Laser arrival measurement tools for SwissFEL
SwissFEL is aiming to produce X-ray pulses from 30 fs down to the attosecond time scale. This requires the compression of the several picosecond long electron bunches produced by a photo-injector to sub-fs level. To achieve this, 40fs accurate injection of the electron bunches into the main linear accelerator is necessary. Therefore high timing accuracy is required from the drive laser of the electron gun. Furthermore fs scan capability is foreseen for the experimental stations of the FEL. The ultra-short pulse pump-probe lasers therefore need to exhibit outstanding, below 10fs short term jitter relative to the X-rays. Timing tools for both the electron gun laser and for the experiments are developed. The former is based on electro-optical modulation of the optical reference at 1560nm by a signal produced from the gun laser at 260nm, a concept similar to beam arrival monitors in the linear accelerator, with an expected resolution below 20fs. The latter will use spectrally resolved cross-correlation technique to determine relative jitter between the optical reference and the laser used at the experiments at 800nm, with fs resolution. These systems will be complemented by electron and X-ray timing tools. In this paper we present the general concept for the laser arrival time measurement and correction, with first results obtained on a Ti: sapphire chirped pulse amplifier system. Shot to shot, short term jitter and long term timing drift measurements are presented, with discussion on the sources of the noise. Plans for the feedback stabilization and the resolution and limitation of the systems are also covered.