Abstract
The ability to generate small transverse emittance is perhaps the main limiting factor for the performance of high-gain x-ray free-electron lasers (FELs). Noting that beams from an rf photocathode gun can have energy spread much smaller than required for efficient FEL interaction, we present a method to produce normalized transverse emittance at or below about $0.1\text{ }\ensuremath{\mu}\mathrm{m}$, which will lead to a significantly shorter length undulator as well as a lower electron beam energy for an x-ray FEL project. The beam manipulation consists of producing an unequal partition of the initially equal emittances into two dissimilar emittances by a flat-beam technique and exchanging the larger transverse emittance with a smaller longitudinal emittance. We study various issues involved in the manipulation. In particular, a new emittance exchange optics we found enables an exact emittance exchange necessary for this scheme.
Highlights
Accelerated beams often need to be manipulated in phase space to optimize their efficiency for application
For a 1-A free-electron lasers (FELs) driven by a 15-GeV electron beam, the normalized emittance of the matched electron beam is about 0:2 m
We show that the manipulated beam improves the performance of x-ray FELs significantly
Summary
Accelerated beams often need to be manipulated in phase space to optimize their efficiency for application. With the typical rms bunch length of z 20 m (about 4 kA of peak current at a 1-nC bunch charge) and taking an increased energy spread of 1 10ÿ4 at 15 GeV, the normalized longitudinal emittance is about z 60 m This order-of-magnitude energy spread increase is necessary to suppress the microbunching instability associated with the bunch compression process
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