ISIS makes neutron beams by firing bunches of protons at very high speeds into a tungsten target. Neutrons are released from atoms in the target which can be channeled to neutron instruments for experiments.
At ISIS, proton bunches travel around 150 metres from the synchrotron accelerator to reach the target. During that time, they fly at 84% light speed inside a metal tube that is under vacuum. Powerful magnets keep the proton bunches squeezed into shape and steer them so they hit the face of the target which is around 40-50 mm in diameter. The beam itself has a diameter of about 30 mm, so accurate delivery of this beam onto the target is critical.
When ISIS is operating at full power to target station 1, the beam delivers around 240 kilowatts of heat into the tungsten target. Cooling water is pumped through channels cut into the target to remove the heat and prevent it getting too hot. Even so the operating temperature can still rise to around 200 degrees centigrade.
Just before the target, a metal beam window separates the vacuum of the proton beam line from the target area. The window was installed in 1984 when ISIS was being built and every proton ever fired at the target has passed through this window. Targets at ISIS are typically changed every few years, but the proton beam exit window has never been changed. After an evaluation, it was decided that a new beam exit window should be installed to avoid the risk of unexpected failure.
A team of 60 people worked for six months to install new magnets and beam exit window into the extracted proton beamline to target station 1
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The beam exit window is made of two 3mm thick sheets of the alloy Inconel separated by a 2 mm sheet of flowing cooling water since nearly 2 kilowatts of heat is lost into the window from the proton beam as it passes through. The window is mounted onto the end of a tube about 3 metres in length that is inserted through a hole in the 4 metre thick steel and concrete shielding that surrounds the target.
Inconel is very resistant to corrosion and is well-suited for extreme environments. Calculations showed that the damage from the proton beam over 25 years of service would have caused each metal atom to have been knocked out of position by around 30 displacements per atom. As atoms are knocked out of position, the mechanical properties of the metal are changed, and studies have found that this can lead to softening and a loss of ductility.
The refit was planned in great detail over several years since special long-reach tools, machines and shielding flasks had to be designed and constructed, movable thick steel shields had to be deployed,and rehearsals of the whole refurbishment were made using a full full-scale mock-up of the work area.
A team of heavy lifting experts, known locally as ‘The Heavy Gang’, worked tirelessly stripping away layer upon layer of concrete and steel weighing hundreds of tonnes. After 4 weeks of continuous lifting, a large hole had been opened up in the roof of the proton beam tunnel and the magnets and proton beam exit widow could be seen.
Old equipment was stripped out, and the new beam exit window and pipe were installed.
Three new large aperture quadrupole magnets were installed, each magnet having an individual vacuum vessel to allow easy replacement. This arrangement matches the modern design of the proton beam to target station 2.
To improve control of the proton beam position, a dipole steering magnet was designed to sit inside one of the quadrupole magnets. The geometry of the coils for this steering magnet were carefully designed using computer simulations to use the steel poles of the surrounding quadrupole magnet to produce a dipole field. This hybrid magnet allows both the quadrupole and dipole magnetic fields to be individually controlled, giving a high degree of flexibility in a compact space. The beam spot can now be steered independently onto the main neutron-producing target and onto the intermediate muon-producing target which lies upstream of the neutron target.
With the refurbished proton beam exit window and beam line complete, the Heavy Gang rebuilt the roof of the proton beam tunnel to allow proton beams to be fired at the target.
New magnets and beam exit window installed into extracted proton beamline to target station 1
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