Picosecond and Nanosecond Laser Ablation of Mixed Tungsten/Aluminum Films

In order to extend the investigation of laser-assisted cleaning of ITER-relevant first mirror materials to the picosecond regime, a commercial laser system delivering 10-ps pulses at 355 nm at a frequency of up to 1 MHz has been used to investigate the ablation of mixed aluminum (oxide)/tungsten (oxide) layers deposited on polycrystalline and nanocrystalline molybdenum as well as nanocrystalline rhodium mirrors. Characterization before and after cleaning using scanning electron microscopy and spectrophotometry shows heavy dust formation, resulting in a degradation of the reflectivity. Cleaning using 5-ns pulses at 350 and 532 nm, on the other hand, proved very promising. The structure of the film remnants suggests that in this case buckling was the underlying removal mechanism rather than ablation. Repeated coating and cleaning using nanosecond pulses is demonstrated.