The performance requirements for next generation electron accelerators put ever increasing demand on the photocathode performance, where it fundamentally limits the achievable beam quality. Metal photocathodes are limited by their high work function and relatively low quantum efficiency, necessitating the use of high powered deep UV lasers. Metal oxide thin film interfaces have been shown to reduce the work function of the underlying metal photocathode, whilst maintaining the ease of use, high durability and fast response time. This leads to an improvement in quantum efficiency and spectral response to desirable incident laser sources. We present the characterisation of a thin film yttria (Y2O3) enhanced Au photocathode at various film thicknesses. Quantum efficiencies were measured at 265 nm along with surface compositions via X-ray photoelectron spectroscopy.

During the last run, the CLARA accelerator* ran with a 2.5 cell 10 Hz S-band RF gun which had a modified back plate to allow the use of INFN-style photocathode pucks. Previously this gun had used a solid wall back plate that also acted as the photocathode**. This presentation describes the different photocathodes that were used during the run and the various methods employed to prepare them for use. An initial cathode which was based on a solid Mo puck with the thin film of Cu grown using magnetron sputtering was seen to give high initial QE but a very fast degradation rate. Subsequent cathodes were hybrids with a Mo body and a solid copper tip for the active area. Several cathodes prepared using alternative techniques were employed, giving varied initial QE and lifetime. The final cathode used had satisfactory QE and a long enough lifetime to deliver a six month period of beam exploitation for external facility users. * D. Angal-Kalinin, et al, ‘Design, specifications, and first beam measurements of the compact linear accelerator for research and applications front end’ Physical Review Accelerators and Beams 23 (2020) 044801 ** T.C.Q. Noakes, et al, ‘Photocathode preparation and characteristics of the electron source for the VELA/CLARA facility’ Proceedings of the International Particle Accelerator Conference 2018 (IPAC-18), THPMK063, 2018, Vancouver, Canada

The generation of high--brightness electron beams is a crucial area of particle accelerator research and development. Photocathodes which offer high levels of quantum efficiency when illuminated at visible wavelengths are attractive as the drive laser technology is greatly simplified. The higher laser power levels available at longer wavelengths create headroom allowing use of manipulation techniques to optimise the longitudinal* and transverse** beam profiles, and so minimise electron beam emittance. An example of this are bi-alkali photocathodes which offer quantum efficiency ~ 10% under illumination at 532 nm. Another solution is the use of modified photoemissive surfaces. Caesium has a low workfunction and readily photoemits when illuminated at green wavelengths (~532nm). Caesium oxide has an even lower workfunction and emits at red wavelengths (~635nm). We present data on our work to create a hybrid copper photocathode surface modified by implantation of caesium ions, measuring the surface roughness and probing its structure using MEIS. We measure the energy spread of photoemitted electrons, the QE as a function of illumination wavelength, and the practicality of this surface as a photocathode by assessing its lifetime on exposure to oxygen.

Over recent years four dedicated facilities have been built at Daresbury Laboratory by a team working on thin film SRF cavities. Firstly, a conventional DC resistance facility allows measurements of critical temperature and residual resistance ratio. In addition, three other facilities were designed in house to address superconducting thin film (STF) characterisation specific to cavities. In a magnetic field penetration facility, a DC parallel magnetic field is applied locally from one side of the sample similar to the field within an RF cavity. The STF behaviour under RF conditions is tested with planar samples using a 7.8 GHz choke cavity with the main advantage of a quick turnaround. The final facility uses a novel idea of split single cell 6 GHz cavities. Such a cavity can be deposited with both planar and cylindrical magnetrons allowing for both deposition techniques to be tested in the same cavity. Also, the results can be compared to choke cavity measurements for planar samples. They can also be inspected easily both visually and with surface analysis instrumentation. All facilities are based on liquid helium free cryocoolers to simplify operation, safety and maintenance.

The relatively high transition temperature of A15 superconducting materials makes them a potential alternative to Nb for radio-frequency applications. We present PVD deposition of one A15 material, V$_3$Si, on Cu and sapphire substrates. The surface structure and composition of the films were characterised via SEM and EDX. The superconducting properties were investigated using a field penetration facilty, four point probe and SQUID magnetrometry. Analysis showed that the composition was slightly Si rich by a few percent with a granular suface structure. Despite this superconductivity was observed on both Cu and sapphire substrates with critical temperatures of 12.8\,K and 14\,K. Field penetration measurements were conducted through two different facilities.

Many current accelerators use cavities that are manufactured as two half cells that are electron beam welded together, across the peak surface current of the cavity. This weld can limit the performance of Thin Film (TF) coated cavities by causing an increase in the surface resistance. Many problems with the coating process for TF Superconducting Radio Frequency (SRF) cavities are also due to this weld. TF SRF cavities can perform as well as bulk niobium cavities if the cavity is manufactured seamlessly, without any weld, however, they are much more difficult and expensive to manufacture. A cavity with a split parallel to the direction of the electric field, would not need to be welded. These cavities are easier to manufacture and coat. Thus, different coating techniques may be used leading to new materials and multilayer coating options which may allow SRF cavities to operate at better parameters than current state of the art cavities. TF SRF cavities have been developed for use in particle accelerators, as they have many advantages over normal conducting and bulk niobium cavities. One such advantage is that SRF TF cavities have a lower surface resistance, below the critical temperature, than NC cavities and a higher thermal conductivity than bulk niobium cavities leading to a more uniform temperature of the superconductor. This work discusses development and testing of longitudinally split seamless TF SRF cavities at Daresbury Laboratory

NEG-coated chambers have been adopted as the beam ducts for large particle accelerators and synchrotron light sources for the sake of the lower yields of the photon stimulated desorption (PSD) and the photoelectrons (PE) from the NEG films in addition to their pumping performance. Measurement of the photoelectron yield (PEY) was performed at the BL19B (PSD) beamline of the 1.5 GeV Taiwan Light Source (TLS) which simultaneously measures the PSD-yield. An aluminium cathode was inserted in the tubes and a positive bias of voltage for extraction of the photoelectrons applied. The PEY was obtained by dividing the photoelectron current by the photon flux of the synchrotron radiation. Measurements of the PEY include various types of NEG-coated stainless steel tubes and the bare tubes of titanium and aluminium alloys for the comparison. The experimental system and the results will be described in this presentation.

Non-evaporable getter (NEG) coated vacuum cham-bers are widely used as a vacuum solution in modern particle accelerators. In the development and testing of new NEG coatings to produce better vacuum, the pumping properties are evaluated. In this paper, Test Particle Monte-Carlo Simulations are created to investigate whether small bends in sample tubes may affect the results of pumping speed measurements, and therefore lead to a set of inaccurate results. With the preference to move towards smaller beam emittance in new accelerators, the required aperture of the beam vacuum chamber is getting smaller as well. The chambers are thus becoming more delicate (less mechanically stable), and able to be bent, therefore creating the risk that when NEG-coated samples are created, a bend in the tube is skewing the results. Findings have shown that a bend of less than 1° could lead to a change in results by a factor of 10 in a sticking probability, which is a severe difference that cannot be ignored. The results have a strong correlation with the molecular beaming area from the bottom to the top of the modelled tubes. In future, it will be important to define how straight a tube must be to obtain accurate pumping property measurements.