Physicists Develop Table-Top Accelerator to Compete With the Big Guys

With popular devices shrinking with every new model, physicists at the University of Texas at Austin weren't going to be outdone, together developing a tabletop particle accelerator capable of generating energies and speeds previously reached only facilities hundreds of meters long and costing millions of dollars to build.

The results, published in Nature Communications, represent a major milestone in the advance toward the day when mult-gigaelectronvolt (GeV) laser plasma accelerators are as standard as computers in research laboratories around the world.

"We have accelerated about half a billion electrons to 2 gigaelectronvolts over a distance of about 1 inch," Mike Downer, professor of physics in the College of Natural Sciences, said in a news release. "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."

Furthermore, Downer explained, the electrons from the current 2 GeV accelerator model can be converted into "hard" X-rays as bright as those from large-facilities and that with further refinement, they could even drive an X-ray free electron laser, the brightest X-ray source currently available to scientists.

To have an X-ray laser available to chemists and biologists everywhere would be transformative, the scientists argue, given that they could use the bright X-rays to study the molecular basis of matter and life with atomic precision without having to travel to a large, national facility.

In addition, according to Downer, the X-rays that such machines will one day be able to produce are of femtosecond duration, the time scale on which molecules vibrate and the fastest chemical reactions occur.

"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" he explained.

In order to generate the energetic electrons that produce the X-rays, Downer and his colleagues used an acceleration method known as laser-plasma acceleration, which involves a brief but intensely powerful laser pulse into a puff of 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."

To explain, Downer put it in terms of what would happen if a person threw a motorboat into a lake with its engine on.

The boat (or laser) makes a splash, then creates a wave as it moves through the lake at high speeds. During that initial splash, however, some droplets (charged particles) break off and get caught in the wave and accelerate by surfing on it.

"At the other end of the lake they get thrown off into the environment at incredibly high speeds," said Downer. "That's our 2 GeV electron beam."

The idea for a laser-plasma acceleration originated in the 1970s and scientists have been experimenting with the concept since the early 1990s. Due to weak lasers, however, the field of study remained stuck at a maximum energy of 1 GeV for years.

All this changed, however, when Downer and his colleagues were able to use the Texas Petawatt Laser, one of the most powerful lasers in the world, to push past this barrier and enable them to use gases that were 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."

Based on their experiment, Downer said he expects 10 GeV accelerators, which could do the X-ray analyses biologists and chemists want, of a few inches in length to be completed in the next few years, with 20 GeV accelerators of a similar size arriving within a decade.

"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."