Forward directed ion acceleration in a LWFA with ionization-induced injection

In this work we present an experimental study where energetic ions were produced in an underdense 2.5 × 10¹⁹ cm^-3 plasma created by a 50 fs Ti:Sapphire laser with 5 TWs of power. The plasma comprises 95% He and 5% N₂ gases. Ionization-induced trapping of nitrogen K-shell electrons in the laser-induced wakefield generates an electron beam with a mean energy of 40 MeV and ~1 nC of charge. Some of the helium ions at the wake-vacuum interface are accelerated with a measured minimum ion energy of He¹⁺ ions of 1.2 MeV and He²⁺ ions of 4 MeV. The physics of the interaction is studied with 2D particle-in-cell simulations. These reveal the formation of an ion filament on the axis of the plasma due to space charge attraction of the wakefield-accelerated high-charge electron bunch. Some of these high-energy electrons escape the plasma to form a sheath at the plasma-vacuum boundary that accelerates some of the ions in the filament in the forward direction. Electrons with energy less than the sheath potential cannot escape and return to the plasma boundary in a vortex-like motion. This in turn produces a time-varying azimuthal magnetic field, which generates a longitudinal electric field at the interface that further accelerates and collimates the ions.