Abstract
Laser-driven ion acceleration is often analyzed assuming that ionization reaches a steady state early in the interaction of the laser pulse with the target. This assumption breaks down for materials of high atomic number for which the ionization occurs concurrently with the acceleration process. Using particle-in-cell simulations, we have examined acceleration and simultaneous field ionization of copper ions in ultra-thin targets (20–150 nm thick) irradiated by a laser pulse with intensity 1 × 1021 W/cm2. At this intensity, the laser pulse drives strong electric fields at the rear side of the target that can ionize Cu to charge states with valence L-shell or full K-shell. The highly-charged ions are produced only in a very localized region due to a significant gap between the M- and L-shells’ ionization potentials and can be accelerated by strong, forward-directed sections of the field. Such an “ionization injection” leads to well-pronounced bunches of energetic, highly-charged ions. We also find that for the thinnest target (20 nm) a push by the laser further increases the ion energy gain. Thus, the field ionization, concurrent with the acceleration, offers a promising mechanism for the production of energetic, high-charge ion bunches.
Highlights
We find that high-Z Cu+20‒27 ions are ionized and accelerated mainly by the sheath electric field through the target normal sheath acceleration (TNSA) mechanism on the target rear surface
We find that the TNSA acceleration can be further enhanced in the case of the 20 nm target due to the target movement caused by the radiation pressure acceleration
It is conceivable that the “s” subshell of higher atomic number materials (e.g. the first subshell of the Ag M-shell) could be targeted
Summary
The ionization energies of copper, shown, illustrate the feature we aim to exploit in this work: there is a substantial gap in ionization potential that exists between different shells
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