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Karlsruhe Institute of Technology (KIT) is doing research and development in the field of megawatt-class radio frequency (RF) sources (gyrotrons) for the Electron Cyclotron Resonance Heating (ECRH) systems of the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Fusion Power Plant that will follow ITER. In the focus is the development and verification of the European coaxial-cavity gyrotron technology which shall lead to gyrotrons operating at an RF output power significantly larger than 1 MW CW and at an operating frequency above 200 GHz. A major step into that direction is the final verification of the European 170 GHz 2 MW coaxial-cavity pre-prototype at longer pulses up to 1 s. It bases on the upgrade of an already existing highly modular short-pulse (ms-range) pre-prototype. That pre-prototype has shown a world record output power of 2.2 MW already. This paper summarizes briefly the already achieved experimental results using the short-pulse pre-prototype and discusses in detail the design and manufacturing process of the upgrade of the pre-prototype toward longer pulses up to 1 s.
Heavy rain on snow often leads to disastrous damages in torrent watersheds. In January 1998 a project was started to investigate “runoff and infiltration characteristics of different alpine soil/vegetation units under snow cover”. One aim was to determine the runoff rates from snow-covered, differently cultivated soil/vegetation units (pastures, dwarf-shrub stands, forests), especially under conditions of sparse snow cover and frozen soil. Differences in runoff formation between artificially snowed skiing areas and plots with natural snow cover were also of interest. Heavy rain (intensity 100 mm h–1) was simulated on four plots by using a transportable spray irrigation installation. The investigations showed runoff coefficients of 0.4–0.7. The lowest runoff values were found where the soil under snow cover was not frozen, but even on these stands the runoff coefficient exceeded 0.4. Snow depth and runoff delay are strongly correlated (R2 = 0.8). An increment in snow of 10 cm is followed by a runoff delay of 3.6 min. Divergences from this coherence are due to the composition of the snowpack, especially in case of artificial snow. This result is encouraged by runoff simulations performed for the irrigated plots by use of a hydrological model.
A number of laser facilities coming online all over the world promise the capability of high-power laser experiments with shot repetition rates between 1 and 10 Hz. Target availability and technical issues related to the interaction environment could become a bottleneck for the exploitation of such facilities. In this paper, we report on target needs for three different classes of experiments: dynamic compression physics, electron transport and isochoric heating, and laser-driven particle and radiation sources. We also review some of the most challenging issues in target fabrication and high repetition rate operation. Finally, we discuss current target supply strategies and future perspectives to establish a sustainable target provision infrastructure for advanced laser facilities.
In this paper, we describe the development of an International Space Station experiment, BioRock. The purpose of this experiment is to investigate biofilm formation and microbe–mineral interactions in space. The latter research has application in areas as diverse as regolith amelioration and extraterrestrial mining. We describe the design of a prototype biomining reactor for use in space experimentation and investigations on in situ Resource Use and we describe the results of pre-flight tests.
Models of spherical dynamos are considered which involve the full interaction between the magnetic field and the motion of an incompressible conducting fluid. In the basic equations magnetic field and fluid velocity are expanded in series of certain decay modes. In this way these equations are reduced to an infinite set of ordinary first-order differential equations for the coefficients of these expansions. The behaviour of dynamos can then be studied by integrating a finite set of these equations numerically. Some first results obtained in this way are presented for mean-field models in which the growth of the magnetic field due to the α–effect is limited by large-scale motions generated by Lorentz forces.
We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80–200 keV electron beams.
The role of self generated magnetic fields in the transport of a heat wave following a nanosecond laser irradiation of a solid target is investigated. Magnetic fields are expected to localize the electron carrying the heat flux but at the same time are affected in their evolution by the heat flux itself. We performed simultaneous measurements of heat wave propagation velocity within the target and magnetic fields developing on the target surface. These were compared to results obtained by numerical magneto-hydrodynamic modeling, including self-generated B fields. The comparison shows that longitudinal heat flow is overestimated in the simulations. Similarly, but most notably, the radial expansion of the magnetic fields is underestimated by the modeling. The two are likely linked, the more pronounced radial drift of B-fields induces a rotation of heat flux in the radial direction, and corresponding longitudinal heat flux inhibition. This suggests the need for improving present modeling of self-generated magnetic fields evolution in high power laser-matter interaction.
As physical activity may modify the effect of the apolipoprotein E (APOE) ε4 allele on the risk of dementia and Alzheimer's disease (AD) dementia, we tested for such a gene–environment interaction in a sample of general practice patients aged ⩾75 years.
Data were derived from follow-up waves I–IV of the longitudinal German study on Ageing, Cognition and Dementia in Primary Care Patients (AgeCoDe). The Kaplan–Meier survival method was used to estimate dementia- and AD-free survival times. Multivariable Cox regression was used to assess individual associations of APOE ε4 and physical activity with risk for dementia and AD, controlling for covariates. We tested for gene–environment interaction by calculating three indices of additive interaction.
Among the randomly selected sample of 6619 patients, 3327 (50.3%) individuals participated in the study at baseline and 2810 (42.5%) at follow-up I. Of the 2492 patients without dementia included at follow-up I, 278 developed dementia (184 AD) over the subsequent follow-up interval of 4.5 years. The presence of the APOE ε4 allele significantly increased and higher physical activity significantly decreased risk for dementia and AD. The co-presence of APOE ε4 with low physical activity was associated with higher risk for dementia and AD and shorter dementia- and AD-free survival time than the presence of APOE ε4 or low physical activity alone. Indices of interaction indicated no significant interaction between low physical activity and the APOE ε4 allele for general dementia risk, but a possible additive interaction for AD risk.
Physical activity even in late life may be effective in reducing conversion to dementia and AD or in delaying the onset of clinical manifestations. APOE ε4 carriers may particularly benefit from increasing physical activity with regard to their risk for AD.
Whether late-onset depression is a risk factor for or a prodrome of dementia remains unclear. We investigated the impact of depressive symptoms and early- v. late-onset depression on subsequent dementia in a cohort of elderly general-practitioner patients (n = 2663, mean age = 81.2 years).
Risk for subsequent dementia was estimated over three follow-ups (each 18 months apart) depending on history of depression, particularly age of depression onset, and current depressive symptoms using proportional hazard models. We also examined the additive prediction of incident dementia by depression beyond cognitive impairment.
An increase of dementia risk for higher age cut-offs of late-onset depression was found. In analyses controlling for age, sex, education, and apolipoprotein E4 genotype, we found that very late-onset depression (aged ⩾70 years) and current depressive symptoms separately predicted all-cause dementia. Combined very late-onset depression with current depressive symptoms was specifically predictive for later Alzheimer's disease (AD; adjusted hazard ratio 5.48, 95% confidence interval 2.41–12.46, p < 0.001). This association was still significant after controlling for cognitive measures, but further analyses suggested that it was mediated by subjective memory impairment with worries.
Depression might be a prodrome of AD but not of dementia of other aetiology as very late-onset depression in combination with current depressive symptoms, possibly emerging as a consequence of subjectively perceived worrisome cognitive deterioration, was most predictive. As depression parameters and subjective memory impairment predicted AD independently of objective cognition, clinicians should take this into account.
The discovery of strong visible photoluminescence at room temperature from porous silicon has triggered new hope that light-emitting devices compatible with existing Si-technology might become possible. We first review the luminescence behavior observed in silicon-based materials such as amorphous Si, microcrystalline Si, or SiO2. We then critically discuss the present model for the luminescence from porous silicon based on quantum confinement in view of the growing experimental evidence for the importance of both hydrogen and oxygen to obtain efficient luminescence from this material. We propose an alternative explanation based on the presence of siloxene (SieO3H6) in porous silicon which is corroborated by experimental results obtained with photoluminescence, Raman and IR spectroscopy. An important aspect is that siloxene can be prepared by methods different from anodic oxidation, and one particular technique will be described together with possible ways to tune the luminescence energy.
Let G be a group and let A be a fixed point free group of automorphisms of G. It is shown that the centraliser near-ring MA(G) has at most one nontrivial ideal. Conditions on the pair (A, G) are given which force MA(G) to be simple. It is shown that if a nonsimple near-ring MA(G) exists, then A and G have unusual properties.
It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In the last decennium more and more evidence has been found that the observed mix of small and complex, stable and highly transient species in space is the cumulative result of gas phase and solid state reactions as well as gas-grain interactions. Solid state reactions on icy dust grains are specifically found to play an important role in the formation of the more complex “organic” compounds. In order to investigate the underlying physical and chemical processes detailed laboratory based experiments are needed that simulate surface reactions triggered by processes as different as thermal heating, photon (UV) irradiation and particle (atom, cosmic ray, electron) bombardment of interstellar ice analogues. Here, some of the latest research performed in the Sackler Laboratory for Astrophysics in Leiden, the Netherlands is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions and vacuum ultraviolet irradiation of interstellar ice analogues at astronomically relevant temperatures. It is shown that solid state processes are crucial in the chemical evolution of the interstellar medium, providing pathways towards molecular complexity in space.
The optical properties of porous Si (p-Si) are compared to those of siloxene and its derivatives in order to gain more insight into the mechanism of the luminescence observed in p-Si. We report new results of photoluminescence (PL), photoluminescence excitation (PLE), time-dependent and pressure-dependent photoluminescence, and optically detected magnetic resonance (ODMR). Important information about the structural, electronic, and microscopic nature of the two classes of materials are deduced from these experiments. Annealed siloxene and p-Si show very similar properties, suggesting that siloxene-related structures, e.g. electrically isolated Si6-rings, might be responsible for the luminescence in p-Si. The Si-planes in as-prepared siloxene, with their green luminescence, are metastable and are readily oxidized into red-luminescent siloxene configurations.
We compare, in this study, the photoassisted processes for silicon deposition using both a low pressure mercury lamp and an ArF excimer laser for the specific case where the SiH4 gas is sealed in the reaction chamber.
Even though GaAs (110) is the only semiconductor whose surface structure is known with confidence, little is known about its microscopic growth mechanisms. We have used RHEED to study the role of steps in the MBE growth of GaAs on vicinal GaAs(11O) surfaces which were misoriented by less than 2 mrad. After thermally desorbing the initial oxide, 20 layers of GaAs deposited at 700K produced a surface with single atomic-layer steps having an average terrace length of a few hundred Angstroms. Upon annealing to 8OOK, a slow mass migration occurred producing a surface with one thousand Angstrom average terrace lengths and predominantly double layer step heights. The RHEED pattern was nearly instrument limited at in-phase angles of incidence, with little background intensity and bright Kikuchi lines. Subsequent deposition showed only weak oscillations in the RHEED intensity, in contrast to growth on the (001) surface. The period of the observed oscillations indicates that the layer-by-layer growth involves single-layer steps. Growth of as little as 5 atomic layers on a surface with double steps could not be annealed to give a RHEED intensity as great as the first annealed surface. These measurements reconcile previous LEED results with the oxygen adsorption measurements of Ranke. The results clearly show the dominance of steps in the formation of RHEED streaks.
The textured oriented overgrowth (epitaxy) of certain metals evaporated onto substrates consisting of highly oriented ultra thin polymer films of polyethylene (PE), polypropylene (PP) and polybutene-1 (PB-1) has been well known since a few years. However, the origin of the observed epitaxy is not clear at all: graphoepitaxy (i. e. orientation induced by nucleation onto oriented topographic features of the substrate), the formation of a chemical layer (i. e. of metal-methyl groups building up the polymer-metal interface, or simply classic epitaxy (i.e. lattice matching) seem all to be possible explanations for the observed orientations. Here, we used Transmission Electron Microscopy (TEM) and Scanning Force Microscopy (SFM) to investigate the possibility of graphoepitaxial growth on polymeric surfaces. Our investigations show that the morphology of polymeric bulk material determine the topographic properties observed at the polymer surface. The semicrystalline nature of the polymer films leads to polymeric surface steps which are suitable as locations for graphoepitaxial growth. Artificial epitaxy (graphoepitaxy) seems to be the most likely orientation mechanism.
Superparamagnetic behavior is characterized by a thermally fluctuating vector of magnetization, leading to magnetization curves free of hysteresis: it is a property of isolated ferrite particles with sizes below ca. 10 nm. These particles fulfil the condition Kv < kT with K … energy of unisotropy, v … volume of the particle, kT has the usual meaning. To produce a superparamagnetic macroscopic part it is necessary to avoid the interaction of the particles. This can be achieved by coating the particles with a second non-magnetic phase. This special material can be synthesized using the microwave plasma process. Because of the specific interaction of charged particles with an oscillating electrical field, microwave plasmas excel in relatively low reaction temperatures. The low reaction temperature and the electrical charging of the particles in the plasma reduce the probability of agglomeration. Therefore, it is possible to pass the gas stream with the as produced particles through a second reaction zone, where the particles will be coated with a polymer. The thickness of the coating can be adjusted in the range from 1 to 5 nm. The composition of the ferrite kernel is selected either for maximum susceptibility or maximum frequency of application. They may consist either of maghemite, manganese-, manganese-zinc-, or magnesium iron spinell for maximum susceptibility or cobalt-iron spinelle type material for maximum frequency. Polymer coated ferrite nanoparticles can be consolidated by hot pressing with temperatures around 100 °C. Superparamagnetism is shown by static magnetic measurements and Moessbauer spectrometry. Dynamic measurements of the complex susceptibility show interesting properties up to frequencies in the gigahertz range.
A non-contact and non-destructive laser-based acoustic technique called impulsive stimulated thermal scattering (ISTS) is used to measure thicknesses of films in single-layer and multilayer assemblies such as W/Si, Ti/Si, and Ti/SiO2/Si structures. Thicknesses are determined to within a few percent accuracy with a laboratory version of the measurement apparatus, using conventional large-frame lasers and optics, and in a commercial prototype using compact diode-pumped and diode lasers and optics which all fit onto a 1-ft × 2-ft breadboard. ISTS and conventional measurements (profilometry, SEM, and 4-point resistance) are made on the same samples and the results are found to compare favorably.