Electron cloud buildup studies for the LHC

Electron clouds can develop in accelerators operating with positively charged particles. The con- sequences of e-cloud related effects are very important for the operation of the Large Hadron Collider (LHC) at CERN, and for the design of future accelerators including the LHC luminosity upgrade (HL-LHC). High electron densities are generated by an interaction between the beam and the confining chamber. Primary electrons, that can be generated through various mecha- nisms, are accelerated by the beam and impinge on the chamber walls, thereby extracting more electrons from the material. Furthermore they also deposit their kinetic energy in the process, which has to be compensated by the cooling system. Especially in cryogenic environments, as it is the case for a large part of the LHC, high heat loads can pose a serious problem. In order to improve the understanding of the electron cloud, simulation studies are performed with the code PyECLOUD, developed at CERN. The work of the first half of the project is described in the master proposal thesis. Among other tasks, a recomputation of measured heat loads at the LHC was performed for all fills and all cryogenic cells at the LHC since 2015. Higher order magnets, such as sextupoles and octupoles, are part of every cryogenic cell at the LHC. However it has not been possible to model higher order magnetic fields within PyECLOUD prior to this work. Consequently, the software has been extended and simulations of sextupoles and octupoles were performed for the first time. The results are thoroughly compared to those from quadrupole simulations. Similar properties of the e-cloud in sextupoles and quadrupoles can be observed. The modelled electron cloud in the octupoles turned out to be less strong. Furthermore, the modelling of build-up simulations with photoelectron seeding has been studied. Results from laboratory measurements of LHC beam screen surface materials are available. The literature has been reviewed and two sets of simulation input parameters were obtained. One set is a conservative estimate, where high photoelectron yields per photon are assumed, correspond- ing to measurements published for clean, "as-received" samples of copper. The other is a more realistic estimate, taking into account the conditioning of the chamber surfaces by continuous synchrotron radiation from the beam. Simulation results for drift spaces and dipolar magnetic fields are very sensitive to the photoemission parameters. Quadrupoles and higher order magnets on the other hand are barely affected by the presence of photoelectrons. Finally, the modelling of photoemission seeding in PyECLOUD has been extended to study the effects of a delayed time structure of photoelectron generation, of different initial photoelectron energy spectra and