Relaxation dispersion in MRI induced by fictitious magnetic fields

A new method entitled Relaxation Along a Fictitious Field (RAFF) was recently introduced for investigating relaxations in rotating frames of rank ≥ 2. RAFF generates a fictitious field (E) by applying frequency-swept pulses with sine and cosine amplitude and frequency modulation operating in a sub-adiabatic regime. In the present work, MRI contrast is created by varying the orientation of E, i.e. the angle ε between E and the z″ axis of the second rotating frame. When ε > 45°, the amplitude of the fictitious field E generated during RAFF is significantly larger than the RF field amplitude used for transmitting the sine/cosine pulses. Relaxation during RAFF was investigated using an invariant-trajectory approach and the Bloch-McConnell formalism. Dipole-dipole interactions between identical (like) spins and anisochronous exchange (e.g., exchange between spins with different chemical shifts) in the fast exchange regime were considered. Experimental verifications were performed in vivo in human and mouse brain. Theoretical and experimental results demonstrated that changes in ε induced a dispersion of the relaxation rate constants. The fastest relaxation was achieved at ε ≈ 56°, where the averaged contributions from transverse components during the pulse are maximal and the contribution from longitudinal components are minimal. RAFF relaxation dispersion was compared with the relaxation dispersion achieved with off-resonance spin lock T(?ρ) experiments. As compared with the off-resonance spin lock T(?ρ) method, a slower rotating frame relaxation rate was observed with RAFF, which under certain experimental conditions is desirable.     

Probability distribution of the breakdown electric fields of polymer dielectrics

The probability distribution is derived for the breakdown electric field of polymers, treated as inhomogeneous dielectrics. The extremal distribution and the Weibull distribution are shown to be particular cases of the result found. The experimental results found with low-pressure polyethylene and polyethylene, Styroflex, and polyethylene terephthalate films are consistent with the theoretical probability distribution found for the breakdown electric field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 49–55, January, 1971.     

Relaxation processes in non-Debye dielectrics

The specific features of the relaxation processes in non-Debye dielectrics have been investigated. The nature of the difference between the relaxation frequencies of the dielectric constant and dielectric loss (conductivity) has been explained. It has been shown that the average relaxation frequency of the conductivity is considerably (in some cases, by several orders of magnitude) higher than the relaxation frequency of the dielectric constant owing to an increase in the conductivity spectra of the statistical weight of the relaxation processes with short relaxation times.     

Water flows induced by microwave electric fields in microsystems

AC electric fields are of increasing importance for the generation of fluid flows in microsystems. We analyse numerically the use of AC electric fields at microwave frequencies for electro-thermal actuation of water in microdevices. Water is heated because of its significant dielectric loss at microwave frequencies. Buoyancy and dielectric forces actuate in the liquid bulk, and the relative importance between them is studied. The microwave liquid actuation can be used for pure water as well as for water saline solutions, such as bio-fluids. Therefore, it is of interest for the Lab-on-a-Chip technology.     

A model for dielectrics experiencing partial discharges under high electric fields

This work is in the research field of partial discharges to electrical insulating materials. A theoretical model is proposed that simulates the partial discharge's development within a metal (solid insulating material) metal structure. A nonlinear, time-dependent resistance using a new formula simulates the activity of partial discharges in dielectrics and the suggested equivalent circuit is analysed. Comparison of the experimental results and the outcome of the model demonstrates satisfactory consistency. Slight deviations consist of a minor phase shift between the computed and measured waveforms, as well as small oscillations of the waveform measured in the vicinity of the potential steps. These deviations are attributed to a very small parasitic inductance, which is always present in the measuring circuit.     

Propagation of electric fields induced by optical phonons in semiconductor heterostructures

The penetration of electric fields accompanying long-wavelength optical phonos from one region of a semiconductor heterostructure to another is investigated. It is proposed to determine the penetration depth of such fields from the relaxation caused by these fields in quantum dots. By the example of a cylindrical Ge quantum dot embedded in a GaP/GaAs heterostructure, it is shown that the electric fields induced by longitudinal optical phonons can penetrate through the interface between semiconductors at distances of about 100 nm.     

Microscopic local fields in dielectrics

The microscopic polarization produced by a spatially constant external field is calculated from the dielectric matrices of diamond, Si, Ge, α-Sn, MgO and NaCl. Local fields and polarization charges are analyzed in real space. Strong dipoles are induced at the bond sites in semiconductors and on the anions in MgO and NaCl. The effects of the inhomogeneity on the linear and non-linear susceptibilities are discussed.     

Heating liquid dielectrics by time dependent fields

Steady state and time-resolved dielectric relaxation experiments are performed at high fields on viscous glycerol and the effects of energy absorption from the electric field are studied. Time resolution is obtained by a sinusoidal field whose amplitude is switched from a low to a high level and by recording voltage and current traces with an oscilloscope during this transition. Based on their distinct time and frequency dependences, three sources of modifying the dynamics and dielectric loss via an increase in the effective temperature can be distinguished: electrode temperature, real sample temperature, and configurational temperatures of the modes that absorbed the energy. Isothermal conditions that are desired for focusing on the configurational temperature changes (as in dielectric hole burning and related techniques) are maintained only for very thin samples and for moderate power levels. For high frequencies, say ν > 1 MHz, changes of the real temperature will exceed the effects of configurational temperatures in the case of macroscopic samples. Regarding microwave chemistry, heating via cell phone use, and related situations in which materials are subject to fields involving frequencies beyond the MHz regime, we conclude that changes in the configurational (or fictive) temperatures remain negligible compared with the increase of the real temperature. This simplifies the assessment of how time dependent electric fields modify the properties of materials.     

Calculation of electric fields induced near metal implants by magnetic resonance imaging switched-gradient magnetic fields

Electric (E) fields induced near metal implants by MRI switched-gradient magnetic fields are calculated by a new equivalent-circuit numerical technique. Induced E-field results are found for a metallic spinal-fusion implant consisting of two thin wires connected to the metallic case of a current generator as well as for its subsections: a bare U-shaped wire, an insulated U-shaped wire, a cut insulated wire, and a generator. The presence of the metallic implants perturbs the E field significantly. Near the ends of the bare U-shaped wire, the E field is 89.7 times larger than in the absence of the wire. The greatest E field concentration occurs near the ends of the cut insulated wire, where the E field is 196.7 times greater than in the absence of the wire. In all cases, the perturbation of the induced E field by the implanted wire is highly localized within a few diameters of the wire.     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength.     

On the possibility of the nucleation of magnetic vortices and antivortices in magnetic dielectrics using electric fields

The micromagnetic distribution in a dielectric nanoparticle is theoretically considered. It is shown that the presence of an inhomogeneous magnetoelectric interaction in magnetic dielectrics creates the possibility of nucleation of magnetic vortices and antivortices in them using an electric field. The estimation of the critical voltage necessary for vortex creation in particles of high-temperature multiferroic bismuth ferrite yields a value of ∼100 V.     

Calculation of electric and magnetic induced fields in humans subjected to electric power lines

In this work, analysis of the human body exposed to high voltage electric and magnetic fields is presented. The distribution of the electric field is obtained by using Laplace's equation. This relates the surface charge induced on the body to the potential in a reciprocal Laplace problem, which is then calculated by charge simulation method coupled with genetic algorithms to determine the appropriate arrangement of simulating charges inside the human body. The magnetic field intensity along the vertical center line of the human is calculated. Exposure to external electric and magnetic fields at power frequency induces electric field, magnetic field and currents inside the human body. The presented model for simulating electric and magnetic fields are a three dimensional field problem and introduced different types of charges to simulate the different elementary geometrical shapes of human body. The particular strength of the charge simulation method in this application is its ability to allow a detailed representation of the shape and posture of the human body. The results have been assessed through comparison induced current, electric field, magnetic field and there distribution over the body surface, as estimated in other experimental and computational work.     

Kinetics of failure of polycrystalline ferroelectric ceramics in mechanical and electric fields

The kinetics of mechanical failure and electrical breakdown in polycrystalline ferroelectric ceramics was studied under the simultaneous action of an electric field and a mechanical load. A kinetic approach is shown to be preferable as compared to the concepts that treat the failure and breakdown as critical phenomena. The mechanical failure and electrical breakdown are shown to be interrelated. It is found that a weak action of one of the fields retards failure caused by the other field and that the simultaneous action of these strong fields accelerates both the mechanical failure and electrical breakdown. Methods for determining the activation characteristics of both processes only from the failure kinetics in one of these processes are developed.     

Relaxation in polycrystalline ferroelectric ceramics upon the simultaneous impact of mechanical stresses and electric field

The motions of domain walls at low stresses are known to result in the relaxation of structural over-stresses and their balancing, while at high stresses we observe overstress increases and microcrack formation, i.e., the changing of resonance frequencies and emerging of spurious resonances of piezoelectric elements [1, 2]. In view of the above, the influence of electromechanical impacts on ceramics of compositions PZT-19 and PZT-35 are investigated in order to estimate the resonance curve changes.     

Relaxation of post-illumination electric fields in strongly biased high-resistance structures with a single deep impurity level

This paper examines how an electric field relaxes when a discontinuous large carrier-depleting voltage applied to high-resistance symmetric metal-semiconductor-metal (MSM) and metal-insulator-semiconductor-insulator-metal (MISIM) structures having a single impurity level, and how its energy level ɛ _t=E _c−E _t and the tunneling transparency T _n,p of the metal-semiconductor or metal-insulator boundary affect the relaxation. It is shown that the relaxation of the field and the form of its steady-state distribution depend on the ratio of the time constant t _p in the majority-carrier (hole) region to the ionization time τ _t ⁻¹ =α _n (n _*+n ₁)+α _p (p _*+p ₁) of a deep trap in the bulk. This ratio determines the relative contributions of free ρ _p,n and bound charge dnsities ρ _t (where α _n,p is the coefficient for capture by an impurity, and p _*, n _*, p ₁, n ₁ are equilibrium concentrations and Shockley-Read constants in the bulk). For τ _t ≈(τ _t )_max≫t _p ,ρ _t ≫ it is found that ρ _p,n and ρ _t ≫ρ _p,n , which corresponds to a trap energy close to , independent of the value of T _n,p , decaying oscillations arise in the concentration distribution, bulk charge, and field appear in the bulk. The amplitude of these oscillations reaches a maximum at time t≈0.4τ _t. Decreasing the ratio α _p/α_n causes τ _t to deviate from (τ _t)_max. When this happens, the field no longer oscillates; instead, it increases with positive curvature in the cathode portion of the bulk. The quantity T _n,p determines the behavior of the field in the neighborhood of the anode. The value of (dE/dx)₀ is positive for MSM structures (T _n,p ≈1), and negative for MISIM structures (T _n,p ≈0). For transparencies close to a critical value T _n,p ⁰ , the field in the structure remains almost uniform over an impurity ionization time. Fiz. Tverd. Tela (St. Petersburg) 39, 1775–1782 (October 1997)     

Transitions between Zeeman substates of excited He I levels induced by electric radiofrequency fields

Making use of the large tensor polarizabilities of excited He I levels, transitions between Zeeman substates of 1snd ¹ D ₂ levels have been induced by electric radiofrequency fields after ion-impact excitation of He atoms. Resonance signals have been observed near the Larmor frequencyω=ω _L as well as nearω=2ω _L . These signals are interpreted as 2-quantum electric-dipole (2E1) transitions and 1E1 transitions, respectively, between the Zeeman sublevels withM=0 andM=±2. The interaction of this 3-state system {¦M〉;M=?2, 0, +2} with the applied external fields (magnetic fieldH _z and static and alternating electric field?=? ₀+? ₁ cosω t) is discussed. The shape of the resonance signals has been evaluated in the rotating field approximation. In contradiction to the theory, the observed signals show a peculiar narrow structure at the center of the signal which is not yet completely understood. The experimental techniques developed are expected to be useful for investigations of Zeeman-, Stark-, fine and hyperfine splittings of excited He I levels.