Recombination of Protons Accelerated by a High Intensity High Contrast Laser

Short pulse, high contrast, intense laser pulses incident onto a solid target are not known to generate fast neutral atoms. Experiments carried out to study the recombination of accelerated protons show a 200 times higher neutralization than expected. Fast neutral atoms can contribute to 80% of the fast particles at 10 keV, falling rapidly for higher energy. Conventional charge transfer and electron-ion recombination in a high density plasma plume near the target is unable to explain the neutralization. We present a model based on the copropagation of electrons and ions wherein recombination far away from the target surface accounts for the experimental measurements. A novel experimental verification of the model is also presented. This study provides insights into the closely linked dynamics of ions and electrons by which neutral atom formation is enhanced.

Figure 1

(a) Model of ion acceleration and neutralization of the fast ions with a cold bunch of copropagating electrons. (b) Steady state average charge of protons in a plasma formed near the target. (c) Spectrum of all particles (${\mathrm{H}}^{+}$ and H atoms) and fast H atoms (neutrals) retrieved from the TOF.

Figure 2

Neutralization as a function of energy at different experimental conditions. The dashed lines shows neutralization spectrum obtained with the copropagation model.

Figure 3

(a) A schematic of the ion trajectory in the presence of an external magnetic field. The blue and the green (dashed) lines represent the ions and neutral particle trajectories in the field. (b),(c) The central spot of the TPS gated to detect H atoms, without and with a magnetic field, respectively. Change in the shape of the H atom spot indicates a shift in the virtual source position. (d) Line profile in the direction of deflection upon scanning the transverse magnetic field in the $x$ direction. (e) Change in the neutral flux due to the magnetic field applied near the target. The experimental measurements compare well with particle trajectory calculations computed using the pinhole imaging formalism.