External Axial Magnetic Field Research Articles

Influence Mechanism of Cathode Materials on the Transition Process of Arc Modes in Drawn Vacuum Arcs Under External AMF

The objective of this article is to determine the influence mechanism of the cathode materials on the transition process of the arc modes in high-current drawn vacuum arcs under the external axial magnetic field (AMF). Six kinds of pure metal of Cu, W, Mo, Fe, Cr, and Al were selected as the cathode materials. A high-speed charge-coupled device (CCD) video camera was selected to record the arc appearance. The experimental results indicate that the cathode materials had a great effect on the arc modes in the high-current vacuum arcs with the application of AMF. Under the cathode of W and Mo, the arc modes turned to the constriction arc mode and the anode spot arc mode more easily than that under the cathode of Cu, Fe, Cr, and Al, with the increase of the arc current. For various cathode materials and same anode, the critical current density inputting to anode surface for the transform of vacuum arc modes was different. This should be caused by the differences in the constricted characteristics and the ion energy of the vacuum arcs, which took a significant effect on the transition process of the arc modes in the vacuum arcs under AMF. The higher ion energy would lead to a more active anode, which resulted in the positive ion and metal vapor from the anode. Moreover, the cathode materials played different roles in the various stages of the discharging process of the drawn vacuum arcs. The effect of cathode material on the transition process of the arc modes in the high-current vacuum arc mainly took in Stage II, which sustained from the end of the bridge arc column to the arc current peak and dominated the transition process of the arc modes.

Enhanced electron acceleration by a chirped tightly focused laser in vacuum in the presence of axial magnetic field

The paper presents a scheme of vacuum electron acceleration due to tightly focused linearly polarized (LP) frequency chirped laser in the presence external magnetic field. The focused terawatt chirped laser pulse and the external axial magnetic field enable the electron of few MeV initial energy to attain high energy in GeV range, hence, explore the possibility of efficient electron acceleration. This can be achieved by optimizing the focused terawatt chirped laser and axial magnetic field parameters for resonance, which is realized between the electron and electric field of the tightly focused laser pulse. The frequency chirp plays a crucial role in enforcing resonance condition for longer duration and the applied magnetic field strongly enforces the electron to remain in the accelerating phase, thereby, both factors are equally imperative for accomplishing high electron energy gain in GeV. The presence of optimum axial magnetic field (~8.5 MG) along with optimum value of linear frequency chirp for laser intensity 1.38 × 1020 W/cm2 results in electron beam with energy ~5.78 GeV. During acceleration process, electron energy gain is highly sensitive to the initial phase of laser along with the frequency chirp and magnetic field. Electron trajectories in the influence of frequency chirp and magnetic field are also analyzed.

Azimuthal surface waves in cylindrical metal waveguides partially filled by magnetoactive plasma: Analysis of energy transfer

Azimuthal surface waves are well known to be eigenwaves of cylindrical waveguide structures—plasma–dielectric–metal—placed into an external axial static magnetic field. The angular velocity of energy transfer along with the angular group velocity is applied to analyze electromagnetic energy rotation in these structures. Optimal magnitudes of effective azimuthal wavenumbers are determined for which the angular velocity reaches its maximum. The angular velocity is shown to decrease with the increasing absolute value of the azimuthal wavenumber and with the decreasing strength of the external axial static magnetic field.

Enhanced proton acceleration via the leaky light-sail regime by laser interaction with nanofoils in strongly magnetized plasmas

An enhancing proton acceleration scheme is proposed by applying an external axial magnetic field in the leaky light-sail regime when a right-hand circularly polarized laser interacts with a multispecies nanofoil. In this scheme, the external axial magnetic field B0 can not only enhance the intensity of the accelerating electric field but also adjust the spatial distribution of the accelerating electric field. It is shown by 2D3V particle-in-cell simulations that a monoenergetic proton beam could be produced in the case of B0=2. Compared with the case of B0=0, the peak energy increases by 50% and the number improves by a factor of two when the laser intensity is 3.4×1019 W/cm2. Besides, the angle divergence of protons can be greatly improved by manipulating the shape of accelerating electric field from concave to convex under the influence of the B0=2. These results may be helpful to understand the process of particle motion in a strongly magnetized plasma.

Effect of external axial magnetic field on a helium atmospheric pressure plasma jet and plasma-treated water

Potential in biomedical-related applications by atmospheric pressure plasma-treated water gradually increased recently. In order to enhance the generation of reactive species in atmospheric pressure plasma in regards to liquid treatment, this study aims to investigate the effect of external axial magnetic field on a helium atmospheric pressure plasma jet (APPJ) and plasma-irradiated water. Magnetic field strength up to 2.0 T was generated by an electromagnet in Helmholtz configuration. The dielectric barrier discharge-based APPJ is driven by 4.4 slm helium and sinusoidal 30 kHz, 10 k signals. Plasma was mainly characterized by optical emission where vibrational and rotational temperature was estimated by nitrogen second positive system, and electron temperature and density by emission intensity line ratio method. Representative reactive species of nitrite, nitrate, and hydrogen peroxide concentration in the plasma-treated water were also quantified. Results of vibrational temperature and electron density showed a first drop then rise trend which partially corresponds to the reactive species in the plasma-treated water. No significant changes were found in rotational temperature, while electron temperature monotonically increased with the magnetic field.

Control of ordinary and extraordinary waves excitation by an axial magnetic field in motional plasma

The propagation of transverse Electromagnetic (EM) waves along a high energy relativistic electron beam passing through a neutralizing heavy ion background is investigated in the presence of an external axial magnetic field. By making use of the linearized fluid equations coupled with Maxwell’s equations the dielectric tensor and the dispersion relation of excited modes are derived. Since the electron motion is considered relativistic and fluid theory of plasma is used to describe the system, the obtained results are applicable to a vast range of high energy density regime. The mutual interaction between EM and Electron Plasma Waves (EPWs) is studied numerically. In the absence of the axial magnetic field, the EPWs and the Extraordinary Electromagnetic (EOEM) waves interact due to the existence of the self-generated magnetic field. The instability caused by this interaction has already been reported [Phys. Plasma.16, 062107 (2009) ]. We found that applying an axial magnetic field will cause deflection to plasma oscillations which enables coupling between EPWs and Ordinary Electromagnetic (OEM) waves as well. The results show that increasing the external magnetic field strength suppresses the growth rate caused by the coupling of EPWs-EOEM and passes it to the growth of EPWs-OEM. Therefore by adjusting the external magnetic field strength, the growth of EOEM and OEM modes can be controlled.

Numerical study on arc-droplet coupled behavior in magnetic field controlled GMAW process

The metal transfer behavior is one of the most pivotal links of gas metal arc welding (GMAW) for improving the welding quality. Arc impacts the current path and heat input on the droplet surface, in reverse, the metal droplet detachment makes the arc to flicker. It is hard to make the metal transfer process clear with only experimental methods since the limited measuring space and high temperature in the welding region. In this work, with the help of computational fluid dynamics software ANSYS FLUENT, a magnetohydrodynamic model including argon arc plasma and steel wire and the droplet is constructed by using two independent governing equations of gas and metal phase for the mass, momentum, and energy balance. Comparing the magnetic field controlled GMAW with conventional GMAW, the temperature field, current density distribution, electric potential field, static pressure field and also metal vapor mass fraction distribution are highlighted. It is shown that the metal transfer frequency decreases and droplet diameter increases with 180 A welding current when applying an external constant axial magnetic field 0.002 T. So, the arc length decreases before the detaching moment, the current density expands in the arc region and the voltage decreases. The centrifugal movement impedes the downward flow and transfers heat from the droplet center to the outside region.

Analysis of azimuthal magnetorotational instability of rotating magnetohydrodynamic flows and Tayler instability via an extended Hain-Lüst equation.

We consider a differentially rotating flow of an incompressible electrically conducting and viscous fluid subject to an external axial magnetic field and to an azimuthal magnetic field that is allowed to be generated by a combination of an axial electric current external to the fluid and electrical currents in the fluid itself. In this setting we derive an extended version of the celebrated Hain-Lüst differential equation for the radial Lagrangian displacement that incorporates the effects of the axial and azimuthal magnetic fields, differential rotation, viscosity, and electrical resistivity. We apply the Wentzel-Kramers-Brillouin method to the extended Hain-Lüst equation and derive a comprehensive dispersion relation for the local stability analysis of the flow to three-dimensional disturbances. We confirm that in the limit of low magnetic Prandtl numbers, in which the ratio of the viscosity to the magnetic diffusivity is vanishing, the rotating flows with radial distributions of the angular velocity beyond the Liu limit, become unstable subject to a wide variety of the azimuthal magnetic fields, and so is the Keplerian flow. In the analysis of the dispersion relation we find evidence of a new long-wavelength instability which is caught also by the numerical solution of the boundary value problem for a magnetized Taylor-Couette flow.

Study of stability in a liner-on-target gas puff Z-pinch as a function of pre-embedded axial magnetic field

Gas puff Z-pinches are intense sources of X-rays and neutrons but are highly susceptible to the magneto-Rayleigh-Taylor instability (MRTI). MRTI mitigation is critical for optimal and reproducible yields, motivating significant attention toward various potential mitigation mechanisms. One such approach is the external application of an axial magnetic field, which will be discussed here in the context of recent experiments on the Zebra generator (1 MA, 100 ns) at the University of Nevada, Reno. In these experiments, an annular Kr gas liner is imploded onto an on-axis deuterium target with a pre-embedded axial magnetic field Bz0 ranging from 0 to 0.3 T. The effect of Bz0 on the stability of the Kr liner is evaluated with measurements of plasma radius, overall instability amplitude, and dominant instability wavelength at different times obtained from time-gated extreme ultraviolet pinhole images. It was observed that the external axial magnetic field does not affect the implosion velocity significantly and that it reduces the overall instability amplitude and the presence of short-wavelength modes, indicating improved pinch stability and reproducibility. For the highest applied Bz0=0.3 T, the stagnation radius measured via visible streak images was found to increase. These findings are consistent with experiments reported in the literature, but here, the Bz0 required for stability, Bz0=0.13 Ipk/R0 (where Ipk is the driver peak current and R0 is the initial radius), is lower. This could be attributed to the smaller load geometry, both radially and axially. Consistent with other experiments, the cause of decreased convergence cannot be explained by the additional axial magnetic pressure and remains an open question.

The Interaction of a High-Power Sub-Nanosecond Microwave Pulse With Plasma

We present the results of experiments, analytical modeling, and numerical particle-in-cell (PIC) simulations for the propagation of a high-power, sub-nanosecond microwave pulse through a plasma-filled cylindrical waveguide in a parameter regime not studied before. Depending on the experimental conditions, the non-linear interaction of the high-power microwave (HPM) pulse with the plasma results in self-channeling through it or causes the formation of a wakefield. To study this phenomenon, two backward wave oscillators (BWOs), operating in the super-radiant mode at frequencies of 9.6 and 28.6 GHz, were designed and tested. These BWOs were driven by an electron beam (~280 keV, ~1.5 kA, ~5 ns) generated in a magnetically insulated foil-less diode, from which it propagates through a slow wave structure guided by an external axial magnetic field. Microwave pulses of ~0.4-ns width, up to ~500-MW peak power at 9.6 GHz, and up to ~1.2 GW at 28.6 GHz were obtained. These powerful subnanosecond timescale microwave beam pulses were injected into the neutral gas, plasma, or their mixture in various configurations.

Control of the Arc Motion in DC Plasma Spray Torch with a Cascaded Anode

Two common concerns in DC plasma torches are stability of plasma jet and anode erosion. The challenge is how to get a stable plasma jet with minimal anode erosion. This study tackles this question by using either a swirling gas injection or an external axial magnetic field applied to the Oerlikon SinplexPro™ plasma torch. A 3-D, time-dependent MHD model of the plasma torch operation was used to predict the value of the external magnetic field and its effect on the heat flux to the anode and plasma jet stability. The special feature of the model is to couple the gas phase and electrodes that makes it possible to follow the anode temperature evolution. For specific operation conditions (anode of O9 mm, 500 A, Ar 60 NLPM), the model predicted that the maximal value of the azimuthal self-magnetic field inducted by the arc current was 0.055 T; it also showed that an external magnetic field of 0.05 to 0.1 T could make it possible to limit the anode erosion without noticeably disturbing the plasma jet issuing from the plasma torch. We expect this approach to help to better understand the arc behavior in commercial plasma torches and control anode erosion

Improvement of proton acceleration via collisionless shock acceleration by laser-foil interaction with an external magnetic field

Effects of the external intense axial magnetic field on collisionless shock acceleration (CSA) are investigated by using two-dimensional particle-in-cell simulations. Proton beams accelerated by CSA show different properties when left-hand circularly polarized (LHCP) or right-hand circularly polarized (RHCP) lasers are individually applied to a foil target with or without the magnetic field. It can be attributed to the difference of the dispersion relationship for the laser propagating in a plasma. Protons achieve more efficient acceleration when magnetized plasma is irradiated by the RHCP laser compared with the LHCP laser. Furthermore, the effect of different amplitudes of the magnetic field is studied numerically. It shows that the induced electrostatic charge-separation field arises deep in the target with huge strength of the magnetic field. Protons in the upstream are accelerated before the shock arrives, leading to less efficient acceleration. As a result, an appropriate magnetic field should be applied to enhance the CSA regime.

Wetting mechanism and microstructure evolution of TC4/304 stainless steel joined by CMT with an assisted hybrid magnetic field

To obtain excellent wettability under lower heat input, the role of external axial magnetic field was explored in wetting of liquid Cu on Ti and steel plate. The wetting activation energy decreased from 66 kJ/mol to 57.9 kJ/mol for the Cu/TC4 system, and from 78.9 kJ/mol to 51.8 kJ/mol for the Cu/304ss system when using a magnetic field, and the spreading limited factors changed from diffusion-limited to reaction-limited for the Cu/TC4 system. Besides this, the external axial magnetic field affected joint formation and interfacial microstructure: a wrap-around joint was formed on the Ti side and the spreading area of Cu on the steel plate surface was increased. A thick, continuous Ti2Cu3 and TiCu4 compound layer was transformed to a loose layer with multiple phases, consisting primarily of Cu, TiCu, Ti2Cu3, TiCu4, TiFe2, and Ti5Si3 therein. The Ti element segregated at the Cu/Fe interface and formed granular TiFe2 that was evenly distributed in the Fe–Cu solid solution, which transformed the original solid solution interface into a reaction interface. The highest tensile strength of 416 MPa increased by 44% compared to the joint without external magnetic field.

Effect of external axial magnetic field on the early stage instabilities in magnetized cylindrical liners

Based on the resistive MHD theory, the dispersion relations of instabilities are derived in the liner geometry, where a uniform current flows over the cross section. Both the external axial magnetic field and the electrothermal effect are taken into account. It is found that instabilities act as electrothermal instabilities (ETIs) below a critical wavelength, which is increased by the axial magnetic field. Beyond the critical wavelength, the growth rate increases with the wavelength quickly and approaches the maximum which depends on the gradient scale length of the azimuthal magnetic field. When the axial magnetic field is applied, the m = 1 mode has a higher growth rate than the m = 0 mode although the difference between them is relatively small.

Effect of the electron heating transition on the proton acceleration in a strongly magnetized plasma

A novel scheme assisted by an external axial magnetic field is proposed to accelerate and collimate protons when a right-hand circularly polarized laser irradiates on an overdense plasma. We find that the transition of heating electron mode plays an important role in proton acceleration. First, the electrons are accelerated by stochastic heating in the case of no external magnetic field. Second, when the ratio of electron cyclotron frequency in the external magnetic field to the laser frequency is smaller than the relativistic factor ωce/ω0≤γ, the cyclotron resonance absorption can occur and a laser front sharpening mechanism greatly improves the energy conversion from the laser to electrons. Meanwhile, the external magnetic field also restrains electrons' transverse motion. Finally, for ωce/ω0>γ, there is a time delay in the electron heating, which can be divided into two stages. In the case of B = 2, a high quality proton beam can be harvested whose cut-off energy is enhanced by a factor of 4 and beam width is reduced to one fifth of that in the case of B = 0. These results may be helpful to understand the electron heating and proton accelerated process in a strongly magnetized plasma.

Experimental Determination of the Current Constriction Characteristics at the Anode of AMF Vacuum Arcs

Before the formation of the anode spot, the anode current constriction is considered as a result of current constriction on the cathode surface and in the arc column. The objective of this paper is to quantitatively determine the current constriction characteristics by experiments at the anode of vacuum arcs under an axial magnetic field (AMF). In this paper, the radial distribution of the anode current density in a diffused vacuum arc was determined quantitatively by a novel method of four types of split-anode contacts, with various diameters of the central areas. The vacuum arc was in the diffuse arc mode with the arc current ranging from 6-to 14-kA rms. An external AMF was applied at 74 mT. The results indicate that the radial distribution of the anode current density $j_{z}$ is in the form of the three-parameter equation, $j_{z}=aj_{A}e^{(-br\wedge 2)}+c$ , where $j_{A}$ is the average current density on the whole anode surface, $r$ is the radius of the anode contact, and the value of constants $a$ , $b$ , and $c$ depends on the arc current. Moreover, the results reveal the constriction characteristics of the anode current density in high-current vacuum arcs subject to the AMF, which was quite uneven in the radial distribution of the anode current density in the diffuse arc mode. Finally, under various arc currents, the constriction characteristics of the anode current density reveal similar patterns. The current density mainly constricted in the central region, in which the diameter might be only one-third of the whole diameter of the anode contact.

The Influence of an External Coaxial Magnetic Field on the Characteristics of a High-Current ARC in a Vacuum Switch

The influence of an external synchronous axial magnetic field on the characteristics of a vacuum-arc discharge is investigated within the range of discharge currents from 6 to 14 kA. The magnetic field amplitude is regulated in a wide range (from 0 to 190 mT) irrespective of the discharge current, while the specific magnetic field induction (in mT/kA) remains constant during the arc burning time. A critical specific magnetic field induction is determined, which prevents the discharge channel from constriction in the gap. It is shown that an application of an external magnetic field does not give rise to the formation of an anode spot, while the cathode is covered with a large number of microspots evenly distributed across its surface. An analysis of the probe currents demonstrates that the density of the post-arc plasma under these conditions decreases, and its decay is accelerated.

Electron Acceleration by a Chirped Short Intense Laser Pulse in Presence of an External Axial Magnetic Field in Vacuum With Different Phase Values

We investigated electron acceleration by a chirped short intense laser pulse in presence of an external magnetic field. The retained electron energy is very high with frequency chirp on increasing the value of chirp parameter and constant value of laser intensity parameter. Also, the retained electron energy increases on increment of laser intensity parameter. A linear frequency chirp \(\omega (t ) =\omega_0 (1-\alpha t)t\) was considered, here \(\omega _0\) is the laser frequency at \(z=0\) and \(\alpha\) is the frequency chirp parameter. On increasing the chirp parameters corresponding to the magnetic field with phase then the retained electron energy become so high. Also, we study the variation of the relativistic factor gamma \((\gamma)\) and the laser intensity parameter \((a _0 )\); also the variation of the relativistic factor gamma \(( \gamma )\) and the magnetic field \((b_0)\) with different values of the phase, \(\phi=0\), \(\pi /4\) and \(\pi/2\), respectively. As the time duration is increased the energy gain increased.

Effect of axial magnetic field on cold metal transfer arc-brazing of Ti6Al4V to 304L steel

Magnetic intensities from 0 to 18.0 mT and CuNi10 wire were employed in Cold Metal Transfer (CMT) joining of Ti6Al4V to 304 L stainless steel. The axial external magnetic field (EMF) could improve the wetting behavior and modify the microstructure of arc-brazed joints. A rotated bell-shaped arc was formed owing to the Lorentz force acting during the welding process, resulting in short arc length, wide arc width, and low temperature of the melting pool. The liquid metal in the melting pool exhibited an incomplete circular motion on the surface owing to the pulsed plasma shear stress and Lorentz force caused by the radial current and axial magnetic field. With increasing magnetic intensity, the inner layer containing β + π + Ф (NiTi + NiTi2+CuTi) and π + ψ + Ф (NiTi2 + CuTi2 + CuTi) and outer layer containing γ + Δ ((Cu, Ni) + TiNiCu) became thinner and more uniform because of the intense flow and short reaction time at high temperature. The microstructure on the 304 L side of the joints changed from columnar dendrites to coarsened grains. With increasing magnetic intensity, the tensile strength of the joints increased from 270.7 to 387.8 MPa. All the samples fractured at the Ti6Al4V/seam interfacial layer of the joints.

Mechanisms of the Formation of an Electric Arc of a Helical Shape in an External Axial Magnetic Field

A numerical simulation of an open direct-current arc in an external uniform axial magnetic field is performed. A circuit analog of fluctuations of the temperature of electrons is used for the numerical implementation of the open electric arc column of a helical shape. It randomly generates asymmetry in the temperature distribution of electrons and, consequently, other plasma characteristics. It is shown that the formation of the arc column of a helical shape is due to the asymmetric influence of the Ampere force, which shifts the cross sections of the arc relative to the central discharge axis. Furthermore, the cross sections of the arc rotate around the central axis at different speeds. The rotational speed at the initial part from the side of the cathode decreases downstream and increases upon approaching the anode. Such a character of the convective heat transfer causes the formation of a helical arc. If the direction of the external axial magnetic field coincides with the direction of the electric current, then the spatial orientation of the helical arc as a whole is right-handed. Otherwise, there is a left-handed structure of the arc column. The directions of rotation of the arc sections at the cathode and anode sides are opposite each other. A helical shape of the open arc is not constant in time and is periodically destroyed. Two modes of destruction were revealed: first, as a result of bridging the spiral of the helix and, second, as a result of the transformation of several spirals into one. Apparently, an open arc cannot maintain a stable and constant helical shape in an external axial magnetic field.