Electron Bunch Timing with Femtosecond Precision in a Superconducting Free-Electron Laser

High-gain free-electron lasers (FELs) are capable of generating femtosecond x-ray pulses with peak brilliances many orders of magnitude higher than at other existing x-ray sources. In order to fully exploit the opportunities offered by these femtosecond light pulses in time-resolved experiments, an unprecedented synchronization accuracy is required. In this Letter, we distributed the pulse train of a mode-locked fiber laser with femtosecond stability to different locations in the linear accelerator of the soft x-ray FEL FLASH. A novel electro-optic detection scheme was applied to measure the electron bunch arrival time with an as yet unrivaled precision of 6 fs (rms). With two beam-based feedback systems we succeeded in stabilizing both the arrival time and the electron bunch compression process within two magnetic chicanes, yielding a significant reduction of the FEL pulse energy jitter.

Figure 1

Schematic of the free-electron laser FLASH with the locations of the two bunch arrival-time monitors (BAM1, BAM2) and the two bunch compression monitors (BCM2, BCM3). ACC1-ACC6: accelerating modules, BC1 and BC2: bunch compressors.

Figure 2

Operation principle of the electron bunch arrival-time measurement using an electro-optic modulator (EOM).

Figure 3

Top: Measured arrival times of 1500 consecutive electron bunches at two locations (BAM1 and BAM2) in the linac. The bunch charge is 0.8 nC. Bottom: Difference signal of the two BAMs.

Figure 4

Effect of the bunch arrival-time feedback on the arrival-time jitter along bunch train for two different machine settings. A stability of 25 fs could be achieved in the best case (setting 2).

Figure 5

Generation of different arrival-time distributions along the electron bunch train using the bunch arrival-time feedback. Each color indicates a predefined arrival-time slope within a single bunch train.

Figure 6

Effect of the bunch compression feedback on the beam phase fluctuations along the bunch train as measured with the bunch compression monitor BCM3.

Figure 7

Distribution of the FEL radiation pulse energy along the bunch train without and with activated bunch compression and arrival-time feedbacks.

Figure 8

Effect of the arrival-time feedback and the bunch compression feedback on the FEL pulse energy fluctuations along the bunch train.