Stability of droplet-target laser-plasma soft x-ray sources

The spatial stability of microscopic target droplets used for laser-plasma soft x-ray generation in vacuum is investigated. A long-term drift in drop position is characterized with an ultrafast laser-diode imaging system. The drift is experimentally and theoretically shown to be due to a temperature -induced increase in target-liquid viscosity as a result of evaporation. Fi nally, the drift is compensated for and stable, long-term unattended operat ion of the source is demonstrated with an automatic phase-delay drop-to-las er synchronizing system. This is important for future compact lithography a nd microscopy systems. (C) 2000 American Institute of Physics. [S0021-8979( 00)00822-7].