Showing papers on "Plasma channel published in 1989"

High current ion source

Abstract: An ion source utilizing a cathode and anode for producing an electric arc therebetween. The arc is sufficient to vaporize a portion of the cathode to form a plasma. The plasma leaves the generation region and expands through another regon. The density profile of the plasma may be flattened using a magnetic field formed within a vacuum chamber. Ions are extracted from the plasma to produce a high current broad on beam.

Transport of vacuum arc plasma through straight and curved magnetic ducts

Abstract: We report on an experimental investigation of the transport of a metal vapor vacuum arc plasma through straight and curved magnetic solenoids for the case of an iron plasma and for magnetic field strength up to 650 G. We find that the fraction of plasma transported increases with magnetic field strength and saturates at a field at which the ion gyroradius equals the radius of the plasma channel. The magnetic field strength at the cathode location, where the plasma is injected into the solenoid, was found to be important, and for the present work the injection efficiency optimizes at a field of about 150 G.

Numerical simulations of axisymmetric erosion processes in ion‐focused regime‐transported beams

Abstract: Erosion rates for a relativistic electron beam propagating through a low‐density plasma channel in the ion‐focused regime are computed via axisymmetric particle simulation. Steady‐state inductive (Ohmic) erosion rates are found that are in quantitative agreement with new theoretical results, which correctly account for the relativistic dynamics of electrons at the beam head. In cases where the finite emittance of the beam is expected to be the dominant loss mechanism, the beam does not develop steady‐state erosion, as suggested by previous analyses. Asymptotically, such a beam is characterized by a low‐emittance population at the beam head, a long rise length, and an erosion rate that tends toward zero.

Current channel migration and magnetic field penetration in a perfectly conducting plasma with emitting, conducting boundaries

Abstract: In collisionless simulations of the plasma erosion opening switch, a highly conductive plasma allows magnetic field penetration through the entire length of the plasma, to depths almost two orders of magnitude greater than the collisionless skin depth, c/ωpe. Field penetration is accomplished by a narrow (skin‐depth‐like) current channel that migrates through the plasma. The plasma behind the current channel is unable to shield the rising magnetic field from the body of the plasma and allows it to penetrate almost instantly and completely through the plasma up to the current channel. The migration of the channel and the penetration of the field appear to occur in the absence of both Coulomb collisions and instabilities. These unusual features are permitted by the electric field structure in the plasma behind the current channel and the presence of conducting boundaries that can emit electrons.

Improved plasma wave tube

Abstract: A plasma wave tube is described in which a pair of counterpropagating electron beams (6, 8) are injected into a waveguide housing (2) in which a plasma is formed, preferably by an array of fine wire anodes (4). The electron beams couple with the plasma to produce electron plasma waves, which radiate electromagnetic energy for beam voltages and currents above established threshold levels. A rapid control over output frequency is achieved by controlling the plasma discharge current, while the output power can be controlled by controlling the voltage and/or current levels of the electron beams.

Plasma torch with cooling and beam-converging channels

Abstract: An improved plasma torch for the high-frequency capacitive generation of a plasma beam, a special nozzle construction providing a large-length, small-diameter plasma beam. The nozzle includes a first channel for cooling an internal plasma-generating electrode and a second channel for converging the plasma beam.

Study of a railgun plasma armature

Abstract: An experimental and theoretical study of a railgun plasma is presented. The experimental results have been obtained by recording the voltage and current variations as well as the spectral emission of the plasma. The gas pressure, helium, or air in contact with the exploding foil has been varied from 0.1 to 2 MPa. The apparatus consisted of a test chamber simulating a railgun facility. To complete the study, some measurements of the plasma characteristics in the 500-kJ EMA 1 railgun system have been recorded. It is shown that the resistance of the plasma, which is 15 m Omega at 10 kA, decreases to 0.2 m Omega at 500 kA. These results indicate the effect of the energy flow to the walls and that of the ionization potential of the gas in contact with the exploding foil at lower currents. >

Characterization of the time-evolution of a microsecond electron beam diode with anode effects

Abstract: Summary form only. The time evolution of a low-aspect-ratio, microsecond-pulse-length, electron beam diode with anode plasmas, operating at initial current densities of approximately=100 A/cm/sup 2/, has been studied using a variety of experimental and empirical techniques. Of particular interest in this study was the observation of a transition to a roughly constant impedance regime. Very rapid cathode plasma closure velocities from 8-11 cm/s were observed. This expansion led to rapid self-filling of the diode, principally on-axis, causing dense plasma formation from the anode and resulting in the observed impedance characteristics. Due to the low-aspect-ratio diode design, many kiloamperes of electron current were still emitted, which interacted with the plasma that filled the diode from both the cathode and anode. Before self-filling, a low-effective-charge cathode plasma was observed, consisting almost exclusively of components derived from the hydrocarbon surface impurities. Evidence for cathode plasma instabilities was also noted. After self-filling, evidence for a beam-plasma interaction leading to RF emission, beam filamentation, and collective effects was suggested. >

A computational model forz-pinch plasma channels

Abstract: An adaptive grid finite difference scheme has been derived for simulating non-linear and unsteady one dimensional (planar, cylindrical and spherical) fluid flow by adapting to steep gradients in different physical quantities. The scheme is applied to z-pinch plasma channels that are used for ion beam transport in Light Ion Beam Fusion Reactor designs. The axial plasma is subject to joule heating caused by the discharge current ($\sim100kA$) that is used to create azimuthal magnetic fields to confine an ion beam current of $\sim0.5MA$. The radiation emitted by the plasma seems to be vitally important for analyzing the formation of channels, requiring accurate methods that treat radiative transfer coupled to magnetohydrodynamics. The channel formation and ion beam injection is studied by simulations with what we call a 1-D Adaptive Radiation Magnetohydrodynamics (ARMHD) computer code developed for the purpose of this thesis research. ARMHD models the plasma with single-fluid MHD equations, and therefore does not distinguish between the electrons and ions. It solves for the radiation field intensity through the radiative transfer equation using a multi-group discrete ordinate $S\sb{N}$ method. The governing equations are hyperbolic conservation laws (PDEs) transformed to an adaptive grid reference frame that moves in time to follow the high gradients in the solutions. An explicit procedure based on the equidistribution principle is used to move the grid system to avoid the implicit coupling between the physical equations and the grid system. Adaptive gridding seems to be most effective in flows with high gradient regions. For z-pinch plasma channel simulations, the adaption on temperature-momentum proves to be good in terms of the high mesh concentration it provides throughout the channel, where other quantities besides the temperature and momentum are also varying. Also, the mesh spacing in the channel provided with the adaptive gridding seems to be about two times smaller than what was obtained with a lagrangian scheme using the same number of mesh points. Applications of ARMHD indicate the feasibility of using argon and nitrogen for plasma channels. These were ruled out in favor of helium by earlier calculations using a radiative diffusion model. ARMHD calculations treat the thin plasma in the channel more accurately than the diffusion approximation, leading to this different conclusion.

Plasma source characterization and development for the density controlled opening switch

Abstract: A drifting plasma from the PSI flashboard has been characterized using a 76 GHz microwave interferometer, a double floating Langmuir probe and an ion charge collector. Correlation between all three diagnostics has been shown. An electron density of approxi mately 10/sup10/ cm/sup-3/ is observed at a distance of 45 cm from the flashboard surface. Using these diagnostics we have investigated the control of the flashboard plasma. We have identified several important factors which govern the shot-to-shot reproducibility of the plasma and examined the effect of varying the driver waveform. Finally, we have successfully demonstrated external control of the flash board plasma with a fast magnetic coil by achieving a 12 db decrease in ion flux in 100 ns.

The effect of low temperature on the role played by electrodes in vacuum breakdown

Abstract: Electrical discharge in a vacuum is discussed, showing that the electrical breakdown requires the creation of plasma in the interelectrode space. Both electrodes are a source of plasma, but the more important dense plasma is created on the anode. In a system with an insulating spacer, the insulator materials is the source of plasma. Plasma creation from a cooled anode needs more energy. Similarly, cooling of the parts of the insulator where the plasma is created increases the electrical strength. >

Interaction of a spherical high voltage probe with the space environment: electron trapping, current drain and collective process

Abstract: The effects of electron trapping and plasma turbulence on the current-voltage relationship for a positively charged high-voltage spherical probe in a magnetized plasma are studied The results of a study conducted prior to the launch of SPEAR I (Space Power Experiment Aboard Rockets) are summarized and analyzed A primary goal of the SPEAR I experiment was to obtain quantitative measures of the effects of an ambient magnetic field on electron collection by a high-voltage spherical probe Analysis indicates that the structure of the sheath and the I-V relation for a high-voltage probe configured as in the originally planned SPEAR I experiment (with a working plasma contactor) would be substantially influenced by the combined action of trapped electrons and wave-particle scattering processes >

The pseudospark as an electron beam source

Abstract: The pseudospark is a low-pressure, hollow-cathode gas discharge that occurs in a special discharge geometry (pseudospark chamber) in different kinds of gases. A modular pseudospark chamber was built to investigate this discharge type as a source of intense electron beams. At a breakdown voltage of 24 kV and a discharge current of 480 A, an electron beam of 106 A and 13 ns FWHM (full width at half maximum) was extracted through the anode hole into a drift chamber filled with low-pressure gas. Electrical parameters of the circuit, including the plasma channel, were evaluated by monitoring the discharge current waveform. First results of beam profile and emittance measurements of the produced electron beam are presented. At an axial distance of 9 cm behind the anode, an RMS emittance of 55 mm-mrad was measured. The results obtained make it possible to consider the pseudospark discharge a high-brightness electron beam source. >

Investigation of the nonlinear behavior of a weakly-ionized plasma

Abstract: Recent progress on the application of chaos theory to a steady-state magnetized plasma is reported. The research was conducted on a classical Penning discharge, which is inherently turbulent. However, by choosing appropriate boundary conditions, it was possible to produce relatively coherent modes. This was achieved by using a long cylindrical anode and a coaxial cathode. This arrangement creates a constant radial electric field along the axis, which in turn causes an E*B instability to develop in the edge region of the plasma. These modes can be damped or enhanced by varying the plasma parameters. Turbulence data were obtained by using a capacitive probe to measure the potential fluctuations. The study was conducted for the three different plasma cases. In each case a different parameter was varied. The parameters varied included that anode voltage, background pressure, and magnetic field strength. >

A POS hall model

Abstract: A model is presented which explains POS (plasma opening switch) operation on the basis of internal processes in the plasma. These processes include cathode plasma creation, double-layer formation, field penetration of the plasma, and current switching. >

Numerical simulation of collective ion acceleration in the laser controlled collective accelerator

Abstract: In the Laser Controlled Collective Accelerator, an intense electron beam is injected at a current above the vacuum space-charge limit into an initially evacuated drift tube. A plasma channel, produced by time-sequenced, multiple-laser-beam ionization of a solid target on the drift tube wall, provides the necessary neutralization to allow for effective beam propagation. By controlling the rate of production of the plasma channel as a function of time down the drift tube, control of the electron beamfront can be achieved. Initial experiments have demonstrated the controlled acceleration of protons at a rate of 40 MeV/m over a distance of 50 cm, in good agreement with experimental design values. In order to understand and to optimize ion acceleration in this system, the one-dimensional electrostatic PIC code CIA has been developed. Initial results agree with the experimental results as regards rate of sequencing for enhanced acceleration and peak enhanced ion energy. In addition, the code demonstrates the erosion of the strong beamfront electric field as well as the decrease in the number of ions accelerated over the channel length. >

Some unconventional ICF approaches with heavy-ion beams

Abstract: Using a heavy-ion beam of 10 GeV energy allows to create a few cm long plasma channel in a frozen D-T mixture. It is shown that generating currents of

Scaling of a final CERN plasma lens design by a one dimensional magnetohydrodynamic z-pinch model

Abstract: Computations with magnetohydrodynamic z-pinch models have been compared with experimental results obtained from a CERN plasma lens prototype designed for antiproton focusing in the antiproton collector ACOL. The focusing azimuthal magnetic field is generated by a z-pinch discharge. Typical pinch parameters are microsecond pinch times, pinch radii of 20 mm, a 250-mm-long plasma channel, and a maximum current of 300 kA. A model whose analysis includes the motion of shockwaves has been improved by taking into account energy losses due to dissociation and ionization of neutrals and by introducing a variable ionization degree. Parameters extrapolated for a final plasma lens scaling that should fulfil the ACOL requirements, e.g. magnetic field strength of about 4 T and pinch duration of at least 0.5 mu s, are represented in the model. >

Three-halves harmonic emission from a laser filament in a plasma channel

Abstract: A self-trapped laser filament is susceptible to decay-producing radially localized Langmuir waves. A nonlinear interaction of the pump wave with the density oscillation at the Langmuir frequency gives rise to three-halves harmonic emissions. Using a basis-function expansion technique, the emitted power in the backward direction is obtained. It decreases with the increasing size of the filament. >

Measurements and simulation of controlled beamfront motion in the Laser Controlled Collective Accelerator

Abstract: In the Laser Controlled Collective Accelerator, an intense electron beam is injected at a current above the vacuum space charge limit into an initially evacuated drift tube. A plasma channel, produced by time-sequenced, multiple laser beam ionization of a solid target on the drift tube wall, provides the necessary neutralization to allow for effective beam propagation. By controlling the rate of production of the plasma channel as a function of time down the drift tube, control of the electron beamfront can be achieved. Recent experimental measurements of controlled beamfront motion in this configuration are presented, along with results of ion acceleration experiments conducted using two different accelerating gradients. These results are compared with numerical simulations of the system. >

Feasibility investigations of the high power coaxial plasma opening switches

Abstract: This paper advances a tentative plan to use a Z-pinch type plasma focus as ahigh power opening switch and describes the feasibility to use the denseplasma focus (DPF) and the Z-pinch type plasma focus (ZPF) as a high poweropening switch in inductive energy storage. The optimal resistance matchingcondition is analysed with the method of approximately solving electriccircuit. Then the relevant adjusttment and modification for these two switchesare made experimentally.

Recent results from the plasma edge cathode experiment

Abstract: Electron beams produced by high power cold cathode diodes are typically limited to a few microseconds in duration due to plasma closure of the extraction gap. A stationary plasma cathode created by partially intercepting a plasma jet with a material edge can be used to extend the electron beam extraction time. Electrons are extracted normal to the flow of the plasma jet behind the obstacle. Under space charge limited extraction, the plasma boundary will experience negligible electric force since the electric field at the boundary is essentially zero. An electron current density in excess of 100 A/cm/sup 2/ is theoretically possible from a plasma with density n = 5x10/sup13/ cm/sup-3/ and kT/sub e/ = 1 eV. Initial experimental investigation has confirmed the underlying concept and has resulted in an achieved current density of 14 A/cm/sup 2/ for 2.5 /spl mu/sec with an 8 mm extraction gap and 15 kV anode potential.

Plasma heating by collisional magnetic pumping in a steady-state modified Penning discharge

Abstract: The experimental application of collisional magnetic pumping to heat a plasma by a sawtooth magnetic perturbation is reported. Results have been obtained relating to the energy transfer process between the perturbed magnetic field and the parallel and perpendicular energy components of the heated species in a steady-state modified Penning discharge apparatus with a magnetic mirror configuration. Plasma characteristics have been measured, and heating of ions and electrons as a function of magnetic induction, electron number density, and neutral gas pressure has been investigated. >

Resistivity of Flame Plasma in an Electric Field

Abstract: A generalized Ohm's law is obtained for a flame plasma in an electric field for the study of arc resistivity in an electromagnetic launcher (EML). The resistivity of flame plasma is reduced by the particle source, which suggests the injection of premixed combustible fuel into the arc plasma as the particle source in order to reduce the arc voltage. Reduction of the voltage in the arc is desirable to reduce the damage of electrodes in EML since the electric field in the arc plasma energizes charged particles which can bombard the electrodes.

Characterization of a 1- mu s, 1-MA plasma erosion opening switch

Abstract: Summary form only. Experiments have been carried out to characterize the conduction phase of the plasma erosion opening switch (PEOS) driven by a fused capacitor bank. Initial experiments with a PEOS cathode (anode) radius of 5 cm (8.9 cm) used magnetic probes to determine the PEOS current distribution and collimated, visible light detectors and Faraday cups to detect the presence of plasma. Results of these preliminary measurements suggest that non-current-carrying plasma is accelerated ahead of the PEOS current channel, in the direction of the load. This plasma must be accounted for when computing the PEOS center-of-mass displacement and may also have some effect on the load performance. >

Particle and power balances of hot-filament discharge plasmas in a multi-dipole device in the presence of a positively biased electrode

Abstract: The plasma potential is typically assumed to float above an anode potential by an amount that is a few times the electron temperature (T/sub e//e). The difference between the plasma potential and the anode potential can be estimated by considering the particle production and loss. However, it has been reported experimentally that the plasma potential of a steady-state plasma can be more negative than the anode potential with a potential dip ( approximately T/sub e//e) in front of the anode. Particle and power balances have been carried out to estimate the bulk plasma potential of a hot-filament discharge plasma produced in a multidipole plasma device. The bulk plasma potential dependence on the positive DC bias applied to the anode is analyzed, and the predicted characteristics of the plasma potential dependence are compared to the experimental results. A steady-state potential dip in front of the anode has been experimentally observed using emissive probes with the zero emission inflection point method, and the conditions for the potential dip formation have been derived. >

And simulation of controlled beamfront motion in the laser controlled collective accelerator

Abstract: In th? Laser Controlled Collective Accelerator, an intense electron beam is injected at a current, above the vacuum space charge limit into an initiaIly evacuated drift tube. A plasma channel. produced by time-sequenced, multiple laser beam ionization of a solid target on the drift tube wall, provides the nccessary neutralization to allow for effective beam propagation. By controlling the rate of production of the plasma channel as a function of time down the drift tube, control of the electron hcamfront can be achieved. Recent experimental measurements of controlled beamfront motion in this configuration are prcsentcd, along with results of ion acceleration experiments conducted using two different accflrrating gradients. These results arc compared with numerical simulations of the system in nhich both controlled beamfront mot,ion and ion acceleration is observed consistent with both design expectations and experimental results. I. Introcluctiou The Laser Controlled Collect,ive Accelerator concept’-3 reprcscnts an attempt to extend the promising results from “naturitlly occurring” collective ion acceleration experiments to practical accelerators in which the accelerating gradient and distance can be systematically cont,rolled. The concept is similar to that employed in the IFA- and IFA- experiments of Olson4’“, although the actual experimental configurat,ion is q”it,c different. The basic concept behind the experiment is shown in Fig. 1. An intense relativistic electron beam is injectcd through a localized gas cloud int,o an evacuated drift tubes at, a current well above the vaculim space charge limit. A virtlml cathode then forms immediately downstream of the injection point and ions produced within the localized gas cloud are accelerated to modest energies in a manner similar to more conventional collective accelerators. At this point, a channel of plasma is produced in a time sequenced manner down the drift tube by laser ionization of a CH2 target strip located on the drift tube wall. The time sequencing of the plasma channel is achieved by dividing a Q-switched ruby laser pulse into ten approximately equal energy beams and using optical delays to ionize sequentially ten target spots equally spaced down the drift tube. In this manner, the virtual cathode at the beamfront can be carefully accelerated down the drift tube and ions trapped by the strong electric fields at the virtual cathode can be accelerated to high energies in a controlled manner. In this paper we present in section II results of experiments in which controlled beamfront motion has been confirmed for two different accelerating gradients. Results of ion acceleration experiments are also presented. Numerical simulations of the experiments presented in section III confirm both controlled beamfront motion and the controlled acceleration of ions by the moving virtual cathode over significant distances. Conclusions are drawn in section IV.