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.
Highlights
The solenoid scan is one of the most commonly used methods for the in-situ measurement of the thermal emittance of a photocathode in an electron gun [1,2,3,4,5,6,7,8,9]
The photoelectron beam exits the gun at relatively high energy after which it immediately enters the transport line where it is focused by a solenoid onto a screen located at the end of a drift
This paper presents a previously overlooked source of emittance growth due to coupled transverse dynamics aberration that leads to an overestimation of the thermal emittance measured via the solenoid scan
Summary
The solenoid scan is one of the most commonly used methods for the in-situ measurement of the thermal emittance of a photocathode in an electron gun [1,2,3,4,5,6,7,8,9]. This paper presents a previously overlooked source of emittance growth due to coupled transverse dynamics aberration that leads to an overestimation of the thermal emittance measured via the solenoid scan. [16] so this paper presents a systematic study of the thermal emittance overestimation from the coupled transverse dynamics aberrations using the solenoid scan technique. This aberration arises when the beam motion in the x − x0 plane becomes correlated with y − y0 plane causing an emittance growth in the 2D phase space. VI to minimize the coupled transverse dynamics aberrations so as to improve the accuracy of thermal emittance measurement using solenoid scan
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