CERN's groundbreaking Mini accelerator complies with the first test

An experiment at CERN has shown a new way to accelerate electrons to high energies – one that could dramatically reduce the size of future particle accelerators and lower their costs.

The technique is the latest entrant in a hot race to develop a technology called plasma wakefield acceleration. The method uses waves in plasma, a soup of ionized atoms, to push electrons to ever higher energies over distances that are much shorter than those needed in contemporary particle accelerators. Several laboratories have demonstrated plasma wakefield acceleration using two different approaches; most teams use laser beams to create the necessary plasma waves. The latest work is the first to show that protons can also induce the waves and achieve electron acceleration – a technique that can have advantages over the other because protons can carry high energies over long distances.

In this case, researchers turned away from protons that would normally be fed into the Large Hadron Collider (LHC) at CERN, the European particle physics lab near Geneva, Switzerland, and instead in the Wakefield accelerator, the Advanced Wakefield. Experiment (AWAKE). The machine worked as expected and created a consistent bundle of accelerated electrons. "That was a great success for us," says Matthew Wing, physicist at University College London, spokesman for AWAKE. "It essentially says that the method works, and it has never been done before." The work is described in Nature on August 29.

During AWAKE's first experimental run in May, the team accelerated electrons into energies of 2 gigaelectron volts (GeV) of more than 10 meters. In principle, a much larger version of the experiment that took protons from the main ring of the LHC – instead of from its brother with lower energy electrons – could push energies of teraelectron volts (thousands of GeV) into a single kilometer-long phase, Wing says.

"This would be a fascinating achievement because there are currently no laboratory resources available for teraelectron-full electrons," said James Rosenzweig, who heads the research team for laser plasma wakefield at the University of California in Los Angeles.

Accelerated progress

Proton & # 39; awakes & # 39; was first proposed as a potential method for accelerating electrons less than ten years ago. "It's really impressive that we're here – in a few years it will have gone from nothing to a really important experiment," says Mark Hogan, a physicist at the SLAC National Accelerator Laboratory at Menlo Park, California, who is working on a plasma wakefield experiment there.

While the LHC, the world's most powerful accelerator, circles protons in a ring, most of the world's accelerator labs use electron beams, mostly produced by linear accelerators. Most electron accelerators use powerful radiofrequency waves to carry electric fields that push a beam of injected electrons, which get more and more energy when moving in a high vacuum tube. Stronger electric fields accelerate particles over shorter distances, so doubling the field strength means achieving the same energy with a half-long accelerator. But the electric fields in these conventional accelerators can not achieve much more than 100 million volts per meter.

Conversely, electrons in a plasma wakefield accelerator move in a mixture of electrons and positive ions instead of empty space. Until now, there were two methods to achieve this acceleration. One is laser-driven, with the oscillating electric field of a laser pushing the electrons of the plasma sideways, while heavier, positive ions react more slowly and remain essentially in place. The resulting separation of positive and negative charges significantly enhances the electric field of the laser. This can create small areas in the waves where electric fields reach 100 billion volts per meter – 1000 times larger than conventional acceleration. Researchers then inject electron beams that are strategically timed to & # 39; s & # 39; & # 39; surf & # 39; and get energy.

Several laboratories have worked on laser technology, which does not require a large infrastructure, but is limited in the energies that it can give to the injected electrons within a single plasma stage. In principle, the electrons can be transported from such an accelerator to another, where the energy is raised at every step, says Wing. But this has not been tried and can be a huge challenge.

Serial booster

Hogan and others at SLAC have played a pioneering role in another plasma wakefield acceleration technique, which produces plasma waves using separate beams of electrons. One bunch is inserted into the plasma and creates the first wave by repelling the plasma electrons. Another group follows in its wake; while the first bundle transfers energy to the plasma, the second takes a part of that energy back and accelerates. In 2007, Hogan and his colleagues showed that they could give electrons within a radius of 1 meter of plasma as much energy as the historical linear accelerator of SLAC covered the entire 3 kilometer stretch.

Then, in 2009, Allen Caldwell of the Max Planck Institute for Physics in Munich, Germany and his collaborators introduced yet another approach3: inducing the plasma waves with protons instead of electrons. In order to put that proposal into practice, CERN has built the Advanced Wakefield Experiment (AWAKE), a facility of US $ 25 million that occupies an experimental room of 100 meters underground in the ring of the LHC.

AWAKE takes protons from a 6-kilometer ring at CERN, called the Super Proton Synchrotron – which normally feeds the particles in the 27-kilometer ring of the LHC – and shoots them in a 10-meter-long cell filled with rubidium plasma. A specially built smaller accelerator produces electrons that are introduced in the wake of the protons. The acclerator achieved beam energies of 2 GeV and improved versions of this arrangement could produce electrons with energies up to 100 GeV, Wing says. Energies in even the 50-GeV range can be useful for experiments where electrons are shot at a fixed target, Wing adds.

In many applications, energies that are just very high will not be sufficient, Rosenzweig and others warn. Beams must also be of high quality, for example with respect to how electrons in each bunch differ in energy from each other and how tightly packed they are. The accelerated electrons can in turn be used to produce high-intensity laser light. This application can make more compact versions of instruments, such as free electron lasers, that are used in other scientific fields to examine materials and molecules.

But even higher quality jets will be required for fundamental-physical experiments in which particles collide. Hogan says that the next phase of the electron-based project of SLAC, starting in 2019, will focus on demonstrating those improvements to the bundle quality. As far as AWAKE is concerned, these improvements will be further in line, says Hogan. "There are many steps to solve, but they have taken a very important first step."

This article was accepted with permission and was first published on August 29, 2018.

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