Time-Domain Measurement Of A Self-Amplified Spontaneous Emission Free-Electron Laser With An Energy-Chirped Electron Beam And Undulator Tapering

We report, with an unequivocal time-domain measurement, that an appropriately chosen undulator taper can compensate for an electron beam longitudinal energy-chirp in a free-electron laser amplifier, leading to the generation of single-spike radiation close to the Fourier limit. The measurements were taken using the frequency-resolved optical gating technique by employing an advanced transient-grating diagnostic geometry. The reconstructed longitudinal radiation characteristics are compared in detail to prediction from time-dependent three-dimensional simulations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754612]