About twice a decade
the linac at ISIS is opened up for cleaning and maintenance. This is a
rare opportunity to see the inside workings of a vintage linear
Hidden deep in the Oxfordshire countryside, ISIS is a world-leading research centre. It draws in about 2000 visiting scientists every year from all over the world.
At the heart of ISIS is a high powered particle accelerator. About the size of a football pitch it is kept running by a dedicated team of 300 engineers, technicians and physicists.
The accelerator is constantly being developed and upgraded, but some parts rely on technology from the 1950's. Tank 3 of the linac was built in 1956 by the Metropolitan Vickers Company in Manchester.
A drift tube linac is a series of tube shaped electrodes. The beam passes along the centre of the tubes. As it passes between neighbouring tubes it sees a voltage that gives it a kick. Packed inside each tube are electromagnets that squeeze the beam to keep it together.
Tank 3 is normally buried under hundreds of tonnes of concrete shielding blocks, these are required to shield workers from the high doses of x-rays and radiation, produced during the normal operation of the accelerator.
A team of heavy lifting experts use specially designed lifting equipment to gracefully remove the concrete blocks.
The linac normally operates under vacuum, so after switching off the pumps the linac tank must be filled with pure nitrogen so the lid can removed. The tank is 12 metres long and needs the equivalent of about 1000 party balloons to fill it up.
Over 100 large bolts are used to clamp the two halves of the tank together. These and several other fixtures and fittings must be removed before the lid can be craned off.
The linac runs off of a 2 million watt high frequency power supply, so it needs a huge power plug. Jim Loughry Mechanical Section Leader is dismantling it:
"It’s one thing getting the RF into the tank. It plugs inside the tank onto the tank liner so if you tighten up the lid with it in there you pull the liner apart."
The tank lid is so long it needs two cranes to be operated simultaneously to lift it.
As the lid is carefully raised the copper liner is revealed. It is made of copper to allow high frequency currents to resonate. The resonating currents produce the high voltages that kick the beam faster and faster.
The currents cause the copper liner to heat up, so water cooling pipes are used to keep it cool.
Mechanical tuners keep the whole structure resonating at the correct frequency.
Before anyone is allowed to work on the linac the levels of radiation are checked to make sure they are within safe working limits.
Mark Keelan Injector Group Operations Leader inspects the damage to the liner:
"What you can see is the connections between the water pipes and this cylindrical section have pinged off. Not sure if that’s a consequence of the temperature from the operation or its just the solder joints have failed and allowed the temperature to rise, because there’s no current path here."
Over 200 small screws must be removed to separate the two halves of the liner.
Pipe connections and support brackets are all painstakingly dismantled along the entire 12 meter length of the linac.
A custom built lifting frame is needed to support the liner lid and prevent it collapsing under it's own weight. As the lid is removed the drift tubes are revealed.
After checking the radiation levels are safe Mark moves in to inspect the drift tubes:
"What we’re looking at is what looks like little frosty speckles. It looks like high voltage corona or some kind of break-down. Its slightly rough on the surface so what we’re trying to do is polish this out and get back to a clean and shiny surface."
Now the messy work begins. Everyday metal polish and good old fashioned elbow grease are used to clean all the drift tubes. It takes several days to get the linac sparkling clean.
Meanwhile Jim carries out maintenance work to the lids. The copper straps that had pinged off are cleaned up and re-braised. Over the years high levels of x-rays and radiation cause the rubber vacuum seals to harden and crack. These are replaced and the mechanism to isolate the neighbouring tank maintained.
After almost a week's work the linac is ready to be put back together. All the high voltage surfaces have been beautifully polished and everything fixed.
The banding pattern on the liner walls is caused by 55 years of bombardment with radiation and particles. This is purely aesthetic and does not affect the operation of the linac.
The final cleaning task is to give all the surfaces a rub down with pure alcohol to remove any last traces of dust or grease.
The liner lid is lowered back into position and the linac is treated to a new set of rubber o-rings to seal all the water cooling pipes.
After reconnecting everything, the only thing that remains is to grease the giant 25 meter o-ring that seals the tank.
The lid is now closed for another 5 years. This reliable old piece of British engineering still plays a key role in keeping the UK at the cutting edge of science and technology.