To answer the thorny question of how to monitor a particle beam that threatens to destroy any device that dares cross its path, scientists and engineers from the 1960s came up with a brilliantly simple solution. To collect information about the beam's size and position, they built a device that detected traces of the few particles that are left in the vacuum of the beam pipe and ionised by the accelerated beam. 60 years later, a team led by James Storey (leader of the Experimental Areas, Electron Beam, Ionisation and Inelastic Collision Profile Monitors section in the Beam Instrumentation group) revived this concept and boosted it with cutting-edge CERN technology. The installation of this new high-resolution beam monitor in the Proton Synchrotron (PS) last month further prepares this venerable LHC injector for future runs and the High-Luminosity LHC.
Designing beam profile monitors can be tricky as, in measuring the beam size, these devices risk destroying the beam or getting destroyed by the high-energy particles. In recent decades, CERN's answer to this riddle has revolved around the wire scanner, a device that measures the beam size by passing an ultra-thin wire directly through the beam to create a "snapshot" of it. With the development of the Beam Gas Ionisation (BGI) beam profile monitor as part of the LHC Injectors Upgrade (LIU) project, snapshots give way to film-like footage and the quality of the beam is preserved. The technology behind the monitor relies on detecting the residual particles that inevitably pervade the beam pipe's strong vacuum and undergo ionisation as the beam flies through the pipe. The charged particles are directed towards the monitor by electromagnetic fields and directly recorded by Timepix3 hybrid pixel detectors. The beam size is then inferred from the distribution of the detected electrons in real time, and the data organised to generate footage of the beam size evolution.
The development of the BGI beam profile monitor is the result of a collaboration between groups across the ATS sector and the EP department. The device breaks new ground in instrumentation technology in many different ways: the first installation of a hybrid pixel detector directly inside an accelerator's beam pipe, the first continuous observation of individual bunches, and superb image resolution. James Storey, project leader for the BGI monitor, highlights the central role of the Timepix3 chip in this remarkable technological advance: "It's been tremendously exciting to make use of the Timepix3 technology, developed by the Medipix collaboration, to transform this elegant 60-year-old beam diagnostic technique into a state-of-the-art measurement device - which will hopefully provide our accelerator colleagues with a new set of high-speed 'eyes' on the beam."
The high-resolution footage of the beam will also help PS beam operators to prepare ther beams for the future HL-LHC: "Close monitoring of the beam's size upstream in the injectors will be key to ensuring the highest possible luminosity downstream, inside the HL-LHC - we need to keep particle bunches as compact as possible throughout the accelerator chain," explains Hampus Sandberg, who, along with colleague Swann Levasseur, dedicated a whole technical studentship, PhD project and fellowship to the device. For James, Hampus and Swann, the installation of the monitor marks the end of many years of demanding development work ("It is a huge relief to hand things over to the operators," smiles Swann). For CERN, another important milestone on the road to higher luminosity has been reached.
