Wakefield generation in near threshold and/or finite radius ionization plasmas*

The field ionization process in plasma wakefield accelerators (PWFAs) and the associated dynamics of the newly created plasma particles are, so far, insufficiently understood, but are important for the optimization of PWFAs. Here, we study the field ionization of a neutral gas in the vicinity of a high-current particle beam and the associated wakefield excitation in the generated plasma with finite radial extent. The wakefields focus the beam and thereby enhance the ionizing space-charge field and/ or can itself trigger additional ionization during the beam propagation through the gas. We show by means of theory and with 3D particle-in-cell simulations with HiPACE that the near-threshold ionization leads to the generation of exotic wakefields with decoupled longitudinal and transverse field magnitude suitable for the quality- preserving acceleration of witness beams with reduced focusing.

We also investigate the possibility to use this highly nonlinear process as a beam diagnostic, in order to probe the charge density and the beam slice emittance.