Uppsala spectrometer “trains” Stanford’s free-electron laser

15 June 2017

The free-electron laser at Stanford University.

The free-electron laser at Stanford University.

In a major international collaboration led by Imperial College, London, researchers at Uppsala University and elsewhere have found a way to facilitate future measurements using advanced free-electron lasers with a spectrometer.

The Linac Coherent Light Source (LCLS), the free-electron laser (FEL) at Stanford University, California, produces X-ray pulses that are millions of times stronger than other sources.

“With this light, you can do amazing experiments. And this is just the beginning: every radiation period brings new discoveries in basic physics, biology and much else,” says Professor Jan-Erik Rubensson at Uppsala University’s Department of Physics and Astronomy, who took part in the collaboration.

For many measurements, the properties of each X-ray pulse must be known in detail. The best way is to use an X-ray spectrometer. In today’s FELs with 10–100 pulses per second, an X-ray spectrometer is fast enough to capture every pulse. In contrast, the installations of the future, such as the European X-ray FEL (XFEL) in Hamburg and the new LCLS-II X-ray laser facility at Stanford, emit tens of thousands of pulses per second, making it difficult to measure each pulse individually.

In the study, the researchers show that a connection may be found between easily measured electron currents in the laser and the properties of the X-ray pulses measured in the spectrometer – here, at the FEL in Stanford.

“It was far from obvious that this would be possible, since the pulses are generated randomly. Besides accurate measurements, advanced mathematics is required, with ideas derived from artificial intelligence and ‘machine learning’,” says Jan-Erik Rubensson.

The scientists’ success in finding a connection may be important for experiments in other new facilities, where the pulse rate is so fast as to rule out measurement of each pulse.

“All you need is a spectrometer to train the machine in a few pulses, and then you can rely on the rapid current measurement to show all the pulses’ properties.”

The study was published in Nature Communications.

Read Imperial College London’s article on the study.