Measurements and Modeling at the PSI-XFEL 500 kV Low-Emittance Electron Source

Paul Scherrer Institute (PSI) is presently developing a low-emittance electron source for the PSI-XFEL project. The electron gun consists of an adjustable diode configuration subject to pulses of 250 ns (FWHM) with amplitude up to 500 kV from an air-core transformer-based high-voltage pulser. The facility allows high gradient tests with different cathode configurations and emission processes (pulsed field emission and photo emission). In the first stage, the beamline consists of focusing solenoids followed by an emittance monitor. Selected beam characterization measurements from photo cathode operation driven by a 266 nm UV laser system delivering 4 μJ energy during 6.5 ps (RMS) are presented and compared to the results of 3D particle tracking simulations. INTRODUCTION AND MOTIVATION The goal of the PSI-XFEL project is the realization of an X-ray Free Electron Laser (FEL) operating in the wavelength range between 1 and 100 A and producing up to 10 photons per pulse at a repetition rate of 100 Hz. To keep spatial and financial requirements within reasonable limits, the project foresees a compact design featuring a 6 GeV S-band main linac. This compact layout requires a high-brightness electron beam, which in turn calls for a low-emittance source. The strategy chosen for the PSIXFEL project consists in utilizing a high-voltage pulsed diode providing fast acceleration with a special cathode optimized for low emittance (photo cathode or field emitter array). To evaluate various configurations and materials, a test stand has been set up at PSI consisting of a pulser, a laser system and a diagnostic beamline [1]. Figure 1 gives an overview of the pulser and beamline assembly. An important aspect of the test facility, in particular in view of the further advancement of the PSI-XFEL project, is to improve the understanding of the space charge dominated electron beam by way of simulation. Indeed, one of the objectives of the test facility is the validation of our 3D particle tracking code against observations. In this paper we present a set of measurements taken at the test facility and compare it to the result of a 3D particle simulation. Figure 1: Schematic view of pulser (left) and diagnostic beamline, including the emittance monitor (right).