How intense does a burst of energy have to be to cause a frozen object to erupt into flames? The answer, if you're hoping to achieve this quickly, is very. Now imagine that the object is cryogenically cooled to just above absolute zero, and the aim is to super-heat it to form plasma.
This is what Dr. Graeme Scott and his team at the CLF have recently achieved using our own Vulcan petawatt laser. This feat involved the use of pioneering cryogenic targetry jointly developed by the A-SAIL consortium and the CLF and has been considered a world first in the field.
The study used nanostructured cryogenic targets for laser driven particle acceleration, which has shown that deuterium beams can be accelerated in directional cones with a narrow range of kinetic energies. Ion beams with these parameters could find applications in fusion energy or healthcare and the research demonstrates a new mechanism by which ion beam properties can be controlled.
But why did cooling the targets make such a difference?
Cooling targets to cryogenic temperatures of only a few degrees above absolute zero (7 Kelvin, -266 Celsius), allows researchers to freeze nanometre-thin deuterium layers onto a target substrate, thereby creating layered targets in a configuration not obtainable by any other means. On interacting with an intense laser pulse, in less than 1 picosecond (10-12 seconds) the target leaves the realm of cryogenics, entering the highly energetic state of relativistic plasma, with temperatures comparable to the sun's core and accelerating electric fields millions of times larger than in conventional accelerators.
Such a rapid transition allows the plasma accelerator to initially retain the layered structure of the cryogenic target. This allows researchers to study the effects of varying the initial target structure on the properties of the accelerated ion beams, and in this research hydrogen and deuterium layers were used to investigate the simplest case interaction between two ion species.
Having just completed the very complex several week long experiment, Graeme recalled the ups and downs, “There was a moment about three weeks in where David and I were in Target Area Petawatt, and things weren't going as planned."
He explained how, with about two shots a day, the experiment was low repetition-rate and for a few weeks it seemed like the experiment wasn't going to work as intended. This was until one particularly hard day on the experiment. “We were just discussing how we may have to rip apart the experiment and start from scratch, then I went through to the scanning room, and it was just one of those moments – I knew the experiment had worked.
“You couldn't have seen it clearer on the screen – and it came to us at a pretty low time. That was probably the best part of the experiment for me. It was successful after that because we'd found the right parameter range to explore."
Thinking of the future with this “proof of principle" type experiment for a new type of ion acceleration scheme, Graeme would like to use the technique to investigate new approaches to experimentally realise the next generation of ion acceleration mechanisms.
Graeme was also appreciative of the people he worked with on this experiment, “I have worked with the Vulcan team a lot and know them pretty well; we'll go on socials and things like that. Working with your mates; it's pretty easy." He stated. “The user group was also great – it was a big collaboration with a lot of resources and effort invested in it, so it was great to see something so successful come out of it."
To read more about this experiment, please visit: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.204801
The A-SAIL consortium is gratefully supported by EPSRC funding under research grant number EP/K022415/1: Advanced laser-ion acceleration strategies towards next generation healthcare.