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. This year the focus has been on exploiting the targeted suite of advanced analytical equipment which the department has assembled for this work. In addition, significant modification has been made to some of the systems to increase the capability to investigate a wider range of materials and evaluate them 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 for the first time GaAsP, which is an alternative to GaAs giving lower initial quantum efficiency but much greater resilience to degradation from residual gasses in the vacuum system. The system has been additionally enhanced through the addition of a white light source based on a high luminosity Xenon lamp. Using a monochromator a range of wavelengths from 255 nm to 1.1um can now be accessed. This new capability has been qualified by measuring the transverse energy spread of a GaAs photocathode as a function of the wavelength with the results showing the expected dependence. The system is now ready to analyse samples requiring higher energy (UV) photons including Cu, other metals and Cs2Te. Research on metal photocathodes will be important in supporting the VELA and CLARA accelerators at Daresbury.
Multiprobe system – The multiprobe surface analysis system has been upgraded with the addition of a compact UV laser system, similar to that on our ESCALAB Mk. II system, which provides higher luminosity to aid in the measurement of quantum efficiency. In addition, a new controller for the atomic force microscope has also been replaced to ensure the continuing availability of this important technique for characterising surface morphology. The system has continued to provide data on metal and metallic thin film cathodes, with studies this year including Cu, Zr and Pb thin films deposited on both Cu and Mo substrates using magnetron sputtering. Quantum efficiency measurements have shown that it is essentially independent of the substrate used with the best values being obtained for Pb films. It is hoped that thin film deposited photocathode will be usable in the VELA and CLARA accelerators sometime in the near future.
ESCALAB Mk. II – This instrument has been completely rebuilt this year in order to make it compatible with the photocathode pucks which will be 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 high repetition rate gun being developed for CLARA. 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 Cu and other metal photocathodes. An important development this year has been to extend the studies to much larger models, which has required the use of high performance, multi-processor computers, as the computing time typically scales with model size. The advantage of larger models is that it enables the evaluation of the effect of lower coverages of adsorbates, such as O, H and Cs, on both the work function and ultimate quantum efficiency. It is also hoped that the larger model size will allow the effect of surface roughness to be evaluated in the near future.
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