Degradation of thin-film filters irradiated by debris emission of a laser induced plasma

Laser-based X-ray sources like laser-induced plasma (LIP) established as laboratory scale EUV- and soft X-radiation sources in many scientific fields. Concerning the relative low conversion efficiencies of about 0.5% to 1% one has to avoid scattered laser light reaching the X-ray optical setup. For this, thin metal filters with a thickness of a few hundred nanometres are used. Another purpose of the filter is to block high-speed ions, clusters and particles (debris) originating from the plasma, thus protecting the X-ray optics against damage. This is especially true when solid targets are used for plasma generation. Concerning the application of LIP sources, one needs to collect as much radiation as possible. Therefore X-ray optics with a high numerical aperture are required and the shielding filter has to be large and far-off the source or vice versa. Since large thin filters are very fragile, one has to find a compromise between these two parameters to achieve appropriate filter lifetime. In this work we describe the stage of experiments to learn more about the process of debris emission characteristics and the risk of damage to sensitive filters and X-ray optics. The experiments were carried out at a LIP source using a liquid nitrogen jet or an ethanol jet as target. Several types of metal foils are investigated at different distances to the source. Each filter is imaged onto a CMOS-Camera to examine the leakage of scattered laser light by debris-generated pinholes. The analysis of the experiments is carried out particularly with regard to the theoretical X-ray throughput versus lifetime of the different filter types.