Ka Range Research Articles

Effect of Impact Toughness Anisotropy on Brittle Fracture Resistance Characteristics of High-Strength Steels Subjected to Thermomechanical Treatment

The structure and mechanical properties under tension and impact bending of eleven batches of high-strength manganese-containing steels (Mn-steels) containing 0.09 to 0.14 wt.% Ti subjected to thermomechanical rolling were studied in the temperature range from –60 to +20 °С. It was found that the values of the coefficient of impact toughness anisotropy in the range of Ka = 2.0–4.9 increase at higher titanium content and decrease at higher aluminum content. The difference between ductile-to-brittle transition temperatures T50 and T34 for longitudinal and transverse KCV samples increases at higher contents of titanium, aluminum, sulfur, and carbon. This effect is caused by an earlier nucleation and growth of large dimples of ductile fracture around sizable inclusions preferentially located in the direction of rolling.

Asynchronous Magnet–Stator Topologies in a Squirrel-Cage Superconducting Dynamo

For HTS dc magnet applications, a large proportion of the cooling load on the cryocooler is imposed by the external current leads. Removing the need for these current leads would allow these systems to be built using smaller, cheaper cryocoolers and reduce the ongoing operating costs. This can be achieved by using an HTS superconducting dynamo as a fully superconducting dc current source. Superconducting dynamos remove the need for external current leads by providing a wireless method of injecting dc current into a closed superconducting circuit. Most previous work on HTS dynamos has considered devices employing just one coated conductor stator tape, but dynamos employing multiple parallel stator tapes can achieve substantially higher output currents. In this study, we present an HTS dynamo design in which eight parallel HTS stator tapes are arranged in a cylindrical geometry, excited by an axially concentric rotor carrying multiple equally spaced Nd-Fe-B magnets. Here, we focus on the differing behavior observed between the synchronous case when the number of magnet nm is a factor of the stator number ns = 8 (i.e., nm = 1, 2, 4, 8) and the asynchronous case when it is not (i.e., nm = 3, 5). Our results indicate that both synchronous and asynchronous configurations can exhibit output short-circuit currents of up to ~1.3 kA. In both cases, a relative drop in performance is observed for higher values of nm. We find that the asynchronous magnet arrangement does not greatly improve the dynamo performance compared to the synchronous arrangement. At present, the thermal load associated with conduction-cooled current leads has prevented the practical realization of cryocooled HTS magnets in the kilo-amp (kA) range. However, the dynamo device reported here now offers a practically feasible route to novel kA-class portable cryocooled HTS magnet systems.

Filtered Reaction Rate Modelling in Moderate and High Karlovitz Number Flames: an a Priori Analysis

Direct numerical simulations (DNS) of statistically planar flames at moderate and high Karlovitz number (Ka) have been used to perform an a priori evaluation of a presumed-PDF model approach for filtered reaction rate in the framework of large eddy simulation (LES) for different LES filter sizes. The model is statistical and uses a presumed shape, based here on a beta-distribution, for the sub-grid probability density function (PDF) of a reaction progress variable. Flamelet tabulation is used for the unfiltered reaction rate. It is known that presumed PDF with flamelet tabulation may lead to over-prediction of the modelled reaction rate. This is assessed in a methodical way using DNS of varying complexity, including single-step chemistry and complex methane/air chemistry at equivalence ratio 0.6. It is shown that the error is strongly related to the filter size. A correction function is proposed in this work which can reduce the error on the reaction rate modelling at low turbulence intensities by up to 50%, and which is obtained by imposing that the consumption speed based on the modelled reaction rate matches the exact one in the flamelet limit. A second analysis is also conducted to assess the accuracy of the flamelet assumption itself. This analysis is conducted for a wide range of Ka, from 6 to 4100. It is found that at high Ka this assumption is weaker as expected, however results improve with larger filter sizes due to the reduction of the scatter produced by the fluctuations of the exact reaction rate.

Simulation and Development of Rogowski Coil for Lightning Current Measurement

The dielectric strength design of a load device is very important for preventing and reducing accidents caused by lightning current. The size of a lightning current is accurately measured to calculate impulse withstand voltage for designing dielectric strength. An electric current sensor should be designed to secure accuracy and economic viability and in this paper, Rogowski coil thus was selected to measure lightning currents in the 100 kA range. This paper analyzed design elements to make a Rogowski coil, with the help of a MATLAB simulation and parameter values were estimated according to the coil structure (coil diameter in fixed external diameter, number of turns, etc.). In the process, a passive integrator was applied while employing an RC element, and a program-based low pass-filter was implemented to remove noise occurring at the initial part of the current waveform. Experiments were also conducted up to the 8/20 μs 100 kA range with an impulse generator. This paper verifies that the output value of the Rogowski coil is accurate as it stood within 5%, when compared with the standard CT.

Hydrogen Bonding to a Dinitrogen Complex at Room Temperature: Impacts on N2 Activation.

We report an experimental and computational analysis of the effects of hydrogen bonding to a metal dinitrogen complex. A series of H-bond donors over a wide p Ka range (Δ 20) interact with the nitrogen unit of a ReI-(N2) complex at room temperature. Analysis by 15N NMR, IR spectroscopy, association equilibria, and DFT studies indicates that the H-bonding interaction polarizes and weakens the N-N bond. These results provide insight into the role of the secondary sphere residues in nitrogenase enzymes.

Exploring multiple-aliquot methods for quartz violet stimulated luminescence dating

Abstract In the pursuit of extending the dating range of quartz, violet stimulated luminescence (VSL) has been proposed as a possible candidate. VSL samples traps deeper than those accessible by blue stimulated luminescence (BSL), the latter often limited to the past 100 ka. This study investigates 51 samples from three different type sections in the central part of the Chinese Loess Plateau with depositional ages between 1 and 2500 ka. Results indicate that the natural VSL signal grows to ∼2700 Gy, is best fitted with a double saturating exponential plus a constant, and its shape can be reproduced with the MAAD protocol. Using a ∼900 ka sample, the bleaching decay is recorded over 53 days of light exposure in a solar simulator to further understand the origin of the VSL signal, and to test a multiple-aliquot regenerative (MAR) protocol. The results of the MAAD and the MAR protocols are evaluated against the independent age control. For samples between 20 and 900 ka, 74% were in agreement with expected age using the MAAD protocol. The MAR protocol was tested by building dose response curves (DRCs) using two samples with ages of ∼75 ka and ∼900 ka. Ages calculated by interpolating natural signals onto these two DRCs for all samples within the same 20–900 ka range show 45% and 81% agreement, respectively, with independent age control. The MAR protocol obtained a ∼900 ka VSL age in agreement with independent age control within 1σ of joint uncertainties.

Геофизические модельные функции зависимости микроволнового излучения холодного океана от скорости ветра на частотах K- и Ka-диапазонов для угла 55°

Геофизические модельные функции зависимости микроволнового излучения холодного океана от скорости ветра на частотах K- и Ka-диапазонов для угла 55°

Modified surfaces to enhance vertical falling film heat transfer – An experimental and numerical study

In this paper we investigate experimentally and numerically the potential improvement of heat transfer in falling film units by introducing modifications of the heat transfer surfaces. Two different modified surfaces are investigated and the obtained results are compared to the ones acquired on a smooth surface. In our experiments we work with a broad range of operating conditions with Kapitza and Reynolds numbers in the range of Ka = 167–574 and Re 18–423, respectively. We compare local heat transfer measurements conducted at multiple positions and relate them to the film thickness measurements conducted by a laser triangulation scanner. Our experiments identify both cases where the introduction of modified surfaces significantly enhances the heat transfer and conditions where such a measure does not have a significant impact. The observed differences are compared to numerical simulations providing details of the flow structures in the liquid film in different cases. For that purpose, we directly solve the full Navier-Stokes equations in two dimensions, using the volume of fluid (VOF) numerical framework. Our results reveal that the heat transfer can be enhanced by a factor of up to 2.5 due to appearance of significantly enlarged time-dependent recirculation zones occurring behind the surface modifications, as opposed to such zones present in falling film units with smooth heat-transfer surfaces.

Photodriven Deprotonation of Alcohols by a Quinoline Photobase.

Control of proton transfer is relevant to many areas in chemistry, particularly in catalysis where the kinetics of (de)protonation reactions are often rate limiting. Photobases, which are molecules with enhanced basicity in the excited state, allow for control of proton transfer with light and have the potential to be used as functional units in catalytic systems. Alcohols are the feedstock in many catalytic reactions, where their deprotonation or dehydrogenation is often important. We report that the photobase 5-methoxyquinoline can deprotonate a series of alcohols upon excitation by light. We measure both the thermodynamic limits and the relevant kinetics of this process. A series of alcohols and water spanning the p Ka range of 12.5-16.5 were used as the proton donors. First, we show evidence from absorption and emission spectroscopy that photoexcited 5-methoxyquinoline deprotonates all donors more acidic than methanol and fails to deprotonate donors that are more basic. Interestingly, in methanol a quasi-equilibrium between the protonated and unprotonated forms of the photobase is established in the excited state, suggesting that the excited state p Ka of the photobase is near the p Ka of methanol (15.5). Second, using ultrafast transient absorption spectroscopy, we find that the time constants for excited state proton transfer range from a few picoseconds to tens of picoseconds, with faster speeds for the more acidic donors. Such a correlation between the thermodynamic drive and kinetics suggests that the same mechanism is responsible for proton transfer throughout the series. These results are necessary fundamental steps for applying photobases in potential applications such as deprotonation of alcohols for catalytic and synthetic purposes, optical regulation of pH, and transfer of protons in redox reactions.

Multi probes measurements at the PALS Facility Research Centre during high intense laser pulse interactions with various target materials

During the interaction of high intense laser pulse with solid target, a large amount of hot electrons is produced and a giant Electromagnetic Pulse (EMP) is generated due to the current flowing into the system target–target holder, as well as due to the escaping charged particles in vacuum. EMP production for different target materials is investigated inside and outside the target chamber, using monopole antenna, super wide-band microstrip antenna and Moebius antenna. The EMP consists in a fast transient magnetic field lasting hundreds of nanosecond with frequencies ranging from MHz to tens of GHz. Measurements of magnetic field and return target current in the range of kA were carried out by an inductive target probe (Cikhardt J. et al. Rev. Sci. Instrum. 85 (2014) 103507).

AC loss characteristics of CORC® cable with a Cu former

High-temperature superconductors from the REBCO (RE = rare earth) family have attained industrial production and their performance is continuously being enhanced. However, cabling technology for high-current (kA range) cables for magnet technology is still challenging and there are only few cable concepts available (CORC®, Roebel cable, twisted stack cable). Each of them exhibits different characteristics. In this paper we experimentally investigate CORC® cable produced in-house utilizing a copper tube former. Such a former offers a central cooling channel for partial or complete cable cooling by forced flow of coolant. We focus mainly on AC loss due to transporting AC current, an external applied AC magnetic field and their simultaneous action. In the case of transporting AC current we found indications that a large part of the total loss has its origin in eddy currents due to an axial magnetic field. For the investigation of magnetization AC loss, we prepared several samples with different configurations. In this case we found direct evidence for increasing AC loss due to losses in the metallic former. However, we also found that at low field amplitudes the magnetization AC loss of the complete cable is lower than the loss in the bare former. This is caused by shielding of the magnetic field by a superconductor, which was also confirmed by numerical simulations.

Late Quaternary TEX86 paleotemperatures from the world’s largest desert lake, Lake Turkana, Kenya

Wetter climatic conditions prevailed across northern and Equatorial Africa during the terminal Pleistocene until the middle Holocene, a climate event known as the “African Humid Period” (AHP). Although hydrologic changes during this period are well-known, very few records of temperature are available for evaluating the mechanisms and dynamics of climate change during the AHP across tropical Africa. To quantify changes in temperature during the onset and termination of the AHP, the TEX86 temperature proxy was used to generate a 14,000 year record of the surface temperature of Lake Turkana. This biomarker and related measures have been used to reconstruct regional and high-latitude paleotemperatures from the oceans and other continental systems, including other large African lakes. Although Lake Turkana’s climate and hydrology are very different from other large African lakes, our reconstruction exhibits a temperature history that shares a number of climatic trends with previous reconstructions. The TEX86 temperatures from Lake Turkana from 14 to 0.4 ka range from 24.3 to 28.6 °C, with a gradual decrease in temperature from early to late Holocene. This cooling roughly follows northern hemisphere summer insolation, similar to the trend observed in Lake Victoria and other regional records. However, the record from Turkana contains many abrupt temperature shifts not seen in other large lakes in the region. Multi-century-scale fluctuations persist through most of the record, and can be attributed to periods of lake mixing. Larger temperature perturbations are likely associated with changes in evaporation and cloud cover.

$Q$ Bounds for Planar and Ellipsoidal Antennas

The procedure for numerically deriving equivalent circuits for the six lowest-order modes of arbitrarily shaped, electrically small antennas is reviewed. Element values and the effective radius are tabulated for thin prisms, spheroids, and ellipsoids with a wide range of aspect ratios. A self-consistency test for the elements is defined and applied. These circuits allow calculation of the internal and external electric and magnetic reactive energies and thus establish lower bounds for Q of antennas with these shapes as a function of their electrical radius, ka. Electromagnetic simulations on a family of meander dipoles and a planar loop provide Q values that are compared with the bounds over a range of ka up to one.

Globus-M plasma edge modeling with B2SOLPS5.2 code

The edge plasma of five Globus-M discharges was modeled by the B2SOLPS5.2 code. Plasma current varied in 114–198 kA range, and all discharges were in the H-mode. The modeled scrape-off layer (SOL) width appeared to be inversely proportional to the plasma current. Such a relation is observed in many other tokamaks. Heat flux to the outer divertor target and radiation power was examined for these discharges. Radiation was found to be responsible for 40% of the energy loss. Energy flux to the low outer divertor target was about 1/3 of the energy loss in the single-null low X-point discharges.

Deposits of the Kazantsevo Transgression (MIS 5) in the Northern Yenisei Region

Dating of marine sediments and faunal remains they contain in stratotype and reference sections by the methods of infrared optically stimulated luminescence (IR-OSL) of K-feldspar, optically stimulated afterglow (OSA) of quartz, electron spin resonance (ESR), and 230Th/U provides new constraints on deposition in the Yenisei mouth during the Kazantsevo interglacial. The luminescence and U–Th ages in the 120–68 ka range and 93–70 ka ESR ages show that the deposition spanned the whole marine isotope stage (MIS) 5. The sediment structures and textures, grain sizes and mineralogy, and faunal records indicate tidal and shelf deposition environments. The sampled assemblages of marine mollusks comprise taxa that typically live in relatively shallow and warm water, as well as abundant subarctic and boreal species, including the Arctica islandica index species. The variations of faunal patterns, more likely, had facies rather than climatic controls, while the sediments were deposited during transgression, in a warm climate, when the area was ice-free.

Location and Sizing of Defects in Coated Metallic Pipes Using Limited View Scattered Data in Frequency Domain

The inverse problem of reconstructing the location and size of defects in a coated metallic pipe from single frequency limited view electromagnetic scattered field is considered. Specifically, the paper addresses the problem of assessing shape changes in the shadow region entailed by limited view data in 2D by operating in an intermediate size parameter (ka) range. The inverse scattering problem is formulated as a non-linear optimization problem solved through genetic algorithm that seeks to minimize in the least-squared sense the difference between measured data and simulated data through iterations of the solution to a forward problem. The hybrid finite element boundary integral formulations were used to solve the forward problem of coated metallic pipe with defects. The proposed inversion methodology was applied for shadow region shape change assessment to determine whether a metal pipe is coated or not, and to reconstruct the location and size of corrosion-like defect, over a range of size parameters. The study is carried out using transverse electric and transverse magnetic polarized fields. To understand the effect of coating on backscattered fields, parametric studies are conducted using numerical data. A range for size parameter \(2.0\le ka\le 4\) was found to produce the highest contrast between defect free pipe and coated metallic pipes with defects in the deep shadow region. The experiments are carried out using vector network analyzer in an anechoic chamber. The inversion results obtained using measured data were found be in good agreement with inversion results obtained using synthetic data. Estimated extent of corrosion in deep shadow region of a coated metallic pipe was found to be within 9 % of actual extent of corrosion.

Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator

Due to their ultra-short duration and peak currents in the kA range, laser-wakefield accelerated electron bunches are promising drivers for ultrafast X-ray generation in compact free-electron-lasers (FELs), Thomson-scattering or betatron sources. Here we present the first single-shot, high-resolution measurements of the longitudinal bunch profile obtained without prior assumptions about the bunch shape. Our method allows complex features, such as multi-bunch structures, to be detected. Varying the length of the gas target, and thus the acceleration length, enables an assessment of the bunch profile evolution during the acceleration process. We find a minimum bunch duration of 4.2 fs (full width at half maximum) with shot-to-shot fluctuation of 11% rms. Our results suggest that after depletion of the laser energy, a transition from a laser-driven to a particle-driven wakefield occurs, associated with the injection of a secondary bunch. The resulting double-bunch structure might act as an elegant approach for driver-witness type experiments, i.e. allowing a non-dephasing-limited acceleration of the secondary bunch in a plasma-afterburner stage.

The Iceland Basin excursion: Age, duration, and excursion field geometry

AbstractThe Iceland Basin geomagnetic excursion coincided with the marine isotope stage (MIS) 6/7 boundary. The age and duration of the excursion, at seven North Atlantic sites with sufficient isotope data, are estimated by matching marine isotope stage (MIS) 7a–7c to a calibrated template. Two criteria for defining the excursion, virtual geomagnetic pole (VGP) latitudes <0° and <40°N, yield excursion durations of 1–4 and 2–5 kyr, respectively. The midpoints of the excursion are in the 189–192 ka range, with a mean of ∼190.2 ka. Although component magnetization directions are generally well defined, rapid changes in field direction during a time of low field intensity are not adequately recorded. During the excursion, VGPs transit southward over Africa and the South Atlantic, reach high southern latitudes at the culmination of the excursion, with partial recovery in relative paleointensity (RPI), and then track northward through the western Pacific. The high southern latitude VGPs, and the recovery in RPI, imply that the Earth's main axial dipole reversed polarity during the excursion, if only for ∼1 kyr; implying that excursions can be manifested globally and are important in millennial‐scale stratigraphy. VGP clustering in the South Atlantic and NW Pacific roughly coincide with maxima in the vertical‐downward component of the modern nondipole (ND) field determined at the Earth's surface, which implies that the ND field became dominant as the geocentric dipole field weakened during the excursion, and also that the ND field configuration is long‐lived on multimillennial timescales.

U–Th dating of broken speleothems from Cacahuamilpa cave, Mexico: Are they recording past seismic events?

Cacahuamilpa cave is one of the largest karst systems in Central Mexico. The cave contains numerous massive speleothems broken and fallen following oriented directions, damaged during cataclysmic geological events. One fallen and two broken speleothems were sampled in the Cacahuamilpa cave for dating the rupture event using measured U–Th disequilibrium ages. A total of eight small carbonate cores were drilled perpendicular and longitudinal to the rupture surface. Results showed three groups of ages (weighted average): 0.95 ± 0.02 ka, 28.8 ± 0.2 ka and 88.0 ± 0.7 ka. This indicates that the construction of the Cacahuamilpa karst system, for which no absolute ages existed before this study, initiated at least since Late Pleistocene. The first two groups of ages might be related to two distinct episodes of intense seismic activity. Calculated minimum horizontal ground acceleration and frequency values of the seismic events needed to create the rupture of the stalagmites dated at 0.95 ± 0.02 ka and 28.8 ± 0.2 ka range between 1.3 and 2.0 m s−2 and between 13.4 and 20.8 Hz, respectively. These parameters are compatible with earthquakes of magnitude equal or higher than 7 M, with an epicentral distance between 50 and 100 km from the Cacahuamilpa cave. The stalagmite rupture dated at 88.0 ± 0.7 ka might result from the invasion of the cave by one of the older lahars deposits of the nearby volcano Nevado del Toluca, and successively fell by gravity instability.

High precision U–Pb zircon geochronology for Cenomanian Dakota Formation floras in Utah

The Cretaceous Dakota Formation on and near the Colorado Plateau represents the time-transgressive shoreline deposits of the Western Interior Seaway and is a rich repository of fossil plants and animals. However, given the basin architecture and depositional setting, correlations between localities are difficult without geochronology. Here we present new high precision ID-TIMS U–Pb zircon dates from five air-fall ash deposits from Westwater (n = 3) and Henrieville (n = 2) that obey stratigraphic superposition and precisely constrain the ages of these two floras. Three air-fall ashes from the Westwater locality record weighted mean Th-corrected 206Pb/238U dates of 97.949 ± 0.037/0.12 Ma, 97.943 ± 0.023/0.12 Ma, and 97.601 ± 0.049/0.13 Ma, providing the first U–Pb dates from this locality. Two ash beds from the Henrieville locality record weighted mean Th-corrected 206Pb/238U dates of 95.070 ± 0.036/0.12 Ma and 94.879 ± 0.032/0.11 Ma. The new dates indicate that the two floras are separated by ca 2.5 Ma and are both within the Cenomanian. The assemblage of fossil plants collected from the mudstone and ash-bed localities at Westwater show no compositional overlap, despite the presence of 28 distinct morphotypes. This lack of compositional overlap is repeated between the mudstone facies from the Westwater and Henrieville localities, where two different morphotypes of “Liriodendron” are the taxa in common. The species incongruence at Westwater is likely due to rapid colonization of early successional plants post deposition of the volcanic ash. For the mudstone floras at Westwater and Henrieville, the different floral composition may be due to rapid species evolution during the rise of angiosperms, the influence of climate in the mid-Cretaceous, the expression of high regional diversity of localities separated by 300 km, or a highly partitioned floodplain vegetation. Estimated sediment accumulation rates from the new radioisotopic dates, combined with existing proxy records for pCO2, suggest that the rise in pCO2 preceding Ocean Anoxic Event 2 (OAE2) began 513 ka (range from 384 to 641 ka) prior to the positive δ13C excursion that defines the event. This estimate from terrestrial rocks is within error of estimates for the timing of changes in δ34Ssulfate (570 ka; range from 420 to 814 ka) in marine sections at the GSSP for the Cenomanian–Turonian boundary. The overlap indicates synchronous perturbation of marine and terrestrial environments related to an increase in pCO2 prior to the onset of OAE2, providing further support for the volcanic initiation hypothesis.