Hierarchically Structured Materials

The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of uranium-nickel alloys for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (C Kα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 44.7 Å (C Kα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. The initial oxidation behavior of sputtered uranium-nickel alloys is similar to pure U so their reflectance was not determined. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required

We have prepared our own very small, Martian test tubes and flasks which possess many of the conditions of Mars’ surface (up to the first three of five conditions listed below) and at a cost well below $100**. We did several experiments of interest to us, namely: lightening on Mars in a jar (12-13 year olds) and what might happen to ice cream on Mars (an 8 year old). Student, amateur, and professional researchers alike need Mars-like conditions in which to test their ideas and to help answer their questions. It has been proposed that vacuum stations like those used in industry and university labs could be employed to produce Mars-like conditions to test student ideas for whole classrooms of children.1 It is our goal to find simpler and lower cost ways to help researchers set up their own small test stations where they can test out their ideas. The test stands need only meet those characteristics of Mars which are pertinent for testing the ideas.

The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the distribution of He+ in Earth's plasmasphere by detecting its resonantly-scattered emission at 30.4 nm. It will record the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Effective imaging of the plasmaspheric He+ requires global 'snapshots' in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view 30 degrees in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84 degrees x30 degrees, which is swept across the plasmasphere by the spin of the satellite. EUV's spatial resolution is 0.6 degrees or similar to 0.1 R-E in the equatorial plane seen from apogee. The sensitivity is 1.9 count s(-1) Rayleigh(-1), sufficient to map the position of the plasmapause with a time resolution of 10 min.

We have developed a new family of EUV multilayer mirror coatings using uranium. Using this approach we have coated a set of six mirrors for the EUV Imager, a component of the IMAGE mission. This mission is a Medium Explorer (MIDEX) program, which is scheduled for launch early in 2000. The EUV Imager will study the distribution of He+ in the Earth's plasmasphere by detecting its resonantly scattered emission at 30.4 nm (41 eV) and will produce images of the structure and dynamics of the cold plasma on a global scale. There is, however, a bright emission at 58.4 nm (21 eV), which comes from neutral helium in the earth's ionosphere which also must be blocked. These photons are at too high an energy to filter with aluminum but at too low an energy to have negligible reflectance from most materials commonly used in EUV mirrors. Thus, a multilayer system which satisfied two optical functions, high reflectance (greater than 20%) at 41 eV and low reflectance (less than 2%) at 21 eV, were designed and successfully fabricated. Such mirrors with dual optical functions in the soft x-ray/EUV had not previously been designed or built. These specifications were particularly challenging because many materials have higher single layer reflectances at 58.4 nm than at 30.4 nm. Essentially, the mirror must have low reflectance at 21 eV without loss of reflection at 30.4 nm. This was accomplished. The top part of the multilayer, which reflects well at 30.4 nm, also acts as antireflection layers at 58.4 nm. In the past, multilayers usually have consisted of periodic bilayers. We have explored the use of a periodic mirrors in place of the standard periodic designs. Along the way we have created the computational tools, which include genetic algorithms, to optimize selection of materials and thicknesses. We are currently in the process of building up an EUV characterization system and developing a general way of measuring the optical constants of air-sensitive thin films. We discuss the other material and fabrication challenges faced, which include: (1) The high absorption of almost everything in the EUV. This means that only a few interfaces in a multilayer will contribute to its reflectance. (2) Surface contamination and corrosion. (3) The deposition on flight mirrors that are highly curved (f equals 0.8).