BETH
Contact:
Dr. Michael Vogel, +49 271 740 2594, michael.vogel@uni-siegen.de or michael.vogel@cern.ch
High Brightness Electron beams generated from novel THermal resistant photocathodes
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Overall scope of the project
Prospective light sources such as the free-electron laser (FEL), as well as synchrotron- and THz radiation sources, require injectors featuring high brightness elctron beams. Typically, X-rays originating from FELs show high intensities and short wavelengths. Consequently, they allow for the study of amorphous materials as well as the investigation of chemical reactions steps. Operating such electron injectors requires photocathodes providing features such as high quantum efficiency (QE), long lifetime, low thermal emittance, and low dark current. Unfortunately, til date, no material for photocathodes was identified satisfying all of the above mentioned requirements. Furthermore, the existing strategies are only able to achieve maximum QEs of about 20%.
Therefore, this collaboration strives to introduce a new materials system, which is able to potentially address all shortcomings of current approaches while at the same time providing a significantly increased QE. The base material of the work proposed here will be gallium nitride (GaN), as GaN based photocathodes feature potential QE values of up to 50%. Additionally, its large direct band gap of 3.39 eV, provides an enormous potential in the ultraviolet (UV) spectral regime. With respect to its application in the context of the SRF- and NC RF-guns utilized in this collaboration and the challenging laser-shaping in the UV-range, however, the performance of GaN needs to be modified. In this context two concepts will be pursued. First, the diffusion length of the excited electrons within the bulk material (GaN) and consequently the QE will be increased by p-type doping with magnesium (Mg). Secondly, the band gap of GaN will be directly tailored for the specific laser wavelength utilized in the applications by introducing indium (In). Consequently, the goal of this project is the investigation and optimization of the materials system InGaN:Mg as a novel film based photocathode approach. Aside from the obvious advantages of a large QE and the option of a tunable band gap, this material offers a high thermal conductivity. The latter will help in the reduction of the effect of cathode heating at high laser powers that limits the QE and lifetime of a photocathode. Further aspects of the work proposed here are fundamental studies of the mechanisms associated with the QE itself as well as options to improve it by means of surface roughness or any potentially applied periodic nanostructures on the surface. The project is based on an already existing closed collaboration between experts in the fields of accelerator physics, surface physics, theoretical electrical engineering and semiconductor theory. This unique combination of expertise offers the required interdisciplinary environment to tackle such an ambitious goal. In particular, it allows for the evaluation of the performance of the new cathodes under real application conditions. A success will channel into the ability to tailor future light sources featuring performances far beyond today state-of-the-art with respect to their specific requirements/ applications.
Scientific and technical objectives of the project
The main goal of this project is the development and optimization of high brightness InGaN:Mg-photocathodes. Initially, the crystallinity of the films, which are synthesized by physical vapor deposition (PVD) at the University of Siegen, will by optimized by means of a development of suitable interlayers and corresponding substrate pretreatment processes. Subsequently, in order to successfully achieve the operation of such novel photocathodes at different laser wavelengths providing the highest achievable perfomance, the band gap of the material needs to be controlled. Additionally, in order to potentially improve the QE of the cathodes even further, the effect of roughness and any applied periodic nanostructures on the surface will be investigated. The experimental findings will be directly incorporated into the numerical design studies at the TU Darmstadt. The crucial heating effects at high laser powers limiting the performance of the novel photocathodes will be investigated at University of Mainz. Studies of the correlation between the properties at high electrical fields and the material itself, carried out at the TU Dresden, will contribute to a better understanding of the fundamentals of the photoemission process, which is partially still not fully comprehended. Finally, the scientific findings on the basis of a prepared photocathode prototype will channel into the development of a new generation of SRF- and NC RF-guns.
The above mentioned individual tasks of the project associated to the participating groups can be summarized into two key aims:
- The development and characterisation of InGaN:Mg-photocathodes featuring a controllable band gap. Initially, an extensive characterisation of the material properties (e.g.: lattice structure, defects, chemical binding energies and composition) as well as the QE with respect to the synthesis conditions of InGaN:Mg-films as well as any inter-layer strategies is required. Afterwards, the material properties will be adjusted with respect to the in situ measurement of the QE (University of Siegen). Furthermore, the electrical properties at high field strengths will be measured at the TU Dresden with and without illumination to validate the material model. The results from the experimental studies will, in particular, be correlated with simulations regarding the effect of surface topography provided by the TU Darmstadt. Consequently, these simulations will be exploited to derive ideas to improve the performance of the photocathode, which will be used to modify the synthesis process at University of Siegen in an iterative fashion.
- Optimization of the photocathode with respect to heating effects that limits the QE and lifetime of a photocathode. The operation of any novel cathode material is typically limited by the above mentioned heating effects at high laser powers. Hence, the photocathode/inter-layer/substrate system will be refined and improved at the University of Siegen guided by the experimental data collected at the University of Mainz. Furthermore, a novel radar based imaging technology to estimate the electron density during illumination and large scale simulations to estimate the thermal load induced by the cavity leackage effects will be investigated at the TU Dresden and the corresponding results will be incorporated into the considerations (synthesis) at the University of Siegen.
Both strategies include a dedicated investigation of the influence of contamination as this is a common phenomenon caused during synthesis, the transport of the photocathode due to some residual gas adsorption, and the photoemission (PE) process itself throughout operation of the SRF- or NC RF-gun. Hence, the obtained expertise here is essential for the design of new photocathodes. After the successful preparation of a prototype, the properties of such a photocathode will be tested at HZDR, HZB and DESY/PITZ under real conditions. The aim here is to be able to achieve a high photoemission independent of the laser wavelength used based on an accurate control of the associated band gap.