DESIGN AND CALIBRATION OF AN EMITTANCE MONITOR FOR THE PSI XFEL PROJECT

The Paul Scherrer Institute (PSI) intends to realize a compact X-ray Free Electron Laser (XFEL) based on the development of a high brightness, high current electron source. Field emitter arrays (FEA) in combination with high gradient acceleration promise a substantial reduction of transverse emittances by up to one order of magnitude compared to existing electron sources for XFELs. The acceleration concept and the main beam parameters will be demonstrated in a 250 MeV injector LINAC project. A flexible, high resolution emittance meter based on the pepperpot measurement technique has thus been designed to characterize this low emittance beam. The realization of this monitor and the calibration procedure will be presented. THE LOW EMITTANCE GUN PROJECT AND THE ROAD TO THE PSI XFEL A feasibility study to explore the options for an economic Free Electron Laser (FEL) facility in the hard X-ray regime with a target wavelength of 1 Å has been initiated at PSI [1]. Since the FEL radiation wavelength depends only linearly on the electron beam emittance, while it scales quadratically with beam energy, the main focus of activities in the present phase of the PSI XFEL project is directed towards the generation of a low emittance electron beam. In this context, a novel electron source – the so called low emittance gun (LEG) – is under development. It should provide sufficient peak current and up to one order of magnitude higher transverse brightness compared as to state-of-the-art RF photocathode guns. The LEG Test Facility starts operation with a conventional photo-cathode laser gun and concentrates during its first phase on the investigation of high gradient pulsed diode acceleration (up to 1 MV/m over 200 ns) [2] followed by a combined fundamental and 3 harmonic RF cavity [3]. Particle tracking indicates that the combination of DC and RF acceleration provides emittance preservation and sufficient linearization of the longitudinal phase space, which eases bunch compression in order to reach the peak currents required for the XFEL project. A diagnostics section has been designed to allow the full characterization of the electron beam with its main focus on the determination and optimization of transverse emittances and energy spread as critical beam parameters. In parallel to the demonstration of this novel acceleration concept, PSI investigates the properties of laser-induced field emission from single ZrC tips [4] and gated Mo field emitter arrays [4] as possible cathode materials promising normalized beam emittances as low as 0.05 mm mrad [5]. There are two test facilities to be realized at PSI until the end of the year 2010 and before starting with the actual XFEL facility. The LEG test stand is presently being set-up for demonstrating the performance of the novel high brightness electron source. The validity of the acceleration concept (including ballistic and magnetic bunch compression) shall be demonstrated in the 250 MeV XFEL injector test facility. More information on the PSI XFEL Project and its new developments including a full set of beam parameters can be found in [6]. THE LEG TEST FACILITY A mechanical drawing and the schematic overview of the LEG test facility is shown in the upper part of figure 1. For the expected range of beam currents (0.1 5.5 A), the smallest transverse emittances will be reached over a length of 0.5 m. The corresponding beam envelopes are shown in the lower part of figure 1. Figure 1: Mechanical drawing of 1 phase of LEG test facility (upper part). Optics simulations for expected beam current range showing beam envelopes at locations of smallest transverse emittances (lower part). The measuring rage of the emittance monitor is indicated in yellow. The electron beam will be generated by a commercial Nd:VAN laser (Duettino from TimeBandwidth company) providing 20 μJ pulse energy at 266 nm (frequency tripled) and up to 10 Hz repetition rate at pulse widths TUPC25 Proceedings of DIPAC 2007, Venice, Italy Beam Instrumentation and Feedback 198 Profile and Transverse between 15 and 40 ps (FWHM). With a copper cathode (QE ~ 10), beam charges of only 40 pC will be accelerated by the high voltage (500 kV) pulser. An upgrade of the laser system for generating the PSI XFEL design current of 5.5 A (200 pC beam charge) is foreseen for the next phase of the facility when the two frequency (fundamental and 3 harmonic) RF cavity will also be installed, providing acceleration up to 4 MeV. In the 1 phase of the LEG test facility the high gradient pulsed diode acceleration will be tested. In case of an optimized gun geometry and accelerating gradients of > 250 MV/m, normalized transverse emittances as low as 50 nmrad and beam sizes in the order of 0.7 mm (FWHM) are predicted by beam dynamics simulations using Homdyn [7] and the particle-in-cell code CAPONE [8]. Linear space charge forces can be compensated over a wide range of beam currents (0.1 – 5.5 A) by using a total of 5 solenoid magnets. DESIGN CRITERIA FOR THE LEG EMITTANCE MONITOR As indicated in the previous paragraph, the LEG test facility will be operated in different phases over a wide range of beam currents and beam energies, requiring the highest possible flexibility from the beam diagnostics devices. For the 500 keV and 1 MeV cases (beam optics shown in the lower part of figure 1) the emittance monitor needs to cover a measuring range of at least 0.5 m length. At the same time, it should be as compact as possible, since further use in the 250 MeV XFEL injector test facility might be required for cathode material studies. The well known “pepperpot” measurement appears most suited for determining the transverse emittance of a space charge dominated, low energy electron beam. While an exhaustive elaboration of the measurement principle is given in [9], only a short and simplistic explanation of the technique is given in the following using the notation indicated in figure 2.