Abstract
Using an experimental scheme based on a vertically deflecting rf deflector and a horizontally dispersing dipole, we characterize the longitudinal phase space of the beam in the blow-out regime at the UCLA Pegasus rf photoinjector. Because of the achievement of unprecedented resolution both in time (50 fs) and energy (1.0 keV), we are able to demonstrate some important properties of the beams created in this regime such as extremely low longitudinal emittance, large temporal energy chirp, and the degrading effects of the cathode image charge in the longitudinal phase space which eventually leads to poorer beam quality. All of these results have been found in good agreement with simulations.
Highlights
The generation of high brightness beams by rf photoinjectors has been a fundamental advance in electron beam sources, having enabled many novel electron beam applications such as self-amplified spontaneous emission freeelectron lasers (FELs) [1,2], inverse Compton scattering sources [3], and ultrafast relativistic electron diffraction [4,5]
While the physics of high brightness beam sources has been investigated in detail [12,13,14] it is noticeable that much of the theoretical and experimental attention has been given to the evolution of the transverse beam parameters, in particular beam sizes and normalized emittances, in the initial space charge dominated regime
In this paper we report on the characterization of the blow-out regime of rf photoinjector operation by measurements of longitudinal phase space performed at the UCLA Pegasus laboratory
Summary
The generation of high brightness beams by rf photoinjectors has been a fundamental advance in electron beam sources, having enabled many novel electron beam applications such as self-amplified spontaneous emission freeelectron lasers (FELs) [1,2], inverse Compton scattering sources [3], and ultrafast relativistic electron diffraction [4,5]. At the UCLA Pegasus photoinjector laboratory, we have experimentally demonstrated the blow-out regime of rf photoinjector operation. In this scheme, proposed a few years ago [8,9], a very short ( < 100 fs) laser pulse illuminates the cathode and the beam expands under its own self-forces from its initial pancake-like shape to create a nearly ideal uniformly filled ellipsoidal distribution [10]. Only an upper limit has been established for the uncorrelated energy spread (or slice energy spread) of a beam directly after emission and acceleration by an rf photoinjector [15,16]
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