Automatic tuning and control for advanced light sources

Abstract

The next generation of X-ray Free Electron Laser (FEL) advanced light sources allow users to drastically change beam properties for various experiments. The main advantage of FELs over synchrotron light sources is their ability to provide more coherent, brighter flashes of light by tens of orders of magnitude with custom bunch lengths down to tens of femtoseconds. The wavelength of the brighter, more coherent light produced by an FEL is extremely dependent on both the electron beam energy, which must be adjusted between different experiments, and maintaining minimal electron bunch emittance. A large change in beam energy and bunch length usually requires a lengthy manual re-tuning of almost the entire accelerator. Therefore, unlike traditional machines which can operate for months or years at fixed energies, RF, and magnet settings FELs must have the ability to be completely re-tuned very quickly. For example, the Linac Coherent Light Source (LCLS) FEL can provide electrons at an energy range of 4–14 GeV and 1 nC pulses with 300 fs pulse width down to 20 pC pulses with 2 fs pulse width. The next generation of X-ray FELs will provide even bright, shorter wave-length (0.05 nm at EuXFEL, 0.01 nm at MaRIE), more coherent light, and at higher repetition rates (2 MHz at LCLS-II and 30,000 lasing bunches/second at EuXFEL, 2.3 ns bunch separation at MaRIE) than currently possible, requiring smaller electron bunch emittances than achievable today. Therefore, the next generation of light sources face two problems in terms of tuning and control. In parallel with the difficulties of improving performance to match tighter constraints on energy spreads and beam quality, existing and especially future accelerators face challenges in maintaining beam quality and quickly tuning between various experiments. We begin this chapter with a brief overview of some accelerator beam dynamics and a list of control problems important to particle accelerators. In the second half of this chapter we introduce some recently developed model-independent techniques for the control and tuning of accelerators with a focus on a feedback based extremum seeking method for automatic tuning and optimization which can tune multiple coupled parameters simultaneously and is incredibly robust to time-variation of system components and noise.