Dynamically self-optimized beam distributions at the UCLA Pegasus laboratory: Linearity of transverse and longitudinal phase spaces

In this paper we present preliminary experimental results of the recently proposed idea to generate ideal uniformly filled ellipsoidal beam distributions in an rf photoinjector by cathode illumination with a very short laser pulse (∼35 fs rms) and subsequent expansion under the beamʹs own space-charge forces. The great interest for uniformly filled ellipsoidal beam shapes derives from the electromagnetic fields associated with such charge distributions, which depend linearly on the bunch coordinates, thus allowing a perfectly linear, emittance-growth free transport. At the UCLA Pegasus laboratory, where the experiment was performed, we used an X-band deflecting cavity as an electron streak camera to visualize directly on a fluorescent screen the longitudinal beam profiles and verify the space-charge-dominated beam expansion. The transverse quality of the bunch was then characterized by a pepper-pot technique, yielding emittance values close to the thermal limits. The measured phase spaces, both transverse (reconstructed from the emittance measurement) and longitudinal (directly measured using the deflecting cavity on a dispersion screen), are also presented.