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 ($<50\text{ }\text{ }\mathrm{nm}$). These beams are created in rf photoinjectors when the laser is focused on the cathode to very small transverse sizes ($<30\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ 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.
Highlights
A recent trend in the development of high brightness electron beam sources is lower charge, lower emittance beams
In this paper we describe the study of the regime of operation of an rf photoinjector where the laser is focused on the cathode to a very small (& 30 m rms) transverse spot
In this paper we explored for the first time the cigar beam operating mode in an S-band rf photoinjector
Summary
A recent trend in the development of high brightness electron beam sources is lower charge, lower emittance beams. A direct application of ultralow charge ultralow emittance beams is high resolution ultrafast relativistic electron diffraction and microscopy where the beam is used to resolve temporally and spatially small changes in matters [2,3,4]. One of the highest brightnesses reported from a radio-frequency (rf) photoinjector has been the 0.2 mm mrad projected emittance (0.14 mm mrad slice emittance) obtained with 20 pC at SLAC using the Linac Coherent Light Source (LCLS) injector [10] This beam, with far lower charge than the original LCLS design working point, has enabled a new regime of operation of the x-ray laser characterized by fs pulses and short free-electron laser gain length. An important difference is that ultralow beam emittances can be obtained due to the minimization of the thermal emittance contribution
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