Development of a compact hard X-ray split-and-delay line for studying ultrafast dynamics at free electron laser sources

Abstract

The study of condensed matter dynamics on ultrafast timescales is one of the key topics in modern material science research. Hard X-ray free-electron laser sources with extreme peak brightness and ultra short pulses provide excellent conditions for studying ultrafast dynamics in the time domain by employing such techniques as X-ray pump-probe spectroscopy or X-ray photon correlation spectroscopy. However, the intrinsic time structure of FEL sources limits the investigated timescales to 0.2 microseconds or slower. One way of overcoming this limitation is split-and-delay technology. This work presents a new concept for a compact hard X-ray split-and-delay device, enabling such experiments at X-ray FEL sources. The device is designed to split a single X-ray pulse into two fractions introducing time delays from -5 to 815 ps. Accessing such timescales allows to push studies of ultrafast dynamics beyond the intrinsic temporal limit of the X-ray source. The split-and-delay unit is based on Bragg optics and modern technologies for mechanics. Having a compact portable design with dimensions of 60x60x30 cm and a weight of about 60 kg allows to install the device in basically any experimental hutch of a FEL source. The split-and-delay line utilizes a combination of various silicon Bragg optics, arranged in various configurations, enabling the operation in the energy range from 7 to 16 keV. The quality of the beam splitting optics is checked by X-ray topography measurements. A novel method for the split-and-delay line alignment and time delay calibration using a infrared laser setup is developed and successfully used. The infrared setup allows a temporal pre-alignment with a precision better than 22 ps without the need for X-rays. The performance of the split-and-delay setup is checked by measuring the throughput and the delay times with the use of Si(111), Si(220) and Si(422) optics at 7 keV and 9 keV photon energies. Delay times are measured, ranging from 130 ps to 716 ps. The average uncertainty of measured delay times is 16.2 ps. The results show, that ultrafast pump-probe or XPCS experiments can be carried out with the compact split-and-delay line