Parallel Flux Research Articles

Chiral anomaly in Dirac semimetals due to dislocations

The dislocation in Dirac semimetal carries an emergent magnetic flux parallel to the dislocation axis. We show that due to the emergent magnetic field the dislocation accommodates a single fermion massless mode of the corresponding low-energy one-particle Hamiltonian. The mode is propagating along the dislocation with its spin directed parallel to the dislocation axis. In agreement with the chiral anomaly observed in Dirac semimetals, an external electric field results in the spectral flow of the one-particle Hamiltonian, in pumping of the fermionic quasiparticles out from the vacuum, and in creating a nonzero axial (chiral) charge in the vicinity of the dislocation.

On the neutral points in Rayleigh transfer

In this paper we consider the dependence of the existence and position of the neutral points on the albedo of single scattering and the optical thickness in a Rayleigh scattering plane-parallel homogeneous atmospheres. We use the Chandrasekhar method of discrete ordinates and the method of approximating the Sobolev resolvent function to solve the vector equation of transfer in l- and r-representation. On the basis of many different models of Rayleigh atmospheres we show the behaviour of the neutral points while the parallel incident flux can be both unpolarized or polarized. Our calculations show with high probability that the maximum number of neutral points in a Rayleigh atmosphere is four.

Power deposition on misaligned edges in COMPASS

If the decision is made not to apply a toroidal chamfer to tungsten monoblocks at ITER divertor vertical targets, exposed leading edges will arise as a result of assembly tolerances between adjacent plasma-facing components. Then, the advantage of glancing magnetic field angles for spreading plasma heat flux on top surfaces is lost at the misaligned edges with an interaction occurring at near normal incidence, which can drive melting for the expected inter-ELM heat fluxes. A dedicated experiment has been performed on the COMPASS tokamak to thoroughly study power deposition on misaligned edges using inner-wall limited discharges on a special graphite tile presenting gaps and leading edges directly viewed by a high resolution infra-red camera. The parallel power flux deducted from the unperturbed measurement far from the gap is fully consistent with the observed temperature increase at the leading edge, respecting the power balance. All the power flowing into the gap is deposited at the leading edge and no mitigation factor is required to explain the thermal response. Particle-in-cell simulations show that the ion Larmor smoothing effect is weak and that the power deposition on misaligned edges is well described by the optical approximation because of an electron dominated regime associated with non-ambipolar parallel current flow.

Rapid enhancement of low‐energy (<100 eV) ion flux in response to interplanetary shocks based on two Van Allen Probes case studies: Implications for source regions and heating mechanisms

AbstractInteractions between interplanetary (IP) shocks and the Earth's magnetosphere manifest many important space physics phenomena including low‐energy ion flux enhancements and particle acceleration. In order to investigate the mechanisms driving shock‐induced enhancement of low‐energy ion flux, we have examined two IP shock events that occurred when the Van Allen Probes were located near the equator while ionospheric and ground observations were available around the spacecraft footprints. We have found that, associated with the shock arrival, electromagnetic fields intensified, and low‐energy ion fluxes, including H+, He+, and O+, were enhanced dramatically in both the parallel and perpendicular directions. During the 2 October 2013 shock event, both parallel and perpendicular flux enhancements lasted more than 20 min with larger fluxes observed in the perpendicular direction. In contrast, for the 15 March 2013 shock event, the low‐energy perpendicular ion fluxes increased only in the first 5 min during an impulse of electric field, while the parallel flux enhancement lasted more than 30 min. In addition, ionospheric outflows were observed after shock arrivals. From a simple particle motion calculation, we found that the rapid response of low‐energy ions is due to drifts of plasmaspheric population by the enhanced electric field. However, the fast acceleration in the perpendicular direction cannot solely be explained by E × B drift but betatron acceleration also plays a role. Adiabatic acceleration may also explain the fast response of the enhanced parallel ion fluxes, while ion outflows may contribute to the enhanced parallel fluxes that last longer than the perpendicular fluxes.

Plasma interaction with tungsten samples in the COMPASS tokamak in ohmic ELMy H-modes

This paper reports experimental results on plasma interaction with tungsten samples with or without pre-grown He fuzz. Under the experimental conditions, arcing was observed on the fuzzy tungsten samples, resulting in localized melting of the fuzz structure that did not extend into the bulk. The parallel power flux densities were obtained from the data measured by Langmuir probes embedded in the divertor tiles on the COMPASS tokamak. Measurements of the current-voltage probe characteristics were performed during ohmic ELMy H-modes reached in deuterium plasmas at a toroidal magnetic field BT = 1.15 T, plasma current Ip = 300 kA and line-averaged electron density ne = 5×1019 m-3. The data obtained between the ELMs were processed by the recently published first-derivative probe technique for precise determination of the plasma potential and the electron energy distribution function (EEDF). The spatial profile of the EEDF shows that at the high-field side it is Maxwellian with a temperature of 5 -- 10 eV. The electron temperatures and the ion-saturation current density obtained were used to evaluate the radial distribution of the parallel power flux density as being in the order of 0.05 -- 7 MW/m2.

Discovery of the optically bright, wide separation double quasar SDSS J1442+4055

Optically bright, wide separation double (gravitationally lensed) quasars can be easily monitored, leading to light curves of great importance in determining the Hubble constant and other cosmological parameters, as well as the structure of active nuclei and halos of galaxies. Searching for new double quasars in the SDSS-III database, we discovered SDSS J1442+4055. This consists of two bright images (18-19 magnitudes in the r band) of the same distant quasar at redshift z = 2.575. The two quasar images are separated by about 2.1 arcsec, show significant parallel flux variations and can be monitored from late 2015. We also found other two double quasar candidates, SDSS J1617+3827 (z = 2.079) and SDSS J1642+3200 (z = 2.264), displaying evidence for the presence of a lensing object and parallel flux variations, but requiring further spectroscopic observations to be confirmed as lensed quasars.

Electromagnetic Performance Analysis of Novel Flux-Regulatable Permanent Magnet Machines for Wide Constant-Power Speed Range Operation

Two novel structures of permanent magnet (PM) machine, namely a hybrid excitation flux modulation machine (HEFMM) and a variable flux memory machine (VFMM), which have excellent field-weakening capability, are presented in this paper. The HEFMM incorporates the advantages of parallel hybrid excitation structure and flux modulation structure, so as to increase the torque density as well as increase the constant-power speed range. Inspired by the HEFMM, aiming to further improve the efficiency of machine, the VFMM with aluminum-nickel-cobalt (AlNiCo) PMs in the inner stator which can be magnetized by the current pulse of the direct current (DC) windings is developed. With double-stator structure, flux modulation effect in both machines can be employed to realize the hybrid excitation and regulate the air-gap flux density readily. The operation principle is illustrated and the static and steady performances of the machines are analyzed and compared with time stepping finite element analysis, which validates the effectiveness of the proposed designs.

Sediment flux from the morphodynamics of elongating linear dunes

Although dunes are very common bedforms in terrestrial sand seas, the description of linear dune growth, either by extension or lateral accretion, is still hindered by our limited understanding of the underlying mechanisms. Therefore, sand flux estimates from remote imagery rely essentially on the migration speed of barchan dunes, but not on the dynamics of linear dunes. Here we use ∼50 yr of high-resolution aerial and satellite imagery of the Tenere desert (Niger), the world's largest source of mineral aerosols, to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration. As they elongate from topographic obstacles in a zone of low sediment availability with multimodal winds, these elongating lee dunes are ideal to isolate and quantify linear dune growth only by extension. Using similar conditions in a numerical model, we show how deposition downstream of low hills may result in nucleation and development of bedforms. From elongation we derive the local sand flux parallel to the linear dune crests. This study shows that the morphodynamics of linear dunes under complex wind regimes can also be used for assessing sediment flux and wind conditions, comparably to the more-established method of using sand flux estimates perpendicular to the barchan dune crests in zones of unidirectional wind.

Bi-Maxwellian electron energy distribution function in the vicinity of the last closed flux surface in fusion plasma

The first-derivative probe technique was applied to derive data for plasma parameters from the IV Langmuir probe characteristics measured in the plasma boundary region in the COMPASS tokamak and in the TJ-II stellarator. It is shown that in the COMPASS tokamak in the vicinity of the last closed flux surface (LCFS) the electron energy distribution function (EEDF) is bi-Maxwellian with the low-temperature electron fraction predominating over the higher temperature one, whereas in the far scrape-off layer (SOL) the EEDF is Maxwellian. In the TJ-II stellarator during NBI heated plasma the EEDF in the confined plasma and close to the LCFS is bi-Maxwellian while in the far SOL the EEDF is Maxwellian. In contrast, during the ECR heating phase of the discharge both in the confined plasma and in the SOL the EEDF is bi-Maxwellian. The mechanism for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. The comparison of the results from probe measurements with ASTRA package and EIRENE code calculations suggests that the main reason of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms. Results for the electron temperatures and densities obtained by the first-derivative probe technique in the COMPASS tokamak and in the TJ-II stellarator were used to evaluate the radial distribution of the parallel power flux density. It is shown that in the SOL the radial distribution of the parallel power flux density is a double exponential. It is pointed out that in the calculations of the parallel power flux density at the LCFS the energy losses from ionization mechanisms must be taken into account.

Hybrid-Excited Doubly Salient Synchronous Machine With Permanent Magnets Between Adjacent Salient Stator Poles

A novel stator hybrid-excited, parallel flux path, synchronous machine of doubly salient topology is proposed. It has the novel features of: 1) hybrid dc field and permanent magnet (PM) excitation in the stator; 2) magnets placed in the slots between adjacent salient stator poles; and 3) the magnetic poles of PMs arranged, such that the flux premagnetizes the stator, but it is in a direction to oppose the dc excitation flux. The electromagnetic characteristics of the machine are analyzed on open-circuit and load. Since the new machine topology is developed from the variable-flux machine (VFM), a comparison of their electromagnetic torque and machine losses is conducted. The average electromagnetic torque of the hybrid-excited machine can be increased by 18% for fixed copper loss in comparison with the VFM due to the reduction of magnetic saturation in the stator. It is also shown that at high temperatures some risk of PM demagnetization exists when excited with dc and armature currents due to fringing flux that exists between the stator and the rotor poles. However, this affects only a very small area of the PMs. The performance of the hybrid-excited machine is predicted by a 2-D finite-element analysis and experimentally validated.

Studying the spatial variability of methane flux with five eddy covariance towers of varying height

In this study, the spatial representativeness of eddy covariance (EC) methane (CH4) measurements was examined by comparing parallel CH4 fluxes from three short (6m) towers separated by a few kilometres and from two higher levels (20m and 60m) at one location. The measurement campaign was held on an intensively managed grassland on peat soil in the Netherlands. The land use and land cover types are to a large degree homogeneous in the area.The CH4 fluxes exhibited significant variability between the sites on 30-min scale. The spatial coefficient of variation (CVspa) between the three short towers was 56% and it was of similar magnitude as the temporal variability, unlike for the other fluxes (friction velocity, sensible heat flux) for which the temporal variability was considerably larger than the spatial variability. The CVspa decreased with temporal averaging, although less than what could be expected for a purely random process (1/N), and it was 14% for 26-day means of CH4 flux. This reflects the underlying heterogeneity of CH4 flux in the studied landscape at spatial scales ranging from 1ha (flux footprint) to 10km2 (area bounded by the short towers). This heterogeneity should be taken into account when interpreting and comparing EC measurements. On an annual scale, the flux spatial variability contributed up to 50% of the uncertainty in CH4 emissions. It was further tested whether EC flux measurements at higher levels could be used to acquire a more accurate estimate of the spatially integrated CH4 emissions. Contrarily to what was expected, flux intensity was found to both increase and decrease depending on measurement height. Using footprint modelling, 56% of the variation between 6m and 60m CH4 fluxes was attributed to emissions from local anthropogenic hotspots (farms). Furthermore, morning hours proved to be demanding for the tall tower EC where fluxes at 60m were up to four-fold those at lower heights. These differences were connected with the onset of convective mixing during the morning period.

Kapitza resistance at segregated boundaries in β-SiC

The impact of grain-boundary segregation on the high-temperature Kapitza resistance of doped β-SiC using non-equilibrium molecular dynamics simulation is investigated. In particular, low-angle, symmetric tilt grain boundaries are examined to assess the roles of dopant concentration and dopant/matrix interaction strength in determining the resistance. For relatively weak interaction strengths, dopant clustering predominates, and the Kapitza resistance increases significantly for small changes in dopant concentration. As the dopant/matrix interaction strength is increased, dopant layering is observed with a concomitant gradual increase in resistance with concentration. The different interaction strength regimes are investigated by mapping the spatial distribution of boundary temperatures and by quantifying the degree of spatial ordering at a boundary. It was found that dopant clustering leads to a heat flux parallel to the grain-boundary plane and to significant boundary disorder, partly explaining the observed increase in Kapitza resistance at the boundary.

Quantum Interference Effects Theoretically Found in the Photodissociation Processes of the Second Absorption Bands of ICl and IBr Molecules.

Some quantum interference effects are exposed directly in experiments, but others are not and remain just hidden and thus require thorough theoretical analysis to be exposed. In this respect, the second absorption bands of IX (X = Cl, Br) molecules show an interesting behavior in the photofragment anisotropy of the lowest I((2)P3/2)+X((2)P3/2) product channel; it changes from strongly parallel distribution on the shorter wavelength side to strongly perpendicular distribution on the longer wavelength side. Because the responsible perpendicular third Ω = 1 (1(III)) excited state correlating adiabatically to this product channel has only a weak absorption, the parallel component flux yielding the same products must be comparatively weak, even though the responsible parallel excitations to the 0(+)(III) and/or 0(+)(IV) excited states have strong absorptions. In the present theoretical study, the branching ratios and the anisotropy parameters have been obtained using the spin-orbit configuration interaction method combined with the quantum mechanical wavepacket and semiclassical approaches. Significant quantum interference effect between the two dissociative de Broglie waves on the 0(+)(III) and 0(+)(IV) potential curves has been found through the avoided crossing, and the weak parallel flux to the ground-state product channel has been explained by their destructive interference character. This interference effect has weak excitation energy dependence due to rather unique behavior of their almost parallel potential curves from the Franck-Condon to avoided crossing regions.

Carpet thermal cloak realization based on the refraction law of heat flux

Conventional studies on the design of a (carpet) thermal cloak are based on the coordinate transformation techniques, which are complicated and hard to realize with natural materials. Here we show another feasible approach to design the carpet thermal cloak through the rotated alternatively stacking materials based on the refraction law of heat flux in analogy to that of light. To make the consequent bending of heat flux parallel to the profile of the cloaking region, three design rules to realize the carpet cloaking effect by such configurations are provided and validated. The boundary condition tolerances of the carpet thermal cloak are examined as well.

Electron pressure balance in the SOL through the transition to detachment

Upgrades to core and divertor Thomson scattering (DTS) diagnostics at DIII-D have provided measurements of electron pressure profiles in the lower divertor from attached- to fully-detached divertor plasma conditions. Detailed, multistep sequences of discharges with increasing line-averaged density were run at several levels of Pinj. Strike point sweeping allowed 2D divertor characterization using DTS optimized to measure Te down to 0.5eV. The ionization front at the onset of detachment is found to move upwards in a controlled manner consistent with the indication that scrape-off layer parallel power flux is converted from conducted to convective heat transport. Measurements of ne, Te and pe in the divertor versus Lparallel demonstrate a rapid transition from Te⩾15eV to ⩽3eV occurring both at the outer strike point and upstream of the X-point. These observations provide a strong benchmark for ongoing modeling of divertor detachment for existing and future tokamak devices.

Far scrape-off layer particle and heat fluxes in high density – High power scenarios

The far scrape-off layer transport is studied in ASDEX Upgrade H-mode discharges with high divertor neutral density N0,div, high power across the separatrix Psep and nitrogen seeding to control the divertor temperature. Such conditions are expected for ITER but usually not investigated in terms of turbulent SOL transport. At high N0,div and Psep the H-mode discharges enter a regime of high cross-field particle and power transport in the SOL which is accompanied by a significant change of the turbulence characteristic analogous to the transition from conductive to convective transport in L-mode. Parallel particle and power flux densities of several 1023m−2s−1 and 10MWm−2 have been detected about ∼40 to 45mm outside the separatrix mapped to the outer mid-plane. The particle flux fall-off length reached up to 45mm. This paper presents for the first time an empirical condition to enter the high transport regime in H-mode and the relation of this regime to changes in the filamentary transport.

Narrow heat flux channels in the COMPASS limiter scrape-off layer

The ITER first wall is designed for start-up and ramp-down in limiter configuration. The wall panels are toroidally shaped in order to spread the incident parallel power flux q|| uniformly, assuming a single decay length λq whose value is not known from first principles. In order to study the scaling of q|| with plasma parameters, infra-red viewing of specially-designed limiters has been used on the COMPASS tokamak in ∼100 discharges with scans in Ip, ne and for all combinations of magnetic field and Ip directions. The IR measurement clearly shows that in addition to the main SOL heat flux profile with λq>40mm, a steep gradient (λqnear=4±2mm) dominates q|| near separatrix. This appears independently of limiter shaping, insertion with respect to neighbors and incident field-line angles. Good agreement is found between the measured λqnear and the prediction of a heuristic drift-based model.

Numerical evaluation of heat flux and surface temperature on a misaligned JET divertor W lamella during ELMs

A series of experiments has been performed on JET to investigate the dynamics of transient melting due to edge localized modes (ELMs). The experiment employs a deliberately misaligned lamella in one module of the JET bulk tungsten outer divertor, allowing the combination of stationary power flux and ELMs to transiently melt the misaligned edge. During the design of the experiment a number of calculations were performed using 2D particle-in-cell simulations and a heat transfer code to investigate the influence on the deposited power flux of finite Larmor radius effects associated with the energetic ELM ions. This has been performed using parameter scans inside a range of pedestal temperatures and densities to scope different experimentally expected ELM energies. On the one hand, we observe optimistic results, with smoothing of the heat flux due to the Larmor gyration on the protruding side of the lamella which sees the direct parallel flux—the deposited power tends to be lower than the nominal value expected from geometric magnetic field line impact over a distance smaller than 2 Larmor radii, a finding which is always valid during ELMs for such a geometry. On the other hand, the fraction of the flux not reaching the directly wetted side is transferred and spread to the top surface of the lamella. The hottest point of the lamella (corner side/top) does not always benefit from the gain from the Larmor smoothing effect because of an enhanced power deposition from the second contribution.

Analysis of the temperature of the rail/wheel contact surface using a half-space model and a moving heat source

The model can be used to analyse the thermal processes due to a frictional block or disc braking or to analyse the temperature conditions close to the ril/wheel contact patch, when macroscopic sliding occurs. It is especially useful in the evaluation of rail chill, head check and other contact tribology-based local heat and stress effects. The contact seizure theory of gear drives developed by Blok is based on a simplified approach to the determination of the temperature due a moving heat source generated by sliding friction. In a simplifying step, the heat flux parallel to the surface of the contacting bodies is disregarded. One-dimensional temperature patterns that vary with time are arranged along the line of motion, based on the displacement of the source. This one-dimensional model is applicable if the velocity (i.e. Péclet number) is ‘high enough’. The influence of the width of the source has not been examined. The literature contains analytical results for various heat flux models, and one can make use of them to overcome the restrictions inherent in the one-dimensional theory. However, their rather complex structure makes them unsuited for further analysis, and only the numerical analysis method appears to be fruitful. One approach is to describe the temperature conditions created in a plate, which extends on a half-plane, by heating a part of its edge. The obtained time-dependent temperature results can be arranged into a sequence in a similar manner as in the approach of Blok. This shows the spatial temperature distribution that can be obtained by enhancing the results obtained using the one-dimensional theory by considering the effect of the crosswise dispersed heat. For heat sources of limited width, the cross-corrected approach better fits the reference values given by a full three-dimensional model. In railway traction, the analysis can be most directly applied to gear drives with a localized load pattern.

Geometric dependence of Nb-Bi2Te3-Nb topological Josephson junction transport parameters

Superconductor-topological insulator–superconductor Josephson junctions have been fabricated in order to study the width dependence of the critical current, normal state resistance and flux periodicity of the critical current modulation in an external field. Previous literature reports suggest anomalous scaling in topological junctions due to the presence of Majorana bound states. However, for most realized devices, one would expect that trivial -periodic Andreev levels dominate transport. We also observe anomalous scaling behaviour of junction parameters, but the scaling can be well explained by mere geometric effects, such as the parallel bulk conductivity shunt and flux focusing.