DESY, Hamburg, Germany
Electron bunch lengths in the sub-10 fs regime and charges of a few tens of picocoulombs are parameters required for free-electron lasers [*] and are also a consequence from the intrinsic process in laser-driven plasma wake field acceleration [**]. Since the coherent spectrum of transition radiation of these bunches carries the information on the longitudinal bunch profile in the form factor, the spectroscopy of transition radiation is an attractive method to determine the electron bunch length. A double-prism spectrometer has been developed and demonstrated for the single-stage measurement of mid-infrared transition radiation between 2 μm and 18 μm. The spectrometer facilitates single-shot spectral measurements with high signal-to-noise ratio utilising a line array of mercury cadmium telluride detectors. In this contribution, we present the spectrometer and measurements of electron bunches of the Free-Electron Laser in Hamburg (FLASH) at DESY. The results are compared to established bunch length monitors which are a multi-stage grating spectrometer for transition radiation and a transverse deflecting structure accessing the longitudinal phase space of the electron bunches directly.
*J. Rönsch-Schulenburg et al., Proceedings of FEL 2014, TUB04 (2014), to be published
**O. Lundh et al., Nature Physics 7, 219–222 (2011)
