Inside the ISIS synchrotron with Dr John Thomason by isisneutronmuon
The science behind the synchrotron
The synchrotron is an extremely complicated machine. However, as Dr Thomason explained: "It all comes down to some very simple physics. If you have charged particles, like the ones we have here at ISIS- we start off with H- ions and then move onto protons-if you want those particles to go faster, you put them in an electric field. The bigger the electric field is, the faster they’ll go.”
“If you want them to bend, you put them in a magnetic field- the stronger the magnetic field is, the more they’ll bend. Those are really the two pieces of physics the whole accelerator is based on. All you’ve got to do is apply those in the right way.”
“We have some big yellow magnets here, these are called dipole magnets, and they are used to bend the beam round in a circle. These magnets are really quite big-each one weighs 33.5 tonnes, is 4 1/2 metres long, and it is about as tall as you or I” said Dr Thomason
Their magnetic fields are used to bend charged particles, and their field strength is around 10,000 times stronger than the magnetic field produced by the Earth
“There are also some other magnets here, which are green- those are called quadrupole magnets. The beam going round the synchrotron is made up of protons, and every proton has exactly the same charge, and they all try to repel each other. If we didn’t do anything about that, they would gradually drift apart and be lost. So we need some means of focussing them back onto the centre of the synchrotron, and these quadrupole focussing magnets are the way we do that.”
Injection and extraction
There are two particularly important straights on the synchrotron, one used for injection of the beam into the synchrotron, and one used for the extraction of the beam from the synchrotron.
A pipe with a diameter of around 10cm brings a beam of H- ions (a hydrogen + two electrons) in from the linear accelerator (Linac). How and why they are changed from H- ions to H+ ions (protons) is explained by Dr Thomason: “The beam that is circulating around the synchrotron is protons, and if we were to try to feed protons into that beam from the Linac, the incoming beam from the Linac would be repelled by the beam already going round”
“H- ions have the opposite charge to H+ ions, and so they get attracted into the beam, so it’s much more easy to build up intensity”
Building up intensity
Intensity is built up using ‘stripping foil’, made out of aluminium oxide. It is very thin-0.25 microns thick- about 1000 times thinner than ordinary kitchen foil.
The foil is very flimsy and very difficult to handle. It can also be lost if there is a vacuum problem- simply by being blown away.
However, “The stripping foil is very efficient at stripping electrons off H- -about 97% efficient in fact. That process of using what’s called multi-turn charge injection allows us to build up very very big intensities of protons inside the machine.”
A big kick
The kicker magnet is used to take the beam out of the synchrotron, and into the extracted proton beamlines.
“On the very last turn around the synchrotron, it provides a kick to move it out of the normal vertical plane that it goes in that keeps it going around the synchrotron, and kicks it by just a few degrees, just enough so it will go into the next magnet which is called a septum magnet, and that septum magnet will sweep the beam up, and out of the extracted proton beamline.”
With the addition of the second target station, there are now two extracted proton beamlines. Four out of every five pulses produced in the synchrotron go to Target Station One, but one out of five goes to Target Station Two. This has meant that some other magnets are needed in order for that to be achieved.
These are two more kicker magnets, which kick the beam now horizontally,” said Dr Thomason “this swoops the beam into another beamline that goes down into proton beamline 2.”
Powering the magnets
The magnets are powered by an alternating current, similar to mains current that supplies domestic homes. However, the current at ISIS is much more powerful.
“If every magnet was perfect, that would get the beam around. But, of course every magnet is not identical to every other one, so we have to have some more magnets, red ones, which run just after the green focussing ones. Those are called trim quadrupoles, and those can be programmed to make up for any small differences that there are between the magnets.”
Another important set of components are the RF acceleration systems. These are long copper tubes, around 2m in length. They have an electrical field inside them, which is phased just right, so that a kick is given to the particles as they pass through.
“Although that kick isn’t enormous, they see it each time they go through, and they go through something like 10,000 times during the acceleration cycle. That means they can get more and more energy as they go around, and they eventually end up with an energy corresponding to 84% of the speed of light.”
The whole acceleration cycle takes about 10 milliseconds (one hundredth of a second), and it repeats 50 times every second. It takes approximately 10,000 turns around the synchrotron to get up to the speed needed, and one turn takes an average of a microsecond (one millionth of a second).
“Once around the synchrotron is 163m. From the ion source where they are produced, to the target, it is something like 2000km,so it is a bit like (for each of those acceleration cycles), having gone from here to Aberdeen and back again.”
The nerve centre of ISIS
To run all of the equipment in the synchrotron hall requires a large number of power supplies.
“For each of the acceleration systems, there are four or five really big power supplies. There are probably 10,000 different knobs you could tweak on ISIS, which would actually make a difference.”
The ISIS main control room is the nerve centre of the facility, and is where every part of ISIS can be monitored and controlled.
There is a four-man crew in the control room at any one time, 24 hours a day, for 365 days a year. They are around to monitor the facility, and to fix problems that may arise. In addition to this, experts are on call 24 hours a day, who are able to come in and fix the instruments they have responsibility for.
“If your kit breaks at 3 o’clock in the morning, then you have to come and mend it at 3 o’clock in the morning, because this is really an operational factory. It is not an experimental facility in the same way as CERN is, so this is really relying on having as much availability of the machine as possible.”