A Simple Method for Timing an Xfel Source to High-power Lasers

We propose a technique for timing an XFEL to a highpower laser with femtosecond accuracy. The same electron bunch is used to produce an XFEL pulse and an ultrashort optical pulse that are, thus, naturally synchronized. Cross-correlation techniques will yield the relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser with femtosecond resolution. Technical realization will be based on an optical replica synthesizer (ORS) setup to be installed after the final bunch-compressor. The electron bunch is modulated in the ORS by an external optical laser. Travelling through the main undulator, it produces the XFEL pulse. Then, a powerful optical pulse of coherent edge radiation is generated as the bunch passes through a long straight section and a separation magnet downstream of the main undulator. Relative synchronization of these pulses is preserved using the same mechanical support for X-ray and optical elements transporting radiation to the experimental area, where single-shot cross-correlation between optical pulse and pump-laser pulse is performed. We illustrate our technique with numerical examples referring to the European XFEL. For a more extensive treatment and references we redirect the reader to [1]. TIMING SYSTEM DESCRIPTION With the realization of x-ray free electron lasers (XFELs), pump-probe experiments will be used to monitor time-dependent phenomena with femtosecond accuracy and atomic resolution. Relative synchronization of radiation pulses from XFEL and optical laser on the femtosecond level can be relaxed when information on the temporal jitter between XFEL pulse and pump-laser pulse is kwown. In fact, if the time shift between pump and probe pulse can be measured on-line with femtosecond time-resolution, jitter can be used to randomly sample various time-delays in the pump-probe experiment, which are subsequently sorted up. Based on this observation, we propose a concept of time-arrival monitor allowing measurement of relative delay between FEL and optical pulses on a fs time scale. Elements of the system are an optical modulator and an optical radiator (see Fig. 1). Operation of the optical modulator A laser pulse is used to modulate the electron energy at the same wavelength by interaction in a short modulator-undulator. Subsequently, the electron bunch passes through a dispersion section, where the energy modulation induces a density modulation at the seedlaser wavelength λ. The amplitude of density modulation at the exit of the chicane approaches ai = R56(Δγ)i/( γ0) exp [ − 〈 (δγ)2 〉 R56/(2γ 2 0 2) ] , so that the current is I = I0[1 + ai cos(ψ)]. Here ψ = ω[z/vz(γ0) − t] is the modulation phase (vz is the longitudinal velocity of a nominal electron, ω = 2πc/λ, c is the speed of light in vacuum, t is the time), and I0 is the unmodulated electron beam current. Moreover, R56 is the compaction factor of the dispersion section, (Δγ)i sin(ψ) and 〈 (δγ)2 〉1/2 are the energy modulation and the rms uncorrelated energy spread of the electron bunch in units of the rest mass. Finally, = λ/(2π) is the reduced modulation wavelength. The modulator to be used is the already foreseen optical replica modulator. In the case of the European XFEL (see Fig. 2) γ0 4 · 103, (2 GeV), 〈 (δγ)2 〉1/2 2 (1 MeV rms uncorrelated energy spread), (Δγ) i 1, (0.5 MeV modulation), and λ = 400 nm (second harmonic of a Ti:Si laser). A value R56 30μm leads to a modulation amplitude ai 0.1. The density modulation reaches an amplitude of about 10%. Following the dispersion section the bunch goes through the main linac and the collimator. Finally, it enters the x-ray undulator. Since a high-current (about 5 kA) electron beam is transported, the initial density modulation produces an energy modulation due to longitudinal impedance caused by space-charge fields through the linac. Energy and density modulation can be shown to obey [1]