Electromagnetic radiation in the frequency range between about 100 GHz and 5 THz can be used for spectroscopy and microscopy, but it is also promising for security screening and even wireless communication. In the present thesis a planar photoconducting large-area THz radiation source is presented. The device exhibits outstanding properties, in particular high THz field strength and generation efficiency and large spectral bandwidth with short THz pulse length. The THz emission is based on acceleration and deceleration of photoexcited carriers in semiconductor substrates. A metallic interdigitated structure at the surface of semi-insulating GaAs provides the electrodes of an Auston switch. In a biased structure photoexcited charge carriers are accelerated. Hence electromagnetic waves are emitted. An appropriately structured second metallization, electrically isolated from the electrodes, prevents destructive interference of the emitted waves. The structure investigated here combines several advantages of different conventional photoconducting THz sources. First, it provides high electric acceleration fields at moderate voltages owing to the small electrode separation. Second, the large active area in the mm2 range allows excitation by large optical powers of some mW. Optical excitation with near-infrared femtosecond lasers is possible with repetition rates in the GHz range. The presented results point out the excellent characteristics regarding the emitted THz field strength, average power, spectral properties, and easy handling of the interdigitated structure in comparison to various conventional emitter structures. Various modifications of the semiconductor substrate and the optimum excitation conditions were investigated. In the second part of this thesis the dynamic conductivity of GaAs/AlxGa1-xAs superlattices in an applied static electric field was investigated with time-resolved THz spectroscopy. The original goal was to explore whether the predicted effect of gain of electromagnetic radiation at THz frequencies is present in such structures. Superlattice samples were grown according to the experimental requirements, which include high specific resistance and sufficient THz transparency. The characterization of the superlattices by Fourier transform infrared spectroscopy and photoluminescence spectroscopy confirms the pronounced miniband properties of the bandstructure. Furthermore indications of Bloch oscillations were found by transport measurements. However, we could not measure a change of the dynamic conductivity when the electric field was toggled. Specific reasons for this and related experiments of other groups are discussed. / Elektromagnetische Strahlung im Frequenzbereich zwischen etwa 100 GHz und 5 THz wird für verschiedene Anwendungen wie Spektroskopie und Mikroskopie genutzt, kann aber auch für Sicherheitstechnik oder sogar Datenübertragung interessant sein. In der hier vorgestellten Forschungsarbeit wird eine großflächige fotoleitende THz-Strahlungsquelle beschrieben, die sich durch eine große THz-Feldstärke und große spektrale Bandbreite auszeichnet. Die THz-Emission basiert auf der Beschleunigung und Verzögerung fotogenerierter Ladungen in Halbleitersubstraten. Eine metallische Interdigitalstruktur auf der Oberfläche von semi-isolierendem GaAs bildet die Elektroden eines Fotoschalters. Ist an diese Struktur eine Spannung angeschlossen, werden optisch generierte Ladungsträger beschleunigt und strahlen elektromagnetische Wellen ab. Eine geeignet strukturierte und isolierte zweite Metallisierung verhindert destruktive Interferenzen der abgestrahlten Wellen. Die vorgeschlagene Struktur vereinigt dabei die Vorteile verschiedener herkömmlicher fotoleitender THz-Quellen. Einerseits ermöglicht der kleine Elektrodenabstand große elektrische Felder zur Beschleunigung fotogenerierter Ladungen schon bei moderaten Spannungen. Andererseits kann die große aktive Fläche von einigen mm2 mit großen optischen Leistungen im Bereich einiger mW angeregt werden. Die optische Anregung mit Nahinfrarot-Femtosekunden- Lasern kann mit Wiederholraten bis in den GHz-Bereich geschehen. Bedingt durch die Eigenschaften der Anregungspulse entstehen kurze spektral breite THz-Pulse. Die vorliegenden Ergebnisse verdeutlichen die hervorragenden Eigenschaften der Interdigitalstruktur im Vergleich zu verschiedenen herkömmlichen Geometrien bezüglich der Feldstärke der abgestrahlten Wellen, der mittleren Leistung und der spektralen Eigenschaften. Dabei ist die Struktur sehr einfach zu handhaben. Es wurden verschiedene Modifikationen des Substrates und die optimalen Bedingungen der optischen Anregung untersucht. Der zweite Teil dieser Arbeit behandelt die Erforschung der dynamische Leitfähigkeit von GaAs/AlxGa1-xAs-Übergittern in Abhängigkeit von einem elektrischen Feld mit Hilfe der zeitaufgelösten THz-Spektroskopie. Es sollte geklärt werden, ob der vorhergesagte Effekt der Verstärkung elektro-magnetischer Strahlung in solchen Strukturen möglich ist. Dazu wurden Übergitterproben gemäß den experimentellen Anforderungen hergestellt. Zu den Vorgaben gehört ein hoher spezifischer Widerstand und ausreichende Transparenz im THz-Bereich. Die Charakterisierung der Übergitter mit Fotolumineszenz- und Fourier-Transformations-IR-Spektroskopie bestätigte die ausgeprägten Minibandeigenschaften der Bandstruktur. Hinweise auf Bloch-Oszillationen wurden durch Ladungstransportmessungen gefunden. Dennoch war eine Änderung der dynamischen Leitfähigkeit beim Schalten des elektrischen Feldes nicht messbar. Gründe dafür und ähnliche Experimente anderer Gruppen werden diskutiert.

In Durchlauföfen zur Erwärmung von Gütern werden die Materialien zu einem großen Teil entweder auf Schienen und Hubbalken oder mittels Transportrollen durch die Öfen transportiert. Die Transportrollen sind auf den ersten Blick ein Widerstand bezüglich des Wärmeübergangs aus dem Ofenraum unterhalb der Rollen an die Unterseite der Güter. Eine Vergleichsmessung zeigte allerdings, dass die Stoffe auf den Transportrollen durch die zusätzliche Wärmeabgabe der Transportrollen an die Unterseite der Werkstoffe schneller erwärmt werden als auf Schienen. Im Rahmen dieser Arbeit wurde ein mathematisches Modell entwickelt, das die Guterwärmung in einem Rollenofen abbildet. Nach der experimentellen Verifizierung des mathematischen Modells wurden die Transportrollenparameter ermittelt, die die Guterwärmung in einem Rollenofen beschleunigen. Je geringer Rollendurchmesser und Rollenteilung sind und je höher die Gutgeschwindigkeit ist, umso mehr verbessern Rollen die Guterwärmung.

Darmstadt, Techn. Universiẗat, Diss., 2007.

Darmstadt, Techn. Universiẗat, Diss., 2007.

Zugl.: Stuttgart, Univ., Diss., 2007.

No description available.

Synchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation. These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields. This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments. Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters. Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated. In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation. Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken. The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy. The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses. In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations. A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields.

The advantages of contemporary particle injectors are high bunch charges and good beam quality in the case of normal conducting RF guns and increased repetition rates in the one of DC injectors. The technological edge of the concept of superconducting radio frequency injectors is to combine the strengths of both these sides. As many future accelerator concepts, such as energy recovery linacs, high power free electron lasers and certain collider designs, demand particle sources with high bunch charges and high repetition rates combined, applying the superconductivity of the accelerator modules to the injector itself is the next logical step. However, emittance compensation — the cornerstone for high beam quality — in case of a superconducting injector is much more challenging than in the normal conducting one. The use of simple electromagnets generating a solenoid field around the gun’s resonator interferes with its superconducting state. Hence, it requires novel and sophisticated techniques to maintain the high energy gain inside the gun cavity, while at the same time alleviating the detrimental fast transverse emittance growth of the bunch. In the case of the ELBE accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, a superconducting electron accelerator provides beam for several independent beamlines in continuous wave mode. The applications include IR to THz free electron lasers, neutron and positron generation, to Thompson backscattering with an inhouse TW laser, and hence, call for a flexible CW injector. Therefore, the development of a 3.5 cell superconducting electron gun was initiated in 1997. The focus of this thesis lies on three approaches of transverse emittance compensation for this photoinjector: RF focusing, the installation of a superconducting solenoid close to the cavity’s exit, and the introduction of a transverse electrical mode of the RF field in the resonator. All three methods are described in theory, examined by numerical simulation, and experimentally reviewed in the particular case of the ELBE SRF Gun II at HZDR and a copy of its niobium resonator at Thomas Jefferson National Laboratory, Newport News, VA, USA.

This thesis is devoted to the investigation of the phenomenon of quantum radiation -- i.e. the conversion of the (virtual) quantum fluctuations of a quantised field into (real) particles owing to the influence of external conditions. For that purpose a canonical particle (and thereby vacuum) definition is presented for a quantum field in the presence of specific external conditions. Utilising this set-up the number of Rindler particles in the Minkowski vacuum is calculated explicitly where the Unruh effect is recovered. Focusing on the gravitational collapse of an object the number of created particles accounting for the Hawking effect is derived and the dependence of the results on the dynamics of the collapse is discussed. Furthermore the influence of finite initial temperatures is investigated for a weakly time-dependent perfectly conducting cavity (dynamical Casimir effect), a dynamical dielectric medium, and the Friedmann-Robertson-Walker metric. Finally the problems arising from the consideration of interacting fields are outlined by means of a simple example.

Ship tracks are regarded as the most obvious manifestations of the effect of anthropogenic aerosol particles on clouds (indirect effect). However, it is not yet fully quantified whether there are climatically relevant effects on large scales beyond the narrow ship tracks visible in selected satellite images. A combination of satellite and reanalysis data is used here to analyze regions in which major shipping lanes cut through otherwise pristine marine environments in subtropical and tropical oceans. We expect the region downwind of a shipping lane is affected by the aerosol produced by shipping emissions but not the one upwind. Thus, differences in microphysical and macrophysical cloud properties are analyzed statistically. We investigate microphysical and macrophysical cloud properties as well as the aerosol optical depth and its fine-mode fraction for the years 2005–2007 as provided for by retrievals of the two Moderate Resolution Imaging Spectroradiometer instruments. Water-cloud properties include cloud optical depth, cloud droplet effective radius, cloud top temperature, and cloud top pressure. Large-scale meteorological parameters are taken from ERA-Interim reanalysis data and microwave remote sensing (sea surface temperature). We analyze the regions of interest in a Eulerian and Lagrangian sense, i.e., sampling along shipping lanes and sampling along wind trajectories, respectively. No statistically significant impacts of shipping emissions on large-scale cloud fields could be found in any of the selected regions close to major shipping lanes. In conclusion, the net indirect effects of aerosols from ship emissions are not large enough to be distinguishable from the natural dynamics controlling cloud presence and formation.