LABORATORI NAZIONALI DI FRASCATI
Speaker: S. Mistry (STFC ASTeC, Loughborough University)
Photocathode technology in accelerator science is an exciting and growing field of research and development. Prior to the first implementation of a photocathode in the RF gun operated at Los Alamos National Laboratory in 1985, thermionic cathodes were typically used in electron guns. Switching from the thermionic cathode to a laser driven photocathode offered a monumental improvement in the overall beam quality. With the advent of the photoinjector, the emittance reduced by over a factor of 10, compared with a thermionic injector. Photocathode research is increasingly important to meet the demands of modern accelerators
In linear accelerator driven 4th generation Free Electron Lasers, the final beam quality is set by the linac and ultimately by its photoinjector and photocathode. Therefore, to deliver cutting-edge beam characteristics, linac based sources have stringent requirements particularly with respect to the photocathode used in the photoinjector. Understanding how surface properties of materials influence photocathode properties such as Quantum Efficiency (QE) and intrinsic emittance is critical for such sources.
Metal cathode R&D at Daresbury Laboratory (DL) is driven by our on-site accelerators VELA (Versatile Electron Linear Accelerator) and CLARA (Compact Linear Accelerator for Research and Applications); the Free Electron Laser test facility at DL. Metals offer the advantage of a fast response time which enables the generation of short electron pulses. Additionally, they are robust to conditions within the gun cavity.
In this work, the effect of different preparation procedures on the surface composition, work function and QE was investigated for a range of metal photocathode candidate materials. For those materials that displayed a QE value greater than that of Cu, thin films were made and deposited, using PVD (Physical Vapour Deposition). A comparison of the surface morphology, QE and composition was made for the films. Surface morphology strongly affects the intrinsic emittance of the cathode and therefore understanding how the roughness of a surface will affect the energy spread of the emitted electrons is critical. I will present an update on the energy distribution measurements for electrons emitted from copper photocathodes as a function of their measured surface roughness as investigated at DL.