Qucosa – Hemholtz-Zentrum Dresden-Rossendorf
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Analytische und experimentelle Untersuchungen zur Modellierung der Fluid-Struktur-Wechselwirkung in einem 2D-Ringspalt
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Advanced emitters and detectors for terahertz time-domain spectroscopy
Get PDFThe idea of terahertz-time-domain spectroscopy (THz-TDS) is to exploit a single cycle, spectrally broad THz radiation pulse to gain insight into the response of matter. Photoconductive devices and nonlinear crystals are utilized in both the generation as well as the coherent detection of THz radiation. The relatively high cost and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems for commercial applications. Er-doped femtosecond fiber lasers operating at 1.55 μm could offer a viable alternative. In this thesis nonlinear crystals and photoconductive emitters are discussed for excitation in the near infrared (NIR) window of between 800 nm to 1550 nm. The main focus of this thesis is a detailed study of substrate materials for an interdigitated photoconductive antenna. Photoconductive antennas with microstructured electrodes provide high electric acceleration fields at moderate voltages because of small electrode separations. The scalability of these devices allows for large active areas in the mm^2 range, which are sufficient for excitation at large optical powers. In comparison with conventional emitter structures, these antennas have more favourable characteristics regarding THz power, spectral properties, and ease of handling. Depending on the utilized substrate material, photoconductive antennas can then be operated using different excitation wavelengths. By employing substrates with short carrier trapping times these antennas can be operated as THz-detectors. Moreover the design of electrode structures for generating radially and azimuthally polarized THz waves are presented. A second topic deals with the signal analysis and signal interpretation of THz pulses transmitted through several material systems. These experiments show the potential for tomographic and spectroscopic applications. The third part deals with THz emission by frequency mixing in nonlinear organic and inorganic crystals. Hereby the focus is on polaritonic phase matching in GaAs. Furthermore, indications of THz tunability by the excitation wavelength were found by utilizing waveguide structures. However, the observed tuning range is much lower then theoretically predicted. Specific reasons for this are discussed
Technologie zur Aktivierung gepreßter Targetmaterialien mit dem Protonenstrahl (12 MeV, 10 µA) des Zyklotrons U-120
Get PDFEs wird eine Technologie für die Aktivierung von gepreßten Pulver-Materialien (bis 500 mg) in einem Protonenstrom (10 µA) beschrieben. Das Verfahren wurde für die Herstellung von 94mTc am Rossendorfer Zyklotron U-120 aus angereichertem 94MoO3-Pulver optimiert. Die Bestrahlung des gepreßten Targetmaterials erfolgt in einem evakuierten und drucküberwachten Targetträger mit dünnem Strahleintrittsfenster. Durch die gute Wärmekopplung des Preßlings mit dem wassergekühlten Targetträger und der heli-umgekühlten Fensterfolie kann das Targetmaterial mit einer absorbierten Strahllei-stung bis zu 40 W belastet werden. Bei einem Folienbruch verhindert der geschlosse-ne Heliumkreislauf das Entweichen von radioaktivem Material. Zur Reduktion der Energieabsorption im Target wird die Dicke des Targetmaterials der Wirkungsquer-schnittsverteilung angepaßt. Nach der Entnahme des Targetträgers aus der Bestrah-lungsanlage verbleibt das Targetmaterial bis zur Aufarbeitung in einem radiochemi-schen Labor in dem geschlossenen Targetträger. The technology described has been developed for safe activation of pressed powder materials (up to 500 mg) in a proton beam (10 µA). The procedure has been optimized for the production of 94mTc with the Rossendorf U-120 cyclotron using enriched 94MoO3. The target material is irradiated inside an evacuated and pressure controlled tar-getholder equipped with a thin entrance window for the proton beam. The target with-stands an absorbed beam power of about 40 W as the result of a good heat coupling between the target material and the water-cooled target holder as well as the helium-cooled entrance window. In the case of a window break the closed helium circuit pre-vents the escape of radioactivity. To diminish the energy absorption in the target the thickness is adapted to the distribution of the cross section. After taking the target holder out of the irradiation equipment, the target remains closed in the target holder
Temperature and void fraction distribution in a side wall heated tank
Get PDFExperiments were performed to investigate heating up processes of fluids in storage tanks under the influence of an external heat source. Storage tanks for fluids are widely used industrial facilities. As a consequence of an external fire, the heat-up of the inventory may lead to the evaporation of the liquid and to release of significant quantities of dangerous gases into the environment. Several boiling tests with water tanks were performed both with heating from the bottom and with heating from the side walls. In different test tanks for the different heating modes the time dependency of the temperature field was determined by thermocouples. In recent tests for investigation of the side wall heating, in addition to the thermocouples the tank was equipped with needle probes for measuring of the local void fraction. The experiments have shown that the liquid inventory behaves very differently depending on the mode of heating. Bottom heating leads to an irregular thermoconvective motion of the liquid, which causes good mixing, so that saturation is reached at all places inside the tank approximately at the same time. The maximum enthalpy of the liquid always remains close to the average value. If the vessel is heated from the side, a stable temperature stratification is observed leading to large temperature gradients. Evaporation can start much earlier than the average temperature reaches saturation. In order to clarify the physical nature of the details of the heating-up and the evaporation process in simple geometrical boundary conditions, a two-dimensional mathematical model was developed, which includes also evaporation and two-phase flow. The measurement of the temperature and of the void fraction makes physical phenomena evident, which could be explained by the own 2D model. The gained experimental results may be used for the validation of boiling models in 3-D CFD codes
Preliminary investigations on high energy electron beam tomography
Get PDFIn computed tomography (CT) cross-sectional images of the attenuation distribution within a slice are created by scanning radiographic projections of an object with a rotating X-ray source detector compound and subsequent reconstruction of the images from these projection data on a computer. CT can be made very fast by employing a scanned electron beam instead of a mechanically moving X-ray source.
Now this principle was extended towards high-energy electron beam tomography with an electrostatic accelerator. Therefore a dedicated experimental campaign was planned and carried out at the Budker Insitute of Nuclear Physics (BINP), Novosibirsk. There we investigated the capabilities of BINP’s accelerators as an electron beam generating and scanning unit of a potential high-energy electron beam tomography device. The setup based on a 1 MeV ELV-6 (BINP) electron accelerator and a single detector.
Besides tomographic measurements with different phantoms, further experiments were carried out concerning the focal spot size and repeat accuracy of the electron beam as well as the detector’s response time and signal to noise ratio
Proceedings of the International Workshop "Innovation Information Technologies: Theory and Practice": Dresden, Germany, September 06-10.2010
Get PDFThis International Workshop is a high quality seminar providing a forum for the exchange of scientific achievements between research communities of different universities and research institutes in the area of innovation information technologies. It is a continuation of the Russian-German Workshops that have been organized by the universities in Dresden, Karlsruhe and Ufa before.
The workshop was arranged in 9 sessions covering the major topics: Modern Trends in Information Technology, Knowledge Based Systems and Semantic Modelling, Software Technology and High Performance Computing, Geo-Information Systems and Virtual Reality, System and Process Engineering, Process Control and Management and Corporate Information Systems