  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.
