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

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.

12 pages, 13 figures

References in corpus (6)

• Direct measurement of sub-10 fs relativistic electron beams with ultralow emittance
• Thermal emittance measurements of a cesium potassium antimonide photocathode
• Commissioning experience and beam physics measurements at the SwissFEL Injector Test Facility
• Exact Cancellation of Emittance Due to Coupled Transverse Dynamics in Solenoids and RF Couplers
• Spatial Control of Photoemitted Electron Beams using a Micro-Lens-Array Transverse-Shaping Technique
• Nano-machining, surface analysis and emittance measurements of a copper photocathode at SPARC_LAB

Cited by in corpus (5)

• Experimental demonstration of the correction of coupled transverse dynamics aberration in an rf photoinjector
• Development and high-power testing of an X-band dielectric-loaded power extractor
• Single shot cathode transverse momentum imaging in high brightness photoinjectors
• Rapid thermal emittance and quantum efficiency mapping of a cesium telluride cathode in an rf photoinjector using multiple laser beamlets
• Eliminating uncertainty of thermal emittance measurement in solenoid scans due to rf and solenoid fields overlap