As advanced light sources generate ever shorter and more intense flashes, it becomes more of a challenge to monitor the properties of those flashes. For FELs and optical lasers, this is particularly important, as their light generation methods are not as stable as those of synchrotrons, necessitating the ability to characterize and control the properties of every flash of light. EUCALL developed a suite of tools that deal with these ultrashort, intense pulses.

Monitoring intensity and shape

In PUCCA, EUCALL scientists developed new methods of monitoring hard X-rays. DESY developed a gas-based monitor that characterizes both high and low intensity pulses at repetition rates keeping up with the megahertz repetition rates of accelerator-based facilities. Scientists learn about the shape of the beam and its propagation along the experiment station using devices called wavefront monitors. A method developed by EUCALL scientists at ESRF uses a simple soft membrane to produce a scattering pattern from a partially coherent hard X-ray beam at two different detectors simultaneously. From a comparison of the two patterns, the wavefront is calculated. In combination with algorithms developed through UFDAC, online wavefront measurements can be performed, allowing users to change their settings on the fly and know about the light that is incoming towards their sample.

Image: The EUCALL gas-based X-ray monitor was set up at the European XFEL’s FXE scientific instrument and tested during the instrument’s commissioning in 2017. The monitor’s multiplier, which boosts the X-ray signal, and the gas inlet, which delivers the rare gas used for the monitoring, are marked. Using this setup, the scientists were able to measure pulse energies at the rate at which the X-ray pulses arrived, which for that experiment was at 1.1 MHz. (Credit: DESY)

Timing tools

Using what is known as a pump–probe experiment, scientists observe time-resolved chemical and physical processes. At FELs and optical laser facilities, these time scales get down to a millionth of a billionth of a second or lower, making atomic vibrations and other movements visible. Performing these experiments using two different sources of light requires what scientists call a “timing tool”: an apparatus measuring when a reaction is set off by a pulse from an optical laser, as well as at what point in its progression a subsequent X-ray pulse images it. One of the timing tools developed by PUCCA measures the delay between “pump” optical lasers and “probe” X-ray pulses by measuring the changes, which are induced by the laser and observed using the X-ray pulses or vice versa, in the reflectivity of flat sheets of a stream of liquid. The result is a time readout that is precise to the needed experimental timescale. A different, accelerator-based approach has been pursued using a technique called terahertz-streaking, which maps the time between pulses as well as the pulse shape and records the data using a fast camera.

Image: The liquid flat sheet that is used for the timing tool, which enables scientists to determine the delay between two laser pulses arriving at the sample region for the experiment. The liquid sheet is made by merging two smaller liquid jets, and the combined jet is thinner than a human hair. The laser pulse and the X-ray pulse distorts the point where the two jets merge, changing the jet’s reflectivity. (Credit: European XFEL)

PUCCA Deliverables

Deliverable 7.1 - Ultimate XGM sensitivities at FEL and ELI sources (Study of the ultimate uncertainty achievable with transparent intensity Monitors at FEL and ELI sources) / Submitted 29 Septemmber 2016

Deliverable 7.2 - Liquid jet capabilities:  flat sheet versus microjet (Report discussing the properties and capabilities of different types of liquid sample jets) / Submitted 31 December 2016

Deliverable 7.3 - THz based arrival time monitor at FEL and ELI facilities (Design Report for the THz based arrival time monitor system)  / Submitted 28 March 2017

Deliverable 7.4 - XGM prototype, calibrated and tested (Prototype of x-ray gas monitor is calibrated and tested) / Submitted 28 March 2018

Deliverable 7.5 - THz-based arrival time monitor (Prototype of THz-based arrival time monitor available) / Submitted 25 September 2018

Deliverable 7.6 - Liquid-jet-based laser X-ray arrival time monitor (Prototype of Liquid-jet-based laser X-ray arrival time Monitor available) / Submitted 25 September 2018

Deliverable 7.7 - CDR to build wavefront sensor for 8-25 keV (Conceptual design Report how to build a wavefront sensor for photon energies from 8 to 25 keV) / Submitted 25 September 2018

Deliverable 7.8 - Wavefront analysis software package (Software package for wavefront analysis available / Submitted 25 September 2018

Deliverable 7.9 - Test of XGM prototype at different light sources (Report on campaign of measurements using the x-ray gas monitor at various advanced laser lights sources) / Submitted 18 September 2018