Understanding Secondary Emission Processes and Beam Matter interactions for Optimization of Diagnostic Wire Grid System in Particle Accelerators

Thin targets, in form of foils, stripes or wires, are widely used in beam instrumentation to measure various beam parameters, such as intensity, position and size. All these monitors can differ in geometry and material. Depending on beam parameters such as intensity, energy, transverse and longitudinal size, the detector can suffer thermomechanical stresses. This can potentially perturb the measurement accuracy and degrade the integrity of the detector. The core of this contribution presents the development of a finite-difference model, developed to simulate the particle-detector interactions and predict the detector material heating and damage. This work has been mainly performed in the context of LINAC4, which is the first accelerator at CERN’s accelerator chain. Some other facilities (CERN PS Booster, CERN SPS, GSI facility) will also be mentioned in this document. However, the majority of the studies and conclusions will refer to the energy range (45 keV - 160 MeV) and detector types available at LINAC4 (SEM grids and wire scanners). To understand the purpose, functionality and limitation of thin target detectors, this document introduces the basic principles of transverse beam dynamics and beam matter interactions, focusing on processes such as: energy deposition, secondary electron emission (SEE), electron backscattering, etc. The thermal model implemented during this work (PyTT) to simulate the thermal evolution of thin target detectors is presented and detailed discussed. To assess the reliability of the simulated results, an experiment performed at LINAC4 is presented. This experiment heavily relied on the theory of thermionic emission to indirectly measure the temperature of the detectors during operation. The performance of the simulation tool is discussed, in terms of results sensitivity to parameter uncertainty. Uncertainties in material parameters, such as the emissivity, yield uncertainties in simulation results. To improve our knowledge of the emissivity values of thin tungsten wires, an experimental setup, based on the calorimetric method, was implemented for this work. The experimental setup, emissivity calculation and results are detailed described. Some examples of how this work has been useful for CERN operations, and other facilities (like GSI) are presented in these pages. This include, beam power limit calculations for the CERN Linac4 and SPS diagnostics, beam intensity and profile measurements at LINAC4, thin foil