Coherent x rays provided by the x-ray free-electron laser (FEL) have kindled strong interest in sophisticated diffraction imaging techniques. At NLCTA, we exploit such techniques for
the diagnosis of the density distribution of the intense electron beams typically utilized in an x-ray FEL
itself. We have implemented this method by analyzing the far-field coherent transition radiation emitted
by a microbunched electron beam. This analysis utilizes an oversampling phase retrieval
method on the transition radiation angular spectrum to reconstruct the transverse spatial distribution of the
electron beam. This application of diffraction imaging represents an advance in the diagnosis of high-brightness beams, as well as the collective
microbunching instabilities afflicting these systems.
The phase reconstruction method uses profile monitors to image the coherent optical transition radiation (COTR) emitted by the e-beam in order to reconstruct the spatial microbunching distribution. To test the proposed method, we generated a laser induced modulation (LIM) at 800 nm in a planar undulator. The experimental layout is shown above. It corresponds to the first part of the EEHG beam line. The 120 MeV beam is sent through an undulator, co-propagating with a resonant 800 nm laser. The resonant interaction generates an energy modulation in the electron beam which is then transformed into density modulation by a subsequent magnetic chicane. The electron beam is finally directed through a metal foil, causing emission of a COTR pulse that is profiled by a CCD camera. The far-field pattern is recorded with a commercial Navitar compound lens focused to infinity. A bandpass filter with a bandwidth of 10 nm is used to eliminate the higher harmonics of the COTR pulse. The seed laser transverse size is large compared to the e-beam, so the microbunching profile is a replica of the transverse shape of the electron beam. In the experiment the beam is uncompressed and thus not notably affected by the space-charge induced microbunching instability. Therefore the measurement can be benchmarked by comparison with a near-field incoherent OTR image obtained with no LIM applied, obtained by directly imaging the OTR screen. An iterative phase reconstruction algorithm is then applied to the far-field images, through which the complex amplitude and phase of the transverse microbunching distribution can be retrieved. The result, shown to the left, closely reproduces the incoherent beam profile. This technique can therefore be used to reconstruct more complicated microbunching geometries, such as those driven by MBI, and other higher-order processes.