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

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 E k i n and bulk ion density n i were characterized for angles of γ = 31 ° to γ = 90 ° from the laser axis in a low-pressure nitrogen atmosphere of 1 × 10 − 2 mbar. 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.