Christopher Gerth

Electron beam profile imaging in the presence of coherent optical radiation effects

High-brightness electron beams with low energy spread at existing and future x-ray free-electron lasers are affected by various collective beam self-interactions and microbunching instabilities. The corresponding coherent optical radiation effects, e.g., coherent optical transition radiation, render electron beam profile imaging impossible and become a serious issue for all kinds of electron beam diagnostics using imaging screens. Furthermore, coherent optical radiation effects can also be related to intrinsically ultrashort electron bunches or the existence of ultrashort spikes inside the electron bunches. In this paper, we discuss methods to suppress coherent optical radiation effects both by electron beam profile imaging in dispersive beamlines and by using scintillation imaging screens in combination with separation techniques. The suppression of coherent optical emission in dispersive beamlines is shown by analytical calculations, numerical simulations, and measurements. Transverse and longitudinal electron beam profile measurements in the presence of coherent optical radiation effects in non-dispersive beamlines are demonstrated by applying a temporal separation technique.

Custom Optomechanics for the Optical Synchronization System at the European XFEL

Free-electron-lasers like the upcoming European XFEL demand highly reliable optical synchronization in the range of a few femtoseconds. The well-known optical synchronization system at FLASH had to be reengineered to meet XFEL requirements comprising demands like ten times larger lengths and raised numbers of optically synchronized instruments. These requirements directly convert to optomechanical precision and have yielded in a specialized design accounting for economical manufacturing technologies. These efforts resulted in reduced spatial dimensions, improved optical repeatability, maintainability and even reduced production costs. To account for thermal influences the heart of the optical synchronization system is based on an optical table made out of SuperInvar. To fully exploit its excellent thermal expansion coefficient, mechanical details need to be taken into account. This work presents the design and its realization of the re-engineered optomechanical parts of the optical synchronization system, comprising mounting techniques, link stabilization units and optical delay lines for high drift suppression.

Operation of Free Electron Laser FLASH Driven by Short Electron Pulses

The program of low charge mode of operation is under development at free electron laser FLASH aiming in single mode radiation pulses. A short pulse photoinjector laser has been installed at FLASH allowing production of ultrashort electron pluses with moderate compression factor of the beam formation system. Here we present pilot results of free electron laser FLASH operating at the wavelength λ = 13.1 nm and driven by 70 pC electron bunches. Relevant theoretical analysis has been performed showing good agreement with experimental results.

Longitudinal bunch profile diagnostics with coherent radiation at FLASH

The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. A novel in-vacuum polychromator (CRISP4) has been developed for measuring coherent radiation in the THz and infrared range. The polychromator is equipped with five consecutive dispersion gratings and 120 parallel readout channels. It can be operated either in short (5-44 μm) or in long wavelength mode (45-430 μm). Fast parallel readout permits the monitoring of coherent radiation from single electron bunches. Due to the large wavelength range covered and the absolute calibration of the device, Kramers-Kronig based phase retrieval allows to online reconstruct a longitudinal bunch profile from the measured coherent radiation spectrum. The device is used as a bunch length monitoring and tuning tool during routine operation at the Free-electron Laser in Hamburg (FLASH). Comparative measurements with the transverse deflecting structure show excellent agreement of both methods.