Regions Of Low Field Strength Research Articles

Effect of percolation on the deformation of isotropic electrorheological elastomers under external electric fields

In this study we investigate the influence of the filler's concentration with an emphasis on the percolation of particles on the field-induced deformation of isotropic, disc shaped electrorheological elastomers (EREs) with barium titanate filler. It was found that above the percolation threshold concentration (>34.7 wt%) the magnitude of the fully reversible contraction was enhanced by a factor of ∼10 in the low field strength region (E≈ 0.2 MVm−1). At higher field strength (E= 1.1 MVm−1) the percolation had no effect on the induced strain, and regardless of the particle's concentration it saturated around 1.2%. It was shown that for isotropic EREs the optimal particle loading is just above the percolation threshold. If the filler content exceeds the threshold concentration, then the same deformation can be achieved with much lower field intensity, but even larger particle concentration does not enhance the performance further.

Application and comparison of thermistors and fiber optic temperature sensor reference for ILP measurement of magnetic fluids in double cell magnetic hyperthermia

One of the simplest way to characterize the heating efficiency of magnetic fluids used in hyperthermia treatment is the calorimetric measurement of the specific loss power with direct temperature detection. However, the performance of metallic sensors in an alternating magnetic field is degraded by the self-heating of the probes, and electromagnetic interference can be also significant. In our double cell differential thermometric system these disturbing effects can be compensated. Specific loss power measurements of EMG700 magnetic fluid with negative temperature coefficient thermistors in differential configuration are presented, and control measurements were performed with an optical fiber thermometer in f=470kHz–1020kHz frequency and H=0.13kAm−1–1.19kAm−1 magnetic field strength range. We found that the specific loss power is proportional to the frequency and shows a quadratic dependence on the field strength in the low field strength region, therefore we calculated the intrinsic loss power of the fluid from the measured specific loss power. At this field conditions intrinsic loss power up to 0.53nHm2kg−1 was determined.

An Experimental Study of the Crystallinity of Different Density Polyethylenes on the Breakdown Characteristics and the Conductance Mechanism Transformation under High Electric Field.

In order to study the crystallinity of different density polyethylenes, this paper conducts an experimental study on the transformation of the conductance mechanism under a high electric field. In this experiment, X-ray diffraction (XRD), differentials scanning calorimetry (DSC), direct current (DC) breakdown of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE), as well as the conductivity characteristics under an electric field of 5–200 kV/mm are tested. In addition, the electric field–current density curves of the four kinds of polyethylene are fitted to analyze their conductance transition in non-ohmic regions under different high field strengths, through applying the mathematical formula of a variety of conductance mechanisms. The experimental results are as follows: as the density of polyethylene increases, the crystallinity increases continuously. Moreover, the continuous increase of crystallinity causes the electric conduction flow under the same field strength to decrease significantly. The field strength corresponding to the two turning points in the conductance characteristic curve increases simultaneously, and the breakdown field strength increases accordingly; through analysis, it is found that in the high field, as the electric field increases, the conductance mechanism develops from the ohmic conductance of the low field strength region to the bulk effect of the high field strength region (Poole–Frenkel effect). Then, it develops into the electrode effect to the high field strength (Schottky effect), although the threshold field strength of this conductance mechanism transition increases with the increase of crystallinity.

Properties of the Umbral Filament Observed in Active Region NOAA 12529

Abstract Recent observations of the solar photosphere revealed the presence of elongated filamentary bright structures inside sunspot umbrae, called umbral filaments (UFs). These features differ in morphology, magnetic configuration, and evolution from light bridges (LBs) that are usually observed to intrude in sunspots. To characterize a UF observed in the umbra of the giant leading sunspot of active region NOAA 12529, we analyze high-resolution observations taken in the photosphere with the spectropolarimeter on board the Hinode satellite and in the upper chromosphere and transition region with the IRIS telescope. The results of this analysis definitely rule out the hypothesis that the UF might be a kind of LB. In fact, we find no field-free or low-field strength region cospatial to the UF. Conversely, we recognize the presence of a strong horizontal field larger than 2500 G, a significant portion of the UF with opposite polarity with respect to the surroundings, and filaments in the upper atmospheric layers corresponding to the UF in the photosphere. These findings suggest that this structure is the photospheric manifestation of a flux rope hanging above the sunspot and forming penumbral-like filaments within the umbra via magneto-convection. This reinforces a previously proposed scenario.

Enhanced Electrorheological Properties of Elastomers Containing TiO₂/Urea Core-Shell Particles.

Polar molecule-coated core-shell particles have been used to prepare electrorheological (ER) fluids with high performance. Inspired by those studies, TiO2/urea core-shell structured particles were fabricated and used to prepare novel ER elastomers, whose properties were compared with the ER elastomers with bare TiO2 particles. Particles characterization results illustrate the TiO2/urea particles present little change in size, morphology and crystal structure with respect to the bare amorphous TiO2 particles, while clear core-shell structure is observed. Compared with the bare TiO2 particles filled elastomer, the TiO2/urea particles filled elastomer presents higher dielectric constant, indicating enhanced polarization. The viscoelastic properties of the two elastomers under different strain amplitude, frequency and electric field were tested. The results indicate that the TiO2/urea particles filled elastomer shows higher storage modulus G' and higher relative ER effect within the low field strength region from 0 to 2 kV/mm. Coating polar molecules is an effective method to improve the ER performance for ER elastomers.

Prediction of clamping pressure in a Johnsen-Rahbek-type electrostatic chuck based on circuit simulation

We propose nonlinear resistance models for bulk resistance of a dielectric layer and contact resistance between a wafer and the dielectric layer used in an equivalent circuit model to predict the clamping or residual clamping pressure in Johnsen-Rahbek-type [A. Johnsen and K. Rahbek, J. Inst. Electr. Eng. 61, 713 (1923)] electrostatic chucks. It is found that the bulk resistance is presumed by a model on the basis of the space-charge-limited current model in insulator. It is also found that the contact resistance is expressed by a superposition of Ohmic resistance in the low-field strength region and a resistance on the basis of field emissions in the high-field strength region. In addition, the gap capacitance is determined based on the probability function of asperity height obtained from a surface roughness profile. The clamping pressure is simulated by calculating a charge stored in the gap, combined with the probability function with circuit simulation. Consequently, the calculation agrees with the measurement within 31% difference. It is also found that the calculation can reproduce the dependency of bulk resistance on the clamping pressure and the time response in residual clamping pressure.

Electrophoresis of a bead-rod chain through a narrow slit: A Brownian dynamics study

We use two-dimensional Brownian dynamics simulations to study the electrophoresis of a bead-rod chain through a narrow slit. A constant electric field is assumed to act inside and outside of the slit, and each bead on the chain is assigned a constant uniform charge. We calculate the dependence of the polymer transit velocity on chain length, slit dimensions (width-to-length ratio), and electric-field strength. For sufficiently narrow slits, the transit velocity increases nonlinearly with the applied field for low-field strengths, whereas it increases linearly for high-field strengths. In the low-field strength region and for sufficiently narrow slits, the transit velocity decreases rapidly for small chain lengths and then decreases slowly beyond a critical chain length. As the slit width increases, the transit velocity decreases with chain length in more continuous manner, and for sufficiently large slits the transit velocity becomes independent of chain length as expected. Distributions of the chain end-to-end distances and the translocation times depend strongly on the relative size of the chain to the slit. These results show the sensitivity of the transit velocity vs chain length relationship to the slit dimensions and applied electric-field strength, and suggest that there may be an optimal slit width for a given field strength and vice versa. The results may be useful for microfluidic separations and for understanding the motion of biological polymers through narrow constrictions.

Exploratory studies of flowing liquid metal divertor options for fusion-relevant magnetic fields in the MTOR facility

Abstract This paper reports on experimental findings on liquid metal (LM) free surface flows crossing complex magnetic fields. The experiments involve jet and film flows using GaInSn and are conducted at the UCLA MTOR facility. The goal of this study is to understand the magnetohydrodynamics (MHD) features associated with such a free surface flow in a fusion-relevant magnetic field environment, and determine what LM free surface flow option is most suitable for lithium divertor particle pumping and surface heat removal applications in a near-term experimental plasma device, such as NSTX. Experimental findings indicate that a steady transverse magnetic field, even with gradients typical of NSTX outer divertor conditions, stabilizes a LM jet flow—reducing turbulent disturbances and delaying jet breakup. Important insights into the MHD behavior of liquid metal films under NSTX-like environments are also presented. It is possible to establish an uphill liquid metal film flow on a conducting substrate, although the MHD drag experienced by the flow could be strong and cause the flow to pile-up under simulated NSTX magnetic field conditions. The magnetic field changes the turbulent film flow so that wave structures range from 2D column-type surface disturbances at regions of high magnetic field, to ordinary hydrodynamic turbulence wave structures at regions of low field strength at the outboard. Plans for future work are also presented.

Ion flux from vacuum arc cathode spots in the absence and presence of a magnetic field

Because plasma production at vacuum cathode spots is approximately proportional to the arc current, arc current modulation can be used to generate ion current modulation that can be detected far from the spot using a negatively biased ion collector. The drift time to the ion detector can used to determine kinetic ion energies. A very wide range of cathode materials have been used. It has been found that the kinetic ion energy is higher at the beginning of each discharge and approximately constant after 150 μs. The kinetic energy is correlated with the arc voltage and the cohesive energy of the cathode material. The ion erosion rate is in inverse relation to the cohesive energy, enhancing the effect that the power input per plasma particle correlates with the cohesive energy of the cathode material. The influence of three magnetic field configurations on the kinetic energy has been investigated. Generally, a magnetic field increases the plasma impedance, arc burning voltage, and kinetic ion energy. However, if the plasma is produced in a region of low field strength and streaming into a region of higher field strength, the velocity may decrease due to the magnetic mirror effect. A magnetic field can increase the plasma temperature but may reduce the density gradients by preventing free expansion into the vacuum. Therefore, depending on the configuration, a magnetic field may increase or decrease the kinetic energy of ions.

Reversing-Pulse Electric Birefringence of Poly(γ-methyl l-glutamate) in Hexafluoro-2-propanol

Abstract Reversing-pulse electric birefringence (RPEB) signals of poly(γ-methyl l-glutamate) (PMLG) were measured, for the first time, in hexafluoro-2-propanol at 25 °C and at 535 nm. Quantitative analyses were carried out on the steady-state birefringence and the field-free decay time (<τ>EB) in the wide field strength region and also on the rise, reverse, and decay portions in the low field strength region by taking into account the continuous molecular length distribution of a fractionated PMLG sample. The weight-average length (lw), permanent dipole moment (μw), polarizability anisotropy (Δαw), and the degree of polydispersity (lw/ln) were evaluated by two procedures: one from the analysis of the reverse-transient portion and the other from the field-free decay portion of RPEB signals. These two sets of the parameters should be identical with each other, if the electrooptical and hydrodynamic properties of PMLG do not change during the field-on and field-off processes. However, the length of the PMLG helix is shorter by about 18% in the presence of an electric field than that in the absence of the field. In order to explain this discrepancy, a new concept of reversible conformational transition under the external electric field is introduced.

Dynamic and static cord-structure: A new dielectrophoretic phenomenon

During the dielectrophoresis of cells or other particles it is observed that unusual distributions of the cells appear on the floor of the test chamber. In certain narrow frequency ranges, the distributions are cord-like, with the “cords” running more or less parallel to the electrode faces, but also developing highly branched structures. It is observed that the cells within the cords form “pearl-chains” along the field lines and across the cords. Within the cords, the cells move actively in spiralized paths. This the dynamic cord-structure is considered to be due to the combined action of negative dielectrophoresis, Brownian motion, and gravity. Outside the conditions requisite for dynamic cord formation, the cells are seen to assume conformations reflective of simple negative dielectrophoresis which merely causes them to amass into regions of low field strength. The theory for the effects is presented, together with experimental evidence for the kinetic behavior of the dynamic cord effect.

Klein-Gordon equation with discontinuous mass: An instructive nonlinear classical field theory

The Klein-Gordon equation with a discontinuous field-dependent mass term provides an example of a simple nonlinear classical field theory. This theory is studied to determine if it exhibits a behavior hypothesized in a recently proposed picture of multimeson production. As expected, it turns out that (with a small additional decay term in the equation) the space-time average of the field strength generated by a delta-function source can be made insensitive to the source strength, once it exceeds a certain threshold. When two space-time- separated delta-function sources are present, their independence (or dependence) in producing slow mesons is determined by the ratio of decay time to source separation. If the mass in the region of low field strength is imaginary, then there are stable sourceless ''kink'' solutions in one space dimension. (AIP)

Statistics of electron multiplication in proportional counters

Pulse spectra produced by the release of single primary electrons in a proportional counter have been determined for a variety of gas fillings. The spectra were recorded with sufficient statistical accuracy to define their logarithmic derivatives in order to investigate deviations from the approximately exponential distributions. Two main effects were observed: (1) at small pulse heights there may be a reduced number of pulses compared with the exponential distribution if the electron multiplication commences in a region of low field strength; (2) at large pulse heights there may be an excess of pulses owing to the occurrence of secondary successor avalanches initiated by photoelectric effects. A theoretical distribution has been derived which describes both these effects in terms of two parameters, one of which is characteristic of the ionizing efficiency of the electron at the commencement of the multiplication process while the other is a measure of the mean number of successors per avalanche. These parameters have been determined for each of the observed spectra.