Energy spread blowup by intrabeam scattering and microbunching at the SwissFEL injector

We present high-resolution measurements of the electron beam energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics, and the longitudinal dispersion of the lattice. The measured values are in general an order of magnitude higher than what is predicted by standard simulation codes. The measured dependences indicate that the energy spread blowup is caused primarily by intrabeam scattering and microbunching instabilities, effects not covered in the conventional modeling of radio-frequency injectors. We present further a numerical model that qualitatively reproduces the experimental data and an approach to mitigate the energy spread deterioration. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.

Figure 1

Sketch of the SwissFEL injector and example of an image of a streaked beam. In the sketch, $z$ corresponds to the longitudinal direction along the accelerator, $X$ is the horizontal position, the red boxes indicate rf accelerating structures, the green circle corresponds to the TD, and the magenta star indicates the position of the profile monitor. In the image, the white points and error bars indicate the slice centroids and slice beam sizes, respectively.

Figure 2

High-resolution energy spread measurement. Top: energy scan for $V=0.25\mathrm{MV}$ to determine the energy spread and the measurement resolution at this voltage. Bottom: measured energy spread as a function of the TD voltage to reconstruct the true energy spread. See text for more details.

Figure 3

Measured energy spread as a function of the laser heater and bunch compressor longitudinal dispersion ($R_{56,\mathrm{LH}}$ and $R_{56,\mathrm{BC}}$).

Figure 4

Energy spread measurements as a function of the peak current of the electron beam for two different $R_{56}$ configurations.

Figure 5

Energy spread measurements as a function of the lattice optics for two different $R_{56}$ configurations.

Figure 6

Measured and simulated dependences on the lattice $R_{56}$ (top) and the peak current of the electron beam (bottom).