Smashing particles with Roger Jones

In the first in a new section where we get to quiz the people behind the discoveries - we meet the man who is tasked with stopping the next generation of particle accelerators literally tearing themselves apart

This month, the Large Hadron Collider at CERN is due to start smashing protons into each other at colossal speeds - 99.99% the speed of light to be precise. At full power, trillions of protons will race around the LHC accelerator ring 11,245 times a second – creating some 600 million collisions every second. When the two beams of protons collide, they will generate temperatures more than 100,000 times hotter than the heart of the Sun.

Mind boggling stats – but they don’t frighten accelerator physicist Professor Roger Jones. For he is made of sterner stuff – but there is one thing that does scare him however - ‘wake fields’.

He says that wake fields - the huge electromagnetic forces that occur during the process of acceleration - can cause particles to fly apart and have the potential to destroy the next generation of particle accelerators.

Controlling these forces is the subject of a new paper by the University of Manchester physicist - the challenge, he says, is finding a way to suppress wake fields sufficiently while still maintaining a high acceleration field to perform particle collisions.

So will wake fields be a problem for the LHC then?

Wake fields have been carefully controlled and suppressed in the Large Hadron Collider at CERN. However, physicists are now looking at what comes after the LHC. An electron-positron collider is the natural successor to the LHC and it turns out the wake fields are much more severe in these linear collider machines.

Indeed, acceleration of particles to ultra-relativistic energies over several tens of kilometres in the proposed Compact Linear Collider (CLIC), for example, poses several significant accelerator physics challenges to designers of these immense machines. Beams consisting of several hundred bunches of tightly focused charged particles can readily excite intense wake fields, forcing the bunches to fly apart.

Sounds very destructive – how on earth can you stop it? One approach entails heavy damping, in which the majority of the wake field is sucked out of the collider by structures, known as waveguides, coupled to each cell in the accelerator.

A second approach entails light damping - in which a small portion is removed - in combination with detuning the cell frequencies of the accelerator. Detuning the wake field can be understood by thinking about acoustics. If you have a collection of huge bells all ringing at slightly different frequencies or tones, the amplitude or ‘wave height’ of the overall sound heard will be markedly smaller than that heard if they all ring at the same tone. This method is very efficient and structures built in this manner are known as a Damped Detuned Structures (DDS).

Detuning is perhaps more elegant than heavy damping as it also enables the position of the beam to be determined by the quantity of wake fields radiated by the beam – in this way a DDS accelerator removes the wake fields and has its own built-in diagnostic.

The DDS concept was developed by Prof Jones and colleagues during one and a half decades spent working at the SLAC National Laboratory at Stanford University in the United States. Whilst at the University of Manchester, he has recently developed this method to apply to the next generation of accelerator – the CLIC (Compact Linear Collider) being developed at CERN.

When it comes to the crunch – which method will work the best?

At this stage, both means of wake field suppression should be pursued in order to thoroughly assess their applicability. Experimental testing, using realistic pulse lengths and at the high gradients planned for the linear collider, will be the final test on the suitability of these techniques.

• Prof Jones has undertaken research into wake field suppression over the last 20 years – the last four of which have been spent at The University of Manchester’s School of Physics and Astronomy and at The Cockroft Institute of Accelerator Science and Technology, based at the Daresbury Laboratory in Cheshire.