Modeling Electron Lenses Including Impact Ionization Physics in Warp

In an electron lens a high-current electron beam with a carefully tailored transverse profile is confined by a solenoid magnetic field. Electron lenses have been applied to, or proposed for uses such as hadron beam halo scraping, nonlinear lens elements, electron cooling and space charge compensation. in all cases a multi-amp beam at relatively low energy (normally below an MeV) must be maintained and collective effects mitigated. In the case of space charge neutralization, especially, impact ionization of a background gas must be modeled and accounted for. In this work we examine one electron lens application, a space charge neutralized electron cooling system, designed for an electron-ion collider. The particle-in-cell code Warp is used to study propagation of the electron beam through the solenoid field. Space charge neutralization is provided by impact ionization of a background hydrogen gas, which is also included in the simulations. For optimal cooling it is essential that space charge be sufficiently neutralized such that the magnetized electron trajectories along the magnetic field lines are not disturbed. We show results of simulations studying the buildup of ionized gas in the cooler and resulting space charge neutralization levels. The impact on the electron gyrocenters in the beam is quantified in terms of beam and gas conditions and the stability of neutralization levels is examined.