• C.H. Gough, S. Ivkovic, F. Le Pimpec, M. Paraliev
  PSI, Villigen

As part of the SwissFEL project at Paul Scherrer Institute, an electron gun test stand has been built and operated. The goal is to achieve an exceptionally low emittance beam with a charge of 200pC for XFEL application. The electron gun consists of a High Gradient (HG) pulsed diode followed by an RF acceleration structure. The diode has an adjustable gap and the cathode is pulsed at up to 500 kV. The electrons were extracted either from a near-flat cathode surface or a dedicated photo-source recessed in a hollow cathode surface. For the diode electrtodes, many metals, geometries and surface treatments were studied for their HG and photo emission suitability. Polished metal electrodes, single tips, field emitter arrays and electrodes coated with different Diamond Like Carbon (DLC) types were tested. In particular, we found that DLC coating had useful properties. Surface electric fields over 250MV/m (350 ~ 400kV, in pulsed mode) with negligible parasitic electron emission were achieved; when UV laser illumination was applied to DLC electrodes, it was possible to extract electron bunches of over 60pC at gradients up to 150MV/m.

• F. Le Pimpec, B. Beutner, H.-H. Braun, R. Ganter, C.H. Gough, C.P. Hauri, R. Ischebeck, S. Ivkovic, K.B. Li, M. Paraliev, M. Pedrozzi, T. Schietinger, B. Steffen, A. Trisorio
  PSI, Villigen

In the framework of the SwissFEL project, an alternative electron source to an RF photo-gun was investigated. It consists of a high voltage (up to 500 kV), high gradient pulsed diode system followed by single stage RF acceleration at 1.5 GHz. The electrons are produced from photo-cathodes or from field emitter arrays. The final goal of this accelerator is to produce a 200 pC electron beam with a projected normalized emittance below 0.4 mm.mrad and a bunch length of less than 10 ps. We present comparisons between beam dynamic simulations and measurements, as well as thermal emittance and quantum efficiency (QE) measurements obtained by producing photo-electrons from various metal cathodes.

• C.P. Hauri, B. Beutner, H.-H. Braun, R. Ganter, C.H. Gough, R. Ischebeck, F. Le Pimpec, M. Paraliev, M. Pedrozzi, C. Ruchert, T. Schietinger, B. Steffen, A. Trisorio, C. Vicario
  PSI, Villigen PSI

In the framework of the SwissFEL activities at PSI we developed a powerful UV laser system delivering wavelength-tunable pulses at a central wavelength varying from 260 to 283 nm. The laser system based on a ultra-stable frequency-trippled Ti:sapphire amplifier delivers mJ pulse energy within a duration of 1-10 ps with 1.5 nm spectral width. Temporal flattop pulses are achieved by direct UV shaping with a UV Dazzler and a prism-based stretcher. The system is used to explore thermal emittance and quantum efficiency dependence on photon energy from metallic photo-cathode (Cu and Mo). With pepperpot techniques we have measured the predicted theoretical limit for thermal emittance (0.4 mm.mrad / mm rms laser spot size at 283 nm and 0.6 mm.mrad / mm at 263 nm) for metallic photocathodes.