Low N-values Research Articles

Formulation for solving 1D minimum propagation zones in superconductors

The minimum propagation zone (MPZ) and the minimum local energy release required for a quench (MQE) are two very important parameters in superconductors targeted to coil applications. Usually, MQE is determined from MPZ. According to recent experiments, the closed-form expressions presented for these are not applicable to conductors with low n-values, e.g. BSCCO and MgB2, but a numerical model is required. When various conductor configurations are designed it is very time consuming to measure basic conductor stability parameters, such as MQE, for each conductor. This paper presents a numerical model to determine MPZs for conductors with finite n-values. Therefore, the model gives an efficient tool for conductor manufacturers to estimate what kind of disturbances their conductors can tolerate. According to the results, MQEs for the n-value 20 at the normalized operation currents of 0.5, 0.7 and 0.9 are about 25%, 60% and 100% higher than for the n-value 50.

Iterative EM Design of an MRI Magnet Using HTS Materials

Conventional superconducting MRI magnet electromagnetic (EM) design involves the critical parameters of the magnet field and dimensional requirements, plus the low temperature superconductor (LTS) wire properties. When using HTS material for an MRI magnet, in addition to the conventional characteristics of a superconductor, the wire and the joint resistance need also to be considered. This is mainly due to the relatively low n-values of the BSCCO tape when compared to that of a conventional LTS wire, and it is also due to the more resistive joint nature for HTS joints. This paper discusses an iterative EM design procedure that calculates the resistance of the HTS wire/tape for the entire magnet, which is a strong function of the magnetic fields at different locations in the coils. The resistance is fed back to the regular MRI EM design for optimization, in order to meet resistance target and the ordinary MRI design goals. This iterative design process results in an optimized HTS MRI design with overall low resistance for magnet drifting and power supply requirements.

Biophysical and Kinetic Characterization of HemAT, an Aerotaxis Receptor from Bacillus subtilis

HemAT from Bacillus subtilis is a new type of heme protein responsible for sensing oxygen. The structural and functional properties of the full-length HemAT protein, the sensor domain (1–178), and Tyr-70 mutants have been characterized. Kinetic and equilibrium measurements reveal that both full-length HemAT and the sensor domain show two distinct O 2 binding components. The high-affinity component has a K dissociation ≈ 1–2 μM and a normal O 2 dissociation rate constant, k O2 = 50–80 s −1. The low-affinity component has a K dissociation ≈ 50–100 μM and a large O 2 dissociation rate constant equal to ∼2000 s −1. The low n-value and biphasic character of the equilibrium curve indicate that O 2 binding to HemAT involves either independent binding to high- and low-affinity subunits in the dimer or negative cooperativity. Replacement of Tyr-70(B10) with Phe, Leu, or Trp in the sensor domain causes dramatic increases in k O2 for both the high- and low-affinity components. In contrast, the rates and affinity for CO binding are little affected by loss of the Tyr-70 hydroxyl group. These results suggest highly dynamic behavior for the Tyr-70 side chain and the fraction of the “up” versus “down” conformation is strongly influenced by the nature of the iron-ligand complex. As a result of having both high- and low-affinity components, HemAT can respond to oxygen concentration gradients under both hypoxic (0–10 μM) and aerobic (50–250 μM) conditions, a property which could, in principle, be important for a robust sensing system. The unusual ligand-binding properties of HemAT suggest that asymmetry and apparent negative cooperativity play an important role in the signal transduction pathway.

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R∥/R⊥) whereR∥ andR⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10−3 in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.

V–I transition and n-value of multifilamentary LTS and HTS wires and cables

Abstract For low Tc multifilamentary conductors like NbTi and Nb3Sn, the V–I transition to the normal state is typically quantified by the parameter, n, defined by (ρ/ρc)=(I/Ic)n. For NbTi, this parameterization has been very useful in the development of high Jc wires, where the n-value is regarded as an index of the filament quality. In copper-matrix wires with undistorted filaments, the n-value at 5 T is ∼40–60, and drops monotonically with increasing field. However, n can vary significantly in conductors with higher resistivity matrices and those with a low copper fraction. Usually high n-values are associated with unstable resistive behavior and premature quenching. The n-value in NbTi Rutherford cables, when compared to that in the wires is useful in evaluating cabling degradation of the critical current due to compaction at the edges of the cable. In Nb3Sn wires, n-value has been a less useful tool, since often the resistive transition shows small voltages ∼ a few μV prior to quenching. However, in “well behaved” wires, n is ∼30–40 at 12 T and also shows a monotonic behavior with field. Strain induced Ic degradation in these wires is usually associated with lower n-values. For high Tc multifilamentary wires and tapes, a similar power law often describes the resistive transition. At 4.2 K, Bi-2223 tapes as well as Bi-2212 wires exhibit n-values ∼15–20. In either case, n does not change appreciably with field. Rutherford cables of Bi-2212 wire show lower values of n than the virgin wire.

Thermal decomposition kinetics of amorphous carbonnitride and carbon films

Kinetic thermal degradation of amorphous carbon and carbon nitridefilms is studied. Significantly improved thermal stability wasobserved for films intensified with C-N, C=N, and C≡Nbonds. When the N2% (percentage of nitrogen)in Ar/N2 during film depositionwas varied from 0 to 30 at pressures of 3×10-3 and 16×10-3 Torr, the onset decomposition temperatures increasedfrom 396 to 538 and from 340 to 360 °C, while the apparentactivation energy ΔE at 60% residual weight increasedfrom 149 to 158 and from 96 to 120 kJ mol-1, respectively. Achange in the thermal stability was observed when the N2% reached 50. Thefilms of higher bonding ratio and structural integration showed asingle-step decomposition mechanism. They had ΔE valuesdecreasing with the decomposition process, following convex-trendisothermal weight-loss curves. Films deposited using low-energycarbon plasma had higher contents of loosely bondedmolecules; this resulted in a seemingly two-step decomposition andΔE increasing with weight loss over a certain range. Theyfollowed concave-trend isothermal weight-loss curves. The overalldecomposition mechanism could best be expressed in terms ofnth-order reactions with the value of n closely related to filmstructures and reaction temperatures. Lower n-values are relatedto densely packed structure and higher temperature due to theincreased diffusion barrier of the products.

Limiting length in cooling design of HTS magnets

Regular operation of HTS windings is accompanied by a low-level heat (index loss), generated due to the low n-value of HTS materials. The paper shows that the index loss in conduction cooled magnets causes thermal instability and quench, when the cooling length exceeds certain critical value L*. This index conduction crisis is brought by the abrupt temperature growth of the index loss. The crisis can occur at rather small temperature gradients (of the order of a K), and should be among the major design considerations for a conduction cooled magnet. Stability criterion /spl lambda/ is derived in a one-dimensional conduction model with nonlinear index loss. Practical formulae are suggested defining maximum allowable current vs. cooling length. Effect of limited cryocooler capacity is modeled. The theory is illustrated by examples of a solenoidal coil and a solitary conductor (bus).

Electron binding to valence and multipole states of molecules: Nitrobenzene, para- and meta-dinitrobenzenes

Nitrobenzene anions (NB−) in both valence and dipole bound states are examined using laser (photodetachment) photoelectron and Rydberg electron transfer (RET) spectroscopies. Photoelectron spectroscopy of the valence NB− anion yields a valence (adiabatic) electron affinity of 1.00±0.01 eV. The reaction rates for charge transfer between atoms of cesium and xenon in high Rydberg states [Cs(ns,nd) and Xe(nf )] and NB exhibit a prominent peak in their n-dependencies consistent with the formation of a dipole bound anion having an electron affinity of 28 meV. Para-dinitrobenzene (pDNB) has a zero dipole moment and a large quadrupole moment. RET studies with pDNB show a complex n-dependence. The rate of formation of pDNB− ions exhibits a broad peak at low n-values and a second very broad feature extending to large n-values. The peak at low n is tentatively attributed to charge exchange into a quadrupole bound state (EAqb=25 meV). The absence of field-detachment for these ions suggests that if these are in a quadrupole bound state they are strongly coupled to the valence state. Meta-dinitrobenzene (mDNB) has a large dipole moment and a small quadrupole moment. The observation of a weak but narrow peak at a low n-value in the RET measurement is consistent with electron attachment into a primarily dipole bound state (EAdb=68 meV) which is also strongly coupled to the valence state.

Multitaper Spectral Analysis and Wavelet Denoising Applied to Helioseismic Data

Estimates of solar normal mode frequencies from helioseismic observations can be improved by using multitaper spectral analysis (MTSA) to estimate spectra from the time series, then using wavelet denoising of the log spectra. MTSA leads to a power spectrum estimate with reduced variance and better leakage properties than the conventional periodogram. Under the assumption of stationarity and mild regularity conditions, the log multitaper spectrum has a statistical distribution that is approximately Gaussian, so wavelet denoising is asymptotically an optimal method to reduce the noise in the estimated spectra. We find that a single m-ν spectrum benefits greatly from MTSA followed by wavelet denoising and that wavelet denoising by itself can be used to improve m-averaged spectra. We compare estimates using two different five-taper estimates (Slepian and sine tapers) and the periodogram estimate for Global Oscillation Network Group (GONG) time series at selected angular degrees ℓ. We compare those three spectra with and without wavelet denoising, both visually and in terms of the mode parameters estimated from the preprocessed spectra using the GONG peak-fitting algorithm. The two multitaper estimates give equivalent results. The number of modes fitted well by the GONG algorithm is 20%-60% larger (depending on ℓ and the temporal frequency) when applied to the multitaper estimates than when applied to the periodogram. The estimated mode parameters (frequency, amplitude, and width) are comparable for the three power spectrum estimates, except for modes with very small mode widths (a few frequency bins), where the multitaper spectra broaden the modes compared with the periodogram. At frequencies below 3 mHz, wavelet denoising of the log multitaper power spectra tends to increase the number of modes for which the GONG peak-fitting algorithm converges well. Close to 3 mHz, where all modes are resolved, wavelet denoising makes little difference. At higher frequencies close to the acoustic cutoff frequency, where modes are blended into ridges, wavelet denoising the multitaper spectra reduces the number of good fits. We tested the influence of the number of tapers used and found that narrow modes at low n-values are broadened to the extent that they can no longer be fitted if the number of tapers is too large. For helioseismic time series of this length and temporal resolution, the optimal number of tapers is less than 10.

Influence of microcracks and anisotropic conductivity on critical current and exponent n in silver-sheathed HTS tapes

The influence of filament non-uniformities (microcracks) and anisotropic conductivity on critical current and exponent n of the current-voltage characteristic of HTS/Ag tape conductors was studied numerically by the means of the finite element method. According to the results, non-uniformities reduce the critical current of the conductor, the reduction by a crack being higher in the cases of anisotropic than isotropic conductivity, but their influence on exponent n is negligible. The result suggests that the low n-values of HTS/Ag conductors are due to intrinsic effects such as thermally activated flux creep.

Refrigeration load calculation of a HTSC current lead under AC conditions

A 5 kA/sub rms//50 kV HTSC hybrid AC current lead is developed within a BRITE/EURAM cooperation between Alcatel Alsthom, Hoechst and Siemens. For the optimisation of the conductor geometry computer codes have been realised. For the evaluation of AC losses in the superconducting melt cast processed Bi-2212 part, the Bean model has not turned out to be practicable especially when the current exceeds the I/sub c/(T) curve, This is of course related to the low n-value (n/spl les/10) of the superconducting transition curve in this material. A new calculation based on experimental E(J,B,T) curves allows the evaluation of AC losses in the temperature range of interest. These results are used for the calculation of the maximum stable length and the low temperature heat load which is more than three times smaller than in an all metal DC lead. Together with the 77 K heat load of the normal part of the AC lead, the overall 300 K refrigeration load is calculated.

Mt. Melbourne volcano, antarctica: Evidence of seismicity related to volcanic activity

Characterization of the microseismic activity (M L <2.0) has been performed at Mt. Melbourne since 1990. We recorded a group of low frequency events with common morphological characteristics, i.e., an emerging onset, an unclear second phase and a sharply dropping coda. Spectral analysis of events recorded at more than one station indicates that the seismogram characteristics and spectral content are largely due to source effects. A polarization filter applied to a set of three component data revealed a first phase made up ofP waves followed (after about 0.9–1.4 sec.) by a second phase probably composed ofSH-type waves. Particle motion analysis detected a seismic ray angle direction mainly between N70°E and N110°E and apparent angle of incidence between 35° and 48° for the first phase. The studied seismicity was localized in an area on the eastern slope of Mt. Melbourne Volcano which presents a surface temperature anomaly (Mazzarini andSalvini, 1994). We formulate two hypotheses for the type of earthquakes recorded: 1) long-period events involving active presence of magmatic fluids in the source processes; 2) or the result of fracturing processes (shear?) in a medium characterized by transition between brittle and plastic behaviors. In the latter hypothesis the superficial thermal anomaly may be a symptom of this behavior at depth and is confirmed by the lown values observed for the exponential fit in the codaQ analysis.

5502243 Hydrocarbonylation of dimethyl ether

This study evaluated the separation of particles suspended in power law fluids by applying a 10 mm hydrocyclone. Al2O3 particles were added to an aqueous polyacrylic acid solution to prepare 0.1 vol% suspensions with different flow behaviors. The effects of operating conditions, such as the inlet velocity, pressure drop, split ratio, and fluid flow behavior, on the fluid velocity distribution, particle trajectory, and particle separation efficiency were studied. The study applied a simulation in which the governing equations were coupled using the SIMPLE algorithm; the Reynolds stress model served as a turbulence model. The distributions of the fluid velocity and pressure were simulated using a segregated, steady-state, three-dimensional implicit numerical solver provided by ANSYS 15.0 software. The tangential velocity increased with the flow behavior index, n, at a fixed inlet velocity and split ratio. This effect was attributed to a lower molecular viscosity. The particle trajectories were simulated using a Lagrangian frame. The particle migration in the cylindrical part of the hydrocyclone dominated the particle separation efficiency. A reduction in the fluid n-value engendered a higher effective viscosity and lower tangential velocity, which caused fewer particle rotations and lower centrifugal effects, consequently lowering the opportunity for particles to collect in the underflow. The separation efficiency increased with the inlet velocity and particle size because of a higher centrifugal effect, but the influence of the inlet velocity was extremely small for a low fluid n-value. The total separation efficiency increased by approximately 40% as the n-value increased from 0.51 to 1.00. The simulated particle separation efficiency was highly consistent with experimental data. This verified the reliability of the simulation method and the validity of this method for understanding the effects of fluid flow behavior on particle separation performance.

Ｂｉ系バルク超電導体における交流通電時の交流損失

It is shown that the observed value of the AC loss of a cylindrical BSCCO bulk superconductor with the diameter of 9.6mm, carrying the AC transport current with the frequency of 60Hz, deviates noticeably from the theoretical values estimated from the critical state model. It is clarified that this deviation results from the fact that the internal distribution of flux density, B(r), is quite different from that predicted by the critical state model, when the AC transport current with the frequency, f, of 60Hz is applied to the sample with a large diameter, R, and with a low n-value. This conclusion is based on the numerically simulated results of B(r) obtained by solving the Maxwell equations using the observed Jc=Jc(B) and E(J)=E(0)(J/Jc)n characteristics for the DC electric field, E, the DC transport current density, J, and the DC critical current density, Jc, determined by the criterion of 0.1μV/cm. Finally, a general numerical chart of the AC loss against fR2Jc is presented for the various n-values, for the convenience of the design of the high-Tc current leads.

Combined Effects of Strain-Hardening Characteristics of Materials and Tool Shape on Square Shell Deep-Drawability. Discussion of Optimum n-Value and Blank Shape for Square Shell Deep-Drawability.

The square shell deep-drawability of pure aluminum is studied systematically. Square blanks with various n-values and huni blanks with a low n-value are used in the experiments. The huni blanks are determined by a slip line field method, and give drawn cups with uniform heights. For square blanks, the limiting drawing ratio (LDR) decreases markedly in the following cases. (1) Punch corners are sharp in a higher n-value, (2) A punch is nearly cylindrical in a lower n-value. The maximum LDR is obtained from the appropriate selection of an n-value and punch shape. These facts are explained from drawing force, fracture force and the fractured cups which are classified according to the initiation and propagation of the cracks of the cups. When an n-value is very low, the LDR of the huni blanks is higher than that of the square blanks for each shape of punches, and much higher, in particular, in the nearly cylindrical shapes. In the use of punches with those shapes, wall breakage in the square blanks does not occur in the huni blanks.

Multistage crystallization of tonalitic enclaves in granitoid rocks (Hercynian belt, Spain): implications for magma mixing

The crystallization sequence, nucleation density and crystal index (n = nucleation density/mode) of tonalitic enclaves from different granitoids of the Hercynian orogeny indicate that they are pieces of magma partially crystallized in the interior of synplutonic bodies of basic magma, which was injected into a silicic magma chamber. A rapid cooling stage can be identified from the high nucleation density and high n-values of minerals. A final stage of slow cooling, is identified by a low nucleation density and low n-values. At this stage a residual melt crystallized in thermal equilibrium to the cooling rate of the whole magma chamber. Thermal equilibrium and convection are necessary conditions for mixing. Enclaves can be interpreted as the remaining, non-disaggregated portions of an early injected mafic magma, which in turn underwent hybridization during injection. Their presence indicates the existence of magma mixing processes and the possibility that the host granitoid was a hybrid rock.

Dynamic compaction of loose and hydrocompactible soils on Interstate 90, Whitehall-Cardwell, Montana : Yarger, T L Trans Res RecN1089, 1986, P75–80

The use of dynamic compaction to increase bearing capacity and reduce settlement of loose foundation soils at great depths is becoming more widespread in the United States. The use of dynamic compaction on a recent Interstate highway overlay project in Montana is described. Foundation soils treated were about 20 to 25 ft (6.1 to 7.6 m) of loose dry alluvial sands and silts having low N-values (blows per foot) by the standard penetration test (less than 4 in some cases). Correlation has beed found between known and suspected deposits of hydrocompactible soils and semiarid areas having less than about 12 in. (30 cm) of precipitation a year in Montana. A 15-ton (13.6-metric ton), 6-ft (1.8-m) diameter weight with a drop height of 60 ft (18.3 m) was used for the dynamic compaction process. The compaction was performed even in the winter months through frost layers broken up by ripping equipment. Dynamic compaction was found to be a very cost-effective method for improving deep deposits of loose sand and silt. Cost savings from this methd of compaction compared with feasible alternatives was estimated at $2.6 million. The average increase in N-value of the loose materials following compaction was 5 to 12. Some complaints of structural damage were received from local citizens even though the closest residence was 170 ft (52 m) from the compacted area.

Numerical Analysis of Elastohydrodynamic Contacts Using Power-law Lubricant With Special Reference to Water-based Hydraulic Fluids

A numerical solution of the elastohydrodynamic lubrication problem for pure rolling with non-Newtonian lubricants is outlined. The non-Newtonian rheological model used is a modified power-law, τ = K · γn. At low shear rate, γ γb the lubricant becomes non-Newtonian, τ = K · γn. At high shear rate (γ → ∞) the lubricant becomes Newtonian again with the same viscosity as the base lubricant. By using this rheological model a modified Reynolds' equation is derived. The influence of n-value and γb on the film thickness has been investigated. Plots of the pressure distribution and film thickness within the contact for fluids with G-values are the range 169–986 is presented. These G-values are typical for water-based hydraulic fluids. For fluids with low n-values (n < 0.5) the pressure profile approaches the Hertzian pressure distribution and the minimum film thickness will be that given by the base fluid viscosity. A new minimum film thickness formula for fluids with ...

Structure of microemulsions of AOT and water in dodecane

The dielectric properties of the system water/AOT/dodecane are studied as a function of volume fractionφ of the dispersed phase and molar ration (water/surfactant). Data shows that the spherical model is valid only at lown values or lowφ values. At high concentrations of dispersed phase, one has to consider micellar aggregation or deformation.

One-photon ionization of K 2 and Na 3

One-photon ionization experiments were performed on K 2 and Na 3 in supersonic molecular beams using a frequency-doubled dye laser and a mass spectrometric detection system. Wavelength scans across the ionization potential of K 2 clearly indicate autoionizing Rydberg levels of low n-values and ungerade symmetry dominating the direct photoionization processes. The ionization of Na 3, on the other hand, does not exhibit an expressed structure of autoionizing Rydberg states. Possible explanations are discussed.