Nanometer emittance ultralow charge beams from rf photoinjectors

Abstract

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 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.

Review and demonstration of ultra-low-emittance photocathode measurements

“…Another variation of this method includes using one slit or a grid of small holes (called a "pepper pot"), and a screen instead of two slits. 21,30 It is also possible to measure a time resolved phase-space by using a deflecting cavity in place of the Faraday cup. 31 At high energies, the slit becomes too thick so that it restricts the range of angles and also offers more opportunity to scatter.…”

Femtosecond Electron Imaging and Spectroscopy

“…(1) is valid so long as the aspect ratio of the laser pulse is similar to that of a pancake. For a time-elongated laser profile (the so-called cigar shape), the space charge limit can be violated Li et al, 2012).…”

Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA

“…Methods for measuring single-shot emittance values for ultra-low charge bunches at the RF gun exit have been developed at UCLA [87]. Low emittances have also been measured, again at the RF gun exit, by using the well-known solenoid scan technique [88].…”