Notes on development of solid state laser source for remote Laser blow Off optics cleaning

In fusion experiments, the exposure of optical elements to deposition of sputtered materials is the main cause of performance degradation, alongside with bulk radiation damage in refractive elements. In the case of material deposition, the initial transmission or reflection can be easily restored by chemical cleaning and/or soft polishing. This is typically requires vacuum breaking and long shutdown periods. Especially for optics which are particularly exposed (and hence have a limited lifetime) the possibility of remote cleaning, at least for some cycles, is obviously attractive. The easiest method of remote cleaning is Laser Blow Off, which takes advantages of the greater absorption coefficient of the impurity particles to evaporate them away. Laser Blow Off cleaning has been tested both for transmissive and reflective optics, working in slightly different way. In the case of refractive elements (windows, lenses) the laser is fired through the material, being the exit surface the one which shall be cleaned. Local absorption of laser light on sputtered particles is quite effective in detaching them, either by the pressure of vapors developing at the interface, either because of mechanical stress induced in the impurity coating. In the case of mirrors, the laser beam shines directly over the surface to clean, relying on impurity selective evaporation. As for the experience coming from Nd:Yag laser [1], the desirable energy should be not less than 0.1 J and the pulse length in the 10 ns range or less, practically obtained with cavity Q switching. Lower energy results in excessively small ablation spots, which poses problems of alignment stability, energy uniformity and so on. Ideally, shorter pulses could be beneficial, while the lower limit for Q switched systems is around one ns.