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

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.

Eliminating uncertainty of thermal emittance measurement in solenoid scans due to rf and solenoid fields overlap

“…Some solenoid scan experiments done in the AWA beamline have used the new method introduced in this paper to fit the thermal emittance. The measured thermal emittance of a cesium telluride photocathode is 1.05 ± 0.04 mm mrad/mm, which is in good agreement with the theoretical value [17,18], proving that the new method is appropriate to be employed in solenoid scans when the rf field profile or the distance between the elements is not accurately knowledged.…”

“…The first one is the reduction of the emittance growth in beam transmission. There are several factors that increase the beam emittance thus leading to an overestimation of the thermal emittance, such as (i) nonlinear space charge (SC) effects [7,11], (ii) rf effects in the gun [13], (iii) spherical, chromatic and coupled transverse dynamics aberrations in the * zhenglm14@tsinghua.org.cn solenoid [14][15][16][17][18]. SC effects can be alleviated by using low charge beams while rf effects can be mitigated by using short beams.…”

Analysis of Skew Quadrupole Compensation in RF-Photoinjectors

“…We start with the case of zero phase advance between the corrector and error quadrupole. Many of the results that we present here have already been discussed in [10,11] and the follow-up paper [13].…”

“…The effects of skew quadrupoles acting on the beam distribution are well documented [10], and widely known to impact significantly the transport and diagnosis of high brightness beams [11,12]. Experimental investigation in RF photoinjectors as well as the possibility of correcting the induced correlations has been recently discussed in the literature [13]. In the treatment offered in these papers, one would want to place the corrector element right at the plane of the error.…”

Transverse phase space characterization in an accelerator test facility

“…The difference between the normal mode emittances is expected to be minimised when there is zero solenoid field on the cathode: with increasing field strength (parallel to the longitudinal axis, in either direction) one emittance will increase while the other will decrease. Tuning the machine for optimum performance generally involves minimising the coupling, to achieve the smallest possible emittance ratio [15], and characterising and understanding the coupling as a function of the strength of the bucking coil is thus an important step in machine commissioning. Four-dimensional phase space tomography offers a powerful tool for providing insight into coupling in the machine, and was used to study the dependence of the phase space distribution on the strength of the bucking coil.…”