Torsten Henning

An internally wetted capillary-type electrospray thruster design is presented. The capillary emitters are optimized for fabrication using 3D micro lithography and can achieve sub 10-micrometer capillary diameters with an aspect ratio of over 20. Also provided is a design for a completely modular integrated extraction electrode that comprises an electrode carrier produced by 3D micro lithography and a thin metal film. The electrode orifices, distance to emitters, and size are all customizable thanks to the modularity of the design, which is compatible with any electrospray thruster type. The design provides alignment precision within 5 micrometers of the emitter tip and electrode orifice. While our new electrode achieved reproducible extraction, instability is still present. The data on emission from these emitter-electrode stacks is presented, as well as in situ microscopic optical observation of individual emitters. The images demonstrate emission in multiple extraction modes, microfluidic behaviour of the capillaries in space-like conditions, and interactions of the emission modes with the integrated electrode.

Electrospray emitters are an obvious choice for miniaturized thrusters for a variety of applications on small satellites (e. g., CubeSats), as well as for other micropropulsion purposes. They are inherently small and require a relatively low electric power for operation, and therefore, they fulfill the requirements imposed due to the small volume of CubeSats. Electrospray emitters of the internally wetted capillary type were fabricated by 3D microlithography in the SU-8 photopolymer down to capillary diameters of about 10 μm. Thus, the emitters are an order of magnitude smaller in lateral dimensions than those fabricated by advanced 3D printing methods and still half an order of magnitude smaller than those made by planar photolithography in SU-8. Fabrication methods and process parameters are presented. Furthermore, the preliminary results of the electric characterization of the emission behavior are shown. The experiments show promising results regarding the fabrication quality and extraction behavior.

The transition from old space to new space along with increasing commercialization has a major impact on space flight, in general, and on electric propulsion (EP) by ion thrusters, in particular. Ion thrusters are nowadays used as primary propulsion systems in space. This article describes how these changes related to new space affect various aspects that are important for the development of EP systems. Starting with a historical overview of the development of space flight and of the technology of EP systems, a number of important missions with EP and the underlying technologies are presented. The focus of our discussion is the technology of the radio frequency ion thruster as a prominent member of the gridded ion engine family. Based on this discussion, we give an overview of important research topics such as the search for alternative propellants, the development of reliable neutralizer concepts based on novel insert materials, as well as promising neutralizer-free propulsion concepts. In addition, aspects of thruster modeling and requirements for test facilities are discussed. Furthermore, we address aspects of space electronics with regard to the development of highly efficient electronic components as well as aspects of electromagnetic compatibility and radiation hardness. This article concludes with a presentation of the interaction of EP systems with the spacecraft.

Ion-beam techniques, i.e., ion-beam sputtering for material deposition or ion beam etching for a controlled modification of surfaces, are well established for planar thin-film processing. The primary beam of ions exiting a broadband source typically used exhibits a macroscopic spatial beam profile. In general, the beam profile is considered an undesirable but unavoidable feature of the ion source, as it may introduce inhomogeneities in the thickness of the deposited thin-film or the etched surface. Ways of circumventing these effects are sought, e.g., by using rotating substrates or large ion sources compared to the size of the substrate. Here, we demonstrate that an active control of the spatial beam profile may become advantageous when attempting to achieve homogeneous coatings on nonplanar substrates or to etch nonplanar macroscopic structures, e.g., when coating free-form optics.

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

Electrospray thrusters are a promising micropropulsion technology for CubeSats, among others, because they have the potential to provide high specific impulse. They avoid moving parts and allow a high degree of miniaturization. In combination with fabrication technologies from microelectromechanical systems (MEMS), miniaturization of individual emitters to the range of tens of micrometers brings with it a new concept of constructing and scaling thrusters, namely, by numbering-up the microthrusters to a device of suitable thrust. The current state of the art is briefly reviewed with an emphasis on MEMS technology. Furthermore, preliminary results, which indicate the feasibility of fabricating electrospray emitters in nonsilicon MEMS technology, notably by 3-D microlithography (two-photon lithography) are presented.

Microstructure heat exchanger are well known for their superior heat transfer capabilities and the good temperature management for applications. Most microstructure heat exchangers presented so far consisted of a number of microchannels in a defined arrangement to provide the transfer of thermal power from one fluid to another, mostly in laminar flow regime. In this publication, several crossflow microstructure heat exchangers are presented. The devices design ranges from simple linear microchannels of different dimensions and shape, complex micro column arrangements up to three dimensional flow structures like crossed sinusoidally shaped microchannels, a kind of split-and-recombine structure. By comparing experimental results of the most interesting devices, microstructure devices with good performance can be chosen.

Large-area high-density patterns of quantum dots with a diameter of 200 nm have been prepared from a series of four multiple quantum well structures of different well width (40 Å, 60 Å, 80 Å and 100 Å) by electron lithography followed by ion beam etching. Below-bandgap photomodulated reflectivity spectra of the quantum dot samples and the parent heterostructures were then recorded at 10 K and the spectra were fitted to extract the linewidths and the energy positions of the excitonic transitions in each sample. The fitted results are compared with calculations of the transition energies in which the different strain states in the samples are taken into account. We show that the main effect of the nanofabrication process is a change in the strain state of the quantum dot samples compared with the parent heterostructures. The quantum dot pillars turn out to be freestanding, whereas the heterostructures are in a good approximation strained to the ZnTe lattice constant. The lateral size of the dots is such that extra confinement effects are not expected or observed.