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April 13, 2009

Smaller and More Powerful Particle Accelerators


Proton-driven plasma-wakefield acceleration in a computer simulation accelerate electrond 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.

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."


Proton-driven plasma-wakefield acceleration

Plasmas excited by laser beams or bunches of relativistic electrons have been used to produce electric fields of 10–100 GV m-1. This has opened up the possibility of building compact particle accelerators at the gigaelectronvolt scale. However, it is not obvious how to scale these approaches to the energy frontier of particle physics—the teraelectronvolt regime. Here, we introduce the possibility of proton-bunch-driven plasma-wakefield acceleration, and demonstrate through numerical simulations that this energy regime could be reached in a single accelerating stage.







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.

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.


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