Underpinning research into photocathode materials for use in next generation electron accelerators continues to play an important part in ASTeC's portfolio of scientific investigations. Improving photocathode performance is important to ensure performance, consistency and reliability of operations and thus minimise down time reducing the running cost of an accelerator. The focus is on exploiting the targeted suite of advanced analytical equipment which the department has assembled for this work with capability to investigate a wide range of materials and evaluate them both within the laboratory and in a 'real' accelerator environment. Meanwhile, progress continues to be made on the theoretical modelling of photocathode materials in collaboration with researchers at Imperial College.
TESS – The Transverse Energy Spread Spectrometer, which can measure both transverse and longitudinal energy spread, has been used to characterise III-V photocathode materials, including GaAs and GaAsP. The system has been additionally enhanced through the addition of a white light source and monochromator such that a range of wavelengths from 220 nm to 1µm can now be accessed. The system is now able to analyse samples requiring higher energy (UV) photons including Cu, other metals and Cs2Te. Research on metal photocathodes and Cs2Te will be important in supporting the VELA and CLARA accelerators at Daresbury.
Multi-probe system – The multi-probe surface analysis system has a compact UV laser system, similar to that on our ESCALAB Mk. II system that provides high luminosity to aid in the measurement of quantum efficiency. In addition, the atomic force microscope has been commissioned to provide an important technique for characterising surface morphology. The system has been used to deliver data on metal and metallic thin film cathodes. Single crystal data has been generated for comparison with theoretical modelling of photocathode materials.
Alkali metal Photocathode Preparation Facility (APPF) – Our alkali metal photocathode deposition work is a major part of our photocathode R&D programme. Our APPF includes state-of-the-art deposition sources and in-vacuum diagnostics which we are using to develop a consistent process to provide the CLARA accelerator with high-efficiency caesium telluride photocathodes which emit under UV illumination at 266 nm. In the longer term, we plan to develop a process using this system to manufacture bi-alkali metal photocathodes which emit under illumination at visible wavelengths around 532 nm, so they are literally green.
Metal Photocathode Preparation Facility (ESCALAB Mk. II) – This instrument has been modified to make it compatible with the photocathode pucks which are used in the VELA and CLARA accelerators. A cathode puck transport system has been designed, fabricated and tested and the instrument has also had a new preparation chamber added in addition to the pre-existing analytical chamber for sample characterisation. This new chamber incorporates facilities for sample cleaning including sputtering, annealing and atomic hydrogen cleaning and two new magnetrons for thin film growth. A vacuum suitcase arrangement is provided to allow photocathodes prepared in this system to be introduced into the VELA and CLARA guns. This upgrade will provide a virtually unique capability to evaluate a wide range of different photocathode materials in a 'real' accelerator environment.
Theory Collaboration – This collaboration with Imperial College has continued to progress with further modelling of metal photocathodes. The theoretical model continues to be extended, for example adding the capability to simulate non-zero temperatures. Direct comparisons can be made between the theoretical calculations and measurements made in the laboratory using our instrumentation, in particular TESS. Further work on novel photocathode materials is ongoing.
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