Abstract
We consider a 3D electron bunch at the cathode, modelled via photoemission, with realistic (asymmetric) spatial and temporal distributions for accelerator beam dynamics studies at the European X-ray Free-Electron Laser (European XFEL). A series of measurements are performed for low energy beams in the European XFEL injector. Using modeled 3D electron bunches, beam dynamics simulations, enabled by a three-dimensional (3D) space-charge solver with valid image-charge calculation on the photocathode plane, have shown improved agreements on measured beam properties (e.g., charge, bunch length, and shape) over a large range of variable machine parameters (e.g., cathode drive laser pulse energy, rf gun phase) for injector operation. In addition, the beam dynamics close to the cathode in a strongly space-charge dominated regime is studied in the rf gun. In light of the choice of a suitable machine working point along a so-called emission curve, the impact of the space-charge effect on the longitudinal length and shape of the produced electron bunch is analyzed in comparison to the measurements downstream the injector exit.
Highlights
Electron bunches with a small emittance, small energy spread, and high peak current are essentially required by short-wavelength self-amplified spontaneous emission (SASE) free-electron lasers to generate high-brilliance photon pulses [1]
To obtain such electron bunches at the European XFEL [2], a high-gradient photoemissionbased radio-frequency gun [3] and complex systems of rf driven superconducting linear accelerators are used in conjunction with a multistage scheme of bunch compression [4,5]
A measurement-based modeling approach is proposed to consider a full 3D photoelectron bunch with realistic spatial and temporal distributions produced at the photocathode
Summary
Electron bunches with a small emittance, small energy spread, and high peak current are essentially required by short-wavelength self-amplified spontaneous emission (SASE) free-electron lasers to generate high-brilliance photon pulses [1]. This means, an inhomogeneous map of the cathode QE can change the actual spot size of the produced bunch compared to the assumed bunch size which is the same as that of the drive laser pulse This may cause already inconsistencies in the intrinsic bunch size on the cathode plane and alter the obtained results from downstream beam dynamics. A significant degradation in the cathode QE and its homogeneity over the illumination area of the drive laser pulse are expected Another missing term in previous simulations is the modeling of fine features imprinted in the electron bunch produced in the photogun. That more realistic machine parameters (or distributions), given as inputs to the simulation (e.g., cathode QE map, 3D cathode drive laser distributions and pulse energy, rf phases, etc.), are determined through characteristic measurements in this study. Numerical discretization of the above equations follows the same route as described in [9]
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