  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.
