TIME-RESOLVED MEASUREMENTS OF X-RAY-DAMAGE TO OPTICAL COATINGS

Thin film optical coatings are susceptible to damage by high intensity x rays. Time-resolved measurements of this damage are required to bet ter understand the mechanism, so that more rugged coatings can be deve loped. In the present experiment, dark-field shadowgraphy was used to temporally map the x-ray damage across the surface of certain anti-ref lecting (AR) coatings. Two beams from the NRL PHAROS III high power Nd :glass laser system were utilized to generate a point source of plasma x rays, which in turn was used to irradiate and damage the optical co atings. Thin, opaque filters, coupled with permanent magnets and pinho les, were used to shield the optical samples from ultraviolet and char ged-particle damage, respectively. The absolute, time-integrated x-ray fluence was measured with a crystal spectrograph, and also was tempor ally resolved with an x-ray diode. The surface morphology of the damag ed optical samples was examined after each shot visually, and later wi th a profilometer as well as with both scanning electron- and atomic-f orce microscopes. A measured threshold fluence for damage of 0.049+/-3 0% cal/cm(2) agrees very well with a radiation-damage code prediction of 0.046 cal/cm(2). (C) 1997 American Institute of Physics.