The experimental successes delivered by ISIS are a result of the intense pulses of neutrons that the facility provides. Currently, ISIS offers the best neutron source of its kind in the world. An additional, complementary neutron source – the Second Target Station (TS-2) – will soon be ready. However, to achieve the required neutron intensities and energies at both the current and new target stations, the ISIS engineers have been making ingenious modifications to the facility’s infrastructure.
The neutrons are produced by firing short bursts of accelerated protons at a target. When TS-2 starts up, it will receive one proton pulse in five generated in the ISIS acceleration system, so to maintain the same average intensity at Target Station 1, the
pulses will contain many more protons. This is tricky since protons, being electrically charged particles, tend to
repel each other. The answer is to create bigger proton ‘buckets’ during the acceleration – achieved by a cunning approach called dual harmonic acceleration.
The proton pulses are prepared in a circular accelerator called a synchrotron. A proton beam is injected into the synchrotron and then travels around the ring, focused and guided by a series of magnets. The electrically charged particles are accelerated by
radiofrequency (RF) cavities, which generate an electric field via a voltage across a gap. The field frequency is precisely double that at which the protons rotate around the ring so that they get an accelerating kick every time they go past a cavity. It also has to
increase throughout the acceleration cycle to match the changing energies and rotational speeds of the protons. At the same time, the protons are also squeezed into two bunches before being extracted and sent to the target station.
Doubling for success
Until recently, acceleration was provided by six, two-gap RF cavities. But to increase the number of protons in a
pulse, four new RF components have been added. These are designed with shorter accelerating cavities producing
RF at four times protons’ rotational frequency – at so-called harmonic-2 frequency. The effect is to stretch the energy range and RF phase of each bunch around the ring so that they can accommodate more protons. The set-up required careful tuning of
frequencies, timing and magnetic fields to achieve stable operation during the accelerating cycle, which happens 50
times a second. It was demonstrated successfully during 2006 and will ensure ISIS experimental successes continue into the future.
A P Letchford
Research date: July 2007
