Point of View: Take Advance with Wakefield Acceleration

February 25, 2019&ball; Physics 12, 19

A method of accelerating particles, called acceleration on the field of work, has increased its output energy, bringing it closer to its goal of reducing the size of accelerator installations.

The field of plasma wake acceleration is gaining momentum. This method, first proposed in 1979 [1], creates a collective motion of plasma particles, generating a field of acceleration in its wake. The amplitude of this acceleration field is not limited, as it is the case in classical acceleration techniques that use radio frequency pulses. The implication is that wake acceleration can potentially run on much smaller lengths, which would reduce the size (and cost) of accelerator installations. There are different methods for generating wakefields, and now researchers are reporting significant progress for two of these techniques. A method using laser-driven wake fields generated 8 GeV electrons, a new energy record that doubles the previous record. [2]. A different approach, with proton beams as drivers, is not far behind, with recent experiments demonstrating its ability to accelerate electrons up to 2 GeV [3–5]. These two achievements are essential, but the goal of having standby field accelerators will have to wait for researchers to gain more control over the output beams.

What's a plasma wakefield? A useful analogy is a capacitor, where two oppositely charged parallel plates generate an electric field capable of accelerating particles from one plate to another. In a plasma, a similar field may occur when a "driver", such as a laser pulse, separates negatively charged electrons from positively charged ions. This charge separation can remain a stable configuration, called wake field, if the effective size of the conductor is less than the plasma wavelength, which characterizes the length of a coherent response to a displacement of charge. Inside the wake field, the electric field – expressed as a voltage gradient – can reach 1 TV / m. In comparison, conventional accelerators can only reach slopes of 100 MV / m before they can be damaged by an electric shock.

If the plasma wake fields can have gradients of 1 TV / m, one can imagine that a "CERN table version" is possible. However, there is a problem. The wake fields are usually driven by a laser pulse, whose velocity is drastically reduced inside a plasma. At one point, the electrons will advance the accelerated part of the wakefield, like surfers who surpass the waves on which they surf. There are essentially two ways of solving this so-called phase shift problem: either we successively stage several laser-controlled plasma accelerators, or we choose another motor, a group of high energy protons that moves in the plasma at a speed close to that of the screen. light.

If one chooses the laser strategy, each step should give an energy increase of about 10 GeV. [6]. In 2004, researchers obtained the first laser-accelerated electrons with a "non-thermal" energy spectrum peaking at around 100 MeV. [7]. Two years later, the peak of energy reaches 1 GeV [8]. Anthony Gonsalves of the Lawrence Berkeley National Laboratory, California, and his colleagues have successfully accelerated electrons at 8 GeV with lasers [2]This is an important step in achieving the required energy of 10 GeV per floor.

Why did it take more than 10 years to go from 1 GeV to 8 GeV? One of the reasons is the complexity of these experiences. The first-generation laser-plasma experiments worked with a single laser, which created the plasma (by gas ionization) and generated the wake field. The next generation relegated the ionization step to a discharge capillary, which is a gas container usually made of glass, of inner diameter less than that of a human hair. When a high voltage is applied to the capillary, the discharge ionizes the gas, creating a plasma with the lowest density along the axis of the tube, as the walls cool faster than the center (Fig 1, above). This transverse density gradient helps to guide the laser pulse over sufficiently long accelerations, of the order of 10 cm. Gonsalves et al. have improved this design by using a second laser system in addition to the discharge capillary. This second laser acts as a "laser heater", further increasing the density gradient of the transverse plasma, a prerequisite for reaching the 8 GeV level.

As already mentioned, the laser method will require the coordination of several steps, a feat that remains to be accomplished. The alternative approach is to use a single group of high energy protons as the engine. The difficulty here is that the mass of protons is initially much longer than the plasma wavelength, which normally means that no stable wakefield can occur. Fortunately, nature provides a solution: the proton – plasma interactions cause the proton group to spontaneously form a density modulation consisting of "microboons" whose spacing is approximately that of the plasma wavelength. Stable wake fields appear between microbeads (Fig. 1, bottom). To control this so-called automatic modulation, researchers can use a laser-induced ionization front that serves as a "seed" for breaking the proton packet.

In early 2018, Wakefield's advanced experiment (AWAKE) was the first successful demonstration of proton-driven acceleration, using 400 GeV proton packets from the CERN Super Proton Synchrotron. [3]. The researchers showed that they could get a 2 GeV energy boost for the electrons injected as a result of these proton packets. In two follow-up studies published earlier this month, the AWAKE collaboration confirmed that bunches of protons about 10 cm long divide into microbabs 2 mm long, as expected for the auto -seeded modulation. [4, 5]. The acceleration occurs on 10 m, which is about 100 times longer than the acceleration length in the laser – driven experiment. The longer distance is due to the fact that the proton packet experiment requires a lower plasma density and therefore a lower electric field gradient. However, the proton-based scheme may be more appropriate than the laser-based case for accelerating positrons. Positrons could simply be injected into the wake of the proton beam in the same way as the electrons.

The AWAKE and Gonsalves collaboration et al. both have achieved historic results, but much remains to be done. CERN's approach aims at studying the possibilities of a new high-energy electron-positron collider, while the laser method is aimed not only at collider applications, but also at the control of hard X-ray free electron lasers, used example, to determine protein structures. It is really difficult to determine which approach is closest to its goal, simply because both methods are still busy demonstrating acceleration. To move to the next stage of creating a user installation, field acceleration requires high quality and beam stability, which no technique has achieved so far.

We often ask: when was the starting point of a new field? Did lunar exploration, for example, begin with Jules Verne's 1865 novel on a journey to the moon, with the first human being in space in 1961 or with the moon landing in 1969? For acceleration in the field of work, several starting points are possible. In my opinion, the moment "human in space" for the acceleration of the plasma wake took place in 2004, when the researchers managed to reach energies of 100 MeV with spectra of. quasi-monochromatic energy. We still do not know when the "landing on the moon" will take place in the form of a user facility with wakefield accelerator. The key to achieving this goal will be to develop an unprecedented level of control over all relevant parameters. This means the introduction of brand new "buttons" for things like final energy and accelerated particle energy propagation.

This research is published in Letters of physical examination and Nature.

References

1. T. Tajima and J. M. Dawson, "Laser Electron Accelerator", Phys. Rev. Lett. 43267 (1979).
2. A. J. Golsalves et al., "Petawatt laser guidance and electron beam acceleration up to 8 GeV in a laser heated capillary discharge waveguide", Phys. Rev. Let 122084801 (2019).
3. E. Adli et al. (AWAKE Collaboration), "Acceleration of electrons in the wake of the plasma of a group of protons", Nature 561363 (2018).
4. Mr. Turner et al. (AWAKE Collaboration), "Experimental Observation of the Growth of the Plasma Wake by Seeded Self-Modulation of a Proton Group", Phys. Rev. Lett. 122054801 (2019).
5. E. Adli et al. (AWAKE Collaboration), "Experimental Observation of the Modulation of Proton Packets in a Plasma at Different Plasma Densities", Phys. Rev. Lett. 122054802 (2019).
6. C. B. Schroeder, E. Esarey, C. G. R. Geddes, C. Benedetti, W. P. Leemans et al., "Physical considerations for laser-plasma linear colliders", Phys. Rev. ST Accel. beams 13101301 (2010).
7. C. G. R. Geddes, Cs. Toth, J. van Tilborg, E. Esarey, CB Schroeder, D. Bruhwiler, C. Nieter, J. Cary, and WP Leemans, "High-quality electron beams from a field accelerator. Laser wake using plasma channel guidance » Nature 431538 (2004); S. Mangles et al."Mononenergetic relativistic electron beams from intense laser-plasma interactions" 431535 (2004); J. Faure, Y. Glinec, A. Pukhov, S. Kiselev, S. Gordienko, E. Lefebvre, J.-P. Rousseau, F. Burgy and V. Malka, "A laser-plasma accelerator producing beams of 39 monoenergetic electrons » 431541 (2004).
8. W. P. Leemans, B. Nagler, A. J. Gonsalves, Cs. Tóth, K. Nakamura, C.G. R. Geddes, E. Esarey, C.B. Schroeder and S.M.Hooker, "GeV Electron Beams from a Centimetric Accelerator" Nat. Phys. 2696 (2006).

About the author

Florian Grüner is a professor at the University of Hamburg, Germany, where he heads the university's accelerator physics group. He is one of the pioneers of "free electron table lasers", which aim to provide brilliant X-ray pulses with an ultra-compact configuration. This reduction in size is made possible by the use of plasma wake acceleration. As for the applications of such accelerators, he is exploring new medical imaging methods based on compact and brilliant X-ray sources. Prior to his appointment in Hamburg, he was a professor at the University of Munich, Germany, where he began an interdisciplinary work between physics and medical research.

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