Improvement in the laser system for the A0 TTF Photoinjector

The production of high charge and high brightness electron beams places increasingly challenging demands on the drive laser used at the A0 photoinjector in the Fermilab. The IR and UV laser pulse lengths need to be optimized for such purpose. We have experimentally investigated two different ways to change the UV laser pulse length on the cathode; either by changing the bandwidth of the oscillator or by changing the distance between two compression gratings, the UV laser pulse length can be varied in the range of 3ps to 30ps. Also the strong correlation between the UV laser energy and the IR laser pulse length has been studied, and the result is applied to achieve the UV laser energy of 18μJ/pulse. Laser System The FNAL/TTF RF photoinjector has been designed to match the requirements of the TTF accelerator with high bunch charge (10nC), low emittance (<20mm-mrad) and high duty cycle (1%). A solid-state laser system to meet these requirements has been developed at the Fermilab.[1] The laser produces a 1MHz train of up to 800 equalamplitude pulses with up to 1mJ per pulse at the wavelength of 1054nm. The laser pulse train is produced by a phase-stabilized, mode-locked, Nd:YLF oscillator and a fast selection Pockels cell. The pulses are amplified in a Nd:glass amplifier chain consisting of a multipass rod ampifier and a 2-pass rod amplifier. The laser system uses Chirped Pulse Amplification (CPA) to produce 10ps pulses. After the fourth harmonic generation of the laser to 263.5nm, 18μJ per UV laser pulse can be obtained. A schematic diagram of the laser system is shown in Figure 1. A mode-locked Nd:YLF oscillator produces a low energy, continuous pulse train at 81.25MHz. Pulses from the oscillator are stretched and chirped in a 2km fiber. After the fiber, a train of 800 pulses is selected at 1MHz from the oscillator pulse train by a fast, low voltage, lithium FERMILAB-FN-0805-AD