Showing papers in "IEEE Transactions on Plasma Science in 1987"
TL;DR: In this article, an envelope equation is derived to describe the radial evolution of a radiation beam propagating through a plasma, and an expression for a critical laser power is derived for the case of constant density background plasma.
Abstract: An envelope equation is derived which describes the radial evolution of a radiation beam propagating through a plasma. The radiation envelope equation contains a defocusing term due to diffraction spreading and a focusing term due to relativistic oscillations of the plasma electrons. The case of a constant density background plasma is analyzed in detail and an expression for a critical laser power is derived. For powers exceeding the critical power, the radiation envelope oscillates and does not diffract. Under certain conditions the radiation beam propagates through the plasma with a constant radius envelope.
256 citations
TL;DR: A general description of arc cathode spots is given with respect to structure, size, and temporal behavior in this article, where the density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes.
Abstract: A general description of arc cathode spots is given with respect to structure, size, and temporal behavior. The density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes. Extrapolation to the region within the spot indicates plasma densities in excess of 1025 m-3. Recent studies of spot current density suggest values of about 1012 A/m2. In this connection, the problem of the electrical conductivity of the spot plasma is discussed. Finally, experiments on spot lifetime and models of crater formation are reviewed.
113 citations
TL;DR: A review of PEOS research performed at the Naval Research Laboratory (NRL) can be found in this paper, where several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date.
Abstract: This paper is a review of plasma erosion opening switch (PEOS) research performed at the Naval Research Laboratory (NRL). Several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date. Significant power multiplication has been achieved on the Gamble II generator, producing 3.5 TW with less than 10-ns rise time. Switching after nearly 1-?s conduction time has been demonstrated on Pawn, producing a 0.2-TW 100-ns pulse. Scaling the switch to higher current, power, and conduction time should be possible based on theoretical analysis and the favorable results of scaling experiments performed thus far.
102 citations
TL;DR: A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given in this article.
Abstract: A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given. Three types of Cerenkov oscillators are investigated in detail: orotrons, surface wave oscillators and a flimatron (free electron maser (FEM) based on Smith-Purcell radiation). The bremsstrahlung oscillators studied are gyrotrons with TM modes, a ubitron operating at a quasi-critical frequency and cyclotron autoresonance masers. Electrodynamic and electron methods of mode selection provide stable radiation with a reproducible space structure of radiation in all oscillators under study. The radiation power attained 50-100 MW for long and 10-30 MW for short millimeter wavelengths at the efficiency up to 5-10 percent. Various types of oscillators are compared. Promising methods for increasing power and radiation frequency are discussed.
88 citations
TL;DR: In this article, a laser-shadow technique of high time resolution was applied to study the erosion of high-current Cu vacuum arcs in situ, and densities greater than 1012 cm-3 were obtained near current-zero for the diffuse mode.
Abstract: A laser-shadow technique of high time resolution was applied to study the erosion of high-current Cu vacuum arcs in situ. Cathodic processes lead to emission of high-velocity droplets shortly before and after current-zero. Increasing movements of the anodic melt produce large droplets several milliseconds after the arc. The many particles generated are responsible for the slow decay of vapor measured by laser-induced fluorescence (LIF). Densities greater than 1012 cm-3 were obtained near current-zero for the diffuse mode. Because of the optical thickness of the vapor to resonance radiation, radiative transfer had to be considered.
86 citations
TL;DR: In this article, the Andrews-Varey model was extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion.
Abstract: Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs were measured for filat OFHC-Cu contacts (D = 25 mm, d = 7.5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5.5-ms duration with peak values up to 11 kA. In comparison with the fast recovery of diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Post-arc currents are simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The flow of charge carriers to the anode and the shield, which is at the anode's potential, are registered separately. The amount and decay of the residual plasma is evaluated from the measurements of post-arc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.
76 citations
TL;DR: In this article, the self-focusing effect of a relativistic electron or positron beam traversing through a thin slab of plasma in a linearized fluid theory is analyzed.
Abstract: We analyze the self-focusing effect of a relativistic electron or positron beam traversing through a thin slab of plasma in a linearized fluid theory, and show that the effect is very strong The idea of employing this effect for a plasma lens suggested by Chen is then reviewed The self-focusing force can be made uniform over the length of the beam by injecting an appropriate precursor Computer simulations on both thin and thick plasma lenses are presented, which show reasonable agreement with theoretical predictions
67 citations
TL;DR: In this article, experimental data on the erosion rates of a copper cathode in He, Ar, and SF6 from 10-6 to 760 torr are presented. And the work performed by the cathodic-erosion plasma expanding against the gas is found experimentally to be constant, the volume of the expanding plasma cloud being linked to the gas pressure by the relation R3p = constant.
Abstract: Experimental data on the erosion rates of a copper cathode in He, Ar, and SF6 from 10-6 to 760 torr are presented. The work performed by the cathodic-erosion plasma expanding against the gas is found experimentally to be constant, the volume of the expanding plasma cloud being linked to the gas pressure by the relation R3p = constant. These data agree with a redeposition model based on condensation of the metallic vapor produced by the arc on the cathode surface. The redeposited mass on the cathode is found to be proportional to the cube root of the gas mass density.
61 citations
TL;DR: In this article, experimental observations of energetic axial electron beams in a linear Z pinch operating in the pseudospark mode are presented, and evidence of the importance of electrons generated in the cathode recess in the formation of the beams is presented.
Abstract: Experimental observations of energetic axial electron beams in a linear Z pinch operating in the pseudospark mode are presented. The device is driven from a fast Marx generator and allows reproducible production of electron beams over a wide pressure range. Evidence of the importance of electrons generated in the cathode recess in the formation of the beams is presented. An electron beam of high energy which is not associated with formation of the discharge is identified. A second beam of high current density and lower energy associated with gas breakdown is also observed.
56 citations
TL;DR: In this article, the aim of the present computer simulation and its development following the author's very first concept of "dynamic field emission" is also given, considering the laws of conservation of charge, mass, and energy in the complete cathode spot system.
Abstract: After an introductory definition of the vacuum arc cathode spot, its main theoretical aspects are highlighted. Most of the review is being devoted to the E-diagram method as well as to advanced nonstationary cathode spot models. A survey of the aim of the present computer simulation and its development following the author's very first concept of "dynamic field emission" is also given. The cathode spot model subjected to the computer simulation is described in detail, considering the laws of conservation of charge, mass, and energy in the complete cathode spot system. A short survey of the computational procedure is also given here. The main computational results may be summarized as follows: Within the actual model assumptions, a steady-state self-sustaining cathode spot does not exist. On the one hand, plasma-wall interactions with steady-state thermal equilibrium exist below the balance conditions of mass and energy, while on the other hand, real cathode spots that meet the requirements of both mass and energy balance never show thermal equilibrium but are characterized by thermal runaway. Therefore, instability is an inherent feature of cathode spots. Finally, a comparison between computational values and representative experimental results of cathode spot characteristics shows a rather good agreement, further supporting the principal validity of a model devoted particularly to a theoretical specification of the vacuum arc cathode spot.
51 citations
TL;DR: In this paper, the beat wave excitation process is examined analytically in the Eulerian fluid description, and the effects of plasma drifts, harmonics, pump rise time, frequency mismatch, phenomenological damping, plasma inhomogeneities, and two dimensions are discussed.
Abstract: The beat wave excitation process is examined analytically in the Eulerian fluid description. The effects of plasma drifts, harmonics, pump rise time, frequency mismatch, phenomenological damping, plasma inhomogeneities, and two dimensions are discussed. The consistency between the Eulerian and Lagrangian fluid descriptions is verified.
TL;DR: In this article, a short-pulsed intense laser is injected into an underdense plasma to sustain a self-trapped photon channel, which can provide a slow wave structure of the electromagnetic wave that is suitable for high energy acceleration.
Abstract: A short-pulsed intense laser is injected into an underdense plasma to sustain a self-trapped photon channel. With either high-enough intensity or strong-enough focusing the optical beam causes total electron evacuation on the beam axis. Under appropriate conditions this laser and plasma fiber system can provide a slow wave structure of the electromagnetic wave that is suitable for high-energy acceleration.
TL;DR: A review of modern physical models of vacuum arc cathode spots in the discussion of which the main attention has been devoted to the question of the cathode spot current density j is presented in this paper.
Abstract: This paper contains a review of modern physical models of vacuum arc cathode spots in the discussion of which the main attention has been devoted to the question of the cathode spot current density j. Experimental features of current density measurements and possible errors with the use of different techniques are discussed in detail. The physical limitations due to transfer of high current densities through the near-cathode plasma have been considered and the criteria which limit the highest and lowest possible values of current density are suggested. In addition, the possibility of obtaining high current density emission in the " metal-dielectric-vacuum" system was examined.
TL;DR: In this article, a model for the metal-vapor evolution from the cathode of a diffuse vacuum arc is described by a model that starts from the expression for the collisionless expansion into vacuum of atoms originating from an instantaneous point source.
Abstract: The metal-vapor evolution from the cathode of a diffuse vacuum arc is described by a model that starts from the expression for the collisionless expansion into vacuum of atoms originating from an instantaneous point source. The velocity distribution is assumed to be Maxwellian at the moment the atoms are released from the source. By convolution with the vapor generation rate, which is given by an effective erosion rate and the waveform of the arc current, this expression is generalized to yield the atomic density at an arbitrary distance from a point source which emits atoms for a finite period. The result is integrated over the cathode surface which is treated as an extended homogeneous source of vapor. The copper-vapor density was calculated for a vacuum arc driven by a sinusoidal 50-Hz current half-cycle of 500 A rms, for the center of the contact gap. With a vapor temperature of 2000 K and an effective copper-vapor erosion rate of 3 ?/C, the model well describes the measured decay of the copper-vapor density from about 5 × 1017 m-3 at 300 ?s before current-zero to 5 × 1014 m-3 at 400 ?s after current-zero. Comparison with calculations based on the assumption that metal vapor is generated predominantly by molten droplets evaporating in flight indicates that before current-zero the contribution of the droplets to the vapor density is negligibly small, while after current-zero both vapor generation mechanisms produce vapor at the same order of magnitude.
TL;DR: Using a 1313-?F, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, the authors performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass cylindrical foil liners of 2-cm height and 5-cm radius.
Abstract: Using a 1313-?F, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, we have performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass (200-400 ?g/cm2) cylindrical foil liners of 2-cm height and 5-cm radius. This technique employs a coaxial discharge through a plasma armature, which stores magnetic energy over 3-4 ?s and rapidly switches it to an imploding load as the plasma armature exits the coaxial gun muzzle. The current transferred to the load by the PFS has a rise time of less than 0.2 ?s. With 5-MJ stored energy, we have driven fast liner implosions with a current of over 9 MA, obtaining an isotropic equivalent 2.7-TW 0.5-MJ X-ray yield.
TL;DR: In this article, the authors studied several key accelerator issues relevant to plasma-based accelerators, such as maximum beam current, efficiency, and energy spread, and compared these results with one-and two-dimensional particle-in-cell (PIC) computer simulations.
Abstract: Certain key accelerator issues relevant to plasma-based accelerators are studied in this article. Analytic results for important quantities of interest, such as maximum beam current, efficiency, and energy spread will be given. These results will then be compared with one-and two-dimensional particle-in-cell (PIC) computer simulations. Special emphasis will be placed on those schemes that offer not only high accelerating gradients, but also high efficiency and small energy spread.
TL;DR: In this article, the high azimuthal magnetic field of a current-carrying plasma column (z-pinch) can be used to collect and focus high-energy charged particles in accelerators.
Abstract: The high azimuthal magnetic field of a current-carrying plasma column (z-pinch) can be used to collect and focus high-energy charged particles in accelerators The beam optics and the apparent advantages of such a linear lens compared with conventional focusing devices, such as magnets and magnetic horns, are described When a plasma lens shall be operated in routine accelerator runs, the physics of the plasma dynamics must be largely understood and the technology has to be mastered The results of plasma dynamics measurements and of long-term behavior tests with a plasma lens for antiproton collection are reported The problems of the plasma dynamics control and of the plasma-wall interaction are discussed in view of the envisaged performance for antiproton collection
TL;DR: The plasminar flow switch as discussed by the authors utilizes the nonlinear and nonuniform dynamics of a plasma discharge in vacuum to accumulate magnetic energy in times of several microseconds and then release this energy to a load of a few hundred nanoseconds.
Abstract: The plasma flow switch utilizes the nonlinear and nonuniform dynamics of a plasma discharge in vacuum to accumulate magnetic energy in times of several microseconds and then release this energy to a load in times of a few hundred nanoseconds. Experiments have been performed with capacitor banks up to 6 MJ, providing currents in excess of 107 A and peak voltages over 0.5 MV. Theoretical models include simple slug dynamics coupled to lumped-circuit analyses, magnetoacoustic considerations of one- and two-dimensional aspects of the plasma flow, and two-dimensional magnetohydrodynamic code calculations. The present article reviews both experimental and theoretical efforts, discusses the use of the plasma flow switch to drive plasma liner implosions and high-energy ion flows, and indicates directions for plasma flow switch applications to very high current, high-energy inductive pulsed power systems.
TL;DR: In this article, the authors used the isothermal ablation-stabilized arc model of Kovitya and Lowke to predict high-current arcing behavior in tubes made from nylon or boric acid.
Abstract: Using the isothermal ablation-stabilized arc model of Kovitya and Lowke [1], the high-current arcing behavior in tubes made from nylon or boric acid can be predicted from the tube length and radius. In the analyses of such arcs that occur in expulsion fuses and circuit breakers, the physical properties of the vapors formed from the ablated liner materials are required. Values of viscosity, thermal and electrical conductivities, density, specific heat, enthalpy, and sonic velocity for nylon and boric acid plasmas have been calculated for pressures of 1, 10, and 100 atm (1 atm = 101.3 kPa) over the temperature range from 5000-30 000 K.
TL;DR: In this paper, an electron gun using lanthanum hexaboride (LaB6) as a cathode material is studied for use as a robust thermionic emitter at high cathode current densities.
Abstract: An electron gun using lanthanum hexaboride (LaB6) as a cathode material is being studied for use as a robust thermionic emitter at high cathode current densities. It has a standard planar cathode, Pierce-type electron gun design with a space-charge-limited perveance of 3.2 × 10-6 A/V3/2. Thus far it has been operated up to 36 kV in the space-charge-limited regime. The cathode is heated by electron bombardment and radiation from an auxiliary tungsten filament. The total heating requirement is found to be 202 W/cm2 of cathode area at a cathode temperature of 1626°C. These observations are found to be in reasonable agreement with a thermal steady-state power balance model. Beam current distribution measurements are made with a movable collector and Faraday cup, and are found to be in agreement with an electron-gun computer code. The cathode temperature distribution is also measured.
TL;DR: In this paper, the operating characteristics of a six-vane MIT-A6 magnetron were determined and the high-power resonant microwave emission at 2.83 GHz was in good agreement with the?mode oscillation predicted by the Buneman-Hartree threshold.
Abstract: We have determined the operating characteristics of a six-vane MIT-A6 magnetron. The high-power resonant microwave emission at 2.83 GHz is in good agreement with the ?-mode oscillation predicted by the Buneman-Hartree threshold. We studied the magnetron performance as a function of applied voltage, magnetic field, and extraction geometry. Maximum power is obtained with three open vanes. Using a simple energy balance model we obtain good agreement between the computed and measured peak power.
TL;DR: In this paper, a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc was used to study macroparticles mass transport, size distribution, and spatial distribution.
Abstract: Macroparticle mass transport, size distribution, and spatial distribution were studied in a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc. The macroparticle erosion rate was determined to be 105 ?g/C, and together with ionic emission, accounted for most of the cathodic erosion. The number of macroparticles emitted decreased exponentially with macroparticle diameter, with 20-80-?m macroparticles carrying the bulk of the mass transport. Macroparticles are emitted preferentially at an angle of 20° with respect to the cathode surface. In comparison to previous investigations, higher macroparticle erosion rates, a larger proportion of large macroparticles, and a higher emission angle are observed, and the differences are attributed to the large current density used in the present experiment.
TL;DR: In this article, the steady-state electric current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field.
Abstract: The steady-state electric-current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field. This medium models the interelectrode plasma of the diffuse mode metal vapor vacuum arc. The results show the following. a) The electric current and the flux of the poloidal magnetic field are constricted at the anode side of the flowing plasma. Most of the constriction takes place within a boundary layer, with a characteristic length of 1/Rme, where Rme is the magnetic-Reynolds number for axial electron flow. b) The electric-current constriction inversely depends on K?, where K? is the azimuthal surface current density which produces the axial magnetic field. c) The magnetic-pressure profile shows a radial pinch force in most of the interelectrode region, but in the anode boundary layer it is axially directed, thus retarding the plasma flow. d) The peak of the magnetic pressure is at the anode, and its amplitude directly depends on K?. As K? increases, the peak location moves toward the anode center.
TL;DR: In this paper, the authors investigated the stability of the driving electron or positron beam in the plasma wakefield accelerator (PWFA) and found that all of these can be stabilized by introducing thermal energy and an axial magnetic field.
Abstract: We investigate the stability of the driving electron or positron beam in the plasma wakefield accelerator (PWFA). Although the beam is subject to self-focusing, filamentation, and two-stream instability, we find that all of these can be stabilized by introducing thermal energy and an axial magnetic field.
TL;DR: In this paper, the authors present the results of experiments with plasma opening switches (POS's) in which energy delivery to the storage inductor occurs on a microsecond time scale.
Abstract: This review presents the results of experiments with plasma opening switches (POS's) in which energy delivery to the storage inductor occurs on a microsecond time scale. Under these conditions, the POS insures an impedance rate of rise of ~109 ?/s to final values of ~10-20 ?. These parameters allow the POS to be considered as a basis for the creation of unique ~ 1013-W pulsed power generators as well as compact ~1010-W commercial generators.
TL;DR: In this article, the ANTHEM implicit simulation code is applied to the PEOS with the aid of an implicit moment method, allowing for time steps well in excess of the electron plasma period and cells much wider than a Debye length.
Abstract: The plasma erosion opening switch (PEOS) has been studied with the aid of the ANTHEM implicit simulation code. This switch consists of fill plasma injected into a transmission line. The plasma is ultimately removed by self-electrical forces, permitting energy delivery to a load. Here, ANTHEM treats the ions and electrons of the fill plasma and the electrons emitted from the transmission-line cathode as three distinct Eulerian fluids-with electron inertia retained. This permits analysis of charge separation effects, and avoids the singularities that plague conventional MHD codes at low density. E and B fields are computed by the implicit moment method, allowing for time steps well in excess of the electron plasma period ?t >> ?p-1, and cells much wider than a Debye length, ?x >> ?D. Switch dynamics are modeled as a function of the driving electrical pulse characteristics, the fill plasma parameters, and the emission properties of the transmission line walls-for both collisionless and anomalously collisional electrons. Our low-fill-density (ne ? 4 × 1012 electrons/cm3) collisionless calculations are in accord with earlier particle code results. Our high-density computations (ne ? 2 × 1013 electrons/cm3) show the opening of the switch proceeding through both ion erosion and magnetic pressure effects. The addition of anomalous electron collisions is found to diffuse the driving B field into the fill plasma, producing broad current channels and reduced magnetic pressure effects, in some agreement with NRL experimental measurements.
TL;DR: In this article, the maximum ion current that can be extracted as a high-energy beam from a metal-vapor vacuum-arc ion source is considered, and it is shown that this source is an efficient generator of metal ions, an intense flux of which is efficiently transported to the beam extractor.
Abstract: The maximum ion current that can be extracted as a high-energy beam from a metal-vapor vacuum-arc ion source is considered. Results are presented of measurements of the plasma ion current in the metal-vapor vacuum-arc II (MEVVA II) ion source. It is shown that this source is an efficient generator of metal ions, an intense flux of which is efficiently transported to the beam extractor. The maximum metal-ion current that is available for extraction at the extractor location is 5 percent of the arc current. The limitation to the intensity of the metal-ion beam that can be produced by this kind of ion source is found to be in the extractor design.
TL;DR: In this article, the characteristics of a vacuum arc between CuCr contacts under an axial magnetic field have been investigated, and the arcing voltage of the contacts is lower than that of pure copper contacts by 10-20 V from the measurement of the post-arc current and interruption test results, it was found that the arc concentrates and a part of the electrode melts at a rather low current.
Abstract: The characteristics of a vacuum arc between CuCr contacts under an axial magnetic field have been investigated Test samples were made of CuCr contacts, and the arcing voltage was measured The arcing voltage of the CuCr contacts under an axial magnetic field is lower than that of pure copper contacts by 10-20 V From the measurement of the post-arc current and interruption test results, it was found that the arc concentrates and a part of the electrode melts at a rather low current, but that the electrode melting does not affect the interrupting capability Also, the insulation characteristics were measured With respect to voltage conditioning, high-current conditioning improved the breakdown voltage by 50 percent for a 20-mm gap and by 100 percent for a 3-mm gap These test results show that CuCr contacts, used with axial magnetic fields, are promising for use in high-voltage and high-power vacuum interrupters
TL;DR: In this article, the authors reported the generation of high-current-density (20 A/cm2) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 cm in diameter.
Abstract: We report the generation of high-current-density (20 A/cm2) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 cm in diameter. The pulse duration was determined by the energy of the pulse generator and varied between 0.2 ?s and several microseconds, depending on the discharge current. The largest electron beam current (900 A) was obtained with an oxidized aluminum cathode in a helium-oxygen atmosphere. An oxidized magnesium cathode produced similar results, and a molybdenum cathode operated at considerably lower currents. A small-diameter ( 1 kA/cm2) was also observed to develop in the center of the discharge. Electrostatic probe measurements show that the negative glow plasma density and the electron beam current have a similar spatial distribution. Electron temperatures of 1-1.5 eV were measured at 7 cm from the cathode. The plasma density (8.5 · 1011 cm-3 at 450 A) was found to depend linearly on the discharge current. In discharges at high currents a denser and higher temperature plasma region was observed to develop at approximately 20 cm from the cathode. We have modeled the process of electron beam generation and predicted the energy distribution of the electron beam. More than 95 percent of the electron beam energy is calculated to be within 10 percent of that corresponding to the discharge voltage.
TL;DR: In this paper, a slit-type field ion thruster with a closed propellant supply system was demonstrated to fire in any optional direction, requiring, in principle, no gravitational forces.
Abstract: Field emission electric propulsion is the technological application of the principle of liquid metal ion sources as thrusters in electric space propulsion. Research work sponsored by the European Space Agency (ESA) on a slit-type field ion thruster is reported and discussed. The most significant new features of its emission performance are as follows: For the first time, a slit emitter with a closed propellant supply system was fired in any optional direction, requiring, in principle, no gravitational forces. Quantitative data relating the constituents of the residual gas atmosphere to the wetting behavior of the liquid metal propellant and the emission site distribution were obtained. A homogeneous distribution of equally spaced emission sites was observed; the measured spacing is in good agreement with a simple hydrostatic model of wavelike instabilities on electrically stressed surfaces of fluids.