Nanometer emittance ultralow charge beams from rf photoinjectors

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 nm). These beams are created in rf photoinjectors when the laser is focused on the cathode to very small transverse sizes (<30 μm 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.