Short proton bunches rapidly accelerate energetic electrons

Experiments show that short bunches of protons can produce electric fields that are strong enough to accelerate energetic electrons compactly. This discovery could lead to miniaturized high-energy particle accelerators.
Toshiki Tajima is in the Department of Physics and Astronomy, University of California, Irvine, California 92697, USA.

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For almost a century, particle accelerators have revealed the microscopic structure of the Universe in ever-increasing detail. This continual improvement has required progressively higher particle energies and, in turn, larger accelerators (the latest accelerator for such exploration1 has a circumference of 27 kilometres). In conventional accelerators, particles are propelled by electromagnetic waves that are produced by external circuits. To drastically reduce the size of accelerators, scientists are exploring ways to use waves that are instead generated internally, in an ionized gas known as a plasma2. In a paper in Nature, Adli et al.3 report such a method, which makes use of an experiment in which the plasma waves are driven by bunches of protons — much like a motorboat on a lake drives waves in its wake.

The authors demonstrated their method using the Advanced Wakefield (AWAKE) experiment4, which is located at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland. In this experiment, a proton bunch is injected into a plasma and sets electrons bobbing in its wake (Fig. 1). This electron motion generates a spatial modulation in the electric-charge density of the plasma, which in turn produces an electric field known as a wakefield. If another electron is injected into the plasma a short distance behind the proton bunch, it is captured by the wakefield and is accelerated to high energies.

Figure 1 | The AWAKE experiment. a, In the Advanced Wakefield (AWAKE) experiment4, a bunch of protons is injected into an ionized gas known as a plasma. As the proton bunch travels through the plasma, it attracts electrons contained in the plasma, pulling them towards the centre. b, By the time these electrons have reached the centre, the proton bunch has moved on. The electrons overshoot and begin to move outwards. c, The region that the electrons vacated is now positively charged. The electrons start to move inwards again, and the cycle repeats. Adli et al.3 show that if an electron is injected into the plasma a short distance behind the proton bunch, this cycling of positive and negative charge can rapidly accelerate the injected electron.

Because the proton bunch moves at close to the speed of light, the wakefield can be extremely strong. It can even be at the level of the Tajima–Dawson field2, the amplitude of which is several orders of magnitude larger than that of the fields used in conventional accelerators. This is the reason that scientists see wakefield acceleration as a means of substantially miniaturizing particle accelerators.

The amplitude of a proton-driven wakefield can be so large only when the proton bunch and the plasma’s internal clock (in this case, the oscillation period of the plasma waves) are in resonance — a condition that enhances the amplitude of the waves, akin to pushing a child on a swing synchronously with the swing’s oscillation period. This condition is met when the length of the proton bunch matches the wavelength of the plasma waves. The plasma’s ability to sustain strong fields increases when the plasma density is increased, which decreases the wavelength of the waves. Consequently, a stronger wakefield requires a shorter proton bunch.

The main innovation in Adli and colleagues’ work was, therefore, to make the length of the proton bunch as short as possible so that the bunch resonates with the plasma’s internal clock, maximizing the amplitude of the wakefield. The authors achieved this feat using a feature of the plasma known as collective force. Although the electric force produced by each particle in the plasma is small, the collective force generated from all of the particles can be large, and becomes larger as the plasma density is increased2. The authors used this force to chop a long proton bunch into a series of shorter bunches. Because proton bunches are stiff (difficult to deform) at the extremely high particle energies present in the AWAKE experiment, this chopping was possible and effective only by using the plasma’s collective force.

Adli et al. found that the wakefield produced by the short proton bunches could accelerate electrons to energies of up to 2 gigaelectronvolts in a plasma that is only about 10 metres in length. For comparison, at the European X-ray free-electron laser facility (European XFEL) in Germany, electrons are accelerated to energies of up to 17.5 gigaelectronvolts in an accelerator that is about 2 km long (see In addition to providing compact acceleration, the authors’ approach has a key advantage over standard accelerators and other wakefield accelerators. Because the proton bunches are stiff, they maintain their structure and speed. As a result, high-energy electrons can be produced in a single acceleration stage, as opposed to the complex multi-stage process that is needed in other accelerators.

Usually, the higher the energy of a particle beam, the longer it takes to stop (dump) the beam after use. The dumping of high-energy beams has become a serious issue because of the requirement of longer dumping lengths, which in turn increases the production of unwanted radioactive isotopes in the dense materials used for the dumping. The authors show that their accelerated electrons can form a beam of short electron bunches, which would encounter a large collective force if injected into an appropriately prepared plasma. Such a beam could therefore be stopped over a much shorter distance than conventional beams, inducing little radioactivity5. Overall, the authors’ work represents a major step towards the development of future high-energy particle accelerators that use collective force.

Nature 561, 318-319 (2018)

doi: 10.1038/d41586-018-06669-7


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