Non-axisymmetric droplet irradiation effects on ion and extreme ultraviolet light emission of laser-produced plasma light sources

Abstract

Extreme ultraviolet (EUV) light emission and ion emissions in the lateral direction generated by Nd:YAG irradiated droplet targets were studied experimentally under non-axisymmetric droplet irradiation. EUV energy monitors and ion electrostatic probes were positioned symmetrically at fixed positions around the plasma. Bulk ion kinetic energy Ekin and bulk ion density ni were characterized for angles of γ=31° to γ=90° from the laser axis in a low-pressure nitrogen atmosphere of 1×10−2mbar. The EUV light emission reached a maximum level, with an increase of 5%, at γ=60° through a shift of the droplet by around 18% of the droplet diameter. The ion bulk kinetic energies at 31° and 75° also increased by 9% and 15%, respectively, with a small increase in ion density of around 3%. Intentional offset of the droplet target can, therefore, potentially result in higher EUV light output for non-axisymmetric mirror configurations, at the expense of a higher ion load but a decreased droplet fragment load. The ion kinetic energy showed a strongly anisotropic expansion behavior when offsetting the droplet relative to the laser focus, with kinetic energies increasing up to 60% in the lateral direction by offsetting the droplet by more than 0.5 droplet diameters for a lateral range of 31°–90°. The ion density showed a more isotropic change with droplet offset, while maintaining a forward-peaked distribution.