Keeping a close watch over the beams
For the members of the Beam Instrumentation group, everything is down to the wire... the wire in the new beam wire scanner they have just finished assembling, that is. The new model was developed in the framework of the LHC Injectors Upgrade (LIU) project and has been designed to cope with the increase in the performance of the accelerators.
Beam wire scanners measure the transverse profile of the beam: an important value to know when adjusting the parameters of the accelerators. They work in the same way as a cheese wire. The wire passes through the beam, generating a flurry of secondary particles, which are then detected by a scintillator. The data gathered makes it possible to determine the position of the beam and the transverse distribution of the particles.
The accelerator complex has 25 such devices, 17 of which are in the injectors. “Some of them are around 30 years old,” explains project leader Raymond Veness from the Beam Instrumentation group. “With the increase in luminosity, we needed to renew them.”
In 2021, the injector chain will be supplying brighter beams with more intense bunches of particles. The new scanners are much faster and will be able to measure these beams without suffering damage. The wire in the model that will be installed in the SPS moves at 20 metres per second (72 km/h!), which is three times faster than the old version. “What’s more, the new models are more accurate thanks to a completely redesigned mechanical structure and a state-of-the-art control system,” continues Veness. The scanners for the PS Booster and the PS, for example, are 20 times more accurate and able to determine the position of the beam to within around 6 microns. This precision is important, as these monitors provide a reference value for the calibration of all the other beam monitoring systems.
The group is also working on the next generation of devices for measuring the beam profile, notably a device that uses the residual gas present in vacuum chambers, which is currently being tested at the PS. The beam ionises these few molecules of residual gas and the electrons thus freed are detected by a Timepix3 chip. The system is less invasive than other methods and can operate continuously.
In addition to these new devices, a huge programme to upgrade and renovate the beam instrumentation systems is underway. The accelerator complex has over 7000 diagnostic devices, 2500 of which are located inside the vacuum chambers. “Several hundred beam positioning monitors or beam loss monitors in the injectors will be replaced or upgraded,” says Rhodri Jones, the Beam Instrumentation group leader. “For example, we are replacing the whole data-acquisition system for the beam position monitors in the SPS.” The work, which has already begun at the PS Booster and the PS, will continue throughout Long Shutdown 2.