Micron-cubed Particle Beam Induced Plasma Dynamics on Femto to Microsecond Timescales*

High performance accelerators can produce particle beams that are femtosecond in duration and have transverse sizes on the micron scale. To address the challenge of measuring and optimizing such micron-cubed particle beams we propose and have carried out an in-depth analysis of an ionization based technique [1]. In contrast to photo-ionized processes, when using intense particle beams to ionize a gas, very different physics is manifested due to the large unipolar fields of the charged particle beam that impart a significant momentum to the plasma electrons. As they escape with high radial velocities, they leave the ions unshielded. This non-neutral plasma undergoes Coulomb explosion and the resulting dynamics offers new avenues for direct particle beam characterization. By characterizing the tunnel ionization induced species, bunch properties can be retrieved. In addition, the exponential dependency of tunnel ionization on the electron beam's radial space-charge field, and thus its charge density, results in a sensitive monitor that can measure micron cubed electron bunches with unprecedented resolution (few 10’s of nm level). Achieving such resolution is challenging or simply not possible with today’s methods, let alone on a single shot basis that is minimally invasive. Simulations using the WARP particle-in- cell code will be presented of the plasma dynamic phenomena on timescales ranging from the femto- to microsecond. Progress on experimentally implementing this novel diagnostic at the BELLA Petawatt laser facility will be discussed that rely on active plasma lenses to focus GeV level beams to reach the required intensities. [1] R. Tarkeshian et al, PRX 08, 021039, 2018