Experimental Investigation of Superradiance in a Tapered Free-Electron Laser Amplifier

We report experimental studies of the effect of undulator tapering on superradiance in a single-pass highgain free-electron laser (FEL) amplifier. The experiments were performed at the Source Development Laboratory (SDL) of National Synchrotron Light Source (NSLS). Efficiency was nearly tripled with tapering. Both the temporal and spectral properties of the superradiant FEL along the uniform and tapered undulator were experimentally characterized using frequency-resolved optical gating (FROG) images. Numerical studies predicted pulse broadening and spectral cleaning by undulator tapering Pulse broadening was experimentally verified. However, spectral cleanliness degraded with tapering. INTRODUCTION The Source Development Laboratory (SDL) in National Synchrotron Light Source (NSLS) at BNL experimentally demonstrated the superradiance in a single-pass high-gain laser-seeded free-electron laser (FEL) amplifier [1]. Efficiency and spectrum enhancement by undulator tapering for a long-pulse seeded FEL were also experimentally verified at the SDL [2]. After these two achievements, we have started investigating the effect of undulator tapering on superradiance by using a short seed pulse. Both the temporal and spectral properties of the radiation along the uniform and tapered undulator were experimentally characterized using frequency-resolved optical gating (FROG) images. The 3-D time-dependent FEL code GENESIS [3] was used to simulate the superradiance process with and without tapering, and its results were compared against the experimental results. The experiments reported here were carried out at the SDL, which consists of an S-band BNL type rf gun, a 250-MeV linac, a 4-magnet chicane bunch compressor, and a Ti:sapphire laser system that generates both a UV laser for the photoinjector and an IR seed laser. The 100MeV electron beam passes through the 10-meter NISUS planar undulator [4]. The gap of each section (total 16 sections) of this undulator can be separately adjusted. For the tapered undulator experiment, the gaps in the last four sections (~7 m to 10 m) were gradually changed so that the undulator field linearly decreased to the field 6% lower at the undulator exit. The spent electron beam is diverted to a beam dump through a dipole magnet while the output radiation is sent to a diagnostic station equipped with a pyroelectric detector, a spectrometer, and a commercial Grenouille configuration FROG [5]. Table 1: Experimental Parameters