Benchmarking of Electro-Optic Monitors for Femtosecond Electron Bunches

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.

Figure 1

Longitudinal bunch charge profile measured during FEL operation with the transverse-deflecting structure (TDS), compared to the electric bunch field $E_b(t)$ obtained by single-shot electro-optic temporal decoding (EOTD). The bunch head is at the left side. The full time window is 2 ps for the TDS and 12 ps for EOTD. (a) Raw data. For the EOTD data, the background measured without electron beam is also shown. (b) The normalized TDS and EOTD signals, with backgrounds subtracted, are shown on an expanded time scale.

Figure 2

Time calibration of the electro-optic measurements is obtained by duplicating the gate pulse (see text). The base delay $T_0$ is not precisely known, but the additional delay $\Delta T$ can be varied with fs accuracy.

Figure 3

Steps in the comparison between TDS and EOTD signal shapes. (a) From top to bottom: TDS signal, equivalent THz pulse at EO crystal, simulated EO signal. (b) Comparison between simulation and EOTD measurement. The simulated and the measured signals have been scaled to the same peak height.

Figure 4

(a) TDS and EOTD signals of bunches showing a double-peak structure due to a detuned compression system. The simulated EO signal is shown by the smooth curve in the lower panel. (b) Sequence of EOTD signals obtained with the machine tuned for minimum bunch length and the bunch compression feedback switched off, resulting in shot-to-shot fluctuations in bunch shape.