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Lorraine Bobb is coming to the end of her PhD with RHUL in collaboration with the John Adams Institute (JAI), CERN and Cornell University. She has been developing a non-invasive, micron scale, transverse beam size monitor. The prototype that she has designed aims to measure the vertical size of the main beam to within one micron. Existing techniques used at other particle accelerators including the LHC are typically based on measurements taken when the beam passes through a thin screen or wire. The problem with these methods is that the interaction of the beam with the screen or wire can damage the integrity of the beam. For the high luminosities that CLIC aims to achieve, the charge density and high energy of the beam would simply destroy the beam measurement instrumentation and therefore a non-invasive technique is required.
“A non-invasive technique (the laser wire scanner) already exists,” explains Lorraine, “but it’s expensive and technically quite demanding. CLIC needs a cheaper and easier solution.”
Lorraine, with the help of her research group, designed the new instrument, chose the materials that would be used to make it and oversaw the manufacture of the prototype. “Finding a company capable of meeting the precision machining tolerances was a challenge; the instrument, which uses a target like a tuning fork, has two prongs that must have a coplanarity (or flatness) to within tens of nanometres [a human hair is approximately 100,000 nm wide].”
Lorraine installed her prototype at CesrTA, the test accelerator at Cornell University and as she approaches the end of her PhD, she is now analysing the data.
“Early indications suggest that there is a lot of background noise caused by synchrotron radiation when observing shorter wavelengths, and that possibly the instrument needs to be positioned differently to minimise this background. But the project has enabled me to develop a new process for the practicalities of measuring beams non-invasively – getting the beam through the 1mm target aperture on a circular machine, without it hitting the sides, was a challenge.”