Microbunching instability study in a linac-driven free electron laser spreader beam line

Suppression of the microbunching instability is a priority for free electron lasers (FELs) whose goal is production of intense coherent radiation with very narrow spectral bandwidth. The instability can have large gain in multibend switchyard lines that connect the accelerator to multiple undulator lines. This study provides practical guidelines for switchyard optics designs that are largely immune to microbunching growth. A case study of the FERMI FEL switchyard is illustrated, resulting in an improved optics design that provides a unity microbunching gain while preserving the electron beam brightness. The analytical computation of the gain for the improved optics is supported by simulations and by experimental results.

Figure 1

Sketch, not to scale, of the FERMI linac-plus-spreader FEL-2 beam line. This study applies from the exit of L0 to the spreader end. Only the first magnetic bunch compressor, BC1, is active and for these calculations is presumed to give perfect linear compression.

Figure 2

Comparison of MBI gain (top) and energy modulation amplitude (bottom) at the end of the FERMI linac vs the initial density modulation wavelength. The two curves correspond to the separate predictions by the HK-model [22] (dashed line) and the B-model [23] (solid line). The laser heater is off.

Figure 3

Comparison of MBI gain (top) and energy modulation amplitude (bottom) at the end of the FERMI linac vs the initial density modulation wavelength. The two curves correspond to the separate predictions by the HK-model [22] (dashed line) and by the B-model [23] (solid line). For these curves, the laser heater induces 10 keV of rms energy spread before compression.

Figure 4

Top: B-model was applied to the spreader in the presence of different collective effects and with LH off. Bottom: Spectral dependence of energy modulation amplitude ($\mathrm{\Delta }\mathrm{E}$), bunching factor [b($\lambda$)] and gain through the spreader. The dots in proximity of the gain curve are from elegant particle tracking runs.

Figure 5

Gain (top) and energy modulation amplitude (bottom), at the end of the FERMI linac (green) and of the linac-plus-spreader beam line (black). The laser heater is set at 10 keV rms energy spread before compression. The bunching factor (not shown) grows up to 0.4 in the wavelength range $10–50\mu \mathrm{m}$.

Figure 6

Top: Linear optics functions along the FERMI spreader for the present optics design. Middle: Linear optics functions along the spreader for the new optics design. Bottom: ${\mathrm{R}}_{56}$ along the spreader for the present and the new optics design.

Figure 7

Gain (top) and energy modulation amplitude (bottom) at the end of the FERMI linac (green) and linac-plus-spreader beam line (black), for the new spreader optics. The laser heater is set at 10 keV rms energy spread before compression. The bunching factor (not shown) grows up to 0.2 in the wavelength range $10–50\mu \mathrm{m}$. Please compare with Fig. 5.

Figure 8

OTR intensity measured at the end of the FERMI spreader vs LH pulse energy, for the standard (blue dashed) and the new optics configuration (red solid). Error bars are the rms fluctuation of the OTR signal over 500 consecutive shots. In the inset: bunching factor calculated at the end of the linac-plus-spreader beam line with LH off vs final (compressed) wavelength of modulation, for the spreader standard (dashed) and new optics (solid).