Femto-second, ultrashort laser wakefield electron bunch-duration measurements: A prism-based dispersion visible-to-IR spectrometer

A wide-band spectral diagnostic system based on dispersion property of the Zinc Selenide prism, a crystalline material highly dispersive in the near-to-far infrared spectral range, has been studied and developed for the laser wakefield acceleration experiment at LOA for the measurement of few femto-seconds long electron beam. The extensive PIC simulation studies of the colliding-beam LWFA have shown very short electron beam duration of less than 10 femtoseconds. The prism spectrometer diagnostic with highly sensitive Mercury Cadmium Telluride infrared detector and the diffraction-grating spectrometer with a high-resolution imaging visible camera together have the spectral range coverage and resolution capable of detecting ultra-short Coherent Transition Radiation (CTR) generated by interaction of bunch charges with a 100 microns thickness aluminum foil. The beam profile of asymmetric shape then could be extracted from the CTR spectrum by inverse Fourier transformation with Kramers-Kronig relation. The diagnostic system has been tested and calibrated for characterization of blackbody source spectrum and spectral responsivity. The measurement of electron beam duration of few femtoseconds has yet been convincingly shown with high resolution, and the measurements of this kind have important implication in understanding and subsequent successful operation of the future FEL light source with a highly mono-energetic LWFA beam source.