Effect Of Self-magnetic Field Research Articles

Self-magnetic field effects on laser-driven wakefield electron acceleration in axially magnetized ion channel

AbstractIn this paper, we have investigated the relativistic electron acceleration by plasma wave in an axially magnetized plasma by considering the self-magnetic field effects. We show that the optimum value of an external axial magnetic field could increase the electron energy gain more than 40% than that obtained in the absence of the magnetic field. Moreover, results demonstrate that the self-magnetic field produced by the electric current of the energetic electrons plays a significant role in the plasma wakefield acceleration of electron. In this regard, it will be shown that taking into account the self-magnetic field can increase the electron energy gain up to 36% for the case with self-magnetic field amplitude Ωs = 0.3 and even up to higher energies for the systems containing stronger self-magnetic field. The effects of plasma wave amplitude and phase, the ion channel field magnitude, and the electron initial kinetic energy on the acceleration of relativistic electron have also been investigated. A scaling law for the optimization of the electron energy is eventually proposed.

Stability of Space-Charged-Limited Electron Beam Diodes Including Applied- and Self-Magnetic Field Effects

The theoretical analysis of large-area diodes shows that the electron flow in the diode is subject to the transit-time instability (TTI). The TTI can modulate the electron beam energy at the anode and add a transverse beam temperature. Both of these effects can have negative consequences on many diode applications that require a high-quality mono-energetic electron beam. A distributed transmission line model of the diode is developed which allows techniques from microwave engineering to be applied to the unstable diode. The theory suggests that a resistively loaded, periodic structure of slots in the cathode can stabilize the TTI. The results from particle-in-cell (PIC) simulations of a large-area electron-beam diode in 2-D are presented that support this conclusion.

Effect of thermionic cathode heating current self-magnetic field on gaseous plasma generator characteristics.

The performance capabilities of the PINK, a plasma generator with a thermionic cathode mounted in the cavity of a hollow cathode, depending for its operation on a non-self-sustained low-pressure gas discharge have been investigated. It has been shown that when a single-filament tungsten cathode 2 mm in diameter is used and the peak filament current is equal to or higher than 100 A, the self-magnetic field of the filament current significantly affects the discharge current and voltage waveforms. This effect is due to changes in the time and space distributions of the emission current density from the hot cathode. When the electron mean free path is close to the characteristic dimensions of the thermionic cathode, the synthesized plasma density distribution is nonuniform and the cathode is etched nonuniformly. The cathode lifetime in this case is 8-12 h. Using a cathode consisting of several parallel-connected tungsten filaments ∼0.8 mm in diameter moderates the effect of the self-magnetic field of the filament current and nearly doubles the cathode lifetime. The use of this type of cathode together with a discharge igniting electrode reduces the minimum operating pressure in the plasma generator to about one third of that required for the generator operation with a single-filament cathode (to 0.04 Pa).

Towards a fully kinetic 3D electromagnetic particle-in-cell model of streamer formation and dynamics in high-pressure electronegative gases

Streamer and leader formation in high pressure devices is dynamic process involving a broad range of physical phenomena. These include elastic and inelastic particle collisions in the gas, radiation generation, transport and absorption, and electrode interactions. Accurate modeling of these physical processes is essential for a number of applications, including high-current, laser-triggered gas switches. Towards this end, we present a new 3D implicit particle-in-cell simulation model of gas breakdown leading to streamer formation in electronegative gases. The model uses a Monte Carlo treatment for all particle interactions and includes discrete photon generation, transport, and absorption for ultra-violet and soft x-ray radiation. Central to the realization of this fully kinetic particle treatment is an algorithm that manages the total particle count by species while preserving the local momentum distribution functions and conserving charge [D. R. Welch, T. C. Genoni, R. E. Clark, and D. V. Rose, J. Comput. Phys. 227, 143 (2007)]. The simulation model is fully electromagnetic, making it capable of following, for example, the evolution of a gas switch from the point of laser-induced localized breakdown of the gas between electrodes through the successive stages of streamer propagation, initial electrode current connection, and high-current conduction channel evolution, where self-magnetic field effects are likely to be important. We describe the model details and underlying assumptions used and present sample results from 3D simulations of streamer formation and propagation in SF6.

Depairing critical currents and self-magnetic field effects in submicron YBa2Cu3O7−δ microbridges and bicrystal junctions

We report on depairing critical currents in submicron YBa2Cu3O7−δ microbridges. A small-angle bicrystal grain boundary junction is used as a tool to study the entrance of vortices induced by a transport current and their influence on the I–V curves. The interplay between the depairing and the vortex motion determines a crossover in the temperature dependence of the critical current. The high entrance field of vortices in very narrow superconducting channels creates the possibility of carrying a critical current close to the depairing limit determined by the S–S′–S nature of the small-angle grain boundary junction.

Critical current anisotropy in Ag/(Pb,Bi)2Sr2Ca2Cu3O10+xmultifilamentary tapes: influence of self-magnetic field

Two factors affect critical current anisotropy in multifilamentaryAg/(Pb,Bi)2Sr2Ca2Cu3O10+xtapes - the intrinsic material anisotropy and the geometry. Experimentalresults on the magnetic field dependence and anisotropy of the criticalcurrent in a multifilamentary Ag/(Pb,Bi)2Sr2Ca2Cu3O10+x tape after correction for self-magnetic fieldeffects were found to fit the anisotropic Kim relation. Based on this relationa finite-element-method numerical code for solving the nonlinear Poissonequation for vector magnetic potential was adopted. It allowed theexperimental data to be reproduced by back calculation and made possible thestudy of the interplay of self and external magnetic fields in different caseswith well defined physical parameters of the material. The model was used toanalyse the distribution of the critical current in individual filaments aswell as to evaluate the influence of their geometrical arrangements on thecritical current of the tape. The self-field critical current of an individualfilament `extracted' from the tape was compared with the critical current ofthe overall tape. The effect of the self-magnetic field on critical currentdistribution obtained by the cutting method was determined. The criticalcurrents of the tapes with different cross sections were calculated andcompared with experiments and the influence of the self-field was analysed.The anisotropic properties of a low anisotropy architecture of amultifilamentary Ag/(Pb,Bi)2Sr2Ca2Cu3O10+x conductor were studied. The dependence of critical currents(normalized to self-field critical currents) on external magnetic fieldcorrected for the self-field was found to follow nearly the same curves asthose for tapes with different critical current densities (in the range20-70 kA cm-2 in a self-field), which makes the numericalmodel applicable to different tapes.

Critical currents in polycrystalline thin films of high-T c superconductors

It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (= SIS) «sandwich» between each grain leads to a crossover from I c ∼ (1 - T/T c) to I c ∼ (1 - T/T c)3/2 for temperatures near T c (where I c is the critical current, T the absolute temperature, and T c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (= SNS) sandwich the dependence I c ∼ (1 - T/T c)2 is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended.

Applied-B ion diode experiments on the Particle Beam Fusion Accelerator-I

A series of experiments was performed with an Applied-B ion diode on the Particle Beam Fusion Accelerator-I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied-B diode operation from the sub-TW level on single module accelerators to the multi-TW level on a low impedance, self-magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100-TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of the diode impedance behavior and beam quality with electrical drive power is obtained from comparison with lower-power experiments. The diode impedance lifetime was about 10 ns, several times shorter than for lower-power experiments. Azimuthal and top-to-bottom variations of the diode and ion currents were found to be approximately ±10%, compared with an estimated requirement of 5%–7% uniformity to avoid focal blurring by self-magnetic field effects. The ion production efficiency was 80%–90%. However, only 50%±10% of the ion current was carried by protons; the balance was carried by multiply charged carbon and oxygen ions. Activation measurements showed a proton beam energy of approximately 50 kJ. A gas cell filled with 5 Torr of argon was used for beam transport. The macroscopic divergence was 15±10 mrad and the microscopic divergence was 20±15 mrad, values that are similar to those from lower-power experiments. A model of beam focusing is formulated that predicts the proton charge focused onto 0.47-cm radius lithium targets, taking into account beam purity, magnetic bending, small-angle multiple scattering, and intrinsic divergence. The model results and activation measurements of the number of protons focused onto targets agree, and indicate that the spatially averaged (over about 3 cm2) peak focal power was about 0.5 TW/cm.2 The most important limitations on power concentration were found to be the low proton content of the beam, the short impedance lifetime of the diode, and the asymmetric current feed of the accelerator. The short impedance lifetime limited the power coupled to the diode, and caused the voltage at peak ion power to be low, which exacerbates the small-angle scattering problem. The asymmetric feed caused focal blurring through nonuniform self-magnetic bending. At least partly because of the experience gained with low impedance beams during these experiments, the next generation accelerator, the 100-TW Particle Beam Fusion Accelerator-II, has been configured to produce a 25–30-MV Li+ beam rather than a 5-MV proton beam. off

The high-power constricted plasma discharge column I. Theoretical analysis

The equation expressing energy balance between Joule heating and thermal conduction is solved under the assumption that the plasma behaves as a Lorentz gas. This yields two relations for the plasma temperature, one in terms of the electric field E and the column radius R; the other in terms of the current I, the radius R and the ionic charge Z. Elimination of temperature between these relations gives the electrical characteristic Eis proportional toZfraction two-fifthsR−fraction seven-fifthsIfraction two-fifths. The analysis is valid over a range of temperature and density limited at one extreme by radiation losses and at the other extreme by effects of self magnetic field on the transport processes. There is excellent agreement between the temperatures given by this theory and those recently measured spectroscopically for capillary discharges.

The Effect of Self-Magnetic Field on the Galvanomagnetic Effects in Bismuth

The effect of the self-magnetic field on the galvanomagnetic effects in bismuth was investigated using high current pulses at 77°K. Qualitative agreement between experiments and theory is obtained, using the Abeles and Meiboom model for the bismuth band structure.

Focusing in High-Voltage Beam-Type Electron Devices†

ABSTRACT The paraxial electron trajectory equation applicable at relativiatic speeds, including the effects of self-magnetic field and space-charge of the beam is derived. Employing the above, problems concerning beam spreading, electrostatic lenses (unipotential Pierce gun being an example), magnetic lenses, Brillouin flow and beam stability are treated. Most of tho results are presented graphically and also discussed in detail. Applications of tho above results to practical situations is indicated.

Current Limitation in the Low-Pressure Mercury Arc

Measurements on current limitation in the low-pressure mercury arc are reported and a theory is given which depends on the limit to the ion generation set by the flux of neutral atoms passing from the walls into the plasma. The paper refers to low current discharges, where self-magnetic field effects are negligible.