The Influence of the Self-Magnetic Field on the Steady-State Current Distribution in an Axially Flowing Conducting Medium

The steady-state electric current distribution in a multicathode-spot vacuum arc was determined by a solution of the magnetic transport equation subject to various boundary conditions. The inter-electrode region of the arc is modeled as a uniform plasma flowing from the cathode to the anode. Dimensional analysis shows that three parameters determine the magnetic field, and hence the current density which is derived from it: AR-the ratio of the electrode separation to the electrode radius, Rmm-magnetic Reynolds number of the axial material flow, and Rme-magnetic Reynolds number of the axial electron flow. While the anode side of the conducting medium is described as an equipotential surface, the following three cases of boundary conditions for the cathode side are examined: 1) a known current density distribution is assumed over the entire cathode side of the plasma surface; 2) the cathode side is an equipotential surface; and 3) the current is allowed to cross the cathode surface only through a finite number of ring shaped regions. Numerical solutions of the nonlinear magnetic transport equation show a constriction of the current at the anode side for all boundary conditions mentioned. On the other hand, the current moves to the perimeter of the cathode for boundary condition 2). When AR, Rmm, and Rme equal 0.72,-0.16, and 1.73, respectively, and a uniform current density flows at the cathode side, the on-axis current density at the anode is six times larger than its value at the cathode.     

Asymptotic Analysis of the Steady-State Current Flow in a Uniform Multicathode-Spot Generated Vacuum-Arc Plasma Flow

The current distribution in a model multicathode-spot vacuum-arc generated uniform plasma flow is analyzed analytically by studying the asymptotic behavior of the magnetic transport equation in the limit of large electron-flow magnetic Reynolds number. It is found that the radial distribution of the axial current flow imposed at the cathode is maintained throughout most of the interelectrode region, and that a strong constriction of the current flow occurs in a narrow boundary layer adjacent to the anode surface. The results support the general trend observed in previous numerical solutions.     

Voltage of the vacuum arc with a ring anode in an axial magneticfield

The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40° in the zero magnetic field case and approaches a value of about 20° with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found     

Basic Characteristics of Vacuum Arcs Subjected to a Magnetic Field Parallel to Their Positive Columns

As vacuum arcs subjected to a magnetic field parallel to their positive column (an axial magnetic field) spread uniformly over all the electrodes and burn in the interelectrode region, arc voltages of these arcs are low and quiescent. When the magnetic field strength decreases, however, the arc voltage develops a large noise component and electrode melting occurs. Experiments were conducted to investigate the condition of these transition phenomena. As a result of these experiments, it was found that these two phenomena do not always occur simultaneously and that a new explanation for the mechanism of anode spot formation should be considered.     

Magnetic structuring of electrodeposits

Metal electrodeposition reflects the pattern of the magnetic field at the cathode surface created by a magnet array. For deposits from paramagnetic cations such as Co2? or Cu2?, the effect is explained in terms of magnetic pressure which modifies the thickness of the diffusion layer, that governs their mass transport. An inverse effect allows deposits to be structured in complementary patterns when a strongly paramagnetic but nonelectroactive cation such as Dy3? is present in the electrolyte, and is related to inhibition of convection of water liberated at the cathode, in the inhomogeneous magnetic field. The magnetic structuring depends on the susceptibility of the electroactive species relative to that of the nonelectroactive background.     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

Vacuum Arc Plasma Centrfuge for Element and Isotope Separation

Vacuum arcs have been studied extensively in the past several decades with applications primarily in the areas of switching, vacuum remelting, and vapor deposition. Application of the vacuum arc for element and isotope separation has been studied recently and is reviewed in this paper. An arc was produced in a 30-cm-diameter 4-m-long cylindrical chamber with coaxially mounted electromagnets providing a 2.6-m-long constant axial magnetic field of up to 6 kG. The vacuum discharge between a solid cathode and a mesh anode was triggered electrically. A pulse-forming network (PFN) of 70-m? impedance provided nearly constant-current discharge pulses of several kiloamps and 6-12-ms duration. The magnetized plasma column, flowing axially from the anode with a typical velocity of 106 cm/s, rotated nearly as a solid body. This rotation was due to the E × B drift, produced by the axial magnetic field and the radial electric field across the column. A typical rotation frequency was 105 rad/s. The centrifugal effect due to the rotation caused a radial redistribution of ions within the plasma column, thereby producing elemental and isotope enrichment. The separation was observed to increase exponentially with the square of the radius. Enrichments of up to 300 percent were measured in a Cu-Zn plasma. The radial plasma density profile was found to be roughly Gaussian, with central electron densities of about 1013 cm-3. The radial potential profile across the column was measured and found to be parabolic with radius.     

Vacuum Arc Behavior in a Transverse Magnetic Field

Experimental measurements in a vacuum interrupter have shown that the application of a transverse magnetic field results in substantial increases in arc voltage. Photographic studies of the arc column indicate that strong magnetic fields reduce the effective anode area and may lead to severe arc constriction.     

Ionization Waves in Glow Discharges with Non-Uniform Magnetic Fields

The properties of ionization waves self-excited in glow discharges with a non-uniform axial magnetic fields are investigated experimentally. The fundamental frequency remains almost constant over the tube for the variable magnetic fields, while the amplitude decreases with increasing the field. The wavelength becomes longer at the cathode side of the magnetic coil and shorter at the anode side with increasing the local magnetic field. The dependence of the wavelength on the magnetic field is explained by the changes of the axial electric field in the non-uniform magnetic field.     

Coaxial Discharge and Transverse Magnetic Field

The study deals with the effect of an applied transverse magnetic field on the dynamics and parameters of the focused and expanded plasma in a coaxial discharge. The experimental results were found with a 3 kJ Plasma focus device of a Mather geometry. The discharge takes place in hydrogen gas with base pressure of 0.5 Torr. The experiments are conducted with a 10 kV bank voltage, which corresponds to 100 kA peak discharge current with rise time 8 μs. Helmholtz magnetic coils are placed outside the expansions chamber to produce a transverse magnetic field with intensity 280 G perpendicular to the plasma expanded from the coaxial electrodes. The investigations have shown that the plasma flow along the expansion chamber axis is restricted when applying the externally transverse magnetic field and the maximum axial velocity of the expanded plasma is decreased by 33%. X-ray probe has been used to measure the focused plasma electron temperature (T_e). The experimental results and the calculations showed that T_e is decreased from 2.2 keV to 800 eV with the application of a transverse magnetic field. The expanded plasma electron temperature and density have been measured by an electric double probe, the results cleared that the expanded plasma electron temperature is decreased by 2.6 times while its density is increased by 9 times, when a transverse magnetic field is applied.     

Stability of the flow of weakly electro-conductive fluid in the presence of a spiral magnetic field

The stability to small disturbances of the flow in a pipe of annular cross section is considered in the presence of a spiral magnetic field. The investigated duct configuration consists of two infinite coaxial cylinders between which a weakly electroconductive viscous incompressible fluid is placed, which moves under the axial pressure gradient. The azimuthal magnetic field is created by a current flowing through the central cylinder, and the longitudinal magnetic field is created by an external solenoid. The magnetohydrodynamic approximation is used. It is found that the introduction of the azimuthal magnetic field may lead to a flow destabilization as compared to the case of only the longitudinal magnetic field.     

Magnetic field generated in a plasma by a short, circularly polarized laser pulse

We study the generation of a quasistatic magnetic field by a short, circularly polarized laser pulse in a tenuous cold uniform plasma. It is shown that two physical mechanisms are responsible for the generation of the various components of the magnetic field. One mechanism is due to the ponderomotive forces and governs the generation of the azimuthal component of the magnetic field. The other is similar to the inverse Faraday effect (IFE) in a nonuniform plasma and gives rise to axial and radial components of the magnetic field. At moderate radiative intensities, all magnetic field components are proportional to the squared intensity. The spatial structure of the magnetic field depends strongly on the pulse shape and the plasma density. Zh. éksp. Teor. Fiz. 114, 849–863 (September 1998)     

Magnetic studies with μSR at high pressures

A newly developed high pressure-low temperature μSR spectrometer was employed for two types of magnetic studies. Firstly we measured the pressure dependence of the local magnetic field B_μ in Fe, Co and Ni at 77 K up to 7 kbar. From the pressure derivative dlnB_μ/dP the volume derivative dlnB _Hf /dlnV was deduced. In connection with previous room temperature data we calculated the temperature dependence of B_hf at constant volume. It deviates markedly from the temperature dependence of the host magnetization. Secondly, we looked at the pressure dependence of the muonic Knight shift in Sb at 30 K for polycrystalline and single crystal samples. A strong pressure dependence was observed which depends on the orientation of the magnetic field relative to the c-axis. The pressure coefficients of the isotropic and the axial term of the Knight shift were deduced.     

脉冲金属离子等离子体推进器的等离子体生成和传播特性

综述了不同阳极结构脉冲金属离子等离子体推进器的放电特性、等离子体生成及传播特性。首先,讨论了一种带有绝缘套筒的裸阳极推进器结构。对比分析了无、有绝缘套筒的裸阳极推进器的等离子体生成及传播特性的区别。结果表明,绝缘套筒阻碍了阴极近旁带电粒子的径向运动,提高了沿绝缘套筒轴向喷射出去的等离子体的喷射性能。此外,发现采用裸阳极推进器结构放电过程中会有大量带电粒子进入阳极。其次,讨论了一种绝缘阳极推进器结构。结果表明,采用绝缘阳极结构进一步提高了沿绝缘套筒轴向喷射出去的等离子体密度。但是,与裸阳极推进器结构相比,等离子体的生成量减少。再次,讨论了一种微孔绝缘阳极推进器结构。结果表明,与裸阳极推进器结构相比,采用微孔绝缘阳极推进器结构生成的等离子体的密度峰值和传播速度峰值分别提高了12.6倍、3.9倍。最后,分别讨论了一种螺旋阳极推进器结构和一种多阳极推进器结构。结果表明,这两种推进器结构分别利用放电过程中形成的自磁场及电场有效提高了等离子体羽流的定向喷射性能。本研究可以为金属等离子体喷射性能的提高以及脉冲金属离子等离子体推进器的设计提供支持。     

Z箍缩等离子体磁重联现象

分析了磁重联对晕等离子体加速和能量平衡过程的影响。分析表明晕等离子体向轴心的加速过程分为两个阶段:第一阶段晕等离子体在磁压或热压(依赖于丝数)作用下向轴心运动;第二阶段晕等离子体由于磁重联过程被加速到阿尔芬速度,并到达轴心形成先驱等离子体。估算表明重联层的厚度与离子的惯性长度具有相同的数量级,电流片内电子运动和离子的运动是解耦合的。在内爆滞止阶段电荷分离产生强的径向电场,这个电场将磁能转化为等离子体轴向(z方向)动能,内爆动能和轴向动能共同转化为X射线辐射能,以此解释了X射线能量大于内爆动能这一观测结果。分析了磁重联电磁脉冲的性质,对于1 MA驱动条件,电磁脉冲的总能量可达kJ量级。     

The Rippled-Field Magnetron (Cross-Field Free Electron Laser)

Millimeter-wave emission from the rippled-field magnetron (cross-field free electron laser (FEL)) is investigated experimentally and theoretically. In this device, electrons move in quasi-circular orbits under the combined action of a radial electric field, a uniform axial magnetic field, and a radial azimuthally periodic wiggler magnetic field. In excess of 300 kW of RF power is observed in two narrow spectral lines whose frequency can be tuned continuously from ~25 to ~50 GHz by variation of the axial magnetic field. The observations are interpreted as a FEL type of instability, associated with a resonance in the particle motion of a layer of electrons embedded in the dense spacecharge cloud. The resonance is shown to occur when 2kw?0 ? (?>0/?0) ?1 -(?p/?0)2, where kw is the wiggler wavenumber, ?0 is the azimuthal electron velocity, ?0 is the relativistic cyclotron frequency in the axial magnetic field, wp is the relativistic plasma frequency, and ?0 = [1 - (?0/c)2]-1/2 of the resonant electron layer.     

The Interaction of Vacuum Arc Ion Currents with Axia I Magnetic Fields

We summarize a series of experiments in which we measured the distribution of ion currents leaving the interelectrode region of a vacuum arc with Cu electrodes. Ion currents were collected by an arrangement of cylindrical collectors surrounding the arcing space. A Helmholtz coil arrangement surrounding the arcing chamber generated the axial magnetic field. Arc currents ranged from 70 to 2400 A dc.     

Distribution of Peak Temperature and Energy Flux on the Surface of the Anode in a Multi-Cathode Spot Pulsed Vacuum Arc

The distribution of the peak temperature and energy flux on the surface of a steel anode in a pulsed high-current vacuum arc was determined by studying the spatial location of the borderline separating the region of hardened steel, produced by the pulse of energy flux to the anode, and the region of the anode which did not undergo a phase transition. The arc was run between a 14-mm-diameter stainless steel cathode and a 25-mm 4340 steel anode, separated by a 4-mm gap, with peak currents up to 1000 A and 71 ms full-width half-amplitude (FWHA) duration. The phase transition of the steel occurs at 727°C and the above-mentioned borderline is thus the geometrical location of all points which reached a peak temperature of 727°C. The peak anode surface temperature was calculated from the borderline position by approximate solution of the three-dimensional heat conduction equation. The effect of an axial magnetic field on the anode surface temperature and energy flux distribution was also studied showing that with no magnetic field the distribution had a pronounced maximum on the axis of the arc, while with the presence of a magnetic field the distribution became annular with a maximum at about mid-radius. In comparison, the shape of the distribution of the cathode mass deposited by the arc on the anode was uniform without a magnetic field. The peak of the anode temperature and the energy flux amplitude also depended on the magnetic field, first decreasing and then increasing almost linearly with it.     

Physical and theoretical aspects of a new vacuum arc controltechnology-self arc diffusion by electrode: SADE

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment