Multilayer Coatings for High-Energy Astronomical Telescopes

The main application for this PhD research is to contribute to the development of new optic technologies for the next generation of high-energy astrophysical space missions, specifically for the NASA probe-class mission concept: The High Energy X-ray Probe (HEX-P). The research aims to enable the development of X-ray reflecting multilayer coatings to significantly increase the X-ray flux density and spectral purity, with focus on expanding the telescopes focusing performance to 200 keV and above. The project has strong interdisciplinary aspects and brings together the fields of X-ray physics, material science, data science and astronomy, and involves the design, development and characterization of nanometer-thin multilayer coatings. The research will help enable HEX-P and other future space missions to observe some of the most extreme environments of the universe, i.e. shocks in supernovae explosions and merging galaxies, hot plasma around black holes, and the strong magnetic fields of neutron stars.

A systematic method for optimizing the design of depth-graded multilayers is presented, where a Python-based optimization tool has been developed that utilizes a Differential Evolution algorithm, to effectively explore the vast parameter space of a multilayer structure, to find the optimum multilayer structure which enables the highest performance. Platinum (Pt) and Nickel (Ni) based multilayers coatings are compared and are shown to enable high performance of focusing X-ray telescopes to energies up to 200 keV The source code is also used for fitting X-ray reflectometry measurements using the Differential Evolution algorithm.

The simulated performance shows that Ni-based multilayer coatings enables a greater performance for high-energy focusing X-ray telescopes, compared to Pt-based multilayers. However, the current fabricated Ni-based coatings contain high interfacial roughnesses, which will affect the predicted performance of the telescope. To evaluate the roughness in the Ni coatings, the DC magnetron sputtering facility at DTU Space is used to coat and test different types of sputtering collimators and different reactive sputtering gas concentrations of nitrogen and argon. The multilayers are characterized using X-ray reflectometry and X-ray photoelectron spectroscopy.
To computationally predict the performance of HEX-P, a ray-tracing tool has been developed.
The baseline optic specifications provided by NASA are used to 3D construct the telescope,
after which the paths of the photons through the telescope are calculated. Different aperture
structures are simulated in between the optics and focal plane in order to remove noise from
stray light and single-bounce photons.

The last part of this PhD dissertation presents the challenge of coating long X-ray reflective
mirrors, as part of the thin film coating development for the 456 mm long parabolic mirror
used in the Beam Expander Testing X-ray (BEaTriX) facility.