Investigation of the upgrade of a space cryogenic actuator with superconducting coils Integration internship

Abstract

The use of mechanical components in cryogenics introduces constraints and uncertainties as material properties are temperature-dependent. The use of these components becomes even more critical when they are responsible for the motion of a mechanism. In addition, space applications impose strict requirements on the development of such mechanisms as they must function reliably in space environment where maintenance is nearly impossible. In this context, the Liège Space Centre developed a first prototype of a cryogenic actuator capable of switching a mirror in an optical instrument between two stable positions. One of the main features of this mechanism is its passive locking in both positions which lowers energy consumption and minimises energy dissipation during actuation. The actuator includes components such as permanent magnets, copper coils and flexible pivots, the latter designed and 3D printed by the Swiss Centre for Electronics and Microtechnology. This project was initially carried out for the SAFARI far-infrared spectrometer on the SPICA space telescope, an ambitious but now aborted mission. The main objective of this thesis is to build upon the work developed for that mission and propose a new design of the mechanism in anticipation of next-generation cryogenic space missions. In particular, the focus is on replacing the copper coils with superconducting coils to reduce both the mass and energy dissipation of the actuator. Since superconductors can carry higher currents without resistance – and therefore without Joule losses – and higher current reduces the number of turns required to generate the same magnetomotive force, the coils can be made smaller and lighter. Consequently, the magnetic parts of the actuator can be optimised in mass based on the new coil design. To achieve this goal, the present work first presents the initial mechanism design, including its components, switching principle, modelling and the characterisation of its critical parts. Additional experimental magnet characterisation carried out as part of this thesis is presented. The obtained results validate the numerical model of the mechanism. Next, the fundamentals of superconductivity are explored with a particular focus on aspects relevant to the mechanism’s design. Two types of superconductors are identified: one suitable for use below 4 K and another for use below 77 K. Then, the selected superconductors are integrated into the actuator model and the geometry is optimised to reduce the mass of the mechanism’s magnetic parts. For the configuration using high-temperature superconductors (operating below 77 K), the mass is reduced by 88%. For the configuration using low-temperature superconductors (operating below 4 K), the mass is reduced by more than 94%. These mass reductions are achieved by reducing the number of coil turns by a factor of 100 and the mass of the mechanism’s magnetic core by a factor of 10. Finally, the design of a hybrid resistive-superconducting harness architecture is proposed to supply this mechanism. Although no experimental validation with superconductors was performed, this thesis lays the theoretical groundwork for future development of this optimised cryogenic actuator using superconducting coils.