Proton acceleration at ISIS begins with the ion source, which produces H- ions. The ion source is fed with hydrogen gas together with hot caesium vapour. A discharge plasma is formed, and positively charged ions from the plasma are attracted towards the cathode surfaces. The deposition of caesium reduces the work function of the cathode, making it a more efficient donor of electrons to the positively charged hydrogen ions, thus enhancing H- ion production. The H- ions are extracted from the ion source in 200 μs long pulses to form a beam which is then passed through a 90° magnet to remove any electrons. The H- ions acquire an energy of 35 keV across a DC acceleration gap, are focused and monitored in the Low Energy Beam Transport and then passed into the Radio Frequency Quadrupole (RFQ) accelerator.
The RFQ
RFQs use intense radio frequency electric fields to focus, bunch and accelerate particles, and are particularly well suited for use with low velocity ions. The ISIS RFQ operates at 665 keV, 202.5 MHz. Inside the RFQ, four specially shaped electrodes produce an alternating gradient quadrupole electric field for focusing and acceleration. Discrete bunches of H- ions 4.94 ns apart are passed into the linear accelerator (linac).
The Linac
The linac consists of four accelerating tanks in which high intensity radio-frequency (RF) fields accelerate the beam to 70 MeV. Each linac tank has an outer steel wall, which forms a vacuum vessel, and an inner copper liner to make the interior a low loss electromagnetic resonator. RF power at 202.5 MHz is fed to the tank through a coaxial line from a high-power amplifier. The H- beam enters the tank and travels along the axis where it sees a series of drift tubes. The bunches of ions keep in step with the alternating RF electric field, crossing the gaps between the drift tubes when the field is in the correct direction for acceleration, but being shielded inside the drift tubes when the field is in the wrong direction. The linac provides 200 μs long, 22 mA H- pulses which are transported to the synchrotron. Final acceleration of the beam occurs in the synchrotron.
The Synchrotron
Ten dipole bending magnets are used to keep the beam travelling around on a circular orbit of radius 26 m, while quadrupole magnets keep the beam tightly focused. On entry the H- beam is stripped of its electrons by a 0.3 μm thick aluminium oxide stripping foil. The resulting protons are accumulated in the synchrotron over approximately 130 revolutions. The advantage of this charge exchange injection scheme is that a larger number of protons (2.8x1013) can be accumulated in the synchrotron than would otherwise be possible.
Once injection is complete the RF system traps the beam into two bunches and accelerates them to 800 MeV. There are six double-gap, ferrite-tuned RF cavities which provide a peak accelerating voltage of 140 kV per beam revolution. The accelerating field and RF frequency are synchronised with the changing magnetic field in the dipoles to maintain a constant proton beam orbit. The beam makes about 10,000 orbits of the synchrotron as it is accelerated, before being kicked in a single revolution into the extracted proton beam line (EPB), delivering 4 μC of protons in two 100 ns long pulses to the neutron and muon targets. The entire acceleration process is repeated 50 times per second, so a mean current of 200 μA is delivered to the targets.
The ISIS target station uses the high energy protons produced by the ISIS accelerator to generate neutrons by the spallation process and to modify their characteristics to make them useful for neutron scattering experiments. Muons are also produced at ISIS using a thin carbon target in the proton beam before the neutron target.
Target Station 1
The neutron target station has three main parts.
- Target, reflector and moderator assembly in which the neutrons are produced.
- A remote handling cell for maintenance and repair operations.
- A services area containing cooling equipment.
There are 18 beam channels, 9 on each side of the target, which feed the neutron scattering instruments. Neutrons are produced when the 160 kW proton beam from the accelerator hits a metal target. The target is made from a series of thick tungsten plates, (clad with tantalum to prevent corrosion) housed in a pressure vessel. Water cooling channels remove around 90 kW of heat generated in the target.
Four moderators are used to slow down fast neutrons escaping from the target to the lower speeds required for neutron scattering experiments. Two use water at room temperature, one uses liquid methane at 100 K and the fourth consists of liquid hydrogen at 20 K. The different temperatures result in different energy neutron beams. The moderators are small, about 0.5 l, and are surrounded by a water-cooled beryllium reflector which scatters neutrons back into the moderators and doubles the useful flux of neutrons. A remote handling cell is used to replace a target or moderator and to perform any required maintenance. In operation, all components become highly radioactive, and the purpose-built cell is integrated into the target station. The cell has a pair of manipulators on each side, and operations are viewed through large shielding windows and video cameras.
The services area provides water cooling for the target, target pressure vessel, moderators and reflector, plus cryogenic systems for the methane and hydrogen moderators. All these circuits can be remotely monitored.
Target Station 2
The ISIS Second Target Station is a low-power, low-repetition-rate neutron source optimised to maximise the production of long wavelength neutrons. It is required to produce neutrons of two pulse shape varieties.
The first, a wide pulse shape with full width at half-maximum height (FWHM) no bigger than 300 µs and a modest tail, is generated by a coupled moderator; whilst the second is a simple pulse shape de-coupled moderator, with little or no tail, and a width comparable to those of the existing ISIS methane and hydrogen moderators (30–50 µs).
With the physical models for the target and moderators now complete, finalisation of the engineering design for the target/moderator assembly is in progress together with the design of the target services area to handle cooling water and liquid gases.
Muon Target
Muons are produced by colliding the ISIS proton beam with a 10 mm thick carbon target 20 m upstream of the neutron target. Collisions produce pions which decay with a mean lifetime of 26 ns into muons. The muon beam is fully polarised, and this polarisation is maintained as the beam is transported to the muon spectrometers. The muon target uses 2-3% of the proton beam.
