Speaker
Description
In the low energy beam transport (LEBT) of intense ion beam linear accelerators such as LIPAc, beam behavior is significantly influenced by strong space charge forces, so that space charge compensation (SCC) effects[1] of background electron plasma trapped within the beam's potential has an important role. These background electrons are supplied through the ionization of residual gas by the beam and through wall emissions resulting from the beam loss. Although the importance of the SCC effect, time evolution of the background electron plasma and how it is influenced by the external electromagnetic fields composing beam optics are not well understood yet.
To analyse the time evolution of the background plasma under the influence of the beam's electromagnetic fields, we conducted Particle-in-Cell (PIC) simulations using the Warp code, employing a setup that modelled the proton beam commissioning phase of LIPAc (Phase B, Stage 1). The results demonstrated that a positive-polarity electrostatic chopper removes electron plasma from the surrounding drift space and that the spatial extent of this influence is delimited by the magnetic field of the solenoid lenses. However, the simulation results could not reproduce a characteristic transient motion of the beam pulse observed in the experiments while they could reproduce only the saturated beam current. We suspected the influence of effects such as inter-electron collisions and thermalization that were not included in the PIC simulations since it requires much higher computational resources. Next, as a method to naturally capture these effects with some exaggerations, we conducted simulations using a one-dimensional electron fluid model that incorporated insights gained from the PIC simulations. However, even in this case, the characteristic beam pulse was not explained, so that a more complex, unresolved process significantly influences the transient behaviors of plasma and beam.