Conceptual Design Reports

CDR chapter 5 - Photoinjector

The front-end injection systems of the FERMI@Elettra linac produce the high brightness electron beams that define the performance of the FEL and the quality of the x-ray beams delivered to the users. The injector mainly consists of the RF gun, based on the BNL/UCLA 1.6 cell design [1] and scaled to European S-band frequency, its compensation solenoid and two traveling wave S-band rf sections (called SØA and SØB) which accelerate the beam up to 100 MeV. The slice emittance at the end of the injector is specified to be less than 1.5 mm mrad. The injector must provide a linearly current ramped bunch in order to linearize wakefield effects in the linac sections [2]. This  equirement translates into finding the best laser pulse shape at the cathode that produces an electron bunch evolving into the desired current profile along the drift between the gun and the first booster section. Two main bunch configuration have been studied: a 0.8 nC/9 ps long “medium length bunch (MLB)” configuration and a 1 nC/11 ps “long bunch (LB)”. It is shown that the optimum laser temporal profile for both the MLB and the LB regimes is a quadratic ramp, which is transformed to a linear ramp by space charge forces in the injector. As the charge density varies from head-to-tail in the bunch, a best compromise has to be found for the emittance compensation process. As a consequence it is shown that slice emittance as well as charge distribution must be ramped along the bunch (see Paragraph 5.6.1): slice emittance values range from 0.7 to 1.1 mm mrad for the MLB regime and from 0.8 to 1.2 mm mrad for the LB regime while the current increases from 40 A up to 80 A. Furthermore, since a seeded harmonic cascade FEL is very sensitive to shot-by-shot variations in beam characteristics, the effects of jitters on the FEL performance have been studied in the ramped current scenario. In particular, time jitters are critical because they are translated into energy jitter in the two chicanes, thus affecting the FEL output power stability. The main source of time jitter comes from jitter in the drive laser arrival time at the cathode with respect to the gun RF waveform, specified to be equal to or less than 200 fs and corresponding to a 0.1 deg jitter in the RF phase seen by the beam. Hundreds of simulation of the injector output beam quality (Section 5.6.2) with input parameters randomly picked, within tolerance values dictated by present technology and supported by measurements, have been performed; they show that the overall expected rms time jitter value at the injector exit is 350 fs. Sensitivity studies and tolerance budgets concerning injector output current, energy, emittance and injector optics parameters are included in the discussion. Retrieve pdf