Giant “atom smashers” like CERN and SLAC are famous for their ability to accelerate matter to very nearly the speed of light. By slamming together particles like protons and electrons at extremely high speeds, physicists can gain a better understanding of their fundamental nature—and even uncover new particles, like the now-famous Higgs boson. Their wide range of applications and their place in the spotlight mean that an ever-increasing amount of effort is being devoted to making proton and electron accelerators cheaper and more accessible to scientists.
The same, however, can’t be said about antimatter accelerators. As Dr. Aakash Sahai of the Imperial College London explains, although there are just over a dozen DOE-funded national labs based on accelerator tech, “most of the demonstrations and interest in laser-plasma accelerator technology revolves around electrons and protons.” Today there exist somewhere between 200 and 250 electron accelerators worldwide, but the number of accelerators designed to work with the electron’s antimatter counterpart, the positron, can be counted on one hand.
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But Sahai doesn’t think we should discount the value of positron accelerators. To begin with, they could prove to be a pathway to learning more about quarks, the most fundamental of particles that make up our universe. By experimenting with extremely high-energy positrons, it’s possible that we could begin producing quarks artificially, especially the lower-energy varieties. But “these things are not just useful for fundamental physics,” Sahai says. When positron beams are shined onto a metal structure, like an airplane wing, the positrons interact with the electrons present in the metal, ultimately annihilating themselves and emitting gamma rays. “These gamma rays can reveal the structure of the material,” Sahai says.
The only problem is that reliable sources of high-energy positron beams are extremely difficult to come by. Current technology requires miles-long tracks lined with extremely powerful electromagnets, and radiofrequency cavities driven by microwave resonators called klystrons. As Sahai explains, “You can imagine the level of infrastructure required for that is not affordable for regular labs.” While some labs use radioactive decay as a positron source, this doesn’t provide the targeted, high energy beam of positrons needed for many experiments and applications. This concerns Sahai from a scientific perspective: “The fact that these immense tools of discovery have been constrained in these facilities…the person who has a crazy idea can’t test it in the lab.”
However, he thinks he might have a way out of this bind, in the form of his new design for a laser-plasma positron accelerator. Although laser-plasma accelerators already exist for electrons—and they’ve reduced electron accelerators to mere centimeters, rather than the dozens of kilometers typically required—his is the very first design to use plasma-based acceleration to create a positron beam.
Sahai’s accelerator has two stages: creating a high-energy electron flux, then “converting” those electrons to positron-electron showers and steering the spray of positrons into a beam. The first stage uses essentially the same technology as a laser electron accelerator, but positron acceleration using just lasers is something that has never been envisaged.
You may recall that a plasma is more or less a “soup” of positively charged nuclei and their disassociated electrons. Because electrons are so much lighter than the protons and neutrons that form the ions, when a laser beam is applied to the plasma, the radiation pressure from the photons sends the electrons flying—but the ions stay put. This creates a charge imbalance between the positive ions (scattered throughout) and the negative electrons (concentrated on one end), an imbalance that can translate to several GeV per centimeter energy gain (i.e. an electron traveling one centimeter picks up several GeV of energy, accelerating to more than 99% the speed of light).
Sahai says that although the principle is simple, scientists are still working hard to hone this technology. But, he says, that’s because they’re focusing on creating very precise high-energy electron beams. “They want to make an ideal electron beam… but I don’t need a nice electron [beam], I need a flux of electrons.” And even a rough plasma accelerator will give him exactly that.
Next, to get a positron beam, Sahai suggests sending this “electron flux” into an element that falls relatively high on the periodic table, like a metal. When high-energy electron flies into such a material, it is deflected by the nuclei’s electric fields—much like a pinball falling through a pinball machine is deflected by the various obstacles it encounters. As the electrons change their course they’re actually decelerating, which means they have less energy. Since they can’t suddenly lose energy without that energy going somewhere (hello, conservation of energy), it’s carried away by a “virtual” gamma ray. (This is formally known as bremsstrahlung, German for “braking radiation”.)
These gamma rays are actually high enough in energy that they’re quite unstable, and can rapidly convert into a positron-electron pair…and voila! We’ve “converted” the electrons at stage one into low-energy positrons. Now all that’s left is to use a plasma wave to contract and accelerate these errant positrons into a well-defined beam, and we have a source of high-energy antimatter, ready to be studied.
The beauty of this invention stems from its compactness. In contrast to the multiple-miles-long positron accelerators already in existence, this accelerator could conceivably sit on a desktop. In fact, the biggest constraint both in terms of cost and size comes not from the accelerator itself but the laser system that drives it. Sahai says that although only a few labs have the approximately 25 square meter laser system already in place, laser technology is heading down the right path to become accessible enough.
“The lasers are going in the right direction… costs will drop, size will drop, lasers will be more affordable, efficient and produced in higher volumes.” The way things are going, Sahai thinks it could be possible to produce quark reactions on a desktop in 5-10 years time, at a price several orders of magnitude below what positron accelerators presently cost.
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| The Large Electron Positron collider, now defunct since being repurposed for the Large Hadron Collider, is 27 kilometers long. Sahai’s positron accelerator would be mere centimeters. Image Credit: LEP collaboration |
We should note that the work Sahai recently published in Physical Review Accelerators and Beams is not an analysis of a working prototype, but rather a proof-of-concept. After creating his design, Sahai ran a series computational analyses, sometimes running codes that could take days or weeks to finish. He is satisfied enough with the results that he has recently submitted an experimental proposal to prototype this technology.
That’s not to say it’s a perfect solution. While it would certainly be much more affordable and accessible than current accelerators, Sahai acknowledges that his invention is only capable of creating bursts of millions or tens of millions of positrons each, while SLAC produces tens of billions at a time. However, his design does allow for much shorter, tighter pulses, which has important applications for studying chemical synthesis and other processes. Instead, the greatest limiting factor is likely to be the divergence of the beam, which currently is projected at somewhere around 0.29 degrees, while the accepted standard is about 0.057 and calls for novel positron cooling techniques. Additionally, technical challenges will certainly present themselves in the prototyping stage when it comes to aligning each stage properly with the others.
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| In this phase-space graph, you can see the very tight pulse of positrons (the black blob) being hurled out of the looser more divergent “shower”. Image Credit: Aakash Sahai. |
Even so, Sahai is optimistic. These problems are presumably fixable once he’s able to work on a prototype, and he’s confident that this technology will help pave the way to a more accessible, more equitable version of high energy physics that’s amenable to spontaneous experiments.
—Eleanor Hook