Determination of the distribution of air and water in porous media by electrical impedance tomography and magneto-electrical imaging

Monitoring the distribution of water content is essential for understanding hydrological processes in the lithosphere and the pedosphere. The movement of water in unsaturated rock formations and in the vadose zone is influenced by different processes (mainly infiltration, evaporation, percolation and capillary flow) which may be rate determining depending on the actual conditions. The interdependence of these processes also strongly influences the transport and distribution of solutes in the pore space. In order to gain a better understanding of the movement and distribution of water in unsaturated media, systematic investigations with non-invasive or minimal invasive methods appear to be most suitable. Studies on the distribution of electrical conductivity can improve risk analysis concerning waste disposals in general and nuclear waste repositories in particular. Induced polarization and magnetic flux density determined with two highly sensitive accessories yield additional information and may allow for better discrimination of coupled flow processes. Electrical impedance tomography (EIT) with 20 current injection and 48 voltage electrodes was used here to monitor the evaporation of tap water from a container filled with sand under laboratory conditions at 20 °C. The results are compared with data obtained by determining spectral induced polarization (SIP) of sand during desaturation in a multi-step outflow equipment. Infiltration processes and evaporation from sand saturated with 0.01 M CaCl₂ were determined by magneto-electrical resistivity imaging technique (MERIT). The results were obtained from a long-term experiment under controlled conditions.     

Nanosecond Switching and Wavelength Tuning of External-Cavity Laser Diode Using a Reflective Electroabsorption Modulator

We present a novel technique for ultrafast on-off switching and wavelength tuning of an external-cavity laser diode (ECLD) using an intracavity reflective electroabsorption modulator array as the end mirror. on-off and wavelength switching of the ECLD are based upon the electrically induced control of the modulator's reflectivity yielding a modulation of the cavity losses and hence a modulation of the lasing threshold. We experimentally demonstrate on-off switching of selected wavelengths with a contrast in excess of 40 dB. Ultrafast modulation in the nanosecond-regime has been achieved which is close to the fundamental physical speed limit of the ECLD.     

THE PRESENCE OF ABSENCE

The photo artists Michael Reisch and Andreas Gefeller are visualizing environments, man-made landscapes and architectural surroundings that could be perceived as standing in the tradition of documentary landscape and architectural photography or as being created for scientific use. Subtle visual shifts provoke moments of confusion and exceed the physical possibilities of the viewer’s perception in an imaginable extra-pictorial reality. By examining interlacing levels of realities — evoked by picture editing processes of digital and analog photographs — these phenomena will be denominated as Impossible Realities. Both artists practice perplexing “games” with distance and nearness, perceptible “mistakes” and seemingly “hygienic” views, leading viewers to doubt the authenticity — in terms of being not manipulated — of the photographs. The subjects hover between apocalyptic doubts and utopian perceptions of the ideal, stimulating reflections on preshaped ideas and concepts of pure nature, on human-influenced environmental changes and the charged relationship between urban environments and seemingly unspoiled nature (the very present sense of human absence plays an important role). Through the photographic process and the postproduction the images as ideas/imaginations and as material become hybrid constructions that could symbolically stand for the impossible but imaginable realities both artists evoke, varying between documentation and imagination.     

Micromirror device controlled tunable diode laser diode laser

An electronically controlled external cavity laser diode for fast wavelength tuning and multiple-colour lasing is presented. The spectral-tuning is accomplished by a digital micromirror device. Tuning of 47.4 nm with a tuning speed of 0.85 nm/ms was achieved. Simultaneous emission of up to 9 laser lines is demonstrated     

Design,Manufacture, and Characterization of a Carbon Fiber‐Reinforced Silicon Carbide Nozzle Extension

Nozzle extensions made of ceramic matrix composites (CMCs) have shown the potential to replace heavy superalloy nozzles and improve the performance of future upper‐stage and orbital rocket engines. Gas permeability has been reported to be a critical issue during the manufacture for CMC nozzles. This work shows the manufacture of a dense radiation‐cooled C/C‐SiC nozzle demonstrator. A multi‐angle fiber architecture was applied using filament winding technique to reduce the incidence of delaminations during the manufacturing process under high temperatures. Additional efforts were made to improve the final gas tightness and reduce the amount of residual silicon by means of an adapted liquid silicon infiltration process. The manufacture, the material, and structural characterization as well as a finite element analysis of a performed internal pressure test are presented.     

Compact x-ray microradiograph for in situ imaging of solidification processes: bringing in situ x-ray micro-imaging from the synchrotron to the laboratory

A laboratory based high resolution x-ray radiograph was developed for the investigation of solidification dynamics in alloys. It is based on a low-power microfocus x-ray tube and is potentially appropriate for x-ray diagnostics in space. The x-ray microscope offers a high spatial resolution down to approximately 5 μm. Dynamic processes can be resolved with a frequency of up to 6 Hz. In reference experiments, the setup was optimized to yield a high contrast for AlCu-alloys. With samples of about 150 μm thickness, high quality image sequences of the solidification process were obtained with high resolution in time and space.     

Continuous-wave THz imaging

The first THz imaging spectrometer based on continuous-wave THz radiation is presented. The new setup is less expensive than conventional time-domain imaging systems that comprise femtosecond lasers. The system uses a two-colour external-cavity laser diode. Hence it is much more compact as compared to systems based on optically pumped solid-state lasers. As an example we investigate a human liver with metastasis