Using the optical-klystron effect to increase and measure the intrinsic beam energy spread in free-electron-laser facilities

We present a setup based on the optical klystron concept, consisting of two undulator modules separated by a magnetic chicane, that addresses two issues in free-electron-laser (FEL) facilities. On the one hand, it allows increasing the intrinsic energy spread of the beam at the source, which is useful to counteract the harmful microbunching instability. This represents an alternative method to the more conventional laser heater with the main advantage that no laser system is required. On the other hand, the setup can be used to reconstruct the initial beam energy spread, whose typical values in FEL injectors around 1 keV are very difficult to measure with standard procedures.

Figure 1

Schematic layout of the proposed scheme for controlling and measuring the intrinsic energy spread of electron beams.

Figure 2

Simulation results for an initial beam energy spread of 1 keV and a longitudinal dispersion of 10 mm. Top: average beam energy spread (black crosses, left axis) and bunching (blue dots, right axis) along the longitudinal position of the system. Bottom: beam energy spread along the longitudinal position of the bunch for three different locations: $Z=1.8\mathrm{m}$ (before the chicane), $Z=3.50\mathrm{m}$, and $Z=4.3\mathrm{m}$ (end of the second undulator module).

Figure 3

Simulated induced beam energy spread (black, left axis) and radiation energy (blue, right axis) as a function of the chicane’s $R_{56}$ for initial beam energy spreads of 0.5 keV and 2 keV.

Figure 4

Achievable beam energy spread as a function of the initial beam energy spread.

Figure 5

Simulated reconstructed beam energy spread for different initial beam energy spreads.

Figure 6

Output beam energy spread and square root of the output radiation power as a function of the chicane’s $R_{56}$.