Displacement Events Research Articles

Analysis method for magneto-mechanical coupled vibration of the in-vessel structures considering halo current effect and application to HL-2M tokamak

Abstract The tokamak in-vessel structures serve in strong magnetic fields. During plasma disruptions such as the vertical displacement events, the induced eddy current and the injected halo current in in-vessel structures cause the vibration of the plasma-facing components. The vibration is coupled to the magnetic field in the form of the motional eddy current. Such magneto-mechanical coupling effect may have a considerable influence on the dynamic mechanical behavior of the in-vessel structures. This paper presents a numerical approach to analyze this magneto-mechanical coupled problem based on the hybrid method of the finite element and boundary element formulation. The plasma current and the halo current are taken into account in a form of a series of movable current filaments and a pair of current source and sink. The proposed approach is applied to the numerical analysis of a simplified model of the vacuum vessel of HL-2 M tokamak under an l -mode plasma disruption. Simulation results show that the vibration is strongly damped and the maximum vertical displacement is reduced by around 30 % when considering the magneto-mechanical coupling.

Competitive interaction between Frankliniella occidentalis and locally present thrips species: a global review

The most severe outcome of the widespread interspecific competition that occurs between invasive organisms and their local congeners is species displacement. The western flower thrips, Frankliniella occidentalis (Pergande), which originated from western North America, has invaded much of the agricultural world since the 1970s, and in so doing, has become a dominant thrips species in many of the areas it has invaded. Its invasion success and the extent of its distribution in the regions it has invaded can be largely attributed to its superiority in interspecific competition. In some instances, however, F. occidentalis has been less successful in its invasion attempts and has not become dominant in its new environment. Thrips species displacements often arise from interactions of different mechanisms that are mediated by numerous biotic and abiotic factors. In this review, we summarize competitive interaction events that have been documented between F. occidentalis and several species of other locally present thrips, their interaction mechanisms and mediating factors. This review will help to better understand displacement events of thrips species in some areas and to develop management strategies for thrips species with high invasion potential.

Eddy Current Analysis of TF Coil Case for EAST

Plasma major disruptions and plasma vertical displacement events represent two extreme examples of fast transient electromagnetic processes, which will induce large current and electromagnetic loads that exceed those of loads during magnet normal operation and seismic loads. Eddy current and electromagnetic loads of the conducting structures need to be analyzed to verify the mechanical reliability of the tokamak during these transients. An integrated module, which consists of the mechanical design of the three-dimensional computer-aided design (CAD) solid modeling, generation of the finite element modeling, and external parameters input into the module, was integrated to perform the calculation of the electromagnetic loads and power losses. The developed integrated module can be used to calculate the Eddy current, electromagnetic forces, and power losses of the toroidal field (TF) coil-case for EAST tokamak during plasma disruptions and vertical displacement events. The simplified geometric model with 22.5° sector for EAST tokamak was used in view of the toroidal symmetry. The plasma current and position as functions of time are given by the numerical method. In this article, the module integration and evaluation of the electromagnetic loads and power losses on the conducting structures of the EAST tokamak by the finite element methods are described.

A Real-Time Disruption Prediction Tool for VDE on EAST

Vertical displacement event (VDE) is one of the most important events related to disruptions. A real-time disruption prediction tool for VDE is presented in this article. Vertical growth rate is an essential parameter to describe vertical instability, which can be obtained based on the rigid plasma response model. A real-time vertical growth rate calculation system is built, which utilizes graphics process unit (GPU) parallel computation and reflective memory (RFM) boards communication with plasma control system (PCS). Based on real-time vertical growth rate calculation system, an alarm algorithm for detecting vertical instability is integrated into experimental advanced superconducting tokamak (EAST) PCS. It is designed to provide the capabilities of a switching control method and to trigger a warning signal in PCS when uncontrollable vertical instability is detected. Such a disruption prediction tool has been preliminarily applied in the 2019 EAST spring experiment.

Reduction of asymmetric wall force in JET and ITER disruptions including runaway electrons

It was shown that asymmetric vertical displacement event disruptions in JET and ITER do not produce a large asymmetric force on conducting structures surrounding the plasma, provided that the current quench (CQ) time is less than the resistive wall penetration time. This is verified with JET data. In ITER, the CQ is expected to be comparable to the resistive wall penetration time, which gives a small wall force. It is shown that runaway electrons (REs) in JET do not produce a large wall force, and this is verified with JET data. This occurs if the RE current is about half of the initial plasma current. In ITER, the RE current might be comparable to the initial current, and REs might lengthen the CQ time sufficiently to produce a large wall force.

Axisymmetric simulations of vertical displacement events in tokamaks: A benchmark of M3D-C1, NIMROD, and JOREK

A benchmark exercise for the modeling of vertical displacement events (VDEs) is presented and applied to the 3D nonlinear magnetohydrodynamic codes M3D-C1, JOREK, and NIMROD. The simulations are based on a vertically unstable NSTX equilibrium enclosed by an axisymmetric resistive wall with a rectangular cross section. A linear dependence of the linear VDE growth rates on the resistivity of the wall is recovered for sufficiently large wall conductivity and small temperatures in the open field line region. The benchmark results show good agreement between the VDE growth rates obtained from linear NIMROD and M3D-C1 simulations and from the linear phase of axisymmetric nonlinear JOREK, NIMROD, and M3D-C1 simulations. Axisymmetric nonlinear simulations of a full VDE performed with the three codes are compared, and an excellent agreement is found regarding the plasma location and plasma currents, as well as eddy and halo currents in the wall.

FAK regulates dynein localisation and cell polarity in migrating mouse fibroblasts.

Fibroblasts executing directional migration position their centrosome, and their Golgi apparatus, in front of the nucleus towards the cell leading edge. Centrosome positioning relative to the nucleus has been associated to mechanical forces exerted on the centrosome by the microtubule-dependent molecular motor cytoplasmic dynein 1, and to nuclear movements such as rearward displacement and rotation events. Dynein has been proposed to regulate the position of the centrosome by exerting pulling forces on microtubules from the cell leading edge, where the motor is enriched during migration. However, the mechanism explaining how dynein acts at the front of the cells has not been elucidated. We present here results showing that the protein Focal Adhesion Kinase (FAK) interacts with dynein and regulates the enrichment of the dynein/dynactin complex at focal adhesions at the cell the leading edge of migrating fibroblasts. This suggests that focal adhesions provide anchoring sites for dynein during the polarisation process. In support of this, we present evidence indicating that the interaction between FAK and dynein, which is regulated by the phosphorylation of FAK on its Ser732 residue, is required for proper centrosome positioning. Our results further show that the polarisation of the centrosome can occur independently of nuclear movements. Although FAK regulates both nuclear and centrosome motilities, downregulating the interaction between FAK and dynein affects only the nuclear independent polarisation of the centrosome. Our work highlights the role of FAK as a key player in the regulation of several aspects of cell polarity. We thus propose a model in which the transient localisation of dynein with focal adhesions provides a tuneable mechanism to bias dynein traction forces on microtubules allowing proper centrosome positioning in front of the nucleus. We unravel here a new role for the cancer therapeutic target FAK in the regulation of cell morphogenesis.

First wall energy deposition during vertical displacement events on ITER

The beryllium (Be) first wall energy deposition and melt damage profiles resulting from the current quench phase of an unmitigated, 5 MA/1.8 T upward vertical displacement event for ITER are investigated. Time dependent 2D magnetic flux profiles are calculated with the DINA code and used as input for the SMITER 3D field line tracing software. 3D maps of the wetted area and perpendicular heat flux show that the majority of the energy deposition occurs on the upper first wall panels #8 and #9. SMITER simulations predict on the surfaces of upper FWPs #8 and #9 at the end of the ∼450 ms current quench. The surface heat flux maps generated by SMITER are used as input in the MEMOS-U code, which models Be melt formation and dynamics. Simulations reveal peak surface temperatures of ∼2200 K, inward surface damage of ∼0.5 mm in depth, and average melt velocities of ∼2 m s−1. Although VDEs are in principle the easiest disruptive instability to avoid, the analysis demonstrates that any non-mitigated events or intentional VDEs taking place during low Ip, early operational phases of ITER for the purposes of estimating disruption forces, must be kept to a low number.

Dynamics of enhanced gas trapping applied to CO2 storage in the presence of oil using synchrotron X-ray micro tomography

During CO2 storage in depleted oil fields, under immiscible conditions, CO2 can be trapped in the pore space by capillary forces, providing safe storage over geological times - a phenomenon named capillary trapping. Synchrotron X-ray imaging was used to obtain dynamic three-dimensional images of the flow of the three phases involved in this process - brine, oil and gas (nitrogen) - at high pressure and temperature, inside the pore space of Ketton limestone. First, using continuous imaging of the porous medium during gas injection, performed after waterflooding, we observed chains of multiple displacements between the three phases, caused by the connectivity of the pore space. Then, brine was re-injected and double capillary trapping - gas trapping by oil and oil trapping by brine - was the dominant double displacement event. We computed pore occupancy, saturations, interfacial area, mean curvature and Euler characteristic to elucidate these double capillary trapping phenomena, which lead to a high residual gas saturation. Pore occupancy and saturation results show an enhancement of gas trapping in the presence of both oil and brine, which potentially makes CO2 storage in depleted oil reservoirs attractive, combining safe storage with enhanced oil recovery through immiscible gas injection. Mean curvature measurements were used to assess the capillary pressures between fluid pairs during double displacements and these confirmed the stability of the spreading oil layers observed, which facilitated double capillary trapping. Interfacial area and Euler characteristic increased, indicating lower oil and gas connectivity, due to the capillary trapping events.

Simulation of disruptions triggered by Vertical Displacement Events (VDE) in tokamak and leading edge effect in plasma energy deposition to material surfaces

The paper describes two major non-linear properties of the vertical instability of a tokamak plasma, which has a vertical elongation: (a) inductive excitation of surface (edge) currents stabilizing the instability and converting it into fast equilibrium evolution, and (b) creation of a wetting zone without the normal component of the magnetic field when the plasma has contact with material surfaces. Two major disruption effects for both mitigated and non-mitigated disruptions, important for JET and ITER, were considered: (a) excitation of vertical disruption during the current quench (i.e., abnormal plasma current ramp down) and (b) related to the wetting zone, potential leading edge effect in plasma energy deposition to the in-vessel tiles during disruptions. Our considerations together with a 2-D version of the VDE (Vertical Disruption Event) code are based on a new mathematical model, called Tokamak MHD (TMHD), as a replacement for the conventional model, a model that cannot solve numerical problems related to extreme plasma anisotropy and negligible mass. The code includes a 3-D model of surface currents on a thin conductive wall and has a well-specified algorithm for extension to vertical disruptions that excite asymmetric kink modes.

UN Peacekeeping and Forced Displacement in South Sudan

ABSTRACTDoes UN peacekeeping reduce the number of people forcibly displaced by violence? While previous research has found that the presence and size of peacekeeping deployments can reduce violence, little is known about how peacekeepers affect other aspects of civilian protection. Using original data on sub-national events of forced displacement and the location and size of UN troop deployments this study systematically evaluates the criticized efforts of UNMISS in South Sudan, while simultaneously testing hypotheses on peacekeepers and forced displacement. It is hypothesized that increasing numbers of troops affect the flight equation among civilians through the promise of and actual deterrence of violence. These deterrence-based hypotheses are also discussed in relation to the South Sudan context, creating scope conditions for their possible application in this case. The statistical analysis provides, however, no robust evidence for peacekeepers reducing the occurrence or levels of forced displacement, and only weak evidence of displaced congregating in larger numbers around peacekeeping locations. The paper ends by arguing that the theoretical argument provided may still be valid, but that an effect was not feasible to identify in South Sudan where the peacekeeping mission – despite its comparatively large numbers – lacks credible deterrent capacity.

Vertical forces during vertical displacement events in an ITER plasma and the role of halo currents

Vertical displacement events (VDEs) can occur in elongated tokamaks causing large currents to flow in the vessel and other adjacent metallic structures. To better understand the potential magnitude of the associated forces and the role of the so called ‘halo currents’ on them, we have used the M3D-C1 code to simulate potential VDEs in ITER. We used actual values for the vessel resistivity and pre-quench temperatures and, unlike most of the previous studies, the halo region is naturally formed by triggering the thermal quench with an increase in the plasma thermal conductivity. We used the 2D non-linear version of the code and vary the post-thermal quench thermal conductivity profile as well as the boundary temperature in order to generate a wide range of possible cases that could occur in the experiment. We also show that, for a similar condition, increasing the halo current does not increase the total force on the wall since it is offset by a decrease in the toroidal contribution.

Electromagnetic-mechanical coupling method and stress evaluation of the Chinese Fusion Engineering Test Reactor helium cooled solid breeder under a vertical displacement event scenario

The helium cooled solid breeder (HCSB) blanket is one of the candidate blanket schemes proposed for the Chinese Fusion Engineering Test Reactor (CFETR). Plasma vertical displacement events (VDE) are considered to be important for evaluating the structural integrity of the blanket. The change of the magnetic field caused by the movement of the plasma and the rapid loss of current induces extra electromagnetic loads, which will cause significant structural damage to the blanket. This study aims to investigate the electromagnetic-mechanical coupling method and the coupling effects on the HCSB under plasma VDE. Based on the design of the HCSB for the CFETR, a finite element model is established by using the MAXWELL software, in which 69 current filaments are used to describe the plasma. The distribution of the induced current and magnetic field inside the HCSB under the plasma VDE, as well as the electromagnetic force are obtained by the ANSYS finite element software. Meanwhile, the distributions of stress inside each component of the blanket are obtained under electromagnetic-structure load and an evaluation of the stress is carried out. The results show that the existing design meets the requirements when plasma VDE occurs.

RESEARCH OF WHEEL-RAIL WEAR DUE TO NON-SYMMETRICAL LOADING OF A FLAT CAR

Purpose. The paper is aimed at determining the influence of non-symmetrical loading of a flat car on the magnitude of the wear factor of a wheel-rail pair when changing the operation parameters that occur in operation. Methodology. The dynamic loading of the flat car, model 13-401 with typical three-piece bogies is studied using a model of spatial oscillations of a five-car coupling with the help of mathematical and computer simulation. Theoretical calculations are performed for the most dangerous sections of the railway track – small and medium radius curves in the range of permissible speeds. Findings. The indicators of wear of the rolling stock wheels and the rails are analysed on the example of flat cars in the presence of a longitudinal and transverse displacement of the load mass centre relative to the car symmetry centre. To obtain information on the effect of permissible deviations of the arrangement of cargo in the car on the magnitude of the dynamic loading of the wheel-rail contact, the authors performed theoretical studies of the spatial variations of the rail carriage and its interaction with the track. Originality. To determine the wear of the wheel-rail pair, the effect of displacement in two directions from the central axis of symmetry of the load gravity centre was studied, taking into account the value of the travel speed along the curved sections of the small and medium radius using a mathematical model of coupling of five freight cars. Practical value. As a result of the theoretical studies carried out, the authors assessed such factors as wear factor, directional force, and hunting of the wheel set of freight rolling stock in the event of load gravity centre displacement when moving along curved sections of the railway track. To establish the possible cause of intensive wear of the wheels and rails, the following parameters were analysed: lozenging of front bogie side frames; hunting of the left side frame of the front bogie; mutual longitudinal movement of the side frame and axle box of the front wheel set; mutual hunting of the left side frame of the bogie relative to the front wheel set.

The thermal stability of dispersion-strengthened tungsten as plasma-facing materials: a short review

One key challenge for the development of fusion energy is plasma-facing materials. Tungsten-based materials are promising candidates for plasma-facing components (PFCs) in the magnetic confinement nuclear fusion reactors because of their high melt temperature, high-thermal conductivity, high-thermal load resistance, low tritium retention, and low sputtering yield. In fusion reactors, PFCs are exposed to high-thermal flux, because there are some transient events such as plasma disruptions, edge-localized modes, and vertical displacement events (VDEs). Especially, in VDEs, a heat flux of 10–100 MW m−2 with duration of milliseconds-to-several seconds can induce recrystallization and then change the microstructure of tungsten-based plasma-facing materials, leading to instability of microstructures. Then, a significant degradation of material properties is caused such as a reduction of mechanical strength and fracture toughness, a rise in the ductile-to-brittle-transition temperature well, and decrease of irradiation/high-thermal load resistance. Therefore, many efforts were devoted to improve the thermal stability of tungsten-based materials as high as possible, such as oxide dispersion strengthening, carbide dispersion strengthening, and K bubbles dispersion strengthening. Here, the thermal stabilities of various dispersion-strengthened tungsten materials are reviewed by evaluating their recrystallization temperature and the corresponding hardness evolutions. In addition, the possible development trends are proposed.

Rolling circle amplification: A (random) primer on the enrichment of an infinite linear DNA template

AbstractRolling circle amplification (RCA) is a robust enzymatic process in which a circular DNA molecule serves as a template for exponential amplification of molecular targets of interest. Random primers that have bound to DNA sequences across the circularized DNA template initiate copying via the action of a highly processive DNA polymerase. Following a cascade of strand displacement events, long tandem copies of the original template sequence are generated. Because the template molecule is in a circular format, it essentially serves as an infinite linear template. In conjunction with selective enrichment techniques, RCA may prove highly useful in the selectivity and sensitivity of current DNA typing systems. In addition, circularization potentially may allow for amplification of fragmented and low copy number DNA encountered in forensic casework samples that can be subsequently typed.This article is categorized under: Forensic Biology > Forensic DNA Technologies

Micro-scale experimental investigations of multiphase flow in oil-wet carbonates. II. Tertiary gas injection and WAG

Carbonate reservoirs are typically oil-wet, which can lead to low oil production during the oil recovery processes such as base waterflooding. The residual oil in these reservoirs is the target of many EOR techniques such as tertiary gas injection and Water-Alternating-Gas (WAG) flooding. Currently, there is scarce research investigating the pore-scale displacement physics governing the afore-mentioned EOR techniques in oil-wet carbonates. In this study, three sets of miniature core-flooding experiments were performed in carbonate samples at elevated pressure and temperature conditions using a three-phase core-flooding system integrated with a high-resolution X-ray micro-CT scanner. Pore-scale displacement events governing tertiary gas injection and WAG under weakly oil-wet conditions were examined. The prevailing oil mobilization mechanisms during the tertiary gas injection were further investigated under varying wettability conditions. The results revealed that gas-to-oil-to-brine double displacement is the main displacement chain for oil production by tertiary gas injection. During this process, the greater was the degree of oil-wetness of the rock, the larger became the additional oil recovery that could be achieved. This was due to the increase in the frequency of gas-to-oil-to-brine double displacements and the superior connectivity of the oil phase under stronger oil-wet conditions. Furthermore, WAG flooding significantly increased the displacement efficiency of both gas and brine phases because of the shielding effect of the trapped gas ganglia. In the first WAG cycle, oil was produced through a series of direct and double displacements. Multiple displacements started taking place and further contributed to oil recovery as more WAG cycles were implemented.

Simulations of plasma disruptions in ITER due to ingress of Be

ITER plasma will be surrounded by the first wall, which is mostly blankets made of beryllium (Be) and divertor plates made of tungsten (W). When the plasma is subjected to vertical displacement events (VDEs), it could come in contact with these first wall materials—be it in the upper dome during upward-moving VDEs or radial plasma movements and W divertor in the downward-moving VDEs. We investigate in this paper specifically the first case of plasma moving upwards, hitting the top dome and ejecting a small piece of Be. The piece of Be falls freely through the plasma, ablating as it moves down causing the plasma temperature to crash, although in a rather slow process of few hundred milliseconds, finally creating plasma disruption and generating a runway current. This whole scenario is simulated using the tokamak simulation code. We have also carried out preliminary simulation studies of possible mitigation of the runaways generated following the Be ingress through injection of deuterium pellets.

Energy-flux control of the steady-state, creep, and dynamic slip modes of faults

Faults exhibit a gamut of slip styles from stable sliding and creep events to earthquakes. These slip styles are affected by the fault properties, e.g., weakening or strengthening, and the properties of the loading system. Here, we investigate the poorly understood effect of energy-flux to the fault that should equal or exceed the energy-dissipation-rate along the slipping fault. We explore the relationship between energy-flux and slip style in shear experiments along granite and diorite laboratory faults, during which the faults were subjected to controlled energy-flux, and responded spontaneously to it. The monitored evolution of slip-velocity, shear stress, and slip-distance revealed three slip styles that depend on the applied energy-flux: (1) steady-state slip; (2) spontaneous creep events of small displacement with negligible weakening; and (3) spontaneous, unstable events with slip-velocities up to 0.8 m/s, slip-distances up to 0.5 m, and stress-drops up to 1 MPa, which are comparable to observed values of moderate earthquakes. These slip styles are similar in character to those observed along natural faults. We further propose that the rate of energy flow from crustal blocks can control the slip velocity during earthquakes.

Experimental Analysis and Numerical Modeling of Polymer Flooding in Heavy Oil Recovery Enhancement: A Pore-Level Investigation

This study provides new insights into pore-scale displacement events during the simultaneous flow of a low-molecular-weight polymer solution and heavy oil through porous media. Rheological measurements were taken to examine the viscosifying ability of the utilized polymer. The efficiency of the employed solutions in enhancing heavy oil recovery was also investigated using a pore network micromodel. Both macroscopic and microscopic sweep efficiencies were evaluated by analyzing high-resolution pictures continuously captured during the multiphase flow experiments. The rheological measurements revealed that viscosity of the polymer solution is more sensitive to increasing polymer concentration than salinity. Oil recovery experiments disclosed that polymer flooding could yield a recovery factor of about 58% of original oil in-place (OOIP), while the ultimate recovery factor for water flooding is only 35% of OOIP. The macroscopic observations proved that dwindling the formation of viscous fingers during polymer flooding is one of the main responsible factors for the significant improvement of heavy oil recovery. Moreover, the microscopic observations unveiled the noticeable effect of the polymer solution on the enhancement of microscopic sweep efficiency and showed that pulling effects and stripping mechanisms are effective in reducing the saturation of heavy oil at dead ends and pore walls.