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Using simulations, a team of German and Russian physicists have pioneered a new technique for particle acceleration, called proton-driven plasma-wakefield acceleration (PWFA). The technique may one day allow machines a fraction of the size of today’s accelerators to create the highest-energy particles ever.

"This could be a major step forward," says Allen Caldwell of the Max Planck Institute for Physics in Munich, coauthor of the study, which appeared in Nature Physics Sunday. "The dream is that it will lead to much more compact — and therefore much cheaper — electron accelerators."

Progress in particle physics is contingent on the power of particle accelerators, and as particle colliders grow, the price tag and bureaucratic hurdles grow with them. Government pocketbooks are becoming increasingly tight — in December both the U.S. and the U.K. pulled out of the proposed $7 billion International Linear Collider, which may never actually be built. So to continue searching for answers to physics’ greatest questions — dark matter, extra dimensions, supersymmetry — physicists may have to find a fundamentally new way to accelerate particles. Caldwell and his colleagues hope proton-driven PWFA will pave the way.

Giant particle accelerators work by smashing subatomic particles such as electrons or protons together at high energies. This transforms the particles into energy, which then converts back into matter, potentially revealing new particles and advancing understanding of old ones. Over the past half century, particle accelerators have thoroughly probed the lower energy levels. The next frontier is the land of the teraelectronvolt (TeV, or a million million electronvolts).

There are only two ways for accelerators to increase the power: create a stronger electric field, or increase the distance over which particles are accelerated. We’ve already pretty much maxed out the strength of electric fields that can be contained without ripping electrons off the walls and essentially melting the inside of the accelerator. The other option is to create ever larger accelerators.

Building bigger proton accelerators, such as Fermilab’s Tevatron in Illinois and the Large Hadron Collider in Europe, is still possible because protons can be accelerated to very high energies in a circle. But the highest-energy electrons need linear tracks such as that of the SLAC National Accelerator Laboratory or the proposed International Linear Collider.

While proton accelerators are more powerful because of the continuous circular acceleration, electron accelerators are important because they are more precise. This is where plasma-wakefield acceleration may be able to help.

This radically new kind of acceleration skirts the electric field issue by using plasma — gas in which electrons have been ripped from their nuclei. This soup of ionized gas can handle electric fields about a thousand times stronger than can conventional accelerators, meaning the accelerators can potentially be a thousand times shorter.

In PWFA, tightly-packed bunches of electrons are fired into the plasma like bullets from a machine gun, blowing the plasma’s electrons away in all directions leaving the heavier plasma nuclei behind. These positively charged nuclei form a bubble of electron-free plasma behind the particle bullet. The negatively charged expelled electrons are drawn back toward the positively charged bubble.

But as the electrons snap back toward the bubble, they overshoot their original positions. So the particle bullet leaves behind a wake of mispositioned electrons, creating an intense electric field. By riding in this wake, the electrons can reach very high energies in a very short distance.

In 2007, a collaboration between SLAC, UCLA, and USC demonstrated PWFA’s potential: In a single meter, they were able to boost electrons zooming down SLAC’s linear track to twice what they can achieve over the entire two-mile-long accelerator.

But this strategy also has its limits. The maximum energy of the accelerated electrons depends on the energy of the particle bunches. SLAC currently produces the highest-energy electrons of any accelerator, at 50 gigaelectronvolts (GeV, or a thousand million electronvolts).

So Caldwell and his colleagues decided to give plasma-wakefield acceleration a new twist by blasting the plasma with protons instead of electrons. Today’s accelerators can bring protons to much higher energies than they can electrons. Protons at the Tevatron can hit 1 TeV (hence the name), and those at the LHC will be seven times as energetic.

"This would be a tool to transfer that energy from the protons to the electrons, via the plasma, in a single stage," says Caldwell.

In a numerical simulation, the team used proton-driven PWFA to accelerate electron bunches to 500 GeV in 300 meters of plasma. Compare that to the proposed $7 billion International Linear Collider (ILC), which will need at least nine miles to hit the same target, and SLAC’s linear accelerator, which needed 10 times the distance to reach a tenth of the energy. Combining the new proton-driven PWFA with the LHC’s powerful proton beam, Caldwell says it might be possible to accelerate electrons to several TeV, so that physicists can have their power, and their precision too.

"I look forward to watching these ideas continue to develop," says Mark Hogan, a member of the electron-driven PWFA team at SLAC. "There is still a lot of research and development needed to nurture these ideas. But in the not too distant future, we may find that ideas such as this have transformed the field of particle accelerators to make future machines that are both smaller and more affordable to society."

Electron acceleration by proton-driven PWFA is in its earliest theoretical stages — this study is the first to describe the concept — and is far from experimental verification. Perhaps the biggest issue is the proton bunch length, which must be very small to allow the electrons to overshoot and create the wakefield.

"It’s easy to do for electron bunches," says co-author Frank Simon of the Max Planck Institute. "But hadron colliders have bunches that are centimeters in length. We need bunches that are a hundred micrometers in length. We’re still looking at how to test the idea with present technology."

As governments put a stranglehold on spending, advancements in PWFA may be the best hope for refining the discoveries expected to be made at the LHC.

"In the past, opening up energy frontiers allowed us to discover new particles and to understand the basic forces," says Caldwell. "Today, there are new theories around that we want to test which predict new particles. But the basic reason is to just see what’s there."

Citation: "Proton-driven plasma-wakefield acceleration" by Allen Caldwell, Konstantin Lotov, Alexander Pukhov, and Frank Simon. Nature Physics, April 12.

Images: Laser-wakefield acceleration visualization / Lawrence Berkeley National Laboratory.