Electrons surf on the plasma wave

Researchers develop concept for next-generation particle accelerators

In the wake of a proton packet: A proton beam generates a bubble of positive charge in a plasma. This results in such high electric field strengths that it would be much more efficient to accelerate an electron bunch that races behind the bubble than with conventional methods. This principle could make future electron-positron accelerators significantly cheaper. © Frank Simon / MPI for Physics
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Particle physics is a bit like retirement: you have to take care of what is supposed to happen in a few decades. Because the construction of particle accelerators is very complex and must be planned for the long term. That's why Max Planck researchers have already come up with an idea on how the next generation of particle accelerators could work - even before the LHC, currently the world's youngest and largest accelerator, has picked up speed.

While protons clash in the LHC, the scientists of the Max Planck Institute for Physics have proposed a concept with colleagues from the universities of Düsseldorf and Novosibirsk to accelerate electrons. The particles should surf on the suction wave, which generates a proton beam in a plasma. Such an accelerator could complement the findings at the LHC. If it is constructed according to the physicist's suggestion, it should be much cheaper than a similarly powerful electron accelerator, which works with the common technique, the researchers report in the journal "Nature Physics".

Discovery Machine LHC

When particle physicists speak of the LHC, they sometimes call it a discovery machine. The International Linear Collider (ILC), an electron-positron accelerator that physicists would like to build as the next accelerator after the LHC, on the other hand, likes to apostrophize it as a precision machine. "This classification may simplify a bit, but it is not completely wrong, " says Frank Simon, one of the scientists at the Max Planck Institute for Physics in Munich, who have now worked out the proposal for the new type of electron-positron accelerator,

The physicists insist that they discover the Higgs particle with the proton collisions at the LHC. But they can only describe it exactly with an accelerator that chases electrons and positrons of similar energy. With collisions of punctiform electrons and positrons, they know exactly how these elementary particles meet. This is different with collisions of the protons, because protons contain many elementary particles. "In addition to the actually interesting reaction much happens, which makes the analysis more difficult, " says Allen Caldwell, who led the work.

Acceleration in the plasma arc of a proton beam

A precision machine for electron-positron collisions would probably be cheaper to build if it accelerated the electrons in the plasma arc of a proton beam. Energies of a few teraelectronvolt would like to reach particle physicists in this way. On a teraelectronvolt of kinetic energy, it also brings a flying mosquito. In a proton, however, this energy is concentrated in a trillion-fold smaller space. display

The researchers around Caldwell suggest shooting electrons through a plasma behind a proton beam. In fact, the proton beam produces an approximately 100 times stronger electric field in the plasma than can be achieved with the technique used hitherto. For this reason, the electrons in the ILC, for example, pass through a large number of segments, where they are exposed to a field of 30 million volts per meter. On the other hand, electrons surfing the plasma wave of a proton beam could drag a field of billions of volts per meter. "Using our approach, we could accelerate electrons to similar levels of energy over a few hundred meters, and the ILC needs nearly 30 kilometers, " says Caldwell.

Almost at the speed of light through the plasma

The proton beam generates the strong field because it races through the plasma at almost the speed of light, pulling a positively charged bubble on its way. A plasma consists of a soup of positively charged atomic fumes and free flying electrons. If a proton package passes through it, it sucks in the electrons like a vacuum cleaner. But since the protons move at almost the speed of light, they are already in hurry when the electrons reach their orbit. The electrons then shoot past their target, a positively charged bubble opens, at the end of which the electrons slosh back and create a strong negative charge.

Exactly at the strongly negatively charged end of the bubble, the scientists would like to place a small package of electrons. The electrons would then see in front of them the positively charged bubble and behind them the negatively charged electrons and, because of the short distance, a very strong electric field. The force of this field would then accelerate the electrons. However, acceleration does not mean that the particles get faster - a paradox that Frank Simon can dissolve.

"Acceleration here means that the particles absorb energy, " the researcher explains: "They do not become any faster because they fly at almost the speed of light." The electrons then do not set the electrons into a higher tempo but into a larger relativistic mass. This explains the special relativity theory. Accordingly, the rest mass of a particle near the speed of light contributes only a tiny part of its total mass, and by far the largest part makes up the mass, which they gain at almost the speed of light thanks to their motion.

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Protons are easier to recharge

But what is gained, once you have to accelerate protons so high that they can drive electrons? Much of it, Simon and his colleagues are convinced. For one thing, with the LHC there is already a proton accelerator that can generate the desired energy, namely seven teraelectronvolts. "We're already having talks there to test our concept, " says Caldwell. On the other hand, protons are easier to charge with energy than electrons. For example, protons fly on a circular path in the LHC until they have collected enough energy.

This is not possible with electrons, because they release radiation in the roundabout and thus lose a large part of the energy they generate. That's why electrons in the ILC are supposed to fly on a 15-kilometer straight line, but they do not even reach one-twentieth of the energy that protons use to chase the LHC at the end of their journey. "Bringing electrons in a linear accelerator of common operation on energies, as they are achieved at the LHC would cost billions of euros, " explains Simon. The price of an accelerator depends mainly on its length.

Compress proton packet

In practice, however, the concept of Munich physicists will not be easy to implement. "The biggest problem is that the proton beam in the LHC extends a few centimeters, " explains Caldwell. The package must not be longer than the bubble it produces, just a few hundred microns. "So now we're thinking about a way to compress the protracted proton packet, " says Caldwell. A problem that researchers want to solve soon. Because, after all, they have to tackle the problems that an accelerator of the generation after next could have.

(MPG, 12.05.2009 - DLO)