TRANSVERSE ENERGY SPREAD MEASUREMENTS FROM GaAs PHOTOCATHODES AT VARIABLE WAVELENGTHS R. Beech, L.B. Jones ∗ , B.L. Militsyn, T.C.Q. Noakes STFC Daresbury Laboratory, ASTeC & the Cockroft Institute, Warrington WA4 4AD, UK H.E. Scheibler, A.S. Terekhov Institute of Semiconductor Physics (ISP), SB RAS, Novosibirsk 630090, Russia Abstract The Transverse Energy Spread Spectrometer (TESS) is an instrument specially developed at Daresbury Laboratory to measure the intrinsic transverse and longitudinal energy distributions from photocathode materials. Early work on the instrument has focused on its use for the characterisation of GaAs photocathodes such as those commonly used in DC photoinjectors. More recently work has been conducted to extend the range of materials which can be evaluated using this apparatus, in particular by incorporating a monochro- mated white light source (250 - 1000 nm) permitting energy spread measurements on metallic and multi-alkali photo- cathodes. New results are presented using a broadband light source with variable wavelength and spectral width, referred to as a white light source (WLS), to measure the energy spread of a GaAs photocathode across a range of different illumination wavelengths, to evaluate how this excess photon energy translates into photoelectron transverse energy. INTRODUCTION Free Electron Laser facilities require a high brightness elecrton beam for reasons that are well documented [1]. Elec- tron beam brightness in a linear accelerator is fundamentally limited by injector brightness, and this is itself limited by the source beam emittance or the intrinsic emittance of the cathode source. Electron beam brightness will be increased significantly by reducing the longitudinal and transverse en- ergy spread in the emitted electrons, thereby creating a cold beam. The TESS system provides the ability to measure transverse energy, and to make direct comparisons between photocathodes which have been prepared in different ways or experienced different conditions during operation. When using photocathodes, the upper limit on transverse electron energy is determined by the illumination wavelength, the level of electron affinity and the photocathode temperature. Consequently, data from the TESS includes a contribution from the emission angle, and values returned from a TESS measurement place an upper limit on the mean transverse energy (MTE). This equipment is therefore a key enabling step towards discovering new materials which will provide high electron beams for future accelerator facilities [2, 3]. Recent work on the TESS instrument has seen an extension of the range of materials which can be evaluated using this apparatus. ∗ lee.jones@stfc.ac.uk TESS EXTENDED CAPABILITY The TESS system combines a reflection–mode photo- cathode holder under grazing incidence illumination with a retarding-field electron detector and imaging system. The photocathode holder can be electrically biased, and can also be cryogenically cooled to liquid nitrogen temperature. The source and detector have been designed to be symmetric and flat, and contain non-magnetic components. The addition of a mu–metal shield around the source and detector provides screening against external magnetic fields. A full description of TESS can be found in a previous publication [4]. Figure 1: Schematic diagram showing the original experi- mental setup on the TESS system. Previously the TESS system relied on several fixed wavelength laser diodes to provide the photon power necessary to stimulate electron emission, these sources ranged from 532-808 nm. Fig. 1 shows the original experimental setup on the TESS system, with fixed laser diodes in place. The photocathode electrode is mounted on the left-hand side which includes cryogenic feeds. The MCP detector is mounted on the Z-translation stage allowing the drift distance between the cathode source and front grid to be varied between approximately 7.5 and 50 mm. A slot cut into a mu-metal magnetic shield around the cathode electrode and detector permits cathodes to be inserted and removed, and a hole in the shield allows transmission of the photocathode illumination source. Pho- tocathodes are operated in reflection mode, with the beam incident at 71° from the surface normal. The electron beam expands while in flight between the source and detector due to its transverse energy content ε tr , and analysis of the beam footprint returns the Transverse Energy Distribution Curve THPOW016 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 3964 Copyright © 2016 CC-BY-3.0 and by the respective authors 02 Photon Sources and Electron Accelerators T02 Electron Sources