Fig. 1. Comparison of numerical photorecombination continuum emission spectra for a set of elements with atomic numbers Z = 13–16 and compounds sodium chloride and potassium chloride. Aluminum: grey polygon. Silicon: red. Phosphorus: blue. Sulfur: green. Sodium chloride: purple. Potassium chloride: violet. The positions of a few characteristic silicon lines of the resonance series of Ly-like Si (Si XIV, vertical solid lines in the figure) and He-like Si (Si XIII, vertical dashed lines) ions are indicated for clarity. The plasma parameters for each element were chosen individually (Table I). The ion densities n_i were set to be equal to the solid density of the corresponding materials, whereas the electron density and temperature, n_e and T_e, were set to have a fixed sum: neTe+∑NkIk.

Fig. 2. Top view of the experimental scheme. A solid target is irradiated by a p-polarized laser beam reflected from a focusing parabola and a plasma mirror. The front-side spectrometers are in the plane of the laser beam.

Fig. 3. Comparison of experimental data (grey curve) with a model spectrum (red curve). The experimental spectrum corresponds to 2-μm-Si-foil laser-plasma emission. The modeled spectrum is a sum of photorecombination, characteristic, and bremsstrahlung spectra; the last of these is indicated with a dotted blue curve. The orange rectangle indicates the wavelength range that is best suited for X-ray source based on Si (3 Å–4.8 Å).

Fig. 4. Dependences of experimentally measured conversion efficiencies on the thickness of solid foil targets: (a) emission from silicon targets integrated over the wavelength range 4.5 Å–5 Å and (b) emission from aluminum targets integrated over the range 5.15 Å–5.65 Å. The conversion efficiency is the ratio of the deposited laser energy and the energy of the emitted photons.

Table 1. Ion density n_i, electron density n_e, and electron temperature T_e, which determine the emission spectra in Fig. 2. The indicated PCEs of NaCl correspond to Cl only, whereas the PCEs of KCl correspond to K.