Electric Field Shear Research Articles

Trajectories, orbit squeezing and residual zonal flow in a tokamak pedestal

Particle orbits and the Rosenbluth–Hinton residual zonal flow are evaluated for the case of a strong equilibrium radial electric field Er = −(dϕ/dψ)|∇ψ|as observed in a tokamak pedestal. The ordering used is Er ∼ Bpvi/c, where Bp is the poloidal field and vi is the ion thermal speed. The earlier calculations of Kagan and Catto for this regime (2009 Phys. Plasmas 16 056105) correctly describe the effect of a uniform dϕ/dψ, but not the ‘orbit squeezing’ effects associated with electric field shear d2ϕ/dψ2 (Kagan and Catto 2009 Phys. Plasmas 16 099902). The present paper gives the corrected dependence on electric field shear for a quadratic equilibrium potential ϕ(ψ). Analytical results are given for the large-aspect-ratio limit and a numerical approach is outlined for performing calculations at finite aspect ratio.

A quasi‐Bingham model for predicting electrorheological fluid behaviour

Purpose – The purpose of this paper is to present experimental research on the behaviour of a new electrorheological fluid (ETSERF).Design/methodology/approach – The ETSERF is a suspension based on diatomite powders dispersed in silicon oil with a surfactant. A design of experiments is conducted to investigate the effects of electric field strength, particle concentration, surfactant percentage, particle size and shear rate on the efficiency of ETSERFs. The influence of the interactions on shear stresses is analyzed by varying all the combinations of the independent variables. The dielectric properties of the ETSERF are investigated in order to explain the interactions between these independent variables. Furthermore, a quantitative relationship between the dynamic shear stresses and the independent variables is developed.Findings – The relationship provides a very useful explanation for the contributions of each independent variable to the viscosity and yield stress.Originality/value – A new empirical mo...

Simulation of the Radial Electric Field Shear in the Edge Plasma of Small Size Divertor Tokamak

The simulation of the radial electric field shear, which is responsible for L-H transition by means B2SOLPS0.5.2D transport code, gives the dependence of this shear on plasma parameters. Also, as result of uni-directional neutral beam heating, internal transport barrier is formed and ion radial heat flux qir starts to decrease. Furthermore, the dependence of radial electric field shear on ion temperature gradient ITG has also investigated.

Electrorheological behavior of biodegradable modified corn starch/corn oil suspensions

In this study, an electrorheological (ER) effect of the suspensions containing both native starch (S) and modified starch (MS) particles in corn oil (CO) under various externally applied electric field strengths ( E) are reported. To prepare an ER active material, biodegradable starch was partially hydrolyzed and converted to its Li-salt. Both biopolymers (S and MS) were characterized by FT-IR, 1H and 13C NMR, SEM, energy dispersive spectroscopy (EDS), TGA and conductivity measurements. Suspensions of S and MS particles were prepared in CO at concentrations ranging from 5% to 40% by mass. Effects of various parameters such as sedimentation stability, particle size, dispersed particle concentration, electric field strength, shear rate, frequency and temperature onto ER activity of suspensions were investigated. Further, creep behaviors of S/CO and MS/CO suspensions were also investigated.

Synthesis, electrorheology and creep behavior of polyindole/polyethylene composites

In this study, polyindole (PIN) and polyindole/polyethylene (PIN/PE) conducting composites, having various amounts of PIN, were synthesized by chemical polymerization using FeCl 3 as an oxidizing agent and taking the ratio of salt:monomer as 3:1. The samples of PIN and PIN/PE composites were characterized by FTIR, UV–vis, TGA, SEM, Gouy scale magnetic susceptibility, conductivity (1.2 × 10 −3 S cm −1 > σ > 1.96 × 10 −6 S cm −1, at T = 25 °C) and density measurements. FTIR analysis suggested a 2,3-propagation mechanism for PIN formation. The ground milled samples were subjected to particle size analysis by dynamic light scattering (DLS) and a micron-sized particle distribution was obtained. A series of volume fractions ( ϕ = 10–25%) were prepared from the materials in silicone oil (SO) and their sedimentation stabilities determined. The most stable composite [PIN(89%)/PE(11%)] against gravitational sedimentation was subjected to flow-rate measurements under externally applied electric field strength ( E) and an electrorheological (ER) activity was observed; threshold energies ( E t ) were calculated. The effects of volume fraction, shear rate, external E, frequency and temperature onto ER activities of the suspensions were investigated. Enhancement in the electric field viscosities and shear thinning viscoelastic behaviors were observed for all the samples studied. Recoverable viscoelastic deformations were determined from the creep tests under external E.

전단 유동 하에서 전기유변유체의 과도응답 특성

The transient shear stress response of an electrorheological fluid is investigated experimentally. The characteristic time constants of an electrorheological fluid sheared between two concentric cylinders were obtained under various electric field strengths and shear rates. Also, two experimental modes are adopted to investigate the effect of the shear flow on the dynamic behavior of the fluid; one is that the electric field is induced before shearing, and the other is the electric field is induced after shearing. From the difference in the response time between two modes, the cluster formation time were obtained. The response times were decreased with the increase of the shear rate, irrelatively of the electric field strength. The cluster formation time were monotonically increased with increase of shear rate, and thereafter, were converged with a certain value.

A novel mechanism for exciting intrinsic toroidal rotation

Beginning from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers two physically distinct mechanisms by which microturbulence may drive intrinsic rotation. The first mechanism, which emanates from E×B convection of parallel momentum, has already been analyzed [O. D. Gurcan et al., Phys. Plasmas 14, 042306 (2007); R. R. Dominguez and G. M. Staebler, Phys. Fluids B 5, 3876 (1993)] and was shown to follow from radial electric field shear induced symmetry breaking of the spectrally averaged parallel wave number. Thus, this mechanism is most likely active in regions with steep pressure gradients or strong poloidal flow shear. The second mechanism uncovered, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. This novel means of driving intrinsic rotation, while nominally higher order in an expansion of the mode frequency divided by the ion cyclotron frequency, does not depend on radial electric field shear. Thus, while the magnitude of the former mechanism is strongly reduced in regions of weak radial electric field shear, this mechanism remains unabated and is thus likely relevant in complementary regimes.

Effect of cluster formation on dynamic response of an electrorheological fluid in shear flow

Transient shear stress responses of an electrorheological fluid to step-wise electric fields are investigated experimentally. The characteristic time constants of the electrorheological fluid sheared between two concentric cylinders are obtained under various electric field strengths and shear rates. Especially, two experimental modes are adopted to investigate the effect of the cluster formation on the dynamic response of the fluid; one is that the electric field is induced before shearing, and the other is the electric field is induced after shearing. The cluster formation time is obtained from the difference in the response times between the two modes. The cluster formation time is monotonously increased with the increase of shear rate and converged on a constant value.

The enhanced pedestal H-mode in the National Spherical Torus experiment

Typical H-mode pedestal electron and ion temperatures in NSTX range from 100–300 eV. A new operating regime termed the ‘enhanced pedestal’ (EP) H-mode has been observed in which the pedestal ion temperature increases to ∼600 eV in about 50 ms, or one energy confinement time, resulting in a global confinement improvement. Ion temperature gradients as high as 30 keV/m are observed. The regime is correlated with a localized braking of the edge toroidal rotation near the q = 3 surface, in which case the pressure gradient term in the radial force balance becomes dominant over the toroidal and poloidal rotation terms. Coupled with increased rotation just inside the barrier, the radial electric field shear is also increased. An MHD trigger event (large edge localized mode) is common to the formation of the EP H-mode phase, which can occur either during the I p ramp-up or flat-top phases of discharges. The observed characteristics of this scenario are presented.

Analytic theory of L→H transition, barrier structure, and hysteresis for a simple model of coupled particle and heat fluxes

The two-field (pressure/density) model for the L→H transition is extended and analyzed qualitatively. In its original form the model is ambiguous as to the location of the transition within the range of bistability of particle and thermal fluxes. Here, the model is regularized by including (i) hyperdiffusion, (ii) time dependence, and (iii) curvature of the pressure profile. The regularizations (i)–(ii) agree and indicate that the Maxwell rule for the forward and back transition applies, as opposed to the maximum flux forward and minimum flux backward transition rules (which yields hysteresis) as suggested previously. Regarding (i)–(ii), simple models suggest that for a pressure gradient driven electric field shear bifurcation, the basic scale of the pedestal is inexorably tied to the particle fueling depth, which normally is the neutral penetration depth. There is no hysteresis predicted by the local model of transport suppression. However, the effect of pressure profile curvature (iii) changes these results substantially. When it dominates, the curvature effect reduces the transition threshold to the lower end of the range of heating power, which falls within the phase coexistence region for both forward and back transitions. This softens the transition threshold requirements. In this limit, the model with pressure curvature also predicts transitions which occur in regimes of flat density and driven exclusively by the temperature gradient. This allows the pedestal to extend beyond the fueling depth, and also allows some decoupling of density and pressure profiles. In a parameter range where the pressure curvature is less important the transition occurs somewhere between the above two limits.

Reduction of chaotic particle transport driven by drift waves in sheared flows

Investigations of chaotic particle transport by drift waves propagating in the edge plasma of tokamaks with poloidal zonal flow are described. For large aspect ratio tokamaks, the influence of radial electric field profiles on convective cells and transport barriers, created by the nonlinear interaction between the poloidal flow and resonant waves, is investigated. For equilibria with edge shear flow, particle transport is seen to be reduced when the electric field shear is reversed. The transport reduction is attributed to the robust invariant tori that occur in nontwist Hamiltonian systems. This mechanism is proposed as an explanation for the transport reduction in Tokamak Chauffage Alfvén Brésilien [R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)] for discharges with a biased electrode at the plasma edge.

Simultaneous evolution of plasma rotation, radial electric field, MHD activity and plasma confinement in the STOR-M tokamak

Radial electric field shear and poloidal plasma rotation are important factors affecting transport and confinement in tokamaks. Alteration of the electric field and plasma rotation in the vicinity of magnetic islands is also an important factor in tokamak plasma confinement. In the STOR-M tokamak, fast (∼1 ms) simultaneous alterations of the radial electric field, plasma rotation (M‖ = 0–0.4 in the plasma current direction), floating potential fluctuations in the periphery and MHD activity generated by rotating islands have been observed experimentally during normal ohmic discharges. The observed time and magnitude of the changes depend on the average electron density and poloidal beta at the beginning of the discharge. In discharges with high initial poloidal beta these changes are accompanied by a reduction in Hα emission and an increase in the line averaged density. Drastic decreases in Hα and increases in line averaged electron density and estimation of poloidal beta suggest that STOR-M confinement is significantly affected in ohmic discharges without an external additional energy input or biasing. MHD activity in STOR-M is damped when a negative electric field is observed at the limiter region of the plasma edge. MHD frequency is observed to decrease with the negative electric field.

Stochastic magnetic field driven charge transport and zonal flow during magnetic reconnection

Magnetic fluctuation-induced charge transport, resulting from particle transport that is not intrinsically ambipolar, has been measured in the high-temperature interior of a reversed-field pinch plasma. It is found that global resistive tearing modes and their nonlinear interactions lead to significant charge transport, equivalent to the perpendicular Maxwell stress, in the vicinity of the resonant surface for the dominant core resonant mode during magnetic reconnection. Finite charge transport can result in a zonal flow associated with locally strong radial electric field and electric field shear. In the presence of stochastic magnetic field, radial electric field is expected to be balanced by radial electron pressure gradient. Direct measurement of local density gradient is consistent with the formation of radial electric field and the zonal flow.

Mason numbers for magnetorheology

The electric field strength and shear rate dependence of the apparent shear viscosity of electrorheological (ER) suspensions can often be represented by a function of only the Mason number. A Mason number defined for magnetorheological (MR) suspensions by direct substitution of magnetostatic variables for electrostatic variables does not produce a similar collapse of shear viscosity data for MR suspensions. We show that a Mason number defined in terms of the suspension magnetization can be employed to produce a collapse of experimental data at various magnetic field strengths and shear rates. As for ER suspensions, this Mason number can be calculated from experimentally measured quantities.

Nonambipolar Magnetic-Fluctuation-Induced Particle Transport and Plasma Flow in the MST Reversed-Field Pinch

First direct measurements of nonambipolar magnetic fluctuation-induced charge transport in the interior of a high-temperature plasma are reported. Global resistive tearing modes drive the charge transport which is measured in the vicinity of the resonant surface for the dominant core resonant mode. Finite charge transport has two important consequences. First, it generates a potential well along with locally strong electric field and electric field shear at the resonant surface. Second, this electric field induces a spontaneous E x B driven zonal flow.

In situ analysis of bacterial capture in a microfluidic channel

We present a microfluidic approach for the continuous capture of Salmonella Newport cells suspended in a phosphate buffer using externally applied electric fields. The effects of flow rate, applied electric field and wall shear stress on cell capture in the device are analyzed using particle tracking via fluorescent microscopy techniques. Analyzing capture across multiple locations on the electrode surface enabled the estimation of average capture over the entire electrode area as a function of time. The device exhibits approximately a constant capture rate over an extended time frame, which is verified independently using the cell culture methods. An increased capture rate with an increased electric field is observed. The capture rate dependence on the flow rate and capture rate at various locations with different wall shear stress magnitudes does not exhibit statistically significant variations. The capture trends presented in this study can be utilized for designing microfluidic systems for biosensors, designed bacterial bio-films and devices for bacterial sample concentration from large volumes.

Intrinsic rotation and electric field shear

A novel mechanism for the generation and amplification of intrinsic rotation at the low-mode to high-mode transition is presented. The mechanism is one where the net parallel flow is accelerated by turbulence. A preferential direction of acceleration results from the breaking of k‖→−k‖ symmetry by sheared E×B flow. It is shown that the equilibrium pressure gradient contributes a piece of the parallel Reynolds stress, which is nonzero for vanishing parallel flow, and so can accelerate the plasma, driving net intrinsic rotation. Rotation drive, transport, and fluctuation dynamics are treated self-consistently.

Effects of Plasma Confining Potentials and the Associated Radially Sheared Electric Fields on the Plasma Energy Confinement

The effects of the plasma-confining potentials and the associated radially sheared electric fields on the central-cell electron energy confinement are theoretically and experimentally investigated in the GAMMA 10 tandem mirror. In particular, the scaling of the central-cell electron temperatures with electron-confining potentials is studied on the basis of the local energy-balance equation. The obtained theoretical scaling of electron temperatures with electron-confining potentials is then compared with the experimentally observed relation between these two parameters.Recently, by the use of new 0.5-MW level gyrotrons in the plug region, four-time progress in the formation of the ion-confining potential c including a new record of 3 kV has been achieved in a hot-ion mode having bulk-ion temperature Ti = several keV. In the hot-ion mode, intermittent vortex-like turbulent structures are observed in the case without the gyrotron injections; in this case, radially produced weak shear of electric fields dEr/dr and appreciable transverse losses are observed. However, during the application of electron-cyclotron heatings, the associated potential rise produces a stronger shear in the central cell (dEr/dr = several 10 kV/m2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement.In order to investigate the effect of the radially sheared electric fields on the electron energy confinement, the radial profiles of the thermal diffusivity are derived from the local power-balance analysis by the use of the data from the following various diagnostics in the above-described hot-ion mode. The obtained radial profiles of radial electric field and thermal diffusivity imply that the reduction of the thermal diffusivity is associated with the radially produced strong shear of electric fields.

Stability Study of Kelvin-Helmholtz Modes due to Radial Electric Field Shear

The Kelvin-Helmholtz mode instability driven by an equilibrium radial electric field in a cold plasma is studied numerically in cylindrical geometry. The numerical analysis is applicable to both toroidal and mirror plasmas. The growth rate and a threshold in the radial gradient scale length and also the magnitude of the E × B drift frequency for instability are obtained.

Effect of polymer coating on the behavior of an electro–rheological fluid in slit flow

Electro–rheological (ER) effect is the term applied to the rapid and reversible changes in rheological properties of certain suspensions or solutions under the application of an electric field. In this study, the dependence of ER-effects of an ER-fluid in rotational as well as in slit flow on electric field strength, field frequency, shear rate and volumetric flow rate was quantified using the Casson model. In addition, the effects of coating the electrodes with a polymer mesh (trevira) on the behavior of an ER-fluid in slit flow were investigated. The results showed that with the trevira mesh on the surface of the electrodes, the ER-fluid demonstrated greater increase in the pressure drop upon the application of both AC- and DC- field. Compared to the uncoated electrodes, the relative ER-effects for the electrodes with the trevira mesh were 1.12–4.23 times larger. On the other hand, the ER-effect of the ER-fluid used can be increased by installation of trevira mesh without increasing the current levels under constant electric field strength.