a cesium telluride cathode, which shows a thermal emittance overestimation of 35% with a rms laser spot size of 2.7 mm. The paper concludes by showing that the accuracy of the solenoid scan can be improved by using a quadrupole magnet corrector, consisting of a pair of normal and skew quadrupole magnets.