In the ISIS synchrotron, the protons are accelerated using radio frequency (RF) cavities. Six of these, the fundamental cavities, produce the overall 'strength' of the voltage, with the remaining four modifying the 'shape' of the voltage to maximise the total number of protons that can be satisfactorily contained within the synchrotron.
These systems were installed when ISIS was first built and at their core are thermionic valves known as tetrodes. The tetrodes used at ISIS were '4648 tetrodes': a design that is now so rare that there is just one supplier based in the USA, with ISIS being the only user. As the team needed to replace three or four tetrodes each year, the costs and logistics of re-supply were becoming unsustainable.
During the ISIS long shutdown in 2021, the tetrodes in the six fundamental RF systems were replaced with TH558 tetrodes, which are much more common not only in other proton synchrotrons but also in the broadcast industry. This upgrade not only resolved the concerns around supply, but also led to a power saving of 38%. Work has now begun on the second stage of this project: replacing the second harmonic RF systems.
The second Harmonic RF High Power Drives are being replaced with the same tetrodes as the fundamental drives, which makes maintaining them a lot simpler and is an improvement on needing two different parts for the different systems. The upgrade will also continue to further reduce the power consumption of the synchrotron by an estimated 900 kW, which is almost half of the power the system was using before the upgrades. In total, the energy savings of the project will reduce the cost of running the synchrotron by around one million pounds a year.
Most of the parts required for this upgrade are designed and built in house by the ISIS RF section. “This gives us the luxury of having first-hand knowledge of failure mechanisms and annoyances in current designs which we can design out of the new systems," explains Neil Farthing. “This makes life easier for ourselves when we have to work on them – particularly during a night-time callout!"
During the October/November 2024 shutdown, one of the Second Harmonic RF systems was stripped out and refitted with the new system. This was commissioned and is now fully operating ready for the coming user run.
There are two Second Harmonic systems left to be upgraded, and this work will take place during the January and Easter 2025 shutdowns. This will then finish this series of upgrades and full energy saving potential will be achieved.
As well as these high-power RF systems, which act as the muscles of the synchrotron, the team have also been working on the low-level RF system, effectively the brain that controls them. As with a lot of the synchrotron controls, they have been in place since ISIS was built in the 1980s, and this project has enabled the team to set them up in a way that makes them able to run more intelligently. This also contributes to the reduction in energy consumption, as it enables the main control room and machine physicists to adapt the synchrotron to operate differently under different conditions.
Andy Seville, Dave Allen, Rob Mathieson, Neil Farthing, Dave Gibbs, Ryan Allinson (Left to Right) in front of the new control racks.
