Role of charge transfer in heavy-ion-beam–plasma interactions at intermediate energies

In this paper we investigate the influence of the plasma properties on the charge state distribution of a swift heavy ion beam interacting with a plasma. The main finding is that the charge state in plasma can be lower than in cold matter. The charge state distribution is determined by the ionization and recombination rates which are balancing each other out. Both, ionization and recombination rates, as well as atomic excitation and decay rates, depend on the plasma parameters in different ways. These effects have been theoretically studied by Monte Carlo simulations on the example of an argon ion beam at an energy of $4\mathrm{MeV}/\mathrm{u}$ in a carbon plasma. This study covers a plasma parameter space ranging from ion densities from ${10}^{18}$ to ${10}^{23}$ ${\mathrm{cm}}^{-3}$ and electron temperatures from 10 to 200 eV.

Figure 1

Ionization degree of the studied carbon plasma depending on density and temperature according to the Saha model.

Figure 2

Plasma temperature dependence of the ionization cross sections of ${\mathrm{Ar}}^{15+}$ in the $1s^{1}2p^{1}3p^1$ configuration on the example of $n_e={10}^{21}$ ${\mathrm{cm}}^{-3}$. The cross sections are normalized to the value of the direct ionization rate in cold gas of the same density.

Figure 3

Direct ionization rate from the ${\mathrm{Ar}}^{16+}$ $1s^2$ state.

Figure 4

Indirect ionization rate including all exitation and decay processes from and to higher shells.

Figure 5

Effective ionization rate dominated by the “density effect.”

Figure 6

Dependence of the cross section for recombination of an ${\mathrm{Ar}}^{16+}$ projectile at 4 MeV/u in the $1s^2$ configuration for different atomic subshells at a electron density of ${10}^{21}$ ${\mathrm{cm}}^{-3}$. The values of the cross section are scaled to the value for cold gas of the same density.

Figure 7

Direct recombination rate to the ${\mathrm{Ar}}^{16+}$ $1s^{2}2s^1$ configuration.

Figure 8

Indirect recombination rate to the ${\mathrm{Ar}}^{16+}$ $1s^{2}2s^1$ state.

Figure 9

Total effective recombination rate dominated by a temperature dependency.

Figure 10

Mean charge state of an ${\mathrm{Ar}}^{16+}$ ion beam after interacting with 200 $\mu \mathrm{g}/{\mathrm{cm}}^2$ carbon plasma.