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The pioneering mini-accelerator from CERN passes the first test

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

The technique is the latest entry into a hot race a technology called Plasma Wakefield Acceleration. The method uses waves in the plasma, a soup of ionized atoms, to bring electrons over ever shorter distances to ever higher energies, as is the case with today's particle accelerators. Several laboratories have demonstrated plasma wake field acceleration using two different approaches; Most teams use laser beams to generate the required plasma waves. The latest work shows for the first time that protons can also induce the waves and reach electron acceleration ̵

1; a technique that can have advantages over the others because protons can carry high energies over long distances.

In this case, the researchers deflected protons and would normally be fed into the Large Hadron Collider (LHC) at CERN, Europe's Particle Physics Laboratory near Geneva, Switzerland, and instead the Wakefield Accelerator, the Advanced Wakefield Experiment ( AWAKE). The machine worked as expected and produced a consistent beam of accelerated electrons. "It has been a great success for us," says Matthew Wing, a physicist at University College London, AWAKE Deputy Speaker. "Essentially, it means the method works, and it's never been done before." The work is described in Nature on August 29.

In AWAKE's first test run in May, the team accelerated electrons to energies of 2 gigaelectron volts (GeV) over 10 meters. In principle, a much larger version of the experiment that took protons from the main ring of the LHC – and not its lower-energy sibling – could push electrons to energies of teraelectronvolt (thousands of GeV) in a single, kilometer-long, wing

"This would be a compelling achievement as there are currently no laboratory sources of teraelectronvolt electrons available," says James Rosenzweig, head of the Wakefield laser plasma research team at the University of California, Los Angeles.

Accelerated Progress

Proton-waking was first proposed as a possible method for accelerating electrons less than ten years ago. "It's really impressive that we're here – in a few years it has not turned into a truly significant experiment," says Mark Hogan, a physicist at the SLAC National Accelerator Laboratory in Menlo Park, California who is working on a plasma wakefield. Experiment Works

While the LHC, the world's most powerful accelerator, circulates protons in a ring, most accelerator laboratories in the world use electron beams, which are typically generated by linear accelerators. Most electron accelerators use strong RF waves to transport electric fields that push a beam of injected electrons that gain more and more energy when moving in a high vacuum tube. Stronger electric fields accelerate particles over shorter distances. A doubling of the field strength means that the same energies are achieved with a half as long accelerator. But the electric fields in these conventional accelerators can not reach much more than 100 million volts per meter.

Conversely, electrons move in a plasma wakefield accelerator in a mixture of electrons and positive ions instead of empty space. So far, there have been two established methods to achieve this acceleration. One is laser-driven, with the oscillating electric field of a laser shunting 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 greatly strengthens the electric field of the laser. This can create small areas in the waves where electric fields reach up to 100 billion volts per meter – 1000 times larger than conventional acceleration. The researchers then inject electron clusters that are strategically placed to "surf" and gain energy in these regions.

Several labs have been working on laser technology that does not require a large infrastructure, but is limited in the energies to the injected electrons within a single plasma step. In principle, the electrons from such an accelerator could be fed into another accelerator, increasing energy at every step, Wing says. But that has not been tried yet and could be extremely challenging.

Series Booster

Hogan and others at the SLAC have developed another plasma Wakefield acceleration technique that generates plasma waves using separate electron bunches. A bundle is introduced into the plasma and generates the first wave by repelling the electrons of the plasma. Another heap follows him; While the first group transfers energy to the plasma, the second decreases and accelerates some of that energy. In 2007, Hogan and his colleagues showed that they can give electrons within 1 meter of plasma as much energy as SLAC's historic linear accelerator has traveled all its 3 kilometers.

Then, 2009, Allen Caldwell of Max Planck. The Institute of Physics in Munich and its collaborators have proposed a further approach3 : to induce the plasma waves with protons instead of with electrons. To put this suggestion into practice, CERN built the Advanced Wakefield Experiment (AWAKE), a $ 25 million facility located 100 meters underground in an experimental hall inside the LHC ring.

AWAKE picks up protons from a 6-kilometer ring CERN called the Super Proton Synchrotron – which normally injects the particles into the 27-kilometer ring of the LHC – and shoots it into a 10-meter-long cell filled with rubidium plasma. A specially designed smaller accelerator generates electrons that penetrate into the proton wake. The accelerator achieved 2 GeV beam energies, and improved versions of this setup could produce electrons with energies up to 100 GeV, says Wing. Energies even in the 50 GeV range may be useful for experiments in which electrons are shot at a solid target, adds Wing.

In many applications, energies that are simply very high will not be enough, warn Rosenzweig and others. Radiation must also be of high quality – for example, in terms of how electrons in each bundle differ and how densely packed they are. The accelerated electrons could in turn be used to generate high intensity laser light. This application could produce more compact versions of instruments such as free-electron lasers used in other scientific fields to study materials and molecules.

For more fundamental physical experiments, however, even higher-quality rays are needed which particles collide head-on. Hogan says the next phase of SLAC's electronic project, scheduled to start in 2019, will focus on demonstrating these beam quality improvements. As far as AWAKE is concerned, these improvements will be further down, says Hogan. "There are many steps to be taken, but they have taken a very important first step."

This article is reproduced by permission and first published on August 29, 2018.

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