Formation of highly charged ions of ambient gases near laser-heated targets

The observation of spectral lines arising from ${\mathrm{Ne}}^{7+}$ ions in a 0.7-Torr atmosphere surrounding a laser-heated slab target is reported. Such line radiation is mainly emitted from a narrow zone which is located some distance away from the target, depending on the target material used (C, Li, or Al). According to a phenomenological model, the neon gas is initially heated to an electron temperature of $k_B$ $T_e$≊200 eV and ionized to the ${\mathrm{Ne}}^{8+}$ (ground-state) species by supersonic radiative and electronic thermal conduction. The ${\mathrm{Ne}}^{7+}$-line radiation results in this model from three-body recombination of the ${\mathrm{Ne}}^{8+}$ plasma as it rapidly cools, following the cessation of the laser pulse and the overtaking by lower-temperature target plasma.