Electro-optic time profile monitors for femtosecond electron bunches at the soft x-ray free-electron laser FLASH

Abstract

Precise measurements of the temporal profile of ultrashort electron bunches are of high interest for the optimization and operation of ultraviolet and x-ray free-electron lasers. The electro-optic (EO) technique has been applied for a single-shot direct visualization of the time profile of individual electron bunches at FLASH. This paper presents a thorough description of the experimental setup and the results. An absolute calibration of the EO technique has been performed utilizing simultaneous measurements with a transverse-deflecting radio-frequency structure that transforms the longitudinal bunch charge distribution into a transverse streak. EO signals as short as 60 fs (rms) have been observed using a gallium-phosphide (GaP) crystal, which is a new record in the EO detection of single electron bunches and close to the physical limit imposed by the EO material properties. The data are in quantitative agreement with a numerical simulation of the EO detection process.

Time-resolved pump-probe experiments at the LCLS

“…Electro-optic (EO) sampling measured the electron bunch arrival times relative to a synchronized external laser. Re-timing the data based on the measured arrival times then gave sub-100 fs timing resolution even though the actual temporal jitter was hundreds of femtoseconds [10][11][12]. The LCLS has developed an alternative but similar solution based on two resonant RF phase cavities (PCAV-1 and PCAV-2).…”

Extending electro-optic detection to ultrashort electron beams

“…The development of these decoding methods has lead to experimentally measured electron bunches as short as 120 and 60 fs using the electro-optic crystals ZnTe and GaP, respectively [7,10,11]. Unfortunately, this represents what is the experimentally verified lower bound for these decoding methods.…”

“…Recently, this has been adapted to the longitudinal profiling of accelerated charged particle beams. EOS has been used to characterize beams either directly, by measuring their self-fields [2][3][4][5][6][7][8][9][10][11], or indirectly by measuring the THz fields generated from transition radiation diagnostics [12,13]. In the case of the former, the particle beam's longitudinal profile can be deduced from its self-fields due to the fact that for sufficiently relativistic beams the temporal structure of the selffields matches the temporal structure of the beam [14].…”

An ultra-fast linear array detector for MHz line repetition rate spectroscopy

“…The multiplexer and the analog buffer operate at a maximum switching frequency of 60 MHz, therefore limiting the line-rate of the system to 2.7 Mlps. The analog outputs are digitized by a commercial Analog-to-Digital Converter (ADC) 1 . Digital-toAnalog Converters (DAC) provide the reference voltages to the GOTTHARD chip.…”

“…Sub-picosecond spatial resolutions have been achieved [1], therefore making EOSD of high interest for VUV and Xray FELs. Several accelerator machines have commissioned beam diagnostic stations based on EOSD [1]- [4].…”