Electron Sources und Photocathodes
ERLs aim to provide high power electron beams with beam currents of several mA. These currents need to be extracted from electron sources with high demands on emittance and stability of the extracted beam. Electron sources based on photoemission from semiconductor-based photocathodes, for example GaAs, meet those requirements. Project Area A of AccelencE addresses theoretical and experimental studies of the emission process and its relevant parameters.
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Zeitaufgelöste Untersuchungen der Emission polarisierter Elektronen aus GaAs-Photokathoden
Time-resolved investigations on the emission of polarized electrons from GaAs photocathodes
Semiconductor photocathodes are used for a multitude of applications. GaAs is of particular interest for accelerator, nuclear and particle physics since it can be used to produce polarized, high current electron beam, i.a. in ERLs. Great importance is attached to optimizing the emission process. A thin film has to be applied on the GaAs surface to enable efficient electron emission. This so-called activation process and the chemical elements used for it are subject of studies at the test stand Photo-CATCH as part of this project. The experimental analysis includes systematic time-resolved investigations on the emission process as a function of, in particular, quantum efficiency, cathode lifetime, laser pulse length and intensity, or bunch charge.
Author: M.Sc. Maximilian Herbert
Aufbau und Test einer Kühleinrichtung für GaAs-Photokathoden (Match)
Construction and test of a cooling setup for GaAs photocathodes (match)
For high-current applications with spin-polarized electrons emitted from GaAs-photocathodes, it is necessary to maximize the charge lifetime of the cathode to ensure reliable operation. By using a cryogenic subvolume, it is expected to improve the local vacuum conditions around the sensitive cathode surface. Furthermore, the cooling of the cathode itself also allows a higher laser power to be deposited in the material, resulting in higher possible beam currents. To further increase the lifetime, an electrostatic bend is introduced leading to the reduction of ion-backbombardment. Such an electron source is presently being developed at the Photo-CATCH test facility in Darmstadt.
Author: M.Sc. Tobias Eggert
Modellierung und Simulation von auf Photoemission basierenden Elektronenquellen
Modeling and simulation of photoemission based electron sources
Modern electron sources, such as the PITZ photoinjector used at the European XFEL at DESY, reach bunch charges up to one nanocoulomb. Simulation studies of the PITZ photoinjector showed significant deviations from the experimental results. This indicates that commonly used simulation codes do not fully cover the physics of the beam generation process. Especially in sections of low beam energy, the influence of space charge effects on the electron bunch is dominating.
In this project, we develop a relativistic, three-dimensional simulation code that computes the space charge interaction based on the fast multipole method (FMM). The FMM is more flexible regarding the choice of the interaction model and yields maximum accuracy for the near field forces between particles. For this reason, it is very suitable to simulate space charge effects in high brilliance electron sources such as the PITZ photoinjector.
Author: M.Sc. Steffen Schmid
Emission mit hoher Brillanz durch Anregung von GaAs nahe der Bandlücke (Match)
High brightness emission by near band-gap excitation of GaAs (match)
The Small Thermalized Electron source At Mainz (STEAM) is a photoemission source designed to operate at up to 200kV bias voltage using negative electron affinity GaAs photocathodes excited by near band gap energy laser wavelength.
At 100kV the field extraction gradient at the photocathode surface is about two and a half fold of the Mainzer Microtron (MAMI) source allowing to extract high bunch charges and therefore increasing the brightness.
Currents of up to 10mA were reached limited by the power supply at high quantum efficiencies (QE). With decreasing QEs – i. a. due to ion back bombardment, gas desorption in the diagnose beam line – the effect of the surface photovoltage could be investigated thoroughly for sub-millisecond laser pulse lengths.
STEAM also has an improved vacuum system compared to the MAMI source allowing to reach very low ultra high vacuum conditions resulting in a vacuum lifetime of thousands of hours.
Author: M.Sc. Simon Friederich