Abstract
Several parameters determine the performance of free electron lasers: the slice and the projected emittance, the slice energy spread, and the peak current are the most crucial ones. The peak current is essentially obtained by magnetic compression stages along the machine or occasionally assisted by velocity bunching at low energy. The minimum emittance and the alignment of the slices along the bunch are mainly determined in the low energy part of the accelerator (injector). Variations at the per-mille level of several parameters in this section of the machine strongly influence these quantities with highly nonlinear dynamic. We developed a numerical tool to perform the optimization of the injector. We applied this code to optimize the SwissFEL injector, assuming different gun designs, initial bunch lengths and intrinsic emittances. We obtained an emittance along the bunch of 0.14 mm mrad and around 0.08 mm mrad for the maximum and the minimum SwissFEL charges (200 and 10 pC, respectively). We applied the same tool to a running injector, where we automatized the optimization of the machine.
Highlights
The availability of reliable high-brightness electron sources is essential for the successful operation of linac based free electron lasers (FELs)
The first is the emittance of a fraction of the total bunch length along the longitudinal dimension of the beam
The code was used to optimize the SwissFEL [12] injector for various gun designs assuming different intrinsic emittances, laser pulse lengths, beam charges, and using nonstandard configurations for the laser longitudinal shaping
Summary
The availability of reliable high-brightness electron sources is essential for the successful operation of linac based free electron lasers (FELs). If the first accelerating cavity is positioned at a point where the emittance maximum and the envelope waist are, the second minimum is shifted toward the end of the accelerating section, where the space charge forces are for the typical conditions in photoinjectors negligible. If this condition is satisfied the resulting emittance can be minimized. The code was used to optimize the SwissFEL [12] injector for various gun designs assuming different intrinsic emittances, laser pulse lengths, beam charges, and using nonstandard configurations for the laser longitudinal shaping.
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