Axial Dc Magnetic Field Research Articles

DC magnetic field-assisted improvement of textile dye degradation efficiency with multi-capillary air bubble discharge plasma jet.

Axial DC magnetic field-assisted multi-capillary underwater air bubble discharge plasma jet has been used to study the productions of reactive oxygen species. Analyses of optical emission data revealed that the rotational (Tr) and vibrational temperatures (Tv) of plasma species slightly increased with magnetic field strength. The electron temperature (Te) and density (ne) increased almost linearly with magnetic field strength. Te increased from 0.53 to 0.59eV, whereas ne increased from 1.03 × 1015cm-3 to 1.33 × 1015cm-3 for B = 0 to B = 374 mT, respectively. Analytical results from the plasma treated water provided that the electrical conductivity (EC), oxidative reduction potential (ORP), and the concentrations of O3 and H2 O2 enhanced from 155 to 229 µS cm-1, 141 to 17mV, 1.34 to 1.92mg L-1, and 5.61 to 10.92mg L-1 due to the influence of axial DC magnetic field, while [Formula: see text] reduced from 5.10 to 3.93 for 30min treatment of water with B = 0 and B = 374 mT, respectively. The model wastewater prepared with Remazol brilliant blue textile dye and the plasma treated wastewater studied by optical absorption spectrometer, Fourier transform infrared spectrometer, and gas chromatography mass spectrometer. The results show that the decolorization efficiency increased ~ 20% after 5min treatment for the maximum B = 374 mT with respect to zero-magnetic field and, power consumption, and electrical energy cost reduced ~ 6.3% and ~ 4.5%, respectively, due to the maximum assisted axial DC magnetic field strength of 374 mT.

Directional Solidification of Aluminum A360 under Moderate DC Magnetic Field and Electric Current

Metal additive manufacturing is rapidly developing technology, but its application in wider scale is limited by several factors. One of these is expensive raw material, because it requires certain physical properties. Two most popular metal additive manufacturing methods are printing from powder and printing from wire. Wire is usually produced by drawing it from rod. Rod can be produced by directional solidification, which is well known method to study the microstructure formation depending on various parameters during solidification. In this study directional solidification of A360 aluminum alloy with electromagnetic interaction is investigated. Aluminum alloy is induction melted and then directionally solidified into the rod 12-20 mm in diameter. Aim of this work is to investigate the role of axial DC magnetic field and electric current interaction on the grain refinement and mechanical properties of A360 aluminum alloy. It is found that electromagnetic interaction can be the approach to refine the grains, regulate the growth of oriented columnar grains and to improve mechanical properties of the material.

A Predictive Model for Multipactor Discharge in Coaxial Systems Based on Chaos Theory

Multipactor is an electron discharge driven by secondary electron emission (SEE). Recently, a novel theoretical approach for multipactor based on principles from nonlinear dynamics and chaos theory has been introduced and applied to a parallel-plate geometry. In this article, the theory is extended to a coaxial geometry driven by the fundamental transverse electromagnetic (TEM) mode and an axial dc magnetic field. This configuration is often examined in light of multipactor suppression schemes for space-borne applications. Maps that relate the radio frequency (RF) emission phase to the RF arrival phase are developed for both the inner and outer conductors with no a priori assumptions on the electron trajectories (neglecting space-charge effects). By combining the system attractors with secondary emission properties of the conductor material, multipactor growth (or decay) is predicted throughout parameter space under realistic spreads in secondary emission energies and angles. This model is validated against published simulation and experimental results.

Phase Control in Parallel Channels of Shock-Excited Microwave Nanosecond Oscillators

The theoretical premises and experimental results of phase control in high-power microwave oscillators with nanosecond pulse duration are presented. In experiments, two-channel backward wave oscillators (BWOs) for both steady state (100-150 cycles) and super-radiance (SR) mode operation (10-20 cycles) are discussed. For the phase control, the shift of the moment with fastest current rise is provided in the sections of nonlinear transmission lines with axially biased ferrites. The voltage pulse sharpening and shift of group velocity depend on the dc axial magnetic field. In SR mode, two-channel source is capable of producing 2 × 0.3 GW pulses with duration of 2 ns and the center frequency of 10 GHz. The source operates at the repetition rate up to 100 pps with electronic control of the phase in one channel relative to another. The last experiment is carried out using two synchronized compact RADAN-type drivers with two parallel Ka-band BWOs (100 MW, 2 ns, 37 GHz). The controllable shift of interference picture is a proof of the coherency in the aggregated radiation. At the maximum of the pattern in the far zone, the detector indicateds fourfold increase in power density over that measured from single channel.

Anode Spot Formation Threshold Current Dependent on Dynamic Solid Angle in Vacuum Subjected to Axial Magnetic Fields

The objective of this paper is to experimentally determine a relationship between an anode spot formation threshold current $I_{\rm th}$ and a dynamic solid angle in drawn vacuum arcs subjected to axial magnetic fields (AMFs). Experiments are performed in a demountable vacuum chamber, in which a Helmholtz coil is installed coaxially with a pair of butt type contacts. Dynamic solid angle $\Omega$ is related to arcing process, which is subtended by the anode from the cathode center or by a ratio of the anode diameter $D$ to the contact separation length $l$ as arc extinguishes $(D/l)$ in a drawn vacuum arc. Although a traditional definition of solid angle $(D/g)$ is related to a fixed full contact gap $g$ , no arcing process is considered. In the experiments, butt type contacts are used with contact material CuCr25 (25% Cr), and the contact diameters $D$ are 12, 25, 40, 60, and 80 mm. The contact separation length $l$ as arc extinguished is controlled in a range of 12–24 mm determined by an opening velocity with an arcing time fixed at 10 ms. A dc AMF is applied to the pair of contact with the flux density $B_{\rm AMF}$ adjusted from 0 to 122 mT. Arc current in the tests is at a range of 0–30 kA rms at 50 Hz. The results show that anode spot formation threshold current $I_{\rm th}$ is linearly dependent on the dynamic solid angle $D/l$ adjusted either by $D$ or $l$ , respectively. And the threshold current $I_{\rm th}$ is also linearly dependent on the AMF flux density $B_{\rm AMF}$ applied. Contact diameter $D$ shows more significant influence on the threshold current $I_{\rm th}$ than the contact separation length $l$ as arc extinguished. In a given contact diameter $D$ and contact separation length $l$ as arc extinguished, there is a critical peak arc voltage value, at which an anode spot first formed, regardless of arc current $I_{\rm arc}$ and AMF flux density $B_{\rm AMF}$ applied.

Multicavity Magnetron With the “Rodded” Quasi-Metamaterial Cathode

We report on the use of the “rodded” or the longitudinally oriented metal-thin-wire structure as the explosive-emission cathode in high-power multicavity magnetrons. The rodded structure is transparent to the time-varying induced electric field <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$E_{1}$</tex></formula> oscillating orthogonally to the individual rods or wires of the structure. The rods (wires) are longitudinally oriented, i.e., along the magnetron axis and parallel to the external dc axial magnetic field <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $H_{0}$</tex></formula> . The mode pattern established within the magnetron resonant cavity corresponds to one of the TE-like cavity modes of the magnetron operation. This allows defining the rodded structure as the quasi-metamaterial (i.e., not-entirely-metamaterial) structure. Particle-in-cell simulations of the U.S. Air Force Research Laboratory's six-cavity (AFRL-A6) L-band magnetron demonstrate that the rodded quasi-metamaterial cathode allows the magnetron to much more faster select the desired <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$2 \pi/3$</tex></formula> magnetron operating mode within the broader range of the input operating parameters (from 130 to 200 kV at 0.15 T), as compared with the same magnetron working with the traditional smooth cylindrical cathode (from 140 to 160 kV at 0.15 T).

Spatial measurements of electron energy distribution and plasma parameters in a weakly magnetized inductive discharge

Spatial characteristics of plasma parameters such as electron temperature, plasma density, plasma potential, and electron energy distribution (EED) were studied in inductively coupled plasma with an axial dc magnetic field. With dc magnetic field, the measured EEDs in the total electron energy scale are spatially coincided except cutting of the low electron energy part indicating the conserved non-local electron kinetics in an axial direction, even though the dc magnetic field is applied. Spatial distributions of the plasma densities at axial positions have almost same trends with various magnetic field strengths. We also discuss the reduction of the ambipolar potential along the axial direction as the applied magnetic field increased.

Analysis and Design of a Novel Photonic Crystal-Based Sub-mm/THz Backward-Wave Oscillator

A novel Cerenkov-transition based high-frequency/high power backward wave oscillator (BWO) is presented. The proposed device is a defected metallic triangular lattice photonic crystal (PC) structure that has no axial discontinuities. The absence of the axial discontinuity and low axial DC magnetic field requirements allows for simple fabrication process. Isolation between the generated electromagnetic fields and electron beam offers easy field extraction and coupling to the output loads. Design of the proposed device was carried out with a first principles based combined FDTD/PIC simulator developed for this work. Typical design examples presented here demonstrate the potentials of the proposed structure. The PC based structure allows easy tunability and scaling to higher frequencies.

Multipactor in a Coaxial Line Under the Presence of an Axial DC Magnetic Field

The main goal of this letter is the analysis of the multipactor effect within a coaxial waveguide structure when an external axial dc magnetic field is applied. We have designed and manufactured a coaxial waveguide sample that has been immersed within a long solenoid. Numerical and experimental results confirm a significant change in the RF breakdown behavior with regard to the case without the axial dc magnetic field, as well as the existence of single- and double-surface multipactor regimes. Good agreement between theory and experimental data has been found.

Magneto-optical study of domain wall dynamics and giant Barkhausen jump in magnetic microwires

Investigation of surface domain walls motion in Co-rich magnetic microwires has been performed in circular and axial magnetic fields. The dc axial magnetic field acceleration of the domain wall motion related to the influence of the axial field on the structure of the moving domain wall has been discovered. Pulsed axial magnetic field induced unidirectional motion of surface domain wall also has been found.

Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magneticfields

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can alsoact as efficient magnetic shields. The magnetic screening properties under an axial DCmagnetic field are characterized at several temperatures below the liquid nitrogentemperature (77 K). Two main shielding properties are studied and compared with those ofBi-2223, a material that has been considered in the past for bulk magnetic shields. The firstproperty is related to the maximum magnetic flux density that can be screened,Blim; it is defined as the applied magnetic flux density below which the field attenuation measuredat the centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall thickness of5 mm and a large ratio of length over radius,Blim is evaluated to 1 Tat T = 10 K. This valuelargely exceeds the Blim value measured at the same temperature on similar tubes of Bi-2223.The second shielding property that is characterized is the dependence ofBlim with respect to variations of the sweep rate of the applied field,dBapp/dt. This dependence is interpreted in terms of the power lawE = Ec(J/Jc)n and allows us todetermine the exponent n of this E(J) characteristics for Bi-2212. The characterization of the magnetic field relaxation involves verysmall values of the electric field. This gives us the opportunity to experimentally determine theE(J) law in an unexplored region of small electric fields. Combining these resultswith transport and AC-shielding measurements, we construct a piecewiseE(J) law that spans over eight orders of magnitude of the electric field.

Use of second generation coated conductors for efficient shielding of dc magnetic fields

This paper reports the results of an experimental investigation of the performance of two types of magnetic screens assembled from YBa2Cu3O7−δ (YBCO) coated conductors. Since effective screening of the axial dc magnetic field requires the unimpeded flow of an azimuthal persistent current, we demonstrate a configuration of a screening shell made out of standard YBCO coated conductor capable to accomplish that. The screen allows the persistent current to flow in the predominantly azimuthal direction at a temperature of 77 K. The persistent screen, incorporating a single layer of superconducting film, can attenuate an external magnetic field of up to 5 mT by more than an order of magnitude. For comparison purposes, another type of screen which incorporates low critical temperature quasipersistent joints was also built. The shielding technique we describe here appears to be especially promising for the realization of large scale high-Tc superconducting screens.

Current transfer in dc non-transferred arc plasma torches

Fundamentals of current transfer to the anodes in dc non-transferred arc plasma torches are investigated. Specially designed anodes made of three mutually isolated sections and external dc axial magnetic fields of various strengths are utilized to explore the conditions for different diffused and constricted attachments of the arc with the anode. A number of new facts are revealed in the exercise. Under constricted attachment, formation of arc root takes place. Spontaneous and magnetically induced movements of the arc root, their dependence on the arc current and the strength of the external magnetic field, most probable arc root velocity, variation of the root velocity with strength of the applied magnetic field, the effect of swirl on the rotational speed of the arc root are some of the important features investigated. Two new techniques are introduced: one for measurement of the arc root diameter and the other for determination of the negative electric field in the boundary layer over the anode. While the first one exploits the rigid column behaviour of the arcs, the second one utilizes the shooting back of the residual electrons over an arc spot. Sample calculations are provided.

Evolution of an electron energy distribution function in a weak dc magnetic field in solenoidal inductive plasma

We investigated the evolution of the electron energy distribution function (EEDF) in a solenoidal inductively coupled plasma surrounded by an axial dc magnetic field. The increase in the dc magnetic field caused the EEDF to evolve from a bi-Maxwellian to a Maxwellian distribution. At the discharge center, the number of low energy electrons was significantly reduced while the high energy electron population showed little change when a weak dc magnetic field was present. However, at the discharge radial boundary, the high energy electron population decreased significantly with the magnetic field while the change in low energy population was not prominent compared to the discharge boundary. These changes in EEDFs at the boundary and center of the discharge are due to the radial confinement and the restriction of radial transport of electrons by dc magnetic field.

Giant magnetoimpedance of electrodeposited Co/Cu/Co on Ag wires

Giant magnetoimpedance (GMI) was studied in electrodeposited Co/Cu/Co-coated Ag wires with variations in the axial dc magnetic field (0–2 kOe), frequency of ac driving current (1 kHz–100 MHz), thickness of each layer (1.0–25.0 µm) and diameter of the Ag core (47.7 and 120.0 µm). With increasing thickness of coated layers and Ag cores, the maximum GMI ratio increased but the characteristic frequency decreased. Unlike Co-coated Ag wires, the frequency-dependent GMI curves of these multilayered wires were broadened and asymmetric. The GMI behaviours were explained in terms of the distribution of current density and current flow in the coated wires obtained from the finite element method simulation program.

GMI effect in ultra-thin glass-coated Co-rich amorphous wires

In this paper we report novel results on the GMI effect at frequencies between 10 MHz and 500 MHz in ultra-thin Co–Fe-rich amorphous glass-coated microwires with nearly zero magnetostriction constant with metallic nucleus diameter of about 8.5 μm. At 10 MHz the GMI of thin microwires is smaller than in thicker conventional wires and glass-coated microwires, but increasing the frequency GMI effect significantly increases, exhibiting much higher GMI effect at high frequencies and the shape of the dc magnetic field dependence of the GMI ratio is typical for the materials with circular magnetic anisotropy, i.e. with the a maximum at certain dc axial magnetic field. Besides, the glass-coating technology gives rise to the internal stresses due to the difference in the thermal expansion coefficients of the glass coating and metallic nucleus. This difference in the fabrication technique results in the different magnetic anisotropy in the surface and in different frequency dependence of the GMI effect.

Dynamic electromagnetic processes in micro-wires inferred from GMI-characteristics

The penetration depth of the skin-effect has been calculated for a specimen in the form of a piece of amorphous glass-coated magnetic micro-wire of the “non-magnetostrictive” composition Co 67Fe 3.85Ni 1.45Mo 1.7Si 14.5B 11.5 displaying large GMI effect. For these calculations a simple model was applied in which a rough assumption was made that the changes of the real component of the impedance are due only to changes in the effective cross-section of the wire for the AC-current. The evolution of the penetration depth with the applied DC-axial field and frequencies of the AC-current, flowing along the wire, is presented. The so-called Cole–Cole diagrams were also plotted for the same specimen. These diagrams, representing the measured impedance plotted in the complex plane, were obtained for axial DC-magnetic field of selected intensities and circular AC-field generated by an AC-current of various frequencies (1–30 MHz) flowing along the wire specimen. Analysis of the diagrams enabled one to characterize magnetization processes within the wire specimen and also to estimate the median relaxation time of these processes as well as a relative measure of the distribution of relaxation times.

Self-forming, quasi-two-dimensional, magnetic-fluid patterns with applied in-plane-rotating and dc-axial magnetic fields

We report flow instabilities with simultaneous, in-plane-rotating and dc-axial uniform magnetic fields. In uniform dc-axial magnetic fields, a ferrofluid drop in a Hele-Shaw cell with an∼1.0-mm gap forms the familiar labyrinth pattern. With subsequent application of an ∼100-G rms in-plane, 20–40-Hz rotating uniform magnetic field, smooth spirals form. If the rotating magnetic field is applied first, the drop holds together for low dc-axial magnetic fields and no labyrinth pattern develops. If the dc magnetic field is subsequently increased above a threshold value, the ferrofluid drop abruptly transforms into many ferrofluid droplets arranged in a regular pattern.

Ferrofluid spiral formations and continuous-to-discrete phase transitions under simultaneously applied DC axial and AC in-plane rotating magnetic fields

New flows and instabilities are presented for a ferrofluid drop contained in glass Hele-Shaw cells with simultaneously applied in-plane clockwise rotating and DC axial uniform magnetic fields. When a ferrofluid drop is stressed by a uniform DC axial magnetic field, up to ∼250 G in 0.9–1.4 mm gap Hele-Shaw cells, the drop forms a labyrinth pattern. With subsequent application of an in-plane uniform rotating magnetic field, up to ∼100 G rms at frequency 20–40 Hz, smooth spirals form from viscous shear due to ferrofluid flow. If the rotating magnetic field is applied first, the drop is held together without a labyrinth. Gradual increase of the DC axial magnetic field, to a critical magnetic field value, results in an abrupt phase transformation from a large drop to many small discrete droplets. A preliminary minimum magnetization and surface energy analysis is presented to model the phase transformation.

High-Impedance Transmission Line With Hybrid Surface Waves Propagating Near Gyromagnetic Resonance on a Ferrite Cylinder

This work presents a derivation of expressions for the electromagnetic fields propagating at a frequency near gyromagnetic resonance on a ferrite cylinder magnetized by an applied axial dc magnetic field. This system allows propagation by hybrid surface waves only, where the axial electric and magnetic fields are both nonzero. The expressions apply to the transverse impedance, the ratios of the electric field to the magnetic field for the radial and azimuthal components of these waves, and the power transfer impedance, which relates the axial current to the power transferred by these waves. Numerical values, based on the properties of strontium hexaferrite measured at 120 GHz, show that the power transfer impedance may exceed 1 M/spl Omega/. The data suggest that short lengths of such a high-impedance transmission line could be used to increase the output from millimeter-wave mixers.