A Multi-Channel THz and Infrared Spectrometer for Femtosecond Electron   Bunch Diagnostics by Single-Shot Spectroscopy of Coherent Radiation
  The high peak current required in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. In this paper, a frequency-domain diagnostic method is described that is capable of resolving structures in the femtosecond regime. A novel in-vacuum spectrometer has been developed for spectroscopy of coherent radiation in the THz and infrared range. The spectrometer is equipped with five consecutive dispersion gratings and 120 parallel readout channels; it can be operated either in short wavelength mode (5 - 44 um) or in long wavelength mode (45 - 430 um). Fast parallel readout permits the spectroscopy of coherent radiation from single electron bunches. Test measurements at the soft X-ray free-electron laser FLASH, using coherent transition radiation, demonstrate excellent performance of the spectrometer. The high sensitivity down to a few micrometers allows study of short bunch features caused for example by microbunching effects in magnetic chicanes. The device is planned for use as an online bunch profile monitor during regular FEL operation. 
S. Wesch, B. Schmidt, C. Behrens, H. Delsim-Hashemi, P. Schm\"user
physics.ins-det physics.acc-ph