Femtosecond X-rays from tabletop accelerator

‘We have accelerated about half a billion electrons to 2 gigaelectronvolts over a distance of about 1 inch,’ said Mike Downer, professor of physics in the College of Natural Sciences. ‘Until now that degree of energy and focus has required a conventional accelerator that stretches more than the length of two football fields. It’s a downsizing of a factor of approximately 10,000.’

The results, published in Nature Communications, are said to mark a major milestone in the advance toward multi-gigaelectronvolt (GeV) laser plasma accelerators becoming standard equipment in research laboratories.

Downer said he expects 10GeV accelerators of a few inches in length to be developed within the next few years, and he believes 20GeV accelerators of similar size could be developed within a decade.

Downer said that the electrons from the current 2GeV accelerator can be converted into ‘hard’ X-rays as bright as those from large-scale facilities. He believes that with further refinement they could even drive an X-ray free electron laser, the brightest X-ray source currently available to science.

A tabletop X-ray laser would be transformative for chemists and biologists, who could use the bright X-rays to study the molecular basis of matter and life with atomic precision, and femtosecond time resolution, without travelling to a large national facility.

‘The X-rays we’ll be able to produce are of femtosecond duration, which is the time scale on which molecules vibrate and the fastest chemical reactions take place,’ Downer said in a statement. ‘They will have the energy and brightness to enable us to see, for example, the atomic structure of single protein molecules in a living sample.’

To generate the energetic electrons capable of producing these X-rays, Downer and his colleagues employed laser-plasma acceleration, which involves firing a brief but intensely powerful laser pulse into a gas.

‘To a layman it looks like low technology,’ said Downer. ‘All you do is make a little puff of gas with the right density and profile. The laser pulse comes in. It ionizes that gas and makes the plasma, but it also imprints structure in it. It separates electrons from the ion background and creates these enormous internal space-charge fields. Then the charged particles emerge right out of the plasma, get trapped in those fields, which are racing along at nearly the speed of light with that laser pulse, and accelerate in them.’

Scientists have been experimenting with the concept of laser-plasma acceleration since the early 1990s, but they’ve been limited by the power of their lasers. As a result the field had been stuck at a maximum energy of about 1GeV for years.

Downer and his colleagues were able to use the Texas Petawatt Laser to push past this barrier. In particular the petawatt laser enabled them to use gases that are much less dense than those used in previous experiments.

‘At a lower density, that laser pulse can travel faster through the gas,’ said Downer. ‘But with the earlier generations of lasers, when the density got too low, there wasn’t enough of a splash to inject electrons into the accelerator, so you got nothing out. This is where the petawatt laser comes in. When it enters low density plasma, it can make a bigger splash.’

Downer said that now that he and his team have demonstrated the workability of the 2GeV accelerator, it should be only a matter of time until 10GeV accelerators are built. That threshold is significant because 10GeV devices would be able to do the X-ray analyses that biologists and chemists want.

‘I don’t think a major breakthrough is required to get there,’ he said. ‘If we can just keep the funding in place for the next few years, all of this is going to happen. Companies are now selling petawatt lasers commercially, and as we get better at doing this, companies will come into being to make 10 GeV accelerator modules. Then the end users, the chemists and biologists, will come in, and that will lead to more innovations and discoveries.’