A typical image of the ultrashort X-ray burst being generated. An intense laser beam enters a gas cell (from the left), and the resulting X-rays exit though a pinhole (to the right)
(Credit: Brendan Kettle)
20 December 2019
Combining powerful lasers and bright X-rays, Imperial and Science and Technology Facilities Council (STFC) researchers have achieved a demonstration of laser-driven X-ray absorption spectroscopy that will allow a new field of extreme experiments in both fusion science and astrophysics.
The new technique would be able to use a single X-ray flash to capture information about extremely dense and hot matter, such as can be found inside gas giant planets or on the crusts of dead stars.
The same conditions are also found in fusion experiments, which are trying to create a new source of energy that mimics the Sun.
The technique, reported this week in Physical Review Letters, was developed by a team led by Imperial College London scientists working with colleagues including those at the UK’s Central Laser Facility (CLF) at the Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory, and was funded by the European Research Council.
The researchers wanted to improve ways to study ‘warm dense matter’ – matter that has the same density as a solid, but is heated up to 10,000⁰C. Researchers can create warm dense matter in the lab, recreating the conditions in the hearts of planets or crucial for fusion power, but it is difficult to study.
The team used the CLF Gemini Laser, which has two beams – one that can create the conditions for warm dense matter, and one which can create ultrashort and bright x-rays to probe the conditions inside the warm dense matter.
Gemini is a high power, ultra-short pulse laser system delivering dual beams at a rate of one shot every 20 seconds
First author Dr Brendan Kettle, from the Department of Physics at Imperial, said: “We will now be able to probe warm dense matter much more efficiently and in unprecedented resolution, which could accelerate discoveries in fusion experiments and astrophysics, such as the internal structure and evolution of planets including the Earth itself.”
The technique could also be used to probe fast-changing conditions inside new kinds of batteries and memory storage devices.
Previous attempts using lower-powered lasers required 50-100 x-ray flashes to get the same information that the new technique can gain in just one flash. The flashes last only femtoseconds (quadrillionths of a second), meaning the new technique can reveal what is happening within warm dense matter across very short timescales.
In the new study, the team used their technique to examine a heated sample of titanium, successfully showing that it could measure the distribution of electrons and ions.
Lead researcher Dr Stuart Mangles, from the Department of Physics at Imperial, said: “We are planning to use the technique to answer key questions about how the electrons and ions in this warm dense matter ‘talk’ to each other, and how quickly can energy transfer from the electrons to the ions.”
Gemini allows scientists to study the way matter behaves under extreme conditions of temperature and pressure
The Central Laser Facility’s Gemini Laser is currently one of the few places the right conditions for the technique can be created, but as new facilities start operating around the world, the team hope the technique can be expanded and used to do a whole new class of experiments.
Dr Rajeev Pattathil, Gemini Group Leader at the Central Laser Facility, said: “With ultrashort x-ray flashes we can get a freeze-frame focus on transient or dynamic processes in materials, revealing key new fundamental information about materials here and in the wider Universe, and especially those in extreme states.”
‘Single-shot multi-keV X-ray absorption spectroscopy using an ultrashort laser wakefield accelerator source’ by B. Kettle et al. is published in Physical Review Letters.
About the Gemini laser system
Gemini is a high power, ultra-short pulse laser system delivering dual beams at a rate of one shot every 20 seconds. Experiments on Gemini produce bright, coherent x-ray sources, or energetic beams of electrons and protons. The pulses from the Gemini front end are so short that they are like sheets of light energy thinner than a human hair, which in addition can be focused to a spot a few thousandths of a millimetre across. The energy they contain is delivered to a very small target extremely quickly, allowing experimenters to study the way matter behaves under extreme conditions of temperature and pressure.
Gemini is one of the laser systems at the Science and Technology Facilities Council’s Central Laser Facility, located at the Rutherford Appleton Laboratory in Oxfordshire.
Last updated: 20 December 2019