DEGRADATION OF FAR-ULTRAVIOLET REFLECTANCE OF ALUMINUM FILMS EXPOSED TO ATOMIC OXYGEN - IN-ORBIT COATING APPLICATION

The drop in reflectance at the interval 82.6-174.4 nm of ultra high va cuum prepared aluminum coatings when exposed to controlled doses of at omic oxygen with average energy 0.17 eV has been measured by the first time. We show that atomic oxygen produces a much stronger effect on t he far ultraviolet (FUV) reflectance of aluminum that molecular oxygen : for the same relative drop in reflectance of 40% at 82.6 nm, the req uired exposure to molecular oxygen is about 5000 times larger than to atomic oxygen. These measurements allowed us to calculate a higher lim it of the useful lifetime of an aluminum mirror placed in a low earth orbit (LEG, 200 to 700 km altitude; oxygen atoms with energy of 5 eV). This limit was found to be as short as a few hours. To avoid the oxid ation by atomic oxygen a high altitude orbit should be used. A more pr actical alternative may be found by placing the aluminum mirror behind a wakeshield device, a well known proposal, in a LEG. In this case, t he flux of oxygen atoms impinging on the mirror would be strongly redu ced and consequently the lifetime of the mirror greatly increased. We have calculated that the energy of oxygen atoms impinging on the shiel ded mirror would range from 0.07 to 1.8 eV. As the energy of oxygen at oms in our experiment is within that interval of energies, a plausible estimate of the lifetime expected for an in-orbit aluminum coating be hind a wakeshield can be derived from our experimental measurements. F or a wakeshield operating as a free flyer at a 300 km altitude orbit a n aluminum mirror would suffer a negligible drop in its FUV reflectanc e after a time as long as 20 years, which is a stimulating prospective .