Plasma Wakefield Particle Accelerator

If you hear the words 'Particle Accelerator', you might picture the largest machine ever made by man, the LHC (Large Hadron Collider). With dozens of kilometers of its tunnels, large electromagnetic plates and coils, vacuum lines, and delicate hardware, it is 27 km long and 50 to 175 meters below the ground. For the majority of the history of particle physics, everytime we wanted to discover a new elusive particle like the Higgs boson, the motto has been: we need a bigger particle accelerator because a larger particle accelerator until recently meant that it can accelerate the particles to even higher energy. And up until recently, it has been a very successful, albeit very expensive, solution. LHC discovered Higgs Boson, the last undiscovered particle in the standard model, which proved the existence of the Higgs field for which a Nobel prize was awarded in 2013.

Figure 1: CERN LHC complex. Take an immersive tour of CERN's accelerator complex here. Image credits: Arpad Horvath.

Yet with all that investment, we are barely discovering new particles. The LHC can accelerate a particle at 10 TeV (10 Terra electron volt or 10*10¹² electron volt, energy of a flying mosquito concentrated into a few particles, eg. protons, electrons, etc.) which is a mind-blowing amount of energy for a single particle but several orders of magnitude too small to completely explore the depths of the universe and discover new particles. The LHC cost approximately $7.5 billion, and making an even larger particle accelerator will cost much higher. So now the motto is changing to instead make smaller, better accelerators, or to use old ones more efficiently. That's where the Wakefield accelerators enter the scene, as they can accelerate particles to the same energy in a couple of meters for which traditional accelerators take kilometers.

Then why aren't these particle accelerators in use?

Short answer: There are some bottlenecks we have to address before these accelerators become the norm.

Currently, you need a really powerful pulsed laser or a decent sized regular accelerator to drive them. Fundamentally, a Wakefield accelerator is a long tube filled with metallic gas, usually Lithium vapor. There are two ways to turn it into a particle accelerator: either a powerful pulsed laser or a decent sized particle accelerator.

Laser-Powered Wakefield Accelerator

To drive a Wakefield accelerator through this method, you need a really short burst of a really powerful laser pulse. This pulse ionizes the gas into plasma. But more importantly, the pulse leaves a wake behind as electrons and ions get pushed around by the sudden burst of energy.

Think of this as a floating object moving forward, like the boat pictured below. The path left behind by the boat, called the wake of the boat, is very choppy and rough with wavy peaks and troughs.

Image 1: The path left behind by the boat, called the wake of the boat, is very choppy and rough with wavy peaks and troughs. Image credits: Edmont.

In the case of Wakefield accelerator, the waviness instead is the charges, some positive and others negative. These oscillations are in a way similar to the regular particle accelerator. The trick is to time your particle in the tube just behind the light pulse. If we want to accelerate a cloud of electrons, the area just behind the light pulse ends up being very positive, constantly pulling the electrons towards the light pulse. This accelerates electrons to a substantial fraction of the speed of light very quickly and in the process, the light pulse loses energy to the wake, which then transfers the energy to the electrons.

Bottlenecks:

Timing the electrons, however, is critical and if the electrons end up too close or too far from the pulse this process doesn't work properly and the electrons don't accelerate. The process when it works is called self-focusing and the trailing cloud of electrons gets packed together into a sense packet which is extra useful.

Wakefield Accelerator Driven by Another Particle Accelerator

Using a particle accelerator to drive the Wakefield accelerator is similar to using the laser pulse, except here you use a packet of high energy particles (like Protons), already accelerated, in the wake of which another packet of particles (like electrons) can be accelerated. Wakefield transfers the energy from the first packet to the trailing packet, hence accelerating it.

Figure 2: Wakefield acceleration. In the laser-driven case, a high energy laser pulse is fired into a metallic gas (generally Lithium). The light ionizes the gas into plasma, induces charge separation in the plasma, and the electric field from this charge configuration can accelerate trapped electrons. In the particle driven (eg. proton) scenario, a packet of high energy particles is sent into the gas. Image Credits: Rasmus Ischebeck.

Conclusion

Wakefield accelerators are quite powerful tools for particle physicists and the scientists in FACET 1 (Facility for Advanced accelerator and Experimental Projects, Stanford) project have been able to accelerate particles upto 9 GeV higher in just 1.2m of tunnel. For comparison, traditional linear accelerators take over a kilometer for the same. FACET 2 (2019-26), the successor of FACET 1, aims at higher efficiency than its predecessor and it aims to address many key issues in the US roadmap. Many other countries including Britain have also proposed a roadmap of their own. The Wakefield accelerator promises a bright future for particle physics and in the coming years we will hear more about them.

Citation

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2. Grüner, Florian. “Shooting Ahead with Wakefield Acceleration.” Physics, 25 Feb. 2019, https://physics.aps.org/articles/v12/19.

3. “Plasma Wakefield Acceleration.” SLAC National Accelerator Laboratory, https://portal.slac.stanford.edu/sites/ard_public/facet/Pages/rpwa.aspx. Accessed 5 Oct. 2020.

4. Mangles, Stuart. https://indico.cern.ch/event/758617/contributions/3146206/attachments/1751792/2838723/Wakefield_intro.pdf. Accessed 5 Oct. 2020.

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