Modeling and simulation of RF photoinjectors for coherent light sources

Abstract

We propose a three-dimensional fully electromagnetic numerical approach for the simulation of RF photoinjectors for coherent light sources. The basic idea consists in incorporating a selfconsistent photoemission model within a particle tracking code. The generation of electron beams in the injector is determined by the quantum efficiency (QE) of the cathode, the intensity profile of the driving laser as well as by the accelerating field and magnetic focusing conditions in the gun. The total charge emitted during an emission cycle can be limited by the space charge field at the cathode. Furthermore, the time and space dependent electromagnetic field at the cathode may induce a transient modulation of the QE due to surface barrier reduction of the emitting layer. In our modeling approach, all these effects are taken into account. The beam particles are generated dynamically according to the local QE of the cathode and the time dependent laser intensity profile. For the beam dynamics, a tracking code based on the Lienard-Wiechert retarded field formalism is employed. This code provides the single particle trajectories as well as the transient space charge field distribution at the cathode. As an application, the PITZ injector is considered. Extensive electron bunch emission simulations are carried out for different operation conditions of the injector, in the source limited as well as in the space charge limited emission regime. In both cases, fairly good agreement between measurements and simulations is obtained.

Simulation studies on the RF gun saturated emission

“…For comparison, we used the results obtained in the Photo Injector Test Facility at DESY, Zeuthen (PITZ) both in the computer simulation and in experiment [Krasilnikov et al, 2012;Chen et al, 2018;Krasilnikov et al, 2019;Li et al, 2019]. We simulated only first 0.6 cell of PITZ L-band normal conducting 1.6 cell RF gun with a Cs 2 Te photocathode (Fig.…”

Beam dynamics of realistic bunches at the injector section of the European X-ray Free-Electron Laser

“…Particularly in the injector, low energy beams (i.e., ≤130 MeV at the European XFEL) are fragile and sensitive to the change of experimental conditions and machine uncertainties. Discrepancies of beam property parameters in the injector are already observed between measurement and simulation (e.g., in [3,14]).…”

“…A phase change corresponds to the change of the rf field gradient applied on the cathode. This affects properties of the produced photoelectron bunch via different mechanisms (e.g., Schottkylike effect [16], space-charge effects in the cathode region [17]). To fairly compare measurement with simulation, the rf phase needs to be carefully determined with respect to the rf field zero-crossing in the experiment, and the measured phase configuration should be used in the simulation accordingly.…”

Slice energy spread measurement in the low energy photoinjector

“…Simulation values are lower than measurement results by 20% to 30%. This discrepancy can be caused by laser temporal modulations [38], or simplified physics model in ASTRA simulations, such as intra beam scattering effect [17], self consistent space charge modelling close to the saturation photoemission regime [39][40][41].…”