Charge equilibration of laser-accelerated carbon ions in a porous-structure foam target

The equilibrium charge state distribution of laser-accelerated carbon ions traversing a tri-cellulose-acetate (TCA, ${\mathrm{C}}_9{\mathrm{H}}_{16}{\mathrm{O}}_8$) foam target was measured experimentally. The ions were generated through the target normal sheath acceleration mechanism. This allowed us to obtain the equilibrium charge state for a wide energy range near the maximum energy loss within a single laser shot. The foam had a porous structure with 2 mg/${\mathrm{cm}}^3$ volume density, which is between the typical density of gas- and solid-state matter. We found that the measured average equilibrium charge states were significantly underestimated by theoretical models applicable for gas targets, while were in close agreement with both semiempirical formulas and rate equation predictions based on ion-solid interactions. The solid-density fiber filaments in the current foam structure were attributed and demonstrated. The target density effects, which increase the ionization probability through frequent collisions and decrease the electron capture probability, were proven to play an important role in the foam target. Since the foam targets are widely used in laser plasma interaction experiments, our findings are relevant for a broad range of applications.

Figure 1

Experimental setup. A picosecond laser is focused onto a copper foil, generating an intense short pulse of ions through the TNSA mechanism. The energy spectra of carbon ions passing through the foam target and the empty hole were measured with a dual-channel TPS coupled to the Fuji BAS-TR-type image plate (IP).

Figure 2

(a) Typical deflection curves of laser-accelerated carbon ions without target (left) and passing through the foam (right). The converted energy spectra of carbon ions (b) without target and (c) passing through the foam.

Figure 3

Average charge states of carbon ions passing through foam (a) and without any target (b) for two shots. The error concerning energy and charge state originates the energy resolution of the TPS and statistical error, respectively. Charge state distribution evolution of carbon ions passing through foam (c) and without any target (d) versus kinetic energy.

Figure 4

The comparison of measured average charge states with theoretical predictions. The experimental data were averaged over two shots with errors representing the shot-to-shot fluctuations and statistical error. (a) The comparison with semiempirical models. (b) The comparison with predictions through solving rate equations.