Design of non-invasive profile monitors for the ESS proton beam

The European Spallation Source (ESS) will be a research infrastructure dedicated to sciences using neutrons as probes. The source is currently under construction in Lund, Sweden, and will be the world's brightest pulsed source of neutrons. As its name suggests, the production of neutrons is ensured by the spallation process: high energy protons will impinge a tungsten target. To accelerate the protons, a powerful 2 GeV linear accelerator is being built. The accelerator can be split in two parts. A 'hot' part is responsible for acceleration up to 90 MeV. Then a 'cold' part made of superconducting cavities cooled with liquid helium is used to reach the highest energies. The high intensity of 62.5 mA and he long pulse of 2.86 ms repeated 14 times per second, lead to an incredible beam power of 5 MW in average and 125 MW in peak. The knowledge of the beam is therefore mandatory to ensure the commissioning, i.e. the beam tuning in order to achieve a proper and safe functioning of the machine. Different diagnostics will be installed along the accelerator to fulfil these tasks. This thesis deals with the development of a non-invasive transverse profiler for the cold part of the ESS accelerator: the Ionization Profile Monitor (IPM). The thesis focuses on critical aspects of the IPMs to guarantee its feasibility in ESS beam conditions. These monitors are based on the ionization of the residual gas induced by the proton beam inside the beam pipe. A transverse electrical field is generated between both parallel plates of the IPM. The electrons or ions drift, with respect to the electric field, towards a segmented detector allowing the reconstruction of the beam profile in one transverse direction. For a complete transverse profile, it is necessary to add a second profiler tilted by 90 degrees. Several challenges for facing IPM to the ESS conditions, which may compromise their use, are described:- the weak counting rates due to the low ionization cross-sections at high energy (90 to 2000 MeV) and to the low residual gas pressure of 10^-9 mbar,- the electric field homogeneity inside the IPM, which is relevant for insuring a precise profile measurement, was not obvious in the narrow vacuum chambers devoted to them,- the large Space Charge Effect of the beam, distorting the measured profile by deviating the ionization by-product trajectories. This fundamental aspect may compromise the use of an IPM for beam profile measurements. Once these former studies done, we selected the three reliable read-out systems based on:- conductive strips read by a multichannel charge integrator,- micro-channel plates coupled with phosphor screen (pMCP),- a silicon detector developed at CERN and foreseen for the future PS beam profiler. This work was the object of the Preliminary Design Review (PDR 2017/01) marking the beginning of the construction phase of the different prototypes. Preliminary tests discarded the possibility of using silicon detectors due to the low ion energies. Starting from scratch, IPMs, reference monitors and a test bench were designed and installed at the IPHI proton accelerator at Saclay. Close ESS conditions were achieved to validate an IPM solution and our simulations. The test campaigns showed that an MCP is mandatory to detect signal. Moreover, the optical IPM (pMCP + Camera) is the preferred solution since it provides higher sensitivity. Feedbacks from the prototype test campaigns, allows us to deliver an IPM final design presented during the Critical Design Review (CDR 2019/02) leading to the beginning of the production phase. (author)