Jorge Vieira

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Plasma Based Acceleration

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Do you recall the old cathodic ray tube (CRT) television? These devices used to have a tiny electron accelerator inside to make the images at the screen [crt]! X-ray images of our bones and tissues are also produced with the aid of electron accelerators at each hospital. These are just two examples where particle accelerators play an important role in technology and in our society. Particle accelerators are also crucial for scientific advance and they are, perhaps, the largest machines ever built by humankind. The Large-Hadron-Collider is a flagship accelerator that crosses 3 countries, and is used to probe the origins of time, space and matter.

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What if we could make a tinier, yet powerful particle accelerator, that could fit in every laboratory and hospital? This would be a tremendous achievement with many implications in science, technology and society. Plasma based accelerators may play an important role in this direction.

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Conventional particle accelerators are challenging to miniaturize because of material disruption. Increasing the accelerating electric fields inside these devices beyond a few tens of MV/m, which are typical in current particle accelerators, will thus ionize the walls of the accelerator, disrupting acceleration. This disruption creates a plasma. Thus, a plasma accelerator may be at the core of an advanced generation of miniaturized particle accelerators.

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Plasma sustains very large electric fields because they are inherently disrupted. Typical field values can be as high as 1 GV/m, a thousand times higher than conventional. This motivates an intense world-wide research program to use plasmas as an accelerator machine. This work started with the pioneering idea and work Toshiki Tajima and John Dawson at UCLA [tajima'79], that demonstrated how intense lasers could excite plasma waves that could accelerate electrons to high energies in very short distances.

 

The research of Toshiki Tajima and John Dawson on plasma accelerators was a mix of theory and computer simulations. Simulations used very powerful supercomputers at that time, in 1979. Today, these simulations could be performed, much faster, in any mobile phone. The pioneering work of Tajima and Dawson marked the age of computer simulations as true virtual experiments. Their exciting ideas lead to decades of research, where theory, experiments and advanced simulations jointly contributed to advance science and the field of plasma accelerators. Current laser-plasma acceleration experiments can accelerate 10 GeV-class electron bunches in a plasma with less than 20cm [goncalves'19]. Beam driven experiments accelerated 40 GeV electrons in a 40 cm plasma [blumenfeld'07]. In a conventional accelerator, this would require 100's-1000's meters of acceleration.

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There are many leading particle accelerator laboratories with plasma acceleration programs, such as SLAC, DESY and CERN. The AWAKE experiment at CERN is the first that uses proton bunches to accelerate electrons [caldwell'09]. It uses 500 GeV proton bunches from the Super-Proton-Synchrotron (the last acceleration stage before injection into the LHC main ring) to drive relatvistic plasma waves. Sucessful experiments demonstrated the acceleration principle, providing electron acceleration up to 2 GeV in a plasma with 10 meters [adli'18].

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Plasma accelerators have very high acceleration gradients, but the accelerating structures are very small, with a few tens of microns. As a result of this miniaturization, controlling the acceleration is harder than in conventional accelerators, and the bunch quality still needs to be improved. The movie below illustrates how a laser plasma accelerator works.