kA Level Research Articles

Teachers' technological competencies in enhancing teaching and learning in secondary schools in Tanzania

The purpose of this study was to investigate level of teachers’ technological competencies and the effect of teachers’ age, gender, educational level and teaching experience in relation to their technological competencies they hold. Questionnaires were used to collect the data from 139 teachers from selected schools using simple random and purposive sampling procedures. Descriptive and inferential statistics were carried out using SPSS software version 23. The study found that majority of teachers were at the first level of technological competency (KA). It was also found that there was significant difference between teachers’ level of education and technological competencies they hold. However, the study found that there is no significant difference between teachers’ gender, age and working experience and technological competencies they hold at p-value of 0.05, though there were some variations. The study recommends that provision of training among teachers should focus on uplifting teachers from KA level to another levels.

Numerical Analysis of the Convective Heat Transfer Coefficient Enhancement of a Pyro-Breaker Utilized in Superconducting Fusion Facilities

The conductive components of the pyro-breaker in the quench protection system (QPS) have high current density, a large number of electrical contacts and high thermal flux. The water system needs to meet the requirements of cooling and arc extinguishing at the same time. In a previous study, the bottleneck of the steady-state capacity appeared in the barrel conductor of the commutation section, which has a cylindrical cavity. The thermal stability of the commutation section at 100 kA level was simulated in ANSYS/Workbench. The results indicate a certain level of enhancement of the convective heat transfer coefficient of the cavity is required to reach the current capacity. However, the fluid flow inside the cavity is very complex, and the convective heat transfer coefficient is difficult to calculate. In this paper, Computational fluid dynamics (CFD) is applied to the optimization of the cooling water system of the pyro-breaker. By studying the enhancement method of convective heat transfer, optimization of the structure and processing method of the water channel are proposed. The convective heat transfer coefficients of the cylindrical cavity in these optimizations were calculated in CFX. A set of optimizations of the cavity, which can meet the requirements of China Fusion Engineering Test Reactor (CFETR), were obtained and verified by experiments.

Low Resistance Soldering and Installation for a kA Level HTS Flux Pump

The HTS flux pump is a promising device which can charge the HTS magnet for various high field applications. In an HTS flux pump, the superconducting tapes are the circuit connections between the charging loop and the load circuit, and using soldering joints between the superconducting layers is a general method. For a large scale HTS flux pump, the joints should endure the electrical current over kA level, and with durable mechanical structures. In our kA level HTS flux pump, the HTS-copper joint was used to connect the charging loop, the HTS bridge and the potential HTS load. Three methods of HTS-copper soldering were tested and compared for a simple HTS-copper circuit: pure indium wire pre-tinning, IN52/SN48 ribbon direct soldering, and IN52/SN48 ribbon pre-tinning. Experiments show the soldering of IN52/SN48 ribbon pre-tinning archived the lowest joint resistance among these three methods, and the individual resistances were determined. This method was used to solder the joint for the kA level HTS flux pump, and acceptably low resistance was achieved.

A new structure of mechanical switch used for hybrid breaker in quench protection system

High power DC breaker is the most important device in the quench protection system of superconducting magnet. In this paper, a new structure mechanical switch with three levels contacts is presented, which is designed for high current hybrid breaker in quench protection system on China Fusion Engineering Test Reactor (CFETR). The topology and operation logic are discussed, and the prototype of hybrid breaker with 10 kA level has been tested. In addition, the corresponding simulation mode of breaker is built by MATLAB/Simulink. The prototype experimental and simulation analysis prove that the mechanical switch can interrupt the current more quickly and reduce the conducting time of solid-state switch.

Data driven analysis and prediction of MILD combustion mode

Abstract Direct numerical simulation (DNS) data of moderate or intense low-oxygen dilution (MILD) combustion and a planar flame are analysed to identify quantities influencing the unique features of co-existing combustion modes and develop a model to identify them in MILD combustion. The results show the existence of direct relationship between the scalar dissipation and reaction rates in MILD combustion, whereas the correlation is weaker than the planar flame. Also, rotational turbulent motions identified by the enstrophy-strain balance also show a substantial influence on the MILD combustion field, via the principal component analysis, suggesting the mechanism by which the non-flamelet part of reaction zones is controlled. A neural network (NN)-based model was proposed to identify the filtered local combustion mode for MILD combustion fields in LES context. The NN is trained based on DNS data of MILD combustion at two different conditions for different filter sizes. The model assessment was performed using the third MILD condition having a higher Ka and dilution level than those of the two training data sets. Several filter sizes ranging from 0.5 to 2.0 times of a corresponding laminar flame thickness were considered, and also prediction performance of a “zeroth-order model” is compared with that of the NN-based prediction. The assessment shows very high correlation between the NN-based prediction and the mode directly obtained from DNS. The predicted local combustion mode could be used to exploit advantages of both flamelet and non-flamelet-type combustion models to predict co-existing MILD reaction zones.

Exploring the roles of climate and tectonics in the geomorphic evolution of the Chitwan Intermontane valley, Central Himalaya

The Himalayan front exhibits varying convergence rates along its length. This study examines the morphotectonic evolution of the landforms in an intermontane valley (viz., Chitwan dun) present in a relatively rapidly converging segment of the Himalaya. Using field investigations, geomorphic analysis, and geochronology, we demonstrate that five geomorphic levels have developed in the past >112 ka. Four major phases of aggradation occurred in the Chitwan, i.e., >112 ka, ~112–>70 ka, ~70–25 ka, and ~18–11 ka. Tectonic activity on the Jharahi (JHT) and Belani thrusts (BT) at >112 ka and >70 ka respectively resulted in the uplift of the geomorphic units to Level 5 and Level 4. Continuous aggradation in the valley occurred between 70 and 25 ka (Level 3); the tectonic activity during this period is marked by the reactivation of the West Chitwan thrust (WCT) and the Central Churia Thrust (CCT). The Level 3 surfaces were abandoned during the LGM (25–18 ka) and subsequently incised. Enhanced monsoon in the post-LGM period favored aggradation of the Level 2 and 1 geomorphic units. The Level 2 surfaces were abandoned due to activity on the Danda (DT) and Shaktikhor Thrusts (ST) whereas the Level 1 landforms were incised due to climatic changes in the Holocene. Comparing the results of this study with other parts of the Himalaya, we infer that despite variable tectonic convergence rates, major aggradation and incision cycles are uniform. This suggests that Late Quaternary climatic perturbations have strongly influenced the fluvial systems across the Himalaya. However, the role of tectonics is also manifested in the form of multiple levels of surfaces and terraces.

Note: Erosion of W-Ni-Fe and W-Cu alloy electrodes in repetitive spark gaps.

A pair of W-Ni-Fe and W-Cu electrodes were tested under 100 kA level pulsed currents for 10 000 shots, respectively. Surface roughness and morphology characteristics of the two pairs of electrodes were obtained and compared. Experimental results indicated cracks divided the W-Cu electrode surface to polygons while the W-Ni-Fe electrode surface remained as a whole with pits and protrusions. Accordingly, the surface roughness of W-Ni-Fe electrodes increased to ∼3 μm while that of W-Cu electrodes reached ∼7 μm at the end of the test. The results reveal that the W-Ni-Fe alloy has a better erosion resistance and potential to be further applied in spark gaps.

Plasma channel undulator excited by high-order laser modes

The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3 kA level, making such an undulator a potential candidate towards a table-top FEL.

Water level history for Lake Turkana, Kenya in the past 15,000 years and a variable transition from the African Humid Period to Holocene aridity

Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine >30m oscillations in water level were documented between ca. 15 and 4ka. The earliest lake level oscillation between ca. 14.5 and 13ka is not well constrained with water level to at least 70m above the present surface and subsequently fell to at least 50m. Lake level increased to at least 90m between ca. 11.2 and 10.4ka, post Younger Dryas cooling. Water level fell by >30m by 10.2ka, with another potential rise at ca. 8.5ka to >70m above current level. Lake level regressed by >40m at 8.2ka coincident with cooling in the equatorial eastern Atlantic Ocean. Two major >70m lake level oscillations centered at 6.6 and 5.2ka may reflect enhanced convection with warmer sea surface temperatures in the western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5ka and is characterized by highly variable lake level (±>40m), rather than one monotonic fall in water level. This lake level variability reflects a complex response to variations in the extent and intensity of the East and West African Monsoons near geographic and topographic limits within the catchment of Lake Turkana. Also, for this closed lake basin excesses and deficits in water input are amplified with a cascading lake effect in the East African Rift Valley and through the Chew Bahir Basin. The final regression from a high stand of >90m above the present lake began at 5.2ka and water level was below 20m by 4.5ka; and for the remainder of the Holocene. This sustained low stand is associated with weakening of the West African Monsoon, a shift of the mean position of Congo Air Boundary west of the Lake Turkana catchment and with meter-scale variability in lake level linked to Walker circulation across the Indian Ocean.

Non-inductive plasma start-up on NSTX and projections to NSTX-U using transient CHI

Transient coaxial helicity injection (CHI) in the National Spherical Torus Experiment (NSTX) has generated toroidal current on closed-flux surfaces without the use of the central solenoid. When induction from the central solenoid was added, CHI initiated discharges in NSTX achieved 1 MA of plasma current using 65% of the solenoid flux of standard induction-only discharges. In addition, the CHI-initiated discharges have lower density and a low normalized internal plasma inductance of 0.35, as desired for achieving advanced scenarios. Transient CHI will be used for non-inductive plasma start-up on the upgrade to NSTX (NSTX-U) that is now under construction. It will have numerous improvements that significantly enhance CHI start-up capability. The TSC code is now starting to be used for full discharge simulation, which includes solenoid-less start-up with CHI and subsequent non-inductive current ramp-up using neutral beams. These results suggest that the increased injector flux capability of NSTX-U should allow CHI start-up at more than the 400 kA level and that the new tangential neutral beam system on NSTX-U should be able to ramp the current to the 1 MA levels.

Consolidation of the 13 kA Interconnects in the LHC for Operation at 7 TeV

The accident in the LHC in September 2008 occurred in an interconnection between two magnets of the 13 kA dipole circuit. Successive measurements of the resistance of other interconnects revealed other defective joints, even though the SC cables were properly connected. These defective joints are characterized by a poor bonding between the SC cable and the copper stabilizer in combination with an electrical discontinuity in the copper stabilizer. A quench at the 7-13 kA level in such a joint can lead to a fast and unprotected thermal run-away and hence opening of the circuit. It has therefore been decided to operate the LHC at a reduced and safe current of 6 kA corresponding to 3.5 TeV beam energy until all defective joints are repaired. A task force is reviewing the status of all electrical joints in the magnet circuits and preparing for the necessary repairs. The principle solution is to resolder the worst defective joints and, in addition, to apply an electrical shunt made of copper across all joints with sufficient cross-section to guarantee safe 12-13 kA operation at 7-7.5 TeV. In this paper the various actions that have lead to this solution are presented.

Two-Stage Cryocooling Design for Hybrid Superconducting Fault Current Limiter

New concept of two-stage cryogenic cooling system for full-scale hybrid superconducting fault current limiter (SFCL) is proposed and designed with an objective to significantly reduce the capital and operational costs. Even though we have developed and successfully tested a cryocooling system for 300 A level, it is not easy to scale up the same technology to 3¿/22.9 kV/2.5 kA level, mainly because of the heavy cost of GM or Stirling cycle coolers to cover the increased thermal load. It is proposed in this study to employ a large capacity of JT cooler for heat intercept at an intermediate location of current leads. The JT coolers with mixed refrigerant are very reliable and commercially available at an inexpensive price. Detailed cryogenic design of cryostat and heat intercept components is carried out in consideration of thermal characteristics of cryocoolers and electrical insulation. It may be concluded that the two-stage cooling with a JT cooler at 130 K and a GM cooler at 77 K is feasible in practice, and can significantly reduce the capital cost and power consumption.

Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level

New high-precision U/Pb geochronology from volcanic ashes shows that the Triassic-Juras- sic boundary and end-Triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial fl ood volcanism in the Central Atlantic Mag- matic Province to <150 ka. This narrows the correlation between volcanism and mass extinc- tion by an order of magnitude for any such catastrophe in Earth history. We also show that a concomitant drop and rise in sea level and negative δ 13 C spike in the very latest Triassic occurred locally in <290 ka. Such rapid sea-level fl uctuations on a global scale require that global cooling and glaciation were closely associated with the end-Triassic extinction and potentially driven by Central Atlantic Magmatic Province volcanism.

Effect of convection heat transfer on the design of vapor-cooled current leads

A general optimization method for vapor-cooled current leads is presented with taking into account the effect of convection heat transfer and extended surfaces. This analytical work is considered as a unified design method, since one formulation calculates the minimum heat load and the corresponding optimal design condition for arbitrary heat transfer condition, spanning two limiting cases—the zero convection (or conduction-cooled leads) and the perfect heat transfer. It is clearly shown that the augmentation of the convective cooling can reduce the heat load to a certain extent, but the optimum lead parameter required to minimize the heat load for the finite heat transfer may not exist between the two limiting values. A new dimensionless parameter called the Ch number is introduced to conveniently incorporate the convection effect into the optimization. The present method is demonstrated for two specific lead designs that have been recently developed for 10 kA level of applications.

Postglacial rebound at the northern Cascadia subduction zone

Postglacial rebound is the response of the Earth to the decay of ice-sheets. A postglacial rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice-sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.5–14 ka ( 14 C yr ago) and rapid sea level fall (land uplift) at 12–12.5 ka. Modelled mantle viscosity values range from 5×10 18 to 5×10 19 Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (10 18–10 19 Pa s). No lower limit to subduction zone mantle viscosity is apparent from our model, but viscosity values equal to or larger than 10 20 Pa s are definitely ruled out. Our modelled subduction zone viscosity values are smaller than most upper-mantle viscosity estimates derived from postglacial rebound studies of tectonically less-active regions (10 20–10 21 Pa s). The rapid observed uplift at 12 ka requires, in addition to a low mantle viscosity, rapid unloading from a sudden collapse of remaining coastal portions of the southern Cordilleran ice-sheet. The sudden collapse provides 0.18 m of global eustatic sea level rise, approximately 0.7% of the sea level rise associated with melt-water pulse IA. Predictions of a global postglacial rebound model (ICE-3G) with a 10 21 Pa s upper-mantle viscosity were previously applied to geodetic data from this region to isolate signals associated with the earthquake cycle. Owing to the low-viscosity values, and resulting rapid recovery of glacial deformation, our model predicts present-day postglacial rebound uplift rates at least 10 times smaller than ICE-3G (less than about 0.1 mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data.

Development of superconducting mechanical persistent current switch

The authors developed and tested a superconducting mechanical persistent current switch for SMES applications. For the purpose of improving electric current carrying capacity of one contact up to the kA level, multiple rods of NbTi with a copper matrix were selected. Pre-experiments surveyed current carrying capacity and electrical resistance of valuable contact samples, that were different in diameter of NbTi, shape of contacts and surface treatment. Vacuum valves were manufactured on trial with most suitable contacts for this purpose, and tested in liquid helium. A quench current up to 1900-2000 A DC at single valves, and 4570 A DC at four parallel valves, resulted. The electrical contact resistance before quench was lower than the measuring resolution (<1/spl times/10/sup -10/ ohm). Vacuum valve open/close speed was improved using coned disk springs and electromagnetic repulsion coils for the driving mechanism. With repeating open/close cycles, quench currents were observed to decrease. Therefore the closing mechanism should be improved.

High resolution stratigraphy of the Faeroe-Shetland Channel and its relation to North Atlantic paleoceanography: the last 87 kyr

Two piston cores from the Faeroe-Shetland Channel have been investigated for stratigraphic and paleoceanographic correlation with Atlantic Ocean and Norwegian Sea records. The high resolution cores have been analyzed for the downcore distribution of planktic and benthic foraminifera and oxygen isotopes. AMS 14C dates and the position of Ash layers I and II form the basis for the correlation. Core NA81-10 covers the period from isotope stage 3 to Recent (stage 1), while core NA81-04 covers isotope stages 5a to 1. The Holocene is well represented in core NA81-10 by 2 m of sediment, whereas it is only 50 cm in core NA81-04 which is situated directly under the main core of the Norwegian Sea Deep Water outflow. The transition of isotope stage 2 to stage 1 is affected by dissolution and thus is less clearly represented. AMS 14C dates show that Termination IA begins at 16 ka and ends at about the 13 ka level. The Last Glacial Maximum (LGM; defined at 19-16 ka) is characterized by an IRD peak, faunal (planktic foraminifera) maxima (dated at 17.3 and 18.25 ka) and a high percentage of Globigerina quinqueloba. Apparently open water conditions and circulation within the Norwegian Sea prevailed at least from 22.5 to 14.8 ka, during the greatest part of stage 2 (including the LGM) and well onto Termination IA. A preservation maximum is observed throughout stage 2. Isotope stage 3, with a nearly complete record of isotope substages is well represented in both cores. Oxygen isotope depletion peaks correlate with fauna minima and in time to North Atlantic Heinrich events (H5-H1). Faunal data indicate that deep water is generated within the Norwegian-Greenland Sea repeatedly since 26 ka, with stops prior to 26 ka, 22.5–20.8/20.5 ka and between 14.9 and 13 ka (H3, H2 and H1). All isotope depletion peaks since ca. 50 ka are anticorrelated with fauna and are mainly controlled by surface salinity changes. Isotope stages and substages are directly correlated to fauna peaks and temperature between 87 and ca. 50 ka. The Faeroe-Shetland Channel record shows that repeated presences of Atlantic Surface Water and deep convection alternating with convection shutdowns have occurred within the Norwegian Sea-Greenland Sea from the beginning of stages 5a to 1.

Theoretical Study of the Electronic States of Si2C

The ground state and electronically excited singlet and triplet states of Si2C have been investigated by ab initio calculations. For the electronic ground state, rovibrational energy levels have been evaluated by a variational approach using several different potential energy functions. The equilibrium geometry, the centrifugal distortion constants, and the vibrational levels up to 1500 cm-1 are given. The best function yielded an electronic barrier to linearity of 859 cm-1. The changes in the Ka level structures of the rovibrational levels caused by this barrier are predicted to appear between the v2 = 4 and v2 = 6 vibrational states. The results are compared with available experiments and with those of other theoretical studies. The electronically excited singlet and triplet states have been characterized by their potential energy functions. The avoided crossings and conical intersections in the excited states have been located and their influence on the electronic spectra is discussed.

A spectroscopically determined potential energy surface for the ground state of H216O: A new level of accuracy

The potential energy function for the electronic ground state of the water molecule has been obtained by fitting rotation-vibration term values involving J≤14 for 24 vibrational states of H216O together with 25 additional vibrational term values belonging to higher excited states. The fitting was carried out by means of an exact kinetic energy Hamiltonian. It was found that the differences between the exact kinetic energy calculations and calculations with the morbid program (i.e., calculations with an approximate kinetic energy operator) depend only very slightly on the parameters of the potential. This fact allowed us to make an inexpensive fitting using the morbid approach and still get the accuracy obtainable with the exact kinetic energy Hamiltonian. The standard deviation for 1600 term values was 0.36 cm−1. For 220 ground state energy levels the standard deviation was 0.03 cm−1. With the fitted potential, calculations of term values with J≤35 were carried out. This showed the excellent predictive power of the new potential. For the J=20 term values in the vibrational ground state, the deviations from experiment are typically below 0.2 cm−1. The discrepancy for the observed level with the highest Ka value, JKaKc = 20200, is only 0.008 cm−1. The calculated term value for the observed level with the highest J, 35035, deviates 0.1 cm−1 from experiment. Because of the level of accuracy achieved in these calculations, we can for the first time demonstrate the breakdown of the Born–Oppenheimer approximation for the water molecule. The high Ka level calculations allow us to show that the rotational energy level structure in water is at least of a very different nature than the fourfold cluster structures observed for H2Se and calculated for H2S, H2Se, and H2Te.

Conductor designs for the ITER toroidal and poloidal magnet systems

The superconductors for the ITER will utilize the cable-in-conduit Nb/sub 3/Sn strands. The protection and stability requirements for the coils require that the operating currents will be at the 50 kA level and that the cables are composed of hundreds of strands. The design features for the toroidal field solenoid conductors are given, together with their expected performance characteristics. The expected critical properties are given as well as the results of stability and quench computations. The status of the conductor development and testing program is summarised and conductor R&D plans for the future are outlined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>