Foundations: Multilayer coatings for extreme ultraviolet optics and soft x-ray solar physics experiments
Early innovations in the development of multilayer coatings by LLNL researchers led to pioneering solar physics observations.
Multilayer coatings enable telescope designs with near-normal incidence angles in the extreme ultraviolet (EUV) wavebands, offering several advantages over grazing-incidence telescopes. Researchers can construct the coatings to reflect a narrow band with an engineered central wavelength, allowing the imaging of plasma lines emitted by the Sun’s surface (the photosphere) and atmosphere (the corona) with exquisite spectral resolution.
These narrow-band images of the Sun’s plasma emission lines enable quantitative studies of the evolving coronal magnetic field and its plasma. Researchers use the images to improve the understanding of the physics behind the activity displayed by the Sun’s atmosphere, including events such as solar flares and coronal mass ejections, which have implications for human life (telecommunications, electricity grid, air travel, and astronaut safety). It is therefore unsurprising that the arrival of multilayers was a game-changer in the fields of solar physics and astrophysics.
Since the 1980s, LLNL has been a leader in multilayer thin film science, a capability born out of the needs of the x-ray laser programs. Work at LLNL led to the development of multilayer optics that enabled space based EUV and soft x-ray solar telescopes aboard sounding rockets and satellites, including some of the earliest instrumentation for solar observations in these wavebands.
Laboratory partnership leads to multilayer optics observations
An important factor that contributed to the genesis and impact of multilayer-based instrumentation was the availability of high-brightness, precisely calibrated light sources. These facilities provided accurate at-wavelength metrology of multilayer optics during the development and optimization phases as well as the final photometric calibration of the flight optics. The capabilities at the Lawrence Berkeley National Laboratory Center for X-ray Optics (CXRO)—a laser plasma source reflectometer in the early 1990s followed by beamline 6.3.2. at the Advanced Light Source synchrotron—have been established as worldwide standards in EUV metrology. LLNL’s partnership with CXRO has been crucial to ongoing research efforts.
The earliest instances of multilayer optics used for astronomical observations are attributed to CXRO and LLNL instrumenting two sounding rocket experiments that first flew in 1985 and 1987. The multilayers for the former were coated at CXRO and for the latter at LLNL. These instruments include:
- A single-reflection Herschelian multilayer mirror operating at a wavelength of 4.4 nanometers (nm) (Si II ionization line). The instrument successfully took photos of a solar active region. (1985 sounding rocket)
- Two double-reflection Cassegrain telescopes with multilayer mirrors operating at 17.3 nm (Fe IX, Fe X) and 25.6 nm (He II). (1987 sounding rocket)
- A Herschelian mirror operating at 25.6 nm. (1987 sounding rocket)
- Convex multilayer mirrors coupled to a grazing incidence Wolter-Schwarzschild mirror operating at 4.4 nm, 17.3 nm, and 25.6 nm. (1987 sounding rocket)
Continued multilayer development enables new space missions
For over 35 years, LLNL has continued to develop multilayer coatings to enable new EUV solar missions, providing ever more capable optics for sounding rocket experiments and satellite missions. Notable missions and experiments in the 1990s and 2000s include the Multi-Spectral Solar Telescope Array I and II sounding rocket experiments, the Transition Region and Coronal Explorer (TRACE) satellite, and the Solar Dynamics Observatory. Since then, researchers at LLNL have advanced the multilayer technology further, driven by the requirements of photolithography, high energy physics, astrophysics, x-ray radiation detection programs, and future generations of solar physics space missions.
In-band and out-of-band reflectance calibrations of the EUV multilayer mirrors of the atmospheric imaging assembly instrument aboard the Solar Dynamics Observatory | SPIE, 2012
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The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) | The Solar Dynamics Observatory, 2011
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Atomic force microscopy characterization of Zerodur mirror substrates for the extreme ultraviolet telescopes aboard NASA's Solar Dynamics Observatory | Applied Optics, 2007
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Development and testing of EUV multilayer coatings for the atmospheric imaging assembly instrument aboard the Solar Dynamics Observatory | SPIE, 2005
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High resolution imaging with multilayer telescopes: resolution performance of the MSSTA II Telescopes | Optical Engineering, 2000
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A new view of the solar corona from the transition region and coronal explorer (TRACE) | Physics of Plasmas, 1999
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Astronomical observations with normal incidence multilayer optics: recent results and future prospects | Phys. Scr., 1990
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Soft X-ray Images of the Solar Corona with a Normal-Incidence Cassegrain Multilayer Telescope | Science, 1988
A.B.C. Walker, Jr., J.F. Lindblom, T.W. Barbee, Jr., and R.B. Hoover
X-ray Photographs of a Solar Active Region with a Multilayer Telescope at Normal Incidence | Science, 1987
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