Space charge compensation on the low energy beam transport of Linac4

Part of the upgrade program in the injector chains of the CERN accelerator complex is the replacement of the the proton accelerator Linac2 for the brand new Linac4 which will accelerate H$^-$ and its main goal is to increase the beam intensity in the next sections of the LHC accelerator chain. The Linac4 is now under commissioning and will use several ion sources to produce high intensity unbunched H$^-$ beams with different properties, and the low energy beam transport (LEBT) is the system in charge of match all these different beams to the Radio frequency quadrupole (RFQ). The space charge forces that spread the beam ions apart of each other and cause emittance growth limits the maximum intensity that can be transported in the LEBT, but the space charge of intense unbunched ion beams can be compensated by the generated ions by the impact ionization of the residual gas, which creates a source of secondary particles inside the beam pipe. For negative ion beams, the effect of the beam electric field is to expel the electrons to the beam pipe walls, while the positive ions are trapped and start to be accumulated. In this thesis we report the results of simulations to study this space charge compensation (SCC) in a 45 keV H$^{-}$ unbunched beam in the CERN Linac4 low energy beam transport. Beam size and emittance were measured for different gases injected into the beam region to control the degree and speed of the SCC. These results are compared with beam simulations that include the generation and tracking of secondary ions leading to a unique understanding of the transport of the ion beam in some specific cases.