Effect of fast electrons on emission properties of non-equilibrium nitrogen plasma in soft x-ray spectral range

Soft x-ray emission at 2.3–4.4 nm waveband in nitrogen plasma can be observed mostly from 2p-1s and 3p-1s resonant transitions. Ions of nitrogen in plasma emitting in necessary band have a high energy excited state in comparison with plasma temperature in experiments. Discharge and laser produced plasmas used in radiation sources often have non-equilibrium electron distributions with fast electrons. In this study the radiative properties of non-equilibrium nitrogen plasma with fast electrons are examined and the plasma conditions for optimal soft x-ray emission output in the water-window region are explored. Kinetic parameters for non-equilibrium plasma including major inelastic ion interaction processes, radiation characteristics are obtained in approach based on the Hartree-Fock-Slater quantum-statistical model and the distorted wave approximation. The presence of fast electrons in plasma at relatively small portions may have a significant influence on the ion level populations and the line emission intensities. Emission generation efficiency doubles at a temperature of 60 eV with 1% portion of fast electrons with respect to equilibrium plasma and the corresponding spectral efficiency jumps to 5.5%. (paper)