Front End Simulations and Design for the CLARA FEL Test Facility

We present the design and simulations of the Front End for CLARA (Compact Linear Accelerator for Research and Applications), the proposed UK FEL test facility at Daresbury Laboratory. This is based around an S-band RF photocathode gun. Initially this will be the 2.5 cell gun, currently used on VELA facility at Daresbury, which is limited to 10 Hz repetition rate. Later, this will be upgraded to a 1.5 cell gun, currently under development, which will allow repetition rates of up to 400 Hz to be reached. The beam will be accelerated up to 50 MeV with a booster linac which will be operated in both bunching and boosting modes for different operating regimes of CLARA. Simulations are presented for a currently achieved performance of the RF system and drive laser with optimisation of the laser pulse lengths for various operational modes of CLARA. INTRODUCTION CLARA (Compact Linear Accelerator for Research and Applications) is a proposed 250 MeV FEL test facility at Daresbury Laboratory [1]. It will comprise a photoinjector, a four section S-band linac and FEL test area. The first 2 m long linac section serves as a booster or a buncher. CLARA is designed to test out various novel FEL schemes which require different beam pulses varying from ultra short very high current bunches for singlespike SASE to relatively long for seeded FEL. To cover all the range of the bunch lengths, longitudinal compression is going to be provided through both magnetic compression and velocity bunching. For many of these schemes, the required beam parameters can be met by either compression scheme. However, seeded FEL operation can only be met with the magnetic compression as a constant current profile is desired along the bunch, while single-spike SASE can only be met with velocity bunching to a very short bunch with high peak current. A comparison of the compression schemes can be found in [2]. A staged approach is taken to construction and commissioning, with stage one comprising of a photoinjector and booster linac to be installed in 2016. This links in to the existing VELA facility at Daresbury [3] and shares its RF and laser infrastructure. The electron source for CLARA will initially be the 2.5 cell S-band RF gun currently used at VELA. This is limited to 10 Hz repetition rate, at bunch charges of up to 250 pC. The gun is fed from a 10 MW klystron with 8.5 MW of power available at the gun. The maximum beam momentum measured around 5.0 MeV/c. To reach repetition rates of up to 400 Hz, a 1.5 cell High Repetition Rate S-band Gun (HRRG) has been designed and is currently under construction [4]. The VELA photoinjector laser will also be used to drive CLARA, with a beam split in the transport line. The laser pulse has a length of 76 fs rms that allows short low charge electron bunches to be produced. In this paper we analyse the beam parameters which can be obtained with the currently achieved performance of the RF system and laser and investigate the use of a longer pulse laser for the different modes of CLARA. OPERATION OF THE BASELINE INJECTOR WITH REDUCED GUN FIELD The original design of the 2.5-cell gun assumed operation at a peak field of 100 MV/m. However, the measured beam momentum in VELA is lower than was expected based on simulations and measured quality factors [5]. To match the measured beam momentum of 5 MeV/c, simulations show the peak electric field to be 70 MV/m. In this work, beam dynamics simulations were carried out with the ASTRA code. The photoinjector laser spot size was simulated as a 1 mm diameter flat-top with a 76 fs rms Gaussian temporal profile. An intrinsic emittance of the beam from the copper photocathodes assumed to be of 0.9 mm mrad per mm rms as per LCLS measurements [6]. 60 70 80 90 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Gun peak field MV m B un ch le ng th rm s p s Figure 1: Simulated dependence of the length of a 250 pC bunch at the exit of the 2.5 cell gun. Due to the short laser pulse length, the gun operates in the “blow-out” regime, where the space-charge expands emitted bunch. Thus, the electron bunch length in this case is determined by the bunch charge and the rate of acceleration. Fig. 1 shows how the rms length of a 250 pC bunch after the gun depends on the peak field (for the same operational phase). It can be seen then that reducing the gun peak field from 100 MV/m to 70 MV/m, causes Proceedings of FEL2015, Daejeon, Korea MOP057 Electron Beam Dynamics ISBN 978-3-95450-134-2 171 C op yr ig ht © 20 15 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s an increase of almost a factor of two in the electron bunch length. 4 2 0 2 4 0 10 20 30 40 50 60