| Paper | Title | Page |
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| TUPJE055 | The Evolution of the Transverse Energy Distribution of Electrons from a GaAs Photocathode as a Function of its Degradation State | 1748 |
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The brightness of a photoelectron injector is fundamentally limited by the mean longitudinal and transverse energy distributions of the photoelectrons emitted from its photocathode, and is increased significantly if the mean values of these quantities are reduced. To address this, ASTeC constructed a Transverse Energy Spread Spectrometer (TESS)* – an experimental facility designed to measure these transverse and longitudinal energy distributions which can be used for III-V semiconductor, alkali antimonide/telluride and metal photocathode research. We present measurements showing evolution of the transverse energy distribution of electrons from GaAs photocathodes as a function of their degradation state. Photocathodes were activated to negative electron affinity in our photocathode preparation facility (PPF)** with quantum efficiency around 10.5%. They were then transferred to TESS under XHV conditions, and progressively degraded through controlled exposure to oxygen. Data has been collected under photocathode illumination at 635 nm, and demonstrates a constant relationship between energy distribution and the level of electron affinity.
* Proc. FEL ’13, TUPPS033, 290-293 ** Proc. IPAC '10, TUPE095, 2347-2349, Proc. IPAC ’11, THPC129, 3185-3187 |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2015-TUPJE055 | |
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| TUPJE056 | VELA Machine Development and Beam Characterisation | 1752 |
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| Recent developments on the VELA (Versatile Electron Linear Accelerator) RF photo-injector at Daresbury Laboratory are presented. These are three-fold; commissioning/installation, characterising and providing beam to users. Measurements for characterising the dark current (DC), 4-D transverse emittance, lattice functions and photoinjector stability are presented. User beam set ups to provide beam for electron diffraction and Cavity Beam Position Monitor development are summarised. | ||
| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2015-TUPJE056 | |
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| TUPJE058 | Preparation of Polycrystalline and Thin Film Metal Photocathodes for Normal Conducting RF Guns | 1759 |
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| A comparison of quantum efficiency (QE) and work function (wf) measurements of polycrystalline and thin film metal photocathodes for use in NCRF guns, similar to the S-band gun under development for CLARA project at Daresbury, are reported. Cu and Nb thin films were grown onto a Si substrate by magnetron sputtering and subsequently were prepared by annealing and Ar ion sputtering. To determine the surface chemistry, x-ray photoelectron spectroscopy was employed. QE measurements were enabled using a UV laser source giving 266 nm light. Wf measurements were carried out using a kelvin probe and ultraviolet photoelectron spectroscopy. Annealing the Cu thin film to 250°C yielded a QE of 1.2E-4; one order of magnitude higher than the QE for sputter cleaned and post annealed polycrystalline Cu. The optimum QE measurement for Nb thin film was 2.6·10-4, which was found to be comparable to the results obtained for cleaned bulk Nb. Analysis of XPS data of these metals suggest surface composition and surface chemistry are main contributing factors to the QE and WF. | ||
| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2015-TUPJE058 | |
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| TUPWI017 | Single-shot Multi-MeV Ultrafast Electron Diffraction on VELA at Daresbury Laboratory | 2278 |
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Funding: This work was funded by STFC Accelerator based Ultrafast Electron Diffraction (UED) is a technique for obtaining static structures and for studying sub-100 fs dynamic structural changes on the atomic scale. In this paper we present the first electron diffraction results obtained from the VELA accelerator in 2014. The accelerator was operated to provide typically 4MeV/c electron bunches. Diffraction patterns were observed with <<1 pC transported to the detection screen. Single shot and multi-shot accumulated diffraction data are presented from single crystal and polycrystalline samples, including Au, Al, Pt and C. Contamination of the diffraction pattern with dark current contributions is an issue. A variable size aperture directly in front of the sample offers some mitigation, but at the expense of reduced charge contributing to the diffraction pattern. We discuss future developments for electron diffraction on VELA including further beam optimization, measurement of bunch length with a newly installed Transverse Deflecting Cavity, and the planned developments for pump-probe studies. |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2015-TUPWI017 | |
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