In this paper we discuss the generation of a new class of high brightness relativistic electron beams, characterized by ultralow charge (0.1–1 pC) and ultralow normalized emittance ($<50\mathrm{nm}$). These beams are created in rf photoinjectors when the laser is focused on the cathode to very small transverse sizes ($<30\mu \mathrm{m}$ rms). In this regime, the charge density at the cathode approaches the limit set by the extraction electric field. By shaping the laser pulse to have a cigarlike aspect ratio (the longitudinal dimension much larger than the transverse dimension) and a parabolic temporal profile, the resulting space charge dominated dynamics creates a uniformly filled ellipsoidal distribution and the emittance can be nearly preserved to its thermal value. We also present a new method, based on a variation of the pepper-pot technique, for single shot measurements of the ultralow emittances for this new class of beams.

• Received 19 June 2012

DOI:https://doi.org/10.1103/PhysRevSTAB.15.090702

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Published by the American Physical Society