Estimation of emission efficiency for laser-produced EUV-plasmas

摘要

Extreme Ultra Violet (EUV) light source produced by laser irradiation emits not only the desired EUV light of 13 ～ 14 nm (about 90 eV) but also shorter x-rays. For example, emissions around 4 ～ 8nm (about 150 ～ 300 eV) and 1 ～ 2.5 nm (about 0.5 ? 1.2 keV) are experimentally observed from Sn and/or SnO_2 plasmas. These emissions are correspond to the N-shell and M-shell transitions, respectively. From the view point of energy balance and efficiency, these transitions should be suppressed. However, they may, to some extent, contribute to provide the 5p and 4f levels with electrons which eventually emit the EUV light and enhance the intensity. To know well about radiative properties and kinematic of the whole plasma, atomic population kinetics and spectral synthesis codes have been developed. These codes can estimate the atomic population with nl-scheme and spectral shapes of the EUV light. Radiation hydrodynamic simulation have been proceeding in this analysis. Finally, the laser intensity dependence of the conversion efficiency calculated by these codes agrees with that of the corresponding experimental results.