Pulse Shaping At the MAX IV Photoelectron Gun Laser

A motivation for the development of a versatile, programmable source of shaped picosecond pulses for use in photocathode electron gun preinjectors is presented. We present the experimental setup for arbitrary longitudinal pusle shaping of the MAX IV photocathode gun laser. The setup consists of a grating-based Fourier-domain shaper capable of stretching the pulses directly in the UV domain. Preliminary results are presented and discussed. INTRODUCTION AND MOTIVATION The MAX IV Laboratory is a facility for production of synchrotron radiation. It includes two storage rings, which operate at electron energies of 1.5 and 3GeV. The facility operates a short pulse facility, while plans to build a soft X-ray Free Electron Laser (FEL) are at an initial stage. Both rings, the Short Pulse Facility (SPF) [1], and the possible future FEL make use of a 3GeV LINAC [2] for injection. There are two preinjectors at the LINAC, a thermionic and a photocathode electron gun. While either of the guns can be used for ring injection, the Short Pulse Facility and the possible future FEL require the short electron bunches that a photoelectrode gun can deliver. The 1.6-cell, BNL/SLACderived photocathode gun [3] is operated at 2.9985GHz. The gun is followed by an emittance-compensating solenoid. The requirement for effective emittance compensation that the RF phase does not vary significantly while the electron bunch is being created sets the upper limit on the usable laser pulse durations. The maximum variation has empirically been determined to be 10-15◦ and, given the RF frequency used, limits the laser pulse duration to about 10 ps. A lower limit on the pulse duration is set by the need to avoid a Coulomb-repulsion mediated expansion of the electron beam. The laser pulses used to photoionize a machined, copper cathode come from a frequency tripled Ti:Sapphire laser system. The laser system outputs IR pulses at 790 nm, with a bandwidth of 20nm. The tripled pulses, at 262 nm, have a photon energy of 4.7 eV, and a spectral bandwidth of 1.5 nm. The pulses are normally stretched to a duration of duration of 3 or 6 ps, using a combination of a prism stretcher and pulse stacking in birefringent crystals. The pulse energy is about 100 μJ on cathode. The laser system, along with the pulse diagnostics is described in detail in [4]. While the LINAC’s design normalised emittance of <1 μm meets the SPF requirements, a FEL would require a normalised emittance of below 0.4 μm, given a charge of 40 pC, at the FEL entrance. We can currently measure ∗ marija.kotur@maxiv.lu.se <0.98 μm at 50 pC and at beam energy of 260MeV. The emittance of the photoelectron gun is currently limited by the low RF power in the gun and the poor cathode quality. An upgrade of the RF driver and the installation of a new, polished cathode are both planned for 2017. At that point, it is anticipated that the laser transversal and longitudinal profiles are going to be the emittance-limiting factors. The temporal shape of the laser pulses generated through chirped pulse amplification is well-approximated by a Gaussian whose width corresponds to the inverse of its spectral bandwidth. However, sharp rise times are more suitable for an effective emittance compensation, and quasi flat-top pulses with a duration of several ps are commonly used at photocathode guns. However, an increase in the RF field during the creating of the electron cloud leads to an increase in Qe during the laser pulse, suggesting that a down-ramp pulse might lead to a better emittance. Furthermore, due to screening of the cathode by the electron cloud, the emittance might benefit from an up-ramp pulse. The interplay between these effects is expected to be charge dependent.