FIRST TOLERANCE STUDIES FOR THE 4GLS FEL SOURCES

The Conceptual Design Report for the 4th Generation Light Source (4GLS) at Daresbury Laboratory in the UK was published in Spring 2006 [1]. 4GLS features three distinct FEL designs, each operating in a different wavelength range: an externally seeded amplifier operating in the photon energy range 8-100eV (XUVFEL); a regenerative amplifier FEL operating over 310eV (VUV-FEL); an FEL oscillator operating from 2.5200μm (IR-FEL). Preliminary results of tolerance studies for the FEL designs are presented. In particular, the effects of the relative timing offset between the seed pulse of the XUV-FEL and the electron bunch, as well as the effects of electron bunch timing jitter in the VUV-FEL, are presented. INTRODUCTION 4GLS is a 4th Generation Light Source proposed by CCLRC Daresbury Laboratory in the United Kingdom to meet the needs of the ‘low photon energy’ community. The 4GLS facility will combine energy recovery linac (ERL) and FEL technologies. This paper summarises the results of first tolerance studies for the XUV-FEL and the VUV-FEL. For the XUV-FEL, Genesis 1.3 [2] has been used to simulate the effects of a temporal offset between the electron bunch and the seed. For the VUV-FEL simulations, a one-dimensional, time-dependent FEL oscillator code which includes the effects of electron bunch arrival time jitter has been used. The XUV-FEL design [3] consists of an undulator system directly seeded by a tuneable HHG laser source. It is capable of generating short, tuneable, high-brightness pulses of 8-100 eV photons with peak output powers of ~2-8 GW and typical FWHM pulse length < 50 fs. The FEL undulator consists of a lattice of undulator modules separated by beam focusing elements and diagnostics. The first eight undulator modules of the FEL will be planar, while the final five will be of APPLE-II design in order to produce variably elliptical polarised radiation. The VUV-FEL [4] is a regenerative-amplifier-type FEL (RAFEL) [5] designed to deliver intense sub-ps pulses of tuneable coherent radiation in the photon energy range 3– 10eV. A hole-outcoupled low-Q cavity using robust low reflectivity optics provides sufficient feedback to allow high gain type FEL saturation after only a few cavity round-trips. In its standard operating mode the VUV-FEL will generate temporally coherent photon beams with peak power ~500 MW and FWHM pulse lengths of ~170 fs. Cavity length adjustment may allow superradiant operation with enhanced peak powers of ~3 GW and FWHM pulse lengths of ~25 fs. These figures are the maximum values across the full wavelength range. To enable variable polarisation, APPLE-II type undulator sections are employed throughout with a strong FODO focussing lattice and beam diagnostics distributed between sections. XUV-FEL SIMULATIONS 100 eV Pulse Amplifier Lasing A simulation of the XUV-FEL operating at 100 eV is performed, using the CDR parameters for the case of seed/electron bunch synchronism. The full set of planar and variable undulator modules are used with the APPLEII undulators set to helical mode so that circularly polarised radiation is generated. Figure 1 shows the seed pulse of peak power P = 30kW and duration 30 fs FWHM. Also plotted is the electron beam current of peak current Ipk = 1.5kA and duration 626 fs (188 ) FWHM. At the end of the FEL a peak saturated power of Ppk 2.4 GW is shown in Figure 2. A ‘clean’ central seeded region upon a noisier pedestal is seen. The pedestal is the pre-saturation SASE as the shot noise power is only a few tens of watts, the seed power of 30 kW saturates first. 0 5 10 15 20 25 30 35 0 100 200 300 400 500 P o w er ( kW ) 0 200 400 600 80