Control of the Swiss Free Electron Laser: Methods for Precision Control of Pulsed-Mode Accelerator Beams

The SwissFEL is capable of generating high-power, tunable X-ray pulses that are used to study molecular structure and dynamic processes at extremely fast timescales. The FEL effectively acts as a femtosecond photographic flash and will allow researchers to observe the creation of molecules in chemical reactions or study the detailed structure of proteins. The X-rays are generated by accelerating a pulsed beam of electrons through a high-energy, pulsed, radio-frequency field-to an energy of 5.8 GeV-and then a transverse periodic magnetic field. The accelerator is a series of 113 evacuated tubelike structures, energized by 32 high-power (up to 50 MW peak) amplifiers. The amplifiers are switched on for only 1-3 μs as each beam pulse is fired, and the entire process repeats 100 times per second. Precise control of each amplifier is applied every 4.2 ns. The control methods described involve learning the subtleties of the effect of adjustments on the beam pulse and applying these as corrections to the next pulse. Experiments on full-scale amplifiers and shorter beam-lines show that these corrections provide the electron beam quality and repeatability for the complete FEL system.