Space optic instrumentation for the sun observation in the Extreme Ultraviolet (EUV) spectral region is often based on multilayer coating technology. Such coatings have not negligible bandwidth, and therefore, often the detected signal is due to the contribution of different very close spectral lines. In this work we present a study of innovative capping layer structures covering the multilayer coating able to improve the rejection of the unwanted lines and at the same time preserving the reflectivity peak at the working wavelength.
The Multi Element Telescope for Imaging and Spectroscopy (METIS) is a coronagraph onboard of Solar Orbiter. It will perform simultaneous observations at HeII Lyman-α line, HI Lyman-α line and in visible. To achieve such capability, instrument mirrors need to be coated by multilayer (ML) structures with high efficiency at all three spectral ranges. Coatings with higher performances with respect standard Mo/Si are desirable. An instrument prototype of METIS has just flown onboard of a NASA sounding rocket: in this case, optics were coated with Mg/SiC MLs. Better performances have been obtained in terms of reflectivity, but long term stability of this coating is an open problem. Moreover the harsh conditions of the environment met during the Solar Orbiter mission given by plasma particles and high temperature could affect the lifetime of the optical components on the long term. We present the design and reflectivity tests of multilayer structures in which performances improvement is obtained by the use of novel capping layers. All multilayers are tuned at 30.4nm line but the design also maximize the performances at 121.6nm and 500 – 650 nm visible range. Analysis of Solar Orbiter environment have been carried on in order to point out the main damaging sources for the nanostructures. Computer simulations with a devoted software have been performed to preliminary evaluation of the possible instabilities in multilayers. Experimental tests for investigating the effects of the thermal heating and particles bombardments in the reflectivity performances have been planned.
The Bepi Colombo mission is a project of European Space Agency (ESA) devoted to Mercury exploration. Probing of Hermean exosphere by ultraviolet spectroscopy (PHEBUS) is a dual Extreme Ultraviolet and Far Ultraviolet (EUV – FUV) spectrometer, that will fly on Bepi Colombo. It will be devoted to spectral analysis of Mercury. A preliminary optical prototype has been assembled by CNRS – LATMOS and the test of optical subsystems (i.e. entrance mirror and gratings) have been performed in the EUV and FUV spectral range at CNR-IFN LUXOR. Experimental results are presented together with proper simulations.
PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) is a double spectrometer for the Extreme Ultraviolet range (55-155 nm) and the Far Ultraviolet range (145-315 nm) dedicated to the characterization of Mercury’s exosphere composition and dynamics, and surface-exosphere connections. PHEBUS is part of the ESA BepiColombo cornerstone mission payload devoted to the study of Mercury. The BepiColombo mission consists of two spacecrafts: the Mercury Magnetospheric Orbiter (MMO) and the Mercury Planetary Orbiter (MPO) on which PHEBUS will be mounted. PHEBUS is a French-led instrument implemented in a cooperative scheme involving Japan (detectors), Russia (scanner) and Italy (ground calibration). Before launch, PHEBUS team want to perform a full absolute calibration on ground, in addition to calibrations which will be made in-flight, in order to know the instrument’s response as precisely as possible. Instrument overview and calibration philosophy are introduced along with the first lights results observed by a first prototype.
A numerical method to design multilayer coating (ML) is presented. The mathematical tool is based on an "evolutive
strategy" algorithm which provides aperiodic solutions by maximizing input merit functions. It allows the optimization
of any kind of structures, comprising interlayers and capping layers, and modelling also inter-diffusion and interface
roughness. It has been applied to the design of MLs for different applications, as photolithography, space instrumentation
and short pulse preservation/compression. The optimization allows the control of the standing wave distribution inside
the ML. When the EUV radiation interacts with the structure, the superposition of the incident and reflected
electromagnetic wave generates a standing wave field distribution in the ML. An aperiodic design allows the regulation
of the distribution of this field, attributing specific properties to the ML. An experimental technique to recover standing
wave intensity on top of the ML is also cited. The technique is based on electron photoemission measurements, which
allow to determine both reflectivity as well as phase on top of ML. Thanks to this technique, both tests of the ML
performances compliance with expected theoretical ones and of degradation through time can be carried on.
Silicon carbide (SiC) is an attractive material for EUV and soft X-ray optics. CVD-deposited silicon carbide (deposited
at 1400° C on Si substrate) is the best reflective material in the whole EUV interval (with about the 48% of reflectance at
121.6 nm). Despite of this, SiC thin films deposited with PVD techniques, such as magnetron sputtering, on silicon
substrate, do not have the same performances and they undergo to a degradation with time, probably because of some
stoichiometry reason (carbon rich). Depositing stable SiC with PVD techniques is crucial in building ML's, like Si/SiC
and SiC/Mg for soft X-ray applications (such space telescope and photolithography).
We deposited some preliminary samples using the Pulsed Laser Deposition (PLD) and the Pulsed Electron Deposition
(PED) techniques achieving a good reflectance in the whole EUV range (27% at near normal incidence at 121.6 nn) on a
silicon substrate. The higher energy involved in these deposition processes could lead to a film with a stoichiometry
much closer to the target one. The reflectivity of the deposited films has been measured at the BEAR beamline of the
ELETTRA synchrotron in Trieste (Italy) and the optical constants retrieved at six wavelength from 121.6 nm down to 5
nm.
In recent years telescopes based on near normal-incidence multilayer (ML) technology have been employed in many
missions for imaging the Sun at selected EUV wavelengths. Such coatings have not negligible bandwidth, therefore the
detected signal often includes the contribution of unwanted adjacent spectral lines. In this work we propose an
innovative method for designing suitable ML capping layers able to preserve the reflectivity peak of the underneath
structure at the selected wavelength while rejecting the unwanted lines. Theoretical design and experimental results are
presented and discussed.
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