How does one accelerate a particle to almost 300,000 kilometres per second?
To study subatomic structures of material, researchers fire extremely fast particles at them. At the GSI Helmholtzzentrum für Schwerionenforschung – the Helmholtz centre for heavy ion research – in the German city of Darmstadt, charged atoms of particularly great mass are used: heavy ions. These particles have to be accelerated to almost the speed of light. Professor Kester, you are a project leader at the GSI Helmholtzzentrum für Schwerionenforschung. How does one accelerate a particle to almost 300,000 kilometres per second?
216 metre long circular track
“The heavy ions are first sent along a straight acceleration track in what is known as a linear accelerator. This is 120 metres long at our facility and uses electromagnetic waves to accelerate the particles to between 15 and 20 per cent of the speed of light. They then enter the circular accelerator at this speed. In a linear set-up, several acceleration tracks would be needed over the space of many kilometres. The advantage of the 216 metre long circular track is that we only have to accelerate the particles each time they pass by one particular point. A high frequency resonator is used to generate an electric field there every time the particles fly through, thereby transferring more energy to them.
Flying without turbulence
A vacuum inside the accelerator means nothing can disrupt their flight, so the particles become faster and faster. The biggest challenge is to keep the particles on the circular track. To this end we use strong magnetic fields which force the heavy ions onto the track and bundle them. For example, up to 33 billion uranium ions bundled together speed through the circular track – they look like a long thin thread. If we did not use magnetic fields to hold them together, they would crash against the wall and be lost. The particle threads have to pass the resonator up to one million times, by which point they reach 98 per cent of the speed of light.
A single second
The entire process lasts just one second. Because the ions accelerate every time they complete a circuit, we repeatedly have to readjust the magnetic fields which keep them on track. This is done in exact synchrony with the acceleration of the particles, which is why a circular accelerator is also known as a synchrotron.”
Kristine August wrote this article for the July-August 2014 edition of the Helmholtz Association’s research magazine “Helmholtz Perspektiven”.
The Magazine of the Helmholtz Association: www.helmholtz.de