  We compare three techniques for characterising the transverse phase space
distribution of the beam in CLARA FE (the Compact Linear Accelerator for
Research and Applications Front End, at Daresbury Laboratory, UK): emittance
and optics measurements using screens at three separate beamline locations;
quadrupole scans; and phase space tomography. We find that where the beam
distribution has significant structure (as in the case of CLARA FE at the time
the measurements presented here were made) tomography analysis is the most
reliable way to obtain a meaningful characterisation of the transverse beam
properties. We present the first experimental results from four-dimensional
phase space tomography: our results show that this technique can provide an
insight into beam properties that are of importance for optimising machine
performance.
