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Want more neutrons? Build a better proton gun

The UK’s ISIS Neutron and Muon Source (ISIS) at Rutherford Appleton Laboratory (RAL) is one of the world’s leading pulsed neutron and muon sources, but in recent years it has been overtaken by a small number of more powerful machines. To meet future research needs for more powerful neutron sources, accelerator physicists at STFC’s Particle Physics Department (PPD) are developing the technology that might allow the ISIS facility to overtake the current record-holder in the US, the Spallation Neutron Source (SNS). A key part of this advancement will be the ISIS Front End Test Stand (FETS).

At its most basic, ISIS works by accelerating protons (the positively-charged particle that forms the nuclei of hydrogen atoms) and slamming them into a target made of tungsten. The impact dislodges neutrons (the non-electrically-charged particle that, along with the proton, makes up the atomic nuclei of elements heavier than hydrogen) from the target, which are then used to perform matter-probing experiments.

Essential to any accelerator is the ‘front end’. This, as the name suggests, is the first part of the acceleration process. Also known as a ‘proton driver’, the front end is a little like a proton gun that’s responsible for the initial production and acceleration of the proton beam. The intensity and energy of the proton pulses created at the front end dictates the ultimate power of the neutron source – the higher the proton flux (the number of protons per hour), the higher the neutron output, which increases the number and sensitivity of the experiments that can be carried out.

ISIS’s current proton driver operates at a power level of 180kW (0.18MW), which was high enough to ensure that ISIS was the world’s most powerful pulse neutron source for more than 20 years, but FETS is aiming to increase this to 4MW. This is a significant increase over, not just ISIS’s current capability, but also the reigning neutron source power champ, the Spallation Neutron Source (SNS), which operates at 1.4MW.

One of the limiting factors that inhibits the development of high-power proton drivers is something called ‘beam loss. Beam loss happens because particle accelerators use modulated electric fields inside metal cavities to accelerate charged particles. These fields alternate between positive and negatively-charged fields and if the positively-charged protons don’t arrive at the right time, instead of being trapped and accelerated around the accelerator ring, they can be lost. As well as having the inevitable effect of decreasing the power in the proton beam, losing protons can also have an adverse effect on the machine itself.

High-energy protons that are lost will pass through the accelerator’s structure and have nuclear reactions with the materials that make it up. This can create radioactive isotopes that then decay and produce radiation. High levels of beam loss can create enough radiation to make it unsafe for human engineers to work on the machine directly, which means that tasks have to be performed robotically – increasing operation costs.

To avoid this, instead of travelling around the accelerator in a long stream, the proton beams are ‘chopped’ into bunches to ensure they arrive at the right part of the electric field at the right time. However, as the intensity and power of the beam is increased, this task of chopping and timing becomes more difficult because there are more protons, travelling with more energy – meaning that, with each potential loss, there are a lot more protons that can escape.

To overcome these problems, the FETS team has developed two innovative techniques. The first is a 4m long Radio Frequency Quadropole (RFQ), which accelerates, focuses and efficiency bunches the beam. The train of bunches created is then passed to the second, a chopper technique that splits the process into two parts. A ‘fast chopper’ creates a short, clean gap in the bunches that then gives the ‘slow chopper’ time  to kick into action and produce a longer gap. This will allow the beam to be cleanly injected into a future circular accelerator.

The FETS RFQ is made up of four sections that, rather than being welded together (as is standard), are simply bolted together. Because of how complex the RFQ’s structure is, it needs to be ‘tuned’ to ensure it is resonating at the correct frequency and that the accelerating field is the same throughout. This is done using 62 tuners that must be manually calibrated using a method called the ‘beadpull technique’, which literally involves pulling a bead through the structure, seeing how it affects the electric fields and then applying a special mathematical algorithm to work out the tuner positions. The tuning, which is based on feedback from the algorithm and is carried out by hand, is essentially like tuning a violin with a digital tuner – in fact, the process even looks similar.

The team, which includes researchers from the University of Huddersfield, STFC’s Particle Physics Department (PPD), and ISIS, has spent several weeks fixing code and adjusting hardware with the  result the tuning process is now underway. When complete, the next steps will be to deliver full power to the RFQ and start accelerating a beam of protons.

As well as providing a future upgrade for ISIS, FETS could also provide the starting source for the next generation of ‘Neutrino Factories’ – machines that creates beams of neutrinos by smashing protons into a solid target, creating muons that then decay into neutrinos. Neutrinos are enigmatic particles that can pass through matter almost unimpeded, possess virtually no mass and have the bizarre ability to change type literally on the fly. It is thought that studying neutrinos could help answer fundamental questions about the universe.

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Last updated: 30 August 2019


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