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Keeping an ion future particle accelerators

3 December 2018

A team of physicists, working at STFC Rutherford Appleton Laboratory, have built a ‘particle accelerator simulator’ that, instead of being many miles in diameter, can fit on a desktop. It is hoped the device will help scientists to design future generations of particle accelerators and lead to a better understanding of the next frontier of particle physics: the ‘high-intensity’ frontier.

Since the invention of the first particle accelerator in the early 1930s, there has been an almost obsessional quest for power – a relentless march toward creating ever larger, ever more powerful accelerators. The very first particle accelerator, could fit on a table top and had a circumference of about 15 inches, while the Large Hadron Collider (LHC) has a circumference of 17 miles!

Initially designed to probe the fundamental nature of matter itself – by smashing particles together to see what came out and, later, to see if what came out matched the predictions of some theoretical model – they have, in fact, changed the nature of our everyday lives. Today, there are some 35,000 particle accelerators around the world and, while some are dedicated to probing the nature of the Universe, most are used to treat cancer, study biology, create new materials, date archaeological finds… the list goes on.

The problem with the quest for power is that with great power, comes great circumference (to paraphrase Spider-Man’s uncle) and, with great circumference, comes great financial cost. While, there are plans for accelerators even larger than the LHC (if it is built, the Future Circular Collider will boast a circumference of some 60 miles) many scientists are now shifting their focus from high-energy accelerators to high-intensity accelerators.

In a ‘standard’ accelerator, bunches of particles are accelerated in beams that contain relatively few particles. When these bunches are collided, very few of these particles actually smash together because there is so much of gap between them. In a high-intensity accelerator more particles are packed into a smaller area as they are accelerated – travelling in extremely tight bunches – creating a particle beam that is incredibly intense. This means that far more particles are going to collide with each other – it’s like the difference between two people standing and throwing a single pebble at each other and expecting them to collide versus them throwing handfuls of pebbles – high-intensity accelerators literally give you ‘more bang for your buck’.

Unfortunately, high-intensity beams come with their own problems – the biggest of which is the fact that the particles really don’t want to be anywhere near each other. These particles carry an electric charge – in the case of protons, this is a positive charge – and because they all carry the same charge they want to repel each other and push each other away (just as two magnets do when their like poles are placed together). In particle accelerators, this propensity to push away from each other is overwhelmed by the power of the magnets they are passing through – they are literally forced together.

This is fine when you are dealing with only a few particles, but the more particles you try to squeeze into the beam the more difficult it becomes for the magnets to overcome the particle’s mutual repulsion. If you think of the particles as an orchestra, then the magnetic fields act like the conductor trying to keep them all playing together and in tune. But it’s not as simple as that, because each particle wants to be nowhere near any other particle, it’s more like orchestra where each musician is a rival that is trying to outplay the musician next to him. In other words, the greater the number of particles, the greater the number of complex interactions that take place between them and (in the case of the orchestra) the greater the chance of the musicians playing out of tune. As a result, the number of variables the magnets (or the conductor) have to compensate for are staggering.

Unlike in an orchestra, where if a few musicians play out of tune and get kicked out of the orchestra, the worst that might happen is that the brass section sounds a little flat, if a few charged-particles travelling at close to the speed of light get kicked out of the beam and escape the particle accelerator, they carry with them so much energy that the entire machine could be seriously damaged and be forced to shut down. This is not only hugely expensive, but will result in an awful lot of annoyed scientists who are no longer able to do their research.

Ideally, an engineer would know exactly what all those variables are and how they occur before they build their multi-million pound machines. After all, if you know how bunches of particles will react, you can engineer the magnets to compensate for those variables and keep the beam on the straight and (literally) narrow. The problem is so complex that it cannot be tackled by simple mathematical equations or theoretical calculations. A whole bunch of computer simulations – modelling each and every outcome – are required and then that information is used to design the accelerator. Unfortunately, there are so many variables – thousands of components and billions of particles – that it would take even the most powerful computers years of computational time to make the calculations. But building a full size accelerator as a test bed would be time-consuming and expensive.

But what if you could test the behaviour of high-intensity beams without having to build huge (and hugely expensive) machines by using a device that (just like those first particle accelerators) can fit on a table top? This is exactly what a team working at STFC Rutherford Appleton Laboratory are attempting to do with IBEX (Intense Beam EXperiment).

IBEX is designed to do all of the practical testing of intense particle beams without having to build a particle accelerator, working in tandem with computer simulations. The experiment itself is not much to look at – really just a shiny vacuum vessel that sits atop a table with a variety of pipes, wires and electronic gauges coming out of it – but, while it lacks the grandeur of the likes of the Large Hadron Collider, it does have the elegance of simplicity (relatively speaking) and extremely low cost.

At the heart of IBEX is a device known as an ion trap, or ‘linear Paul trap’. Instead of using a beam of charged particles, like protons, it uses charged Argon ions – Argon gas, which is normally neutral, but that has had electrons knocked off to give it a positive electrical charge – and rather than using magnets to confine and accelerate the beam at near-light speed around a ring, it uses electric fields to trap the ions and hold them in place.

These electric fields focus and manipulate the beam in the same way as a particle accelerator, all while keeping the beam stationary. This allows the team to see how the beam behaves in real-time as it moves (without moving) through the simulated magnetic fields of a simulated particle accelerator – an effect that has been described as like being able to ‘ride on top of a particle beam’.

By applying a variety of electric fields they will be able to see how the beams respond and whether or not any ions have escaped the confines of the beam, which, unlike high-energy protons tearing through a multi-million dollar accelerator, isn’t a problem with Argon ions.

IBEX is the result of collaborations. Two team members, Dr Suzie Sheehy and Lucy Martin are from the John Adams Institute for Accelerator Physics, Oxford University while Dr David Kelliher is from STFC. The project itself is the result of a collaboration between STFC, Oxford University and Hiroshima University in Japan.

The project, which started in 2015, is now up and running, taking data and is ready to start delving into the complex science of intense beams. Ultimately, the team hopes that IBEX will be able to simulate types of accelerator that have yet to be built – allowing their designers to ensure that, when it is turned on, all those particle conductors are leading their particle musicians in a perfect symphony.

Last updated: 04 December 2018


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