Transversal diagnostics of low-charge electron bunches at REGAE

A small Linac is operational as an electron source for the ultra-fast relativistic electron diffraction experiment, REGAE (Relativistic Electron Gun for Atomic Exploration), at DESY. Electron bunches, few fs-short, will be used to study structural dynamics of a sample in a time scale comparable to the electron pulse length. In order to confine electrons in a small volume and avoid the space-charge effect, relativistic electrons of 2-5 MeV energy are generated via a photo-injector RF-gun. Furthermore low-charge electron bunches of sub-pico Coulomb are required to keep the beam emittance small. Apart from all these preparations the atomic evolution should be monitored in a shot-to-shot basis. Sophisticated single-shot diagnostics are essential to generate and maintain such electron-bunches. Diagnostics include charge, energy, energy spread and transversal profile measurements. A LYSO scintillator coupled to a detector forms the transversal diagnostics. High-light emission of LYSO crystal in addition to efficient collection of the scintillated light, make the transversal diagnostics very sensitive to the low-charge detection. For this reason the first coupling optics component is located as close as possible to the scintillator. A stand-alone CCD as well as a home-made ICCD can be used as detector. The flexibility of switching between two different detectors provides a wide dynamic range of sensitivity to light. In ICCD mode the detectability of charge goes down to a few electrons per pixel with a significant single-shot S/N ratio. Aside from a breakthrough in low-charge detection in transversal diagnostics, this fulfills the requirements of electron bunch diagnostics at REGAE.