Showing papers in "IEEE Transactions on Plasma Science in 1977"

Vacuum Arc Anode Phenomena

Abstract: This paper briefly reviews anode phenomena in vacuum arcs, specially experimental work. It discusses, in succession, arc modes at the anode, anode temperature measurements, anode ions, transitions of the arc into various modes (principally the anode spot mode), and theoretical explanations of anode phenomena. The two most common anode modes in a vacuum arc are a low current mode where the anode is basically passive, acting only as a collector of particles emitted from the cathode, and a high current mode with a fully developed anode spot. Characteristically this anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. However, other anode modes can exist. A low current vacuum arc with electrodes of readily sputterable material may emit a flux of sputtered atoms from the anode. Usually this sputtered flux will have little effect upon the vacuum arc, but in certain circumstances it could be significant. A vacuum arc doesn't always transfer directly from a low current mode to the anode spot mode. In appropriate experimental conditions, formation of an anode spot may be preceded by the formation of an anode footpoint. This footpoint is luminous, but much cooler than a true anode spot. Finally, (again in appropriate circumstances) several small anode spots may form instead of one large anode spot. With sufficient increase in arc current or arcing time these will usually combine to form a single large active spot.

Anode Phenomena in Triggered Vacuum Gaps

Abstract: Arc voltage - current characteristics and threshold current densities occuring during the formation of anode spots in triggered vacuum gaps are reported. Single pulses of 1.65 ms arcing time, which correspond to switching surge currents, are used in the study with copper and aluminum anodes. The threshold values are 1.75 times the values reported earlier using the longer, 8 mis, arcing time. They are found to depend upon the duration of arcing time as well as upon electrode material, surface conditions, electrode size and contact separation. Lateral inhomogenity in the electrode geometry appears to reduce the threshold value by promoting early formation of anode spots.

Scaling Law for Ablation of a Hydrogen Pellet in a Plasma

Abstract: A scaling law is derived which gives the rate of ablation of a frozen hydrogen pellet immersed in a plasma without the presence of a magnetic field or space charge. Plasma particles penetrate the ablating gas-plasma cloud to the evaporating pellet surface and deliver power to the cloud to drive its expansion. The evaporation rate is determined by a "self-regulating" mechanism. The ablation time is shown to be proportional to a5/3?o-2/3?o-1/3 where a is the pellet radius, ?o is the effective penetration depth of the incident particle into solid hydrogen and ?o is the power flux density of the incident particles. It is found that for ablation in a thermonuclear plasma, the alpha particles significantly affect the ablation rate. For thermonuclear plasma electrons and alpha particles, the energy required to ablate one molecule is shown to be of the order of 10 eV or less. The scaling law is applied to presently available experimental results, and reasonable agreement is found.

Low Voltage Firing Characteristics of a Simple Triggered Vacuum Gap

Abstract: The firing characteristics of the simple triggered vacuum gap (TVG) using lead zirconate titanate as dielectric material in the triggered gap are described. This TVG has a long life of about 2000 firings without appreciable deterioration of the electrical properties for main discharge currents upto 3 kA and is much superior to these made with Supramica (Mycalex Corporation of America) and silicon carbide as used in our earlier investigations. The effects of the variation of trigger voltage, trigger curcit, trigger pulse duration, trigger pulse energy, main gap voltage, main gap separation and main circuit energy on the firing characteristics have been studied. Trigger resistance progressively decreases with the number of firings of the trigger gap and as well as of the main gap. This decrease in the trigger resistance is more pronounced for main discharge currents exceeding 10 kA. The minimum trigger current required for reliable firing decreases with increase of trigger voltage upto a threshold value of 1.2 kV and there-onwards saturates at 3.0 A. This value is less than that obtained with Supramica as dielectric material. One hundred percent firing probability of the TVG at main gap voltages as low as 50 V is possible and this low voltage breakdown of the main gap appears to be similar to the breakdown at low pressures between moving plasma by other workers. and the cold electrodes immersed in it, as reported

Mass Scaling of Permanent Magnet Line Cusp Plasma Leaks

Abstract: Plasma leaks to permanent magnetic produced "full line cusps" are shown to depend on ion mass as (mj?). This result is consistent with hybrid gyroradius (i.e., geometric mean of ion and electron gyroradii) leaks but not with ion gyroradius leaks.

Gridded Cross Field Tube

Abstract: A triode crossed field tube has been operated as a high voltage on-off switch tube. A third, partially transparent (grid) electrode is interposed between the anode and cathode and electrically tied to the cathode by a grid leak resistor. High voltage is first applied to the anode and cathode; the magnetic field is then raised to the conduction level (~ 0.01 Tesla). Ignition does not occur because the magnetic field is too low in the grid-anode gap and the electric field is zero in the cathode-grid gap. Pulsing the grid positive relative to the cathode (~ 1 kV) then results in breakdown of the cathode-grid gap; plasma comunication between the two gaps then fully ignites the tube and closes the main power circuit. Grid ignition has been achieved at 50 kV as well as grid ignition followed by current interruption against 10 kV. These levels were limited by the use of a modified, laboratory tube and not by the physics of the technique.

Investigation of the Anode Boundary Layer of an Atmospheric Pressure Argon Arc

Abstract: This paper is concerned with the thermodynamic state of the plasma in the anode region of a high intensity arc. Spectrometric studies of the boundary layer in front of a plane anode perpendicular to the axis of a wall-stablized, high-intensity arc in argon atmosphere indicate substantial deviations from LTE. Data taken at distances of 0.5 to 2 mm from the anode surface are evaluated with two different but not entirely independent non-equilibrium data reduction techniques in order to obtain radial distributions of electron temperature and electron density. The electron temperature distribution displays an off-axis peak which increases in magnitude with decreasing distance from the anode. Both electron temperatures and electron densities are considerably higher than the corresponding LTE values.

Interaction of a Plasma Beam with a Transverse Magnetic Barrier

Abstract: The interaction of an energetic He plasma beam with a quasi step-like transverse magnetic barrier is reported. When the rate of flow of momentum in the incident beam is less than the pressure of the magnetic field, a collisionless electrostatic shock is observed to form in front of the barrier as a result of the drastic compression of the plasma. The initially cold plasma is substantially thermalized by the shock and the electron density is increased by a factor of four. In the opposite limit the plasma penetrates the barrier in a flute-like manner.

On Current Distribution through the Cross-Section of an Arc Moving in a Narrow Insulating Slot

Abstract: Current distribution across the arc plasma cross-section constructed in a narrow slot between isolating walls and other factors affecting the distribution are considered. Experimental data on current density distribution of the arc moving at subsonic velocity are analyzed. Data available on the mean current density in an arc moving at supersonic velocity and under phase transition conditions on an active surface of the walls (gas-gen erating) are summarized. Possible influence of conditions of heat transfer with walls and pressure inside the column on the extent of non-uniformity of current distribution on the arc cross-section are shown. The problems under consideration are of practical application for avoiding thermal overload of arc chambers of magnetic-blast breakers.

High-Frequency Electrostatic Surface Waves on a Warm Gaseous Magnetoplasma Half-Space

Abstract: Recently, there has been an increase of interest in the theoretical and experimental properties of surface wave propagation. On the theoretical side the development has been based upon fluid or kinetic mode!s. For example, a fluid approach, adopted by Ritchie1, has been extended by Kaw and McBride2 andl has been used by Shivarova et a:3 to account successfully for the behaviour of surface waves on neon gas column discharges in the absence of an external magnetic field. Alternative kinetic field-free approaches have been investigated by Fuchs and Kliewer4, Barr and Boyd5 and Clemmow and Elgin6 In particular, Fuchs and Kliewer4 from a detailed comparison of selfconsistent and Boltzmann equation (kinetic) models with the hydrodynamic (fluid) model have shown that these separate approaches yield, for all practical purposes, identical results for the frequency-wave nunmber dispersion for 0 < K 100 where K is a dimensionless wave number measured in units of cu p/c; up being the p!asma frequency and c being the velocity of light in vacuo, This range of wave numbers more than covers that which is accessihle experimentally in experiments such as those conducted by Shivarova et al3. The damping of surface waves which can he incorporated into these models is either collisionless, collisional or both. The former type arises in the kinetic models whi!e hydrodynamic models possess the latter type. Kliiwer andi Fuchs have shown that collisional damping is of order v/2&p where v is a constant, phenomenological collision frequency and that, in contrast with the Landau damping of bulk modes, collisionless damping is of order 10-4 K being present down to the smallest wave numbers. However, this collisionless damping is quite small compared with collisional damping until K100 and furthermore the results of Kliewer and Fuchs reveal that, for realistic parameters, the real part of the frequency and hence the dispersion equation is not materially affected by the presence of damping. Several cold magneto plasma calculations have been reported such as that of \\/innikova and Gintsburg7 on the propagation of sLurface waves on a semi-infinite magnetoactive plasma which has been extended by Stepanov8 to include both a plane plasma-vacuum interface and the case of a plasma cylinder. The latter calculations are restricted to the retardation regime, and concern waves travelling perpendicuilar to a magnetic field applied parallel to the plamsa boundary. Subsequently Pakhomov and Stepanov9 calcuilated both the highand the low-frequency electrostatic surface wave behaviour for a magnetoactive electronion plasma in which the directions of wave propagation and of the magnetic field were taken to be arbitrary, whilst Ahdel-'Shahid and Pakhomov10 considered a similar problem for plasma layers. Associated problems of surface wave propagation in the retardation regime and in the presence of a magnetic field on a plane vacuum-plasma interface have also heen investigated by Kotsarenko and Fedorrhenko11 and hxy Kotsarenko et al12. The common feature of all these calculations are restrictions that the thermal velocity of the particles in negligible and, with the exception of a qualitative assessment by Stepanov, that the plasma interfaces are sharp. It is clearly of some interest, therefore, to consider any effects produced hy lifting one or both of these restrictions. For example, density gradients can be quite significant in somne devices such as those which involve electron or ion heams13, whereas the elegant experiments of Shivarova et a13 on a neon gas column discharge yield measurements in excellent agreement with a theory based on the assumption that the surface is sharp. However, it is the removal of the cold plasma restriction which has attracted the greatest attention in the most recently reported studies. The majority of these deal with magnetic field-free plasmas but thermal effects for a magnetoplasma have been

Electrodynamic Anomalies in Arc Discharge Phenomena

Abstract: The assumption that two electric particles are insufficient by their mutual action to develop a torque makes the Lorentz law a special case of a more general law of electrodynamics, of importance when electrodynamic closed circuit interactions involve charge carriers of different mass. Experimental anomalies evident in the mutual electrodynamic behaviour of ions and electrons are reviewed. It is argued that there is now sufficient evidence for this more general law of electrodynamics to be given attention by the electrical engineer interested in plasma and discharge phenomena.

Effects of Frequency Averaging on Estimates of Plasma Wave Coherence Spectra

Abstract: The effects of frequency averaging (i. e., data smoothing) on digitally computed plasma-wave coherence-spectra are investigated. Such averaging, or smoothing, of the data is necessary to reduce the variance of the spectral estimators to acceptable levels. However, too much averaging results in a loss of frequency resolution and in an "artificial" reduction of the degree of coherence characterizing each wave in the fluctuation spectrum. This latter problem is the subject of this paper and is concluded that in order to avoid such difficulties, it is necessary: (1) that the averaging bandwidth be small compared to the spectral bandwidth of the wave packet, and (2) that the distance between the two spatial monitoring points be small compared to the coherence length of the wave packet.

A Helical Launching Structure for Alfvén Wave Heating in the Proto-Cleo Stellarator

Abstract: A helical wave launching structure is analyzed to determine the spectrum of Alfven waves that it can excite. Using an ideal MHD plasma model, the effectiveness of this helical coil for producing Alfven wave heating in the Proto-Cleo stellarator is investigated. It is found that significant energy absorption should occur. The amount of absorption and the frequency range over which it is greatest are dependent on the shape of the radial profiles of the confining magnetic field and the plasma density.

Gaseous Fuel Reactor Research

Abstract: Gaseous Fuel Nuclear Reactors are externally moderated and contain the fissile material inside a cavity where it is suspended by fluid mechanics forces. The gaseous phase of the nuclear fuel permits operation of the reactor at temperatures much higher than the melting point of all materials. NASA has originally supported relevant research for space propulsion. The continuation of this work includes now research on power generation on Earth for improved economy and environmental acceptability. In reactor experiments with enriched uranium hexafluoride, UF6, a critical mass of 6 kg is determined. Pressurized UF6 remains chemically stable at temperatures up to 2000 kelvins. The interaction of fission fragments with their gaseous environment causes preferential excitation and ionization, leading to non-equilibrium optical radiation. Powerful fluxes of photons are expected to become a superior mechanism of energy extraction from the fissioning gas or plasma in the reactor. The pumping of lasers solely by fission fragments is realized in a variety of lasants. A near term objective of the NASA gaseous fuel reactor program is a benchmark experiment at 100 kw power and at a gas temperature of 1600 kelvins, demonstrating the feasibility of major advances in reactor technology. A concerted research effort is leading to this experiment. A plasma core cavity reactor for high specific impulse propulsion in space reminas a long range goal.

Numerical Simulation of Autoresonant Ion Acceleration

Abstract: Computational and analytic studies of the Autoresonant Acceleration proposal for collective ion acceleration are presented. Linear theory is reviewed, the electrostatic well depth is estimated nonlinearly, and an electron beam envelope equation is derived and solved. Two-dimensional numerical simulation results are given. Together, these calcualtions demonstrate unneutralized electron beam equilibrium in a diverging magnetic guide field, the behaviour of large amplitude slow cyclotron waves in the beam, and the acceleration of test ions over short distances in the wave troughs. In addition, the computer simulations point up the need for improved understanding of the linear theory of radially inhomogeneous noneutral beams and for methods of suppressing radial modulation at the diode-waveguide interface.

Design of Closed-Cycle MHD Generator with Nonequilibrium Ionization and System

Abstract: A closed-cycle MHD generator topping a steam bottoming plant is analyzed. The combined power plant involves three working fluids in three loops. The MHD loop is investigated more thoroughly since it is the least conventional of the three. Equations are developed to determine the geometric and thermodynamic variables throughout the MHD channel for inlet conditions of mass flow, temperature, pressure, and velocity. Limiting design parameters are output power, channel length, channel aspect ratio, Hall parameter, and interaction parameter. The basic closed-cycle MHD loop working fluid can consist of either argon or helium seeded with cesium. Both non-equilibrium ionization produced by the elevation of the electron temperature from joule heating of the plasma and thermal ionization are considered. Equations used to calculate the electrical conductivity and the elevation of electron temperatures are derived. These equations are coupled with the one-dimentional differential equations applicable to an MHD generator. The chief interest is in determining those MHD channel conditions which result in the most thermodynamically efficient MHD-steam plantcombination. Thus an overall heat balance forthe system is required. Equations are developed to calculate the gas properties at the various stations of the closed loop and to determine the overall efficiency of the cycle. A rather flexible computer program written in Fortran is used to solve the MHD generator equations and to make the overall heat balance. Some typical results presented demonstrate the feasibility and adaptability of the analysis for optimizing the thermal efficiency and the sensitivity of thermal efficiency to various parameters.

New Developments in Laser Diagnostics for Tokamak Plasmas

Abstract: New types of infrared and far-infrared lasers can be used to make otherwise impossible measurements of plasma parameters in the next generation of large tokamaks. Recent progress in the development of such new diagnostic techniques is reviewed.

Computer Simulation of the Periodic Electrostatic Focusing Converter

Abstract: A computer study was made of the various physical processes that take place in the periodic electrostatic focusing, direct energy converter. Computer simulation techniques are described and the results are presented. The effect that space charge, secondary electrons, beam size, charge exchange, and ionization have on conversion efficiency is examined. The results are applied to the design of a reactor. Finally, future plans for studies of direct converters are discussed.

Turbulent Heating Experiments in a Plasma Betatron

Abstract: The various experiments carried out with the University of Saskatchewan plasma betatron are reviewed. The apparatus, having a major/minor radius of 19 cm/3 cm, a toroidal magnetic field B? ? 0.4 T and a heating field E? ? 12 kV/m, has been operated in different modes ? designated as betatron, tokamak and reversed - B?. The characteristics of the turbulently heated plasma and the physical processes occurring in each mode are discussed.

Heterodyne Detection of Pulsed Co2 Laser Light Scattered from Ion Acoustic Waves in a Plasma

Abstract: The small angle scattering of light from a pulsed CO2 laser has been used to measure ion acoustic waves excited in a low density plasma. The laser is a hybrid laser consisting of a high pressure TEA discharge and a low pressure continuous discharge. The scattered light is detected by heterodyning, using laser light as the local oscillator. Two methods of providing the laser light local oscillator have been used. The dependence of the scattered light on the ion acoustic wavelength has been measured. The amplitude of the scattered light compared with the amplitude of the waves in the plasma indicates that the heterodyne mixing efficiency is better than 10 percent.

Progress in Switching Technology for Mets Systems

Abstract: Three distinct sets of switching requirements have emerged from design optimization studies of large superconducting magnetic energy storage systems, such as the METS system to power the adiabatic plasma compression field in the proposed ?-pinch SFTR. Extremely low joule loss cryogenic disconnects are required between storage coils in the liquid helium environment to allow charging the coils in series over a prolonged time, then to isolate the coils for parallel fast discharging into the load. Another switch must break the current in the series charging loop and absorb the energy from the stray inductance. This action will allow the subsequent opening of the cryogenic disconnects under near zero current condition. The current now has been transferred to the many paralled circuits, each containing a high current, high voltage interrupter. The opening and arc commutation of the interrupter starts the energy transfer into the load. The primary activities associated with the cryogenic disconnect have been testing and development of contact materials, configurations, and closing forces for carrying 26 kA with a resistance less than 40 n?, and development of an actuating system that is both reliable and fast acting in a liquid helium environment. The charging loop switch will include a continuous duty switch and a vacuum interrupter. The continuous duty switch resistance can be an order of magnitude larger than that of the cryogenic disconnect because it does not present a refrigeration load.

Stability of Obliquely Modulated Langmuir Waves in Collisional Plasmas

Abstract: The effects of collisions and oblique modulation on Langmuir waves are studied. The nonlinear Schrondinger equation for these waves are derived by using the Krylov-Bogoliubov-Mitropolsky method. The collisions damp the envelope of these waves and the obliqueness of the modulation effects the dispersion of the system. The Langmuir waves are found to modulationally stable as in the collisionless and parallel modulation case. However, the width of the envelope hole state of these waves is found to increase due to the oblique modulation.

A Direct Convertor Based upon Space Charge Effects

Abstract: A device capable of converting directly the kinetic energy of charged particles into electrical energy is considered. The device differs from earlier ones (such as Post's periodic focus electrostatic direct convertor) in that it makes use of the space charge repulsion in a high density charged particle beam. The beam is directed into a monotonic decelerating electrostatic field of a several stage planar finned structure. The collector fins coincide with vacuum equipotential surfaces. Space charge blowup of the beam directs particles onto various collector fins. The energy efficiency of a 4-stage device has been determined using a numerical simulation approach. We find that efficiencies approaching 75 percent are possible. An approximate scaling law is derived for the space charge based direct convertor and a comparison is made to the periodic focus direct convertor. We find the space charge based direct convertor to be superior in a number of ways.

Electrostatic Waves in the Warm Magnetoplasma at the Cyclotron Harmonic Frequencies

Abstract: Mode conversion and collisionless absorption of electromagnetic wave at the cyclotron harmonic frequencies in an inhomogeneous non-Maxwellian magnetoplasma have been studied. Under suitable energy transfer condition the converted electrostatic wave (plasma wave) either grows or damps. The expressions for the growth/damping rates of this wave have been derived and studied at the cyclotron harmonic frequencies. The effect of the temperature anisotropy on the growth/damping rate of the electrostatic wave at the second cyclotron harmonic frequency has been shown. Growth of such electrostatic waves at ionospheric heights may explain the observed upper hybrid resonance (UHR) echoes and noise bands at the second cyclotron harmonic frequency.

Temperature Measurements in Alternating Current Cross-Flow Arcs

Abstract: Using optical techniques appropriate to columns having non-circular cross-section, temperature distributions within an alternating current cross-flow arc column have been obtained. Experiments were conducted upon a 61.1 A rms, 840 Torr argon AC plasma (having 11.2 mm electrode spacing) in a crossflow of 1.1 m/s. Isotherm distributions, obtained in horizontal planes, were generally non-circular with major axis in the direction of flow. Column deflections increased as arc current was reduced. The (characteristic) temperature-time history showed that temperature reached a minimum following current zero.

A Magnet Field System for a High Density Tokamak Reactor

Abstract: A design for a magnet system for a high density high field tokamak reactor is described. In this design stress in the trunk of the toroidal field structure is significantly reduced by elimination of the central ohmic heating transformer. In addition all superconducting coils are operated in the steady state and pulsed fields are generated only by copper coils.

Light Emission from Atom Beam - Plasma Interactions

Abstract: A beam of atoms penetrating a plasma can be considered a line source of contaminants. Atoms excited by the plasma electrons yield information about the spatially resolved electron density and temperature. The light intensity as a function of time and position along the beam can be conveniently observed with an optical multichannel analyzer Since the beam atoms are different from the plasma constituents, the method does not suffer from self-absorption.

Saturation of the Substorm Mode by Magnetic Trapping

Abstract: The purely growing electromagnetic instability, which was proposed by the author in an earlier publication as the onset mechanism for an isolated substorm, is shown to get saturated by the trapping of particles in the magnetic potential of the wave The quasilinear diffusion plays a secondary role and is responsible for the generation of a high energy tail

Electromagnetic Instability in Counterstreaming Collisional Plasmas in a Magnetic Field

Abstract: The stability of low frequency electromagnetic waves is studied for collisional, counterstreaming plasmas situated in a steady magnetic field. Based on the cold fluid equations, it is found that obliquely propagating waves are unstable when the parallel component of the phase velocity is smaller than the electron streaming velocity. The frequency of the instability is of the order of but always smaller than the ion cyclotron frequency. The growth rate is proportional to the electron-ion collisional frequency.