Electron microscopy and trams: Towards a friendly coexistence

Abstract

Electron microscope (EM) imaging capabilities are reduced upon exposure to environmental disturbances like mechanical vibrations and slowly varying magnetic fields, which necessitates stringent threshold limits on these environmental factors, e.g., ≤25-100 nT for magnetic fields [1]. EM labs, often situated at universities, are traditionally found in- or close-to city centres with rather “extreme” electromagnetic conditions. Tram lines, representing an efficient and powerful public transport mode, are DC powered, contributing to that large scale low-frequency, magnetic field fluctuations found in dense urban areas (~400 nT @ 50 m), and by that potentially diminishing imaging- and analytical performance of EMs, see Fig. 1a. Even though there are compensation systems at the instrument side available, they exhibit reduced damping factors when it comes to strong field gradients, are costly and difficult to retrofit. Similar on the tram side, battery- and ground-level power supply based systems can reduce the field emissions but they are also costly and demand specialized tramcars. An effective and economic solution is a passive system that strongly reduces the area of the current loop produced by the catenary and the rails using a special arrangement of bypass cables and additional supply points on the catenary [2,3], see Fig. 1b. By adopting this approach to the dense urban setting of Vienna simulations and calibration tests show that a maximum field of 20 nT per tram at a distance of 100 m should be achievable, see Fig. 2a/b. This “Viennese” variant of a passive field reduction system for tram lines may guarantees that urban public transport demands no longer interfere with researchers’ desire for electromagnetic quietness.