Sputtering of Mo and Ag with xenon ions from a radio-frequency ion thruster.

The goal of this work is to set up an electric propulsion (EP) sputtering test section as a feasibility study for ground-based sputter testing of spacecraft materials with a radio-frequency ion thruster. Such experiments deliver valuable data, which are scarce but highly desired to model EP-based space missions, for example, with the Spacecraft Plasma Interaction System in order to predict the performance and lifetime of spacecraft components. This study assessed if sufficient testing conditions can be met to produce reliable experimental material data in the future. Therefore, the thruster was operated at ion energies of 1.5 and 1.8 keV, and a quartz crystal microbalance (QCM) was installed to detect sputter deposition rates. Molybdenum (Mo) and silver (Ag) were chosen as sputter targets. Wafer substrates served as a passive sampling method to characterize the composition of sputtered material by Rutherford backscattering spectrometry. Additionally, sputtering simulations matching the experimental conditions were performed with the software SDTrimSP. We obtained comparable experimental and computational data, as measured sputter deposition rates lie within the simulated order of magnitude and to some extent show the predicted angular dependence. Analysis of the deposited sputter material revealed the formation of metal oxides, which requires a future adaption of the material specific QCM settings. Furthermore, the cooling system of the QCM sensor head was not sufficient, limiting the comparability of results.

Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer.

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.

Direct-current current transformer for the measurement of an electric propulsion ion beam.

An important task in the development of electric propulsion devices is ground-based testing. In order to characterize electric thrusters, their generated ion beams are regarded as figures of merit and have to be well-known and understood. We present an approach to measure DC ion beam currents generated by electric thrusters by means of an inductive direct-current current transformer. In contrast to commonly used devices like Faraday cups, this device can measure the beam current non-intrusively by means of magnetic field coupling rather than charge collection. This paper shows the development of a prototype sensor which can measure currents in the typical range for electric thrusters with high resolution and satisfactory accuracy. A detailed SPICE model to assist during hardware development is also introduced and verified by test case measurements. The prototype has been tested and validated with a radio-frequency ion thruster. Its readout shows very good agreement with the output of an analytical model which computes a Gaussian-shaped ion beam in the far-field based on experimental input data.

[Spiral CT angiography in demonstration of cerebral aneurysms before and after treatment with titanium clips]. / Spiral-CT-Angiographie zur Darstellung von zerebralen Aneurysmen vor und nach Versorgung mit Titan-Clips.

PURPOSE: To determine the suitability of spiral CT angiography (SCTA) in patients with intracerebral titanium aneurysm clips. MATERIAL AND METHOD: The section parameters were optimised using a phantom. 16 patients were examined preoperatively and 18 following treatment of aneurysms with clips. RESULTS: The resolution, sensitivity and specificity for demonstrating cerebral aneurysms measuring > or = 3 mm was 91.3% and 75% respectively. In 15 patients with intracerebral clips there were no or only minor artefacts; three examinations were of no value because of major metal artefacts. Amongst six aneurysms demonstrated angiographically, two, which measured more than 3 mm, could be shown by SCTA. CONCLUSION: SCTA was able to demonstrate intracerebral aneurysms measuring > or = 3 mm. Titanium clips did not usually affect image quality significantly.