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Plasma Science, IEEE Transactions on

Issue 6 • Date Dec 1993

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Displaying Results 1 - 22 of 22
  • Scaling studies and time-resolved microwave measurements on a relativistic backward-wave oscillator

    Page(s): 714 - 724
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    A relativistic backward-wave oscillator (BWO) operating at a frequency near 8 GHz has been built. The parameters of the 60-ns electron beam driving this microwave source are varied over the ranges 0.8-1.5 MV and 2-10 kA. Several different annular cathodes for launching the electron beam are tried, varying the outer radius and shape. The axial magnetic field guiding the beam through the BWO is varied between 0.6 and 3 T. The power transfer downstream to an output waveguide is investigated as a function of the shape of the transition from the BWO to the waveguide. The scaling of the output power and frequency with these variations is discussed. Time-resolved measurements of 2-ns-long segments of the microwave output are shown. In observations of the microwave signal, it is found that the frequency shifts as the output power envelope passes through a sharp dip. It is proposed that this shift corresponds to a change in the longitudinal operating mode of the BWO View full abstract»

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  • Railgun using plasma initiation separated from the projectile

    Page(s): 755 - 759
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    Preacceleration of a projectile is important for reducing the erosion of the bore surface in a railgun. Gas guns, electrothermal guns, and other railguns are commonly used to preaccelerate the projectile. A new method, called the plasma initiation separated from the projectile (PISP) method is proposed, and its effectiveness is confirmed experimentally. A thin copper wire is placed near the edge of the railgun, and it explodes and forms a plasma that has a fast flow velocity due to the Lorentz force. This fast flowing plasma collides with the projectile, which obtains an initial velocity mainly by the momentum transfer. Since the current increases while only the plasma is accelerated, the driving force of the projectile just after the collision of the plasma with the projectile is large. The PISP method works as an inductive energy storage circuit with an opening switch View full abstract»

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  • Preliminary investigation of power flow and performance phenomena in a multimegawatt coaxial plasma thruster

    Page(s): 625 - 644
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    Preliminary experimental and theoretical research that was directed toward the study of quasi-steady-state power flow in a large, unoptimized, multimegawatt coaxial plasma thruster is summarized. Large coaxial thruster operation is discussed, and the experimental results are evaluated and interpreted with a view to the development of efficient, steady-state, megawatt-class magnetoplasmadynamic (MPD) thrusters View full abstract»

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  • Fast processes on cathode surface resulting in pseudospark discharge

    Page(s): 725 - 730
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    An alternative mechanism for emission during the high-current phase of a pseudospark discharge is presented. A high current density is provided by multiple arc cathode spots which appear due to cathode-plasma breakdown. The model of plasma formation in the main gap of the pseudospark switch is discussed, together with conditions required for the initiation of spot during some tens of nanoseconds, the appearance of a moving double plasma layer, and high-current transport through the plasma-filled diode View full abstract»

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  • A magnetically-guarded electrostatic probe

    Page(s): 785 - 787
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    It is shown that a magnetically guarded hairpin can support a Langmuir probe electrode with reduced disturbance of the plasma and less thermal damage to the probe View full abstract»

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  • Two-dimensional nonequilibrium fluid models for streamers

    Page(s): 684 - 695
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    The self-consistent, two-dimensional fluid simulation for describing nitrogen gas prebreakdown phenomena under atmospheric pressure is presented in cylindrically symmetric geometry. The models of the electron dynamics are characterized either by an equilibrium single-moment equation or by a nonequilibrium three-moment equation. A more accurate flux-corrected transport (MAFCT) technique, which provides a solution with steep and varying gradients in large dynamic ranges, is used to solve the electron fluid equations. Included in the step-by-step presentation are the electron density, the space-charge electric field, the electron average velocity, the electron mean energy, and the electron power deposition from the initial stage to the later stage when the ionizing channel bridges the gap. The differences between equilibrium and nonequilibrium fluid models are discussed in terms of the formation of ionizing channels and the propagation of streamers View full abstract»

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  • A multi-fluid stagnation-flow plasma model with self-consistent treatment of the collisional sheath

    Page(s): 768 - 777
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    A two-temperature, multifluid model of a plasma in stagnation flow against a cooled, electrically biased surface is presented. The model couples bulk fluid motion, species diffusion and convection, electron and bulk energy equations, and net finite-rate ionization with Poisson's equation for the electric field in a generalized formulation. Application of the model to argon flow reveals important interactions between thermal, hydrodynamic, chemical and electrical boundary layers, with implications for current-limiting regimes of arcjet operation. The response of a planar Langmuir probe in contact with a collisional, flowing plasma is examined. Determinations of current-voltage behavior compare well with simple theory, including dependence on incident plasma velocity. Departures from this theory arise from boundary-layer perturbations near the electrode surface, away from free-stream conditions. The computational model incorporates a finite-rate catalytic recombination of ions and electrons at the electrode surface together with a specified current View full abstract»

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  • Mechanism of electrical breakdown of gases at very low pressure and interelectrode gap values

    Page(s): 645 - 653
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    The electrical breakdown mechanism of gases at very low pressures (from 10-4 to 105 Pa) and interelectrode gap values (from 0.1 to 1 mm) is considered. Comparing experimental results and theoretical analysis, the boundaries between areas of different breakdown mechanisms are established. The existence of edge breakdown, which was previously erroneously interpreted by the anomalous Paschen's effect, is explained and proved. The avalanche hypothesis of vacuum breakdown is experimentally proved View full abstract»

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  • High current electron beam generation in a vacuum transient hollow cathode discharge

    Page(s): 778 - 783
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    Experimental observations are presented of prebreakdown electron beam generation in a transient hollow cathode discharge (THCD) in a vacuum. The discharge driver consists of a 400-kV maximum voltage, 25-nF Marx operated at 450-J stored energy coupled to a 120-ns, 1.5-Ω coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd:YAG laser is used to produce a plasma at the back of the cathode surface, inside the hollow cathode region (HCR). It is found that a plasma density of a few 1018 cm-3 in a volume of a few mm3 is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10-200 mtorr range View full abstract»

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  • Spectroscopic characteristics of argon plasma produced by multistage pseudospark discharges

    Page(s): 731 - 738
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    A measurement is made of the spectroscopic characteristics of argon plasma produced by multistage pseudospark discharges in a peculiar geometry which has not yet been reported in literature. High electron temperature, high peak current density, and a high degree of ionization are demonstrated in such a device. The population number density of the upper and lower levels of 488.0, 476.6, 514.5, 351.1, and 363.8 nm transitions in ArII and ArIII are measured. The ultraviolet emission characteristics of pure Ar and Ar/He mixture plasmas are compared. An appreciable increase is observed in peak intensities of ArIII lines when nitrogen is added to Ar View full abstract»

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  • Superheat turbulence in gas discharged plasma

    Page(s): 614 - 618
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    The influence of the electron concentration and temperature fluctuations on local thermodynamic equilibrium (LTE) in a gas-discharge plasma due to superheat turbulence development is analyzed. Data for the noble-gas atmospheric plasmas Ar and He (T=6-18 kK) and air ( T=4-9 kK) are given. It is shown that superheat turbulence causes deviation from LTE when parameter-space gradients are absent. As a result, the influence of superheat turbulence for weakly radiative gases (He, H2) is considerably greater than for strong radiative gases (Ar, Xe, etc). The artificial excitement of superheat turbulence in plasma without any current by means of external electric field fluctuations is demonstrated View full abstract»

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  • Spatial and temporal characteristics of the X-ray emitted by a 1-J 50-ns vacuum discharge

    Page(s): 669 - 675
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    The X-ray emission from a source based on a small vacuum discharge is studied. Two different X-ray pulses whose intensities vary with the anode-cathode distance are identified. Time integrated pinhole images show that the whole anode and the teflon insulator emit X-rays. Some of the X-ray emission is found to originate also in the vicinity of the tungsten anode. The temporal behavior of the X-ray varies with respect to the distance between the anode and the cathode. Three different cathode geometries are tested: hollow cathode, hollow conical cathode, and massive or needle cathode. The spatial distribution of the X-ray sources is obtained by means of a sensitive imaging device. Some X-ray spots that appear in the discharge gap near the anode tip are similar to hot spots found in more powerful discharges View full abstract»

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  • Nonlinear discharge dynamics in small bore current channels

    Page(s): 743 - 754
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    General properties of current and voltage dynamics for discharges having narrow current channels are studied using a generic transmission-line model with active parameters. The line parameters for the inductance and capacitance per unit length are, except for positivity and some physically motivated monotonicity conditions, left as unspecified functions of local line current and voltage. The resistance per unit length is taken to vary inversely with current, thereby assuring dynamical properties common to transient electrical discharges. Emphasis is placed on determining global properties of current front dynamics, i.e., the speed and direction of motion of the free boundary interfaces between instantaneously active (current-carrying) and passive (current-free) regions of the discharge current channel, and the space-time paths across which current reversal occurs. The results are applied to surface electrical discharges initiated at a charge spot View full abstract»

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  • Experimental and numerical studies of sheet electron beam propagation through a planar wiggler magnet

    Page(s): 760 - 767
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    Detailed experimental studies on sheet relativistic electron beam propagation through a long planar wiggler are reported and compared with numerical simulations. The planar wiggler has 56 periods with a period of 9.6 mm. Typically, the wiggler field peak amplitude is 5 kG. The experimental efforts are focused on controlling the deviation of the beam toward the side edge of the planar wiggler along the wide transverse direction. It is found that a suitably tapered magnetic field configuration at the wiggler entrance can considerably reduce the rate of deviation. The effects of the following techniques on beam transport efficiency are discussed: side focusing, beam transverse velocity tuning at the wiggler entrance, and beam spread limiting. High beam transport efficiency (almost 100%) of a 15-A beam is obtained in some cases View full abstract»

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  • Effect of plasma on efficiency enhancement in a high power relativistic backward wave oscillator

    Page(s): 609 - 613
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    A linear theory of the excitation of electromagnetic waves in a plasma-filled backward-wave oscillator driven by an intense relativistic electron beam is presented. It is found that the spatial growth rate of backward-wave instability exhibits a resonant increase for a particular value of fill-plasma density. Results are compared to the results of an experiment by K. Minami et al. (1988) on a high-power backward-wave oscillator View full abstract»

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  • An electrodynamic vacuum arc ion acceleration mechanism based on Ampere's force law

    Page(s): 701 - 713
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    It is known that the motion of cathode spots is of electromagnetic origin, thus leading to methods of steering the spots by controlling the direction of the self and applied magnetic fields. The author discusses the ion flux from the cathode spot, focusing on two well-recognized features of this flux that are difficult to explain. These are the anomalously high energies of the ions, and their highly anisotropic spatial distribution. An electrodynamic acceleration mechanism, based on Ampere's force law, is proposed as an explanation of these phenomena. The theory is shown to be consistent with existing particle production models, and also provides a consistent solution to macroparticle emission and retrograde motion View full abstract»

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  • Field theory of a traveling wave tube amplifier with a tape helix

    Page(s): 654 - 668
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    A self-consistent relativistic field theory for a helix traveling wave tube (TWT) is presented for a configuration in which a magnetized pencil beam propagates through a tape helix enclosed with a loss-free well. A linear analysis of the interaction is solved subject to the boundary conditions imposed by the beam, helix, and wall. The wave equation for the fields within the electron beam corresponds to the Appleton-Hartree magnetoionic wave modes that are of mixed electrostatic/electromagnetic polarization. Hence, the determinantal dispersion equation that is obtained implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. This dispersion equation includes azimuthal variations and all spatial harmonics of the tape helix. Solutions that correspond to both the extraordinary (X) and ordinary (O ) solutions for the Appleton-Hartree modes are found numerically View full abstract»

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  • A gas-insulated railgun

    Page(s): 784 - 785
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    A railgun concept is discussed in which a high-pressure prefill of hydrogen serves as the close-in insulator between the rails. The gas-insulated railgun (GIRG) approach can reduce or eliminate the velocity-limiting problem of growth and separation of the current-carrying armature region behind the projectile, and thus allow higher launch velocities, to be achieved. A hybrid armature appears appropriate for this launcher View full abstract»

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  • Resonant cavity vircator driven by a thermionic cathode electron beam gun

    Page(s): 739 - 742
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    A resonant cavity vircator (virtual cathode oscillator) driven by an electron beam emitted from a broad-area thermionic cathode is tested. Narrow-bandwidth (1.0 MHz at the -3 dB level) excitation of the TM023 mode of a cylindrical resonant cavity is observed at a frequency of 986 MHz with a pulselength of 1.2 μs. The single-cavity-mode excitation is attributed to the constant-voltage and -current electron beam emitted from the thermionic cathode View full abstract»

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  • Formation of a high-current pulsed gas discharge as a source of highly concentrated energy flux to treat metal surfaces

    Page(s): 619 - 624
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    The properties of a novel, pulsed, high-current gas discharge with minimized energy losses are investigated. The discharge provides a highly concentrated energy flux that can be used to treat metal surfaces and to form thin surface layers with desirable properties. A theoretical treatment of the formation of the discharge is presented, and the limitations on its voltage and interelectrode separation length are considered. Experiments are carried out to test the theoretical predictions of the discharge parameters. The experimental results show that more than 80% of the energy input to the discharge from the power supply is delivered to the metal surface View full abstract»

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  • Electrical discharge initiation and a macroscopic model for formative time lags

    Page(s): 676 - 683
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    Formative times in electrical discharges in overvoltaged gaps are analyzed with a model having no spatial dependence and with simple assumptions about discharge channel temperature T and discharge voltage. The model treats the early temporal evolution of the discharge. Specifically, the dissipative voltage drop, V*, across the discharge is taken to be a step function of T. Thermal quasi-equilibrium is assumed in the discharge medium, and it is shown that d(In td)/d(In θ)=-1, i.e., θtd=constant, where θ is the fractional overvoltage and td is the formative time lag, in agreement with measured values of td for much of the experimentally explored range of θ. Highly-time-resolved (~92 ps) experimental data are presented for the first 10 ns of electrical discharge initiation; these data suggest that the authors' model should provide a reasonable representation of t d when td>10-100 ns View full abstract»

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  • Air breakdown times of two-stage spark gaps and their applications

    Page(s): 696 - 700
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    The air breakdown times of second spark formation, geometrically in series with trigatron switching, are studied and compared to that of a triggered spark gap. Despite a large breakdown jitter of ⩾1 μs in the trigatron, the latter is not observed to affect the electrical closure times of the series-connected second spark gap, which is triggered largely as a result of their direct electrical and optical coupling. While most of the multistage spark gaps are designed for fast and precision switching of high voltages, the air breakdown times of the second spark formation find a number of useful applications, particularly in achieving uniform glow discharge in pulsed gas lasers View full abstract»

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IEEE Transactions on Plasma Sciences focuses on plasma science and engineering, including: magnetofluid dynamics and thermionics; plasma dynamics; gaseous electronics and arc technology.

 

 

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