Riding a Plasma Wave

This article was recently printed in Bang!’s Women in Science issue. Bang! would like to apologise to Rob Shalloo for the discrepancies between his final draft and the one which appeared in magazine. Here is Rob’s version:

Particle accelerators have been around for over 100 years, moving from the humble beginnings of a simple cathode ray tube to the colossal 27km long, state of the art Large Hadron Collider, 100m under the Swiss Alps. But did you know that of the 30,000 in operation worldwide, less than 1% of accelerators are used for high energy particle physics research? Almost 90% are used for medical and industrial applications and are worth more than 500 billion dollars to the US economy alone, meaning that even small technological improvements have a large impact.

Why are particle accelerators so big? Linear accelerators are big due to constraints on the accelerating cavities. The material used has a damage threshold; it can only support electric fields up to a critical value. Go beyond it and your cavity starts sparking worse than an arc welder – a problem known as electrical breakdown. This can be avoided by making an accelerator which passes the particles through the accelerating cavities multiple times – a circular accelerator. But these too are large, mainly because turning a particle moving at 99.999996% the speed of light is difficult at best, and using a bigger turning radius makes this practical.

Sticking with linear accelerators, the current state-of-the-art liquid helium cooled superconducting accelerating cavities can add energy to a particle at a whopping 100MeV/m. To put that into perspective, that’s zero to one billion kilometres per hour in a metre (suck it Ferrari). But even with these state of the art machines it would take a linear accelerator approximately 50km to have enough energy to create a Higgs Boson! We are fast approaching a size which is simply unfeasible economically.

So it looks like we need a new way of doing things. If the main problem facing linear accelerators is the sparking of the metal cavities, why not remove the cavities all together? Bear with me here. If we take a material which is already electrically broken down, say a plasma (an energetic gas in which the electrons have been ripped off the atoms), then we can’t break it down any further by accelerating particles from it. Enter Laser Wakefield Acceleration.

Laser Wakefield Acceleration, a concept first proposed in the 1970s, works by firing an intense laser pulse into a plasma. The laser pulse essentially snow-ploughs the electrons out of its way, leaving the ions behind. This separation of positive and negative charges creates a huge electric field which can be used to accelerate particles. Think of the laser like a speedboat and the plasma like a lake. As the speedboat powers its way through the lake it creates a large wake behind it upon which people (or electrons in our case) can surf upon.

Accelerators of this type can add energy to particles at an impressive 100,000 MeV/m (1000 times greater than in conventional particle accelerators). This means reducing your 1km long linear accelerator into something which could comfortably fit onto a lab bench. Manufacturing a more compact, less expensive particle accelerator would benefit industry, medicine and research alike. In particular, being less expensive to build, an accelerator like this could be placed in more hospitals and treatment centres around the world to perform medical imaging and radiation based therapies.

Well then, all that’s left now is for me to say “Give me my Laser Wakefield Accelerator!” Unfortunately, it’s not that simple (pro tip: it is never that simple). There are several big challenges ahead before these new devices could become a viable alternative. First and foremost, these systems require a large laser, with power on the scale of terawatts, to run correctly. Again perspective is helpful; this is comparable to the entire worldwide power consumption. Luckily they run for less than a trillionth of a second. Secondly the repetition rate (number of particle bunches you could accelerate per second) is about one to a tenth – compare this to the thousands of bunches per second achieved by conventional accelerators. Lastly there are problems associated with the beam quality and stability. It is nowhere near the level needed for precision particle physics experiments, though for many medical and industrial applications it isn’t far off. It is clear that there are challenges ahead, but incredible advances have been made in the research and development of these accelerators since they were first proposed in the 1970s. Recent experiments have shown acceleration of the order of 4GeV in a few centimetres. A conventional accelerator would require a tunnel several hundred meters long to achieve the same effect! While they cannot currently replace conventional accelerators, this is an extremely promising lead in the pursuit of the next generation of particle accelerators.