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 [Phys. Rev. Accel. Beams 21, 010101 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.010101]. 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 manuscript 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, showing that the corrected emittance could reach the thermal value for an electron bunch with a low charge and a short length.