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.
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.
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
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.