US3078388A - Method and apparatus for controlling electrical discharges - Google Patents

Method and apparatus for controlling electrical discharges Download PDF

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Publication number
US3078388A
US3078388A US769927A US76992758A US3078388A US 3078388 A US3078388 A US 3078388A US 769927 A US769927 A US 769927A US 76992758 A US76992758 A US 76992758A US 3078388 A US3078388 A US 3078388A
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Prior art keywords
cathode
voltage
discharge
current
anode
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US769927A
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Charles W Hanks
Charles D A Hunt
David A Vance
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Stauffer Chemical Co
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Stauffer Chemical Co
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Priority to LU37811D priority Critical patent/LU37811A1/xx
Priority to NL244036D priority patent/NL244036A/xx
Application filed by Stauffer Chemical Co filed Critical Stauffer Chemical Co
Priority to US769927A priority patent/US3078388A/en
Priority to GB33608/59A priority patent/GB929831A/en
Priority to FR807725A priority patent/FR1239527A/fr
Priority to CH7960059A priority patent/CH386020A/de
Priority to DEST15697A priority patent/DE1208428B/de
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/32—Gas-filled discharge tubes
    • H01J37/32009—Arrangements for generation of plasma specially adapted for examination or treatment of objects, e.g. plasma sources
    • H01J37/32422—Arrangement for selecting ions or species in the plasma
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/10—Regulating voltage or current
    • G05F1/12—Regulating voltage or current wherein the variable actually regulated by the final control device is ac
    • G05F1/32—Regulating voltage or current wherein the variable actually regulated by the final control device is ac using magnetic devices having a controllable degree of saturation as final control devices
    • G05F1/34—Regulating voltage or current wherein the variable actually regulated by the final control device is ac using magnetic devices having a controllable degree of saturation as final control devices combined with discharge tubes or semiconductor devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/135—Circuit arrangements therefor, e.g. for temperature control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/24—Circuit arrangements not adapted to a particular application of the tube and not otherwise provided for
    • H01J37/241—High voltage power supply or regulation circuits
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/32—Gas-filled discharge tubes

Definitions

  • This invention relates to the control of electrical discharges used for heating, melting, and otherwise treating materials in a high vacuum by electron bombardment; and its chief object is to maintain the power expended in such a discharge and the voltage developed across it within desired ranges, irrespective of variations in operating conditions, such as may result from the irregular release of gases or vapors from the treated material, or other changes that may occur.
  • the discharge to be controlled is established within a constantly-pumped vacuum chamber, which is maintained at an average absolute pressure of the order of one micron of mercury or less.
  • the dis charge occurs from a heated, thermionic cathode to an anode, which is usually and preferably the material to be treated but may under certain circumstances be a crucible containing the material.
  • a certain amount of gaseous matter is preferably introduced into the discharge near the anode, sometimes from an external source but usually by the evolution of vapors and gases from the heated material, so that a pressure gradient exists from the discharge outward into the body of the vacuum chamber, and the gas density is greatest immediately adjacent to the anode.
  • the gas in the immediate vicinity of the electrical discharge is believed to be highly ionized; probably, for the most part, by secondary emission of electrons from the anode, although ionization by primary electrons and by photons may occur to some extent.
  • the discharge is diffuse and luminous; gas focusing concentrates it upon the molten anode surface, over which it spreads fairly uniformly.
  • ionized gas near the anode may become so highly conductive that there is relatively little voltage drop across or through it, and it forms a plasma which may be considered a virtual anode, much closer to the cathode than the physical anode formed by the bombarded material.
  • the gas density is much lower, and the resistivity of the gaseous medium is sufiiciently high that several thousand volts can be maintained between the cathode and the virtual anode when the discharge is properly regulated and controlled as herein explained.
  • Such a discharge tends to break down into a low-voltage arc; the present invention inhibits such breakdown, so that the desired, moderately high-voltage discharge can be maintained and controlled at higher power levels than heretofore.
  • the discharge is essentially electronic, of a difiuse, luminous type and the current is approximately equal to the cathode emission.
  • the proximity of the virtual anode to the cathode permits the establishment of a potential gradient high enough to achieve saturation current, limited by cathode emission rather than space charge, at a much lower total voltage than would be required in a gas-free discharge path between the same cathode and the treated material.
  • the voltages that are employed are much greater than can be maintained across an are discharge, and the currents (for equal power dissipated) are correspondingly less.
  • the use of saturated thermionicemission discharges of this diffuse, luminous character therefore permits high-power electric heating without the employment of excessively high voltages or large currents.
  • the high vacuum permits greater purification by evaporation of impurities from the melted material than does conventional arc melting, and the discharge is diifused over the entire surface of the melt, instead of being localized as in a hard core arc.
  • Most of the electric power supplied to the discharge is utilized in accelerating the primary electrons to high velocities.
  • the gaseous medium is sufficiently rarefied that few of the primary electrons experience collisions en route, so that their kinetic energy is mostly expended in bombarding the anode, whereby the anode surface is heated with good efliciency and uniformity.
  • One of the important uses for discharges such as that just described is for high-vacuum melting and casting of metals having a high melting point, a high chemical activity, or both, whose original reduction from their ores leaves them in either powder or sponge form.
  • Examples of such materials are tungsten, titanium, and columbium, among many others. Rods and ingots can also be melted and recast.
  • the raw materials usually contain impurities which can greatly affect the physical characteristics of the metal as finally produced. Many of the impurities commonly present in such materials have vapor pressures higher than that of the desired material at its melting point, so that high-vacuum melting can result in a high degree of purification of the material treated.
  • Successive remeltings and recastings of the material in a high vacuum may be employed for greater purification.
  • the very low gas pressure at which the present process is carried out facilitates the evaporation of the impurities, which are removed by condensation on cooler surfaces and by the vacuum pumps.
  • a greater degree of purification can therefore be accomplished, or an equal degree in fewer remeltings, than is possible under the much higher gas pressures that are necessary to sustain the arcs employed in ordinary arc-melting, even when such melting is possible.
  • Such diffuse, luminous discharges as those described are only metastable at high power levels. If the highly conductive plasma extends itself too near the cathode structure--focusing electrodes, supports, or the cathode itselfand the voltage gradient near the cathode becomes excessive, a localized discharge of much lower resistance shunts the desired diffuse discharge, and if it persists, the localized discharge soon degenerates into a self-sustaining are that restricts the discharge voltage to a relatively low value until the arc is extinguished. Breakdown of this kind may occur as the result of bursts of gaseous matter released from the melt.
  • the nominally cold portions of the cathode structure may in fact become quite hot, and this reduces their work function and renders electron emission easier. They may, however, be cool enough to condense vapors emanating from the melt, and the condensate contaminates their surface. Areas of such contamination on the cathode structure may further reduce the work function where they occur. Particularly if this is in a region of high potential gradient, considerable electron emission may occur from the nominally cold, contaminated surface, and initiate an undesired, low-resistance, local discharge.
  • the present invention provides a method and apparatus for so controlling diffuse, luminous discharges of the type described as to maintain them at maximum efficiency
  • major objects of the invention are to provide a method of control that will maintain both the voltage across a diffuse, luminous discharge and the power dissipated by it constant to within a narrow range of values, that will normally extinguish minor, localized discharges before they can develop into self-sustaining arcs, that will promptly extinguish any arcs that do form and upon their extinction immediately re-cstablish the desired, diffuse,
  • a further object of the invention is to provide apparatus whereby control can be exercised manually until optimum operating parameters have been determined for the treatment of a specific material and by which control can thereupon be transferred to automatic equipment for maintaining optimum operation.
  • operation is initiated by supplying sufficient electrical power directly from a power supply to a thermionic cathode to heat it to a temperature at which it will thermionically emit sufficient electrons to carry the current corresponding to the power desired in the main discharge at the desired operating voltage.
  • voltage is supplied to the cathode-anode circuit to establish the discharge.
  • the current is maintained substantially constant at the established value; initially, the voltage across the discharge path will be higher than the normal operating voltage, but it will fall gradually as the material treated heats and by the evolution of gaseous matter and secondary electron emission establishes a zone of ionization.
  • ionic bombardment of the cathode and other processes tend to increase the electron emission; hence, in accordance with the invention, the
  • the power supplied to heat the cathode directly from the power supply is reduced as a non-proportional function of the voltage between the cathode and anode, which controls the resistance of the discharge path for maintaining a substantially constant average applied high voltage.
  • the directly supplied, cathode-heating power is continuously varied as a non-proportional function of the discharge voltage to maintain the power and voltage of the main discharge both within their desired operating ranges.
  • the emission current is regulated at the power supply to prevent substantial fluctuations in the discharge current
  • the cathode temperature is separately regulated to control the voltage gradient at the cathode for maintaining the average resistance of the discharge path and the average power dissipated at the anode substantially constant.
  • Any cathode structure has a certain thermal capacity, and there is a consequent time lag between changes in the power supplied to it, both directly from the power supply and indirectly from the discharge, and changes in the emission current.
  • a decrease in the resistance of the discharge path caused, for example, by the sudden release of a burst of gas from the melt, is countered by a reduction in the heating current supplied to the cathode, for the purpose of reducing the cathode temperature to increase the voltage gradient at the cathode.
  • the cathode heating current might be reduced essentially to zero, and if the arc persists long enough the cathode may cool below the minimum emission temperature. Then, when the arc breaks the resistance of the discharge path will be very high, the
  • FIG. 1 is a diagram, partly schematic and partly in block form, of equipment embodying this invention
  • FIG. 2 is a partial circuit diagram illustrating in more detail certain equipment symbolized in block form in FIG. 1;
  • FIG. 3 illustrates the general relation between emission current and voltage gradient at a hot cathode
  • FIG. 4 illustrates the general relation to be established between cathode or filament current and applied high voltage for stable operation in the treatment of different materials
  • FIG. 5 illustrates an approximate voltage distribution etween the cathode and anode.
  • FIG. 1 is a greatly simplified diagram, partly schematic and partly in block form, illustrating the elements of a control system for a high-vacuum, electronbombardment furnace, in accordance with the present invention.
  • the discharge within the furnace takes place from a thermionically-emissive cathode 7 to an anode 9, which may be a molten pool of the material treated contained in a conductive crucible it and electrically grounded through the metal walls of the vacuum tank. Electrons emitted by the cathode are accelerated to high velocities by the cathode-anode voltage, and bombard and heat the molten surface of the material within crucible 10. Gaseous matter evolved from the melt becomes ionized and forms a plasma (a highly conductive ionized body of essentially neutral charge) extending outward from the melt.
  • a plasma a highly conductive ionized body of essentially neutral charge
  • the principal voltage drop occurs between the cathode and the plasma; and the chief object of the invention is to control the discharge so as to keep the resistivity of this high-voltage region sufficiently high, and to prevent arcing, and other forms of breakdown, while operating at high power levels.
  • focusing electrodes, heat shields, crucible-cooling means, provisions for continually supplying and withdrawing the material treated, and the like, with which the present invention is not directly concerned, have been omitted for simplicity and clarity.
  • the cathode 7 in a preferred form of the device, is a filament in the form of a single loop of tungsten wire or rod. Current passed through the loop heats the cathode to electron-emitting temperature, and leads for supplying this current are brought in through the sides of the chamber by insulating bushings, symbolized at 11. Cathode-heating current is supplied through a filament transformer 13 which, in the usual arrangement, derives its power from a 60-cycle commercial source. Heating current is controlled by means of a saturable reactor 15, controlled as will be described hereinafter.
  • High voltage between the cathode 7 and anode 9 is provided by a constant-current D.-C. supply, whereby the applied D.-C. voltage is proportional to the resistance of the discharge path between the cathode and the anode.
  • the power used is derived from a three-phase, 60-cycle commercial source. Power from that source is first supplied to a conventional, three-pl1ase constant-current network, preferably of the type known as a Steinmetz constant-current network, indicated generally by the reference character 17.
  • This network comprises three delta-connected legs, each including an inductor 19 in series with a condenser 21 tuned to series resonance at the supply frequency.
  • inductors 19 and condensers 21 are shown as variable, for the sake of simplicity; in practice the condensers preferably take the form of tapped condenser banks and the inductors are also tapped so that the legs can be tuned and the output current adjusted by selection and interconnection of the proper taps.
  • the three-phase input leads 23a, 23b and 230 are connected to the apices of the delta network, so that the input phase vector rotates into the inductor of each leg firsti.e., counterclockwise in network 17 as illustrated in FIG. 1.
  • the output leads 25a, 25b and 250 connect to the junction between the inductor and the condenser of each leg.
  • the voltage developed across the output leads is very nearly proportional to the effective impedances connected across these leads, with the corollary that the current supplied to the output circuit is very nearly constant. For present purposes, no substantial error is involved by considering it to be a constant, and it Will be so treated in what follows.
  • Output leads 25 of the constant-current network connect to the delta-connected, primary 27 of a three-phase, step-up transformer.
  • the secondary 29 of this transformer is star connected, in the example illustrated.
  • the secondary is connected to a rectifier bank comprising, for example, six mercury-vapor rectifiers, collectively designated by the reference character 31. These rectifiers are connected in known manner to lead 35.
  • constant-current supply Various other types are available, and the invention is not limited to the use of any particular type.
  • the arrangement shown is preferred because of its simplicity and high efiiciency.
  • the current can be reduced, when so desired, by manually detuning the network.
  • a constant-current source such as the network 17 as viewed from the discharge through the rectifier bank, will limit the current through the discharge to a definite maximum value even if the cathode and anode are shorted, so that the impedance of the gap and the voltage across it approach zero, and will maintain the current within a few percent of that maximum even if the effective impedance of the gap rises to such a value that the potential required rises to several thousand volts.
  • One side of the rectifier bank is grounded. The other side connects through lead 35 to the secondary of the filament transformer 13 and thence to the cathode 7.
  • an ammeter 37 is preferably provided in the ground lead of the rectifier bank to indicate the discharge current.
  • a voltmeter 39 in series with an external multiplier resistor 40, connects from lead 35 to ground for like purposes.
  • a high resistance voltage divider consisting of resistor 41 and potentiometer 42 in series.
  • Lead 53 connected to the circuit junction between elements 41 and 42, picks off of the voltage divider a small voltage, generally about -200 volts, proportional to the voltage across the discharge in the furnace, and feeds it to a variable-gain D.C. amplifier 47, which is adjustably biased, as hereinafter explained.
  • Output current from the amplifier 47 is supplied to a winding 49 of the saturable-core reactor 15, to supply thereto a current directly related to the voltage across the discharge path within the furnace. Increase in this current, increasing the saturation of the core, decreases the effective reactance of reactor 15 and thereby increases the electrical energy supplied to heat the cathode 7 and thus increases its temperature.
  • a decrease in voltage has, of course, the opposite effect.
  • the conditions existing within the discharge path are not only complex but change from moment to moment owing, in part at least, to variations in volume and position of the ionic plasma.
  • FIG. 5 shows the approximate voltage distribution between the cathode and the anode. Gaseous matter released from the molten anode becomes ionized and forms a low-resistance, ionic plasma adjacent to the anode. Because of its high electrical conductivity, there is little voltage drop across this plasma, and its outer surface,
  • the curves illustrated in FIG. 3 are somewhat related to the familiar current-voltage curves of a diode, carried into the current-saturation range, and are necessarily only rough approximations in view of the very complex nature of the phenomena under consideration.
  • the currents are plotted as ordinates; because, however, the size and position of the ionic plasma, constituting a virtual anode, is variable, and in varying changes the configuration as well as the actual length of the discharge path through the relatively high-resistance region, the abscissas represent the voltage gradient in the immediate vicinity of the cathode and not the total voltage across the discharge.
  • the gradient varies as a direct, non-linear function of the total voltage and an inverse, non-linear function of the length of the high-resistance zone between the cathode and the plasma.
  • the cathode emission current varies with the voltage gradient at the cathode in approximately the manner shown.
  • the gradient at the cathode is substantially zero, or even slightly negative.
  • saturation the point commonly called saturation is reached, where nearly all of the electrons emitted by the cathode are drawn over to the anode.
  • a positive voltage gradient can be obtained fairly close to the cathode, and thereby, due to many, complexly interrelated factors, not requiring discussion here, a small but significant increase in current over the saturation value can be obtained.
  • the cathode is operated in this beyondsaturation region. Of course, if this is pushed too far and the voltage gradient becomes too large, breakdown and arcing will occur.
  • Emission current also varies as a function of cathode temperature.
  • curve A applies to one cathode temperature
  • curve B applies to another, somewhat higher cathode temperature. If the cathode becomes too cool, no substantial emission occurs, in the high vacuum under consideration, until the voltage gradient becomes so high as to cause almost immediate breakdown. Hence, the cathode must be kept above the minimum temperature for thermionic emission, which depends on the cathode material.
  • the dotted line C of FIG. 3 indicates the current supplied by the constant-current network 1'7; it has a slightly negative slope, indicating the slight decrease in current as the effective impedance across the discharge gap increases from zero to a relatively high value.
  • ionic currents are very small in comparison to the electronic currents; therefore, the supply current and the emission current must be approximately equal.
  • the cathode temperature corresponds to curve A
  • the operating point is the intersection of curves A and C, and the voltage-gradient will be that corresponding to the abscissa of point X.
  • An increase of cathode temperature to that corresponding to curve B will drop the voltage-gradient cordinate to point Y.
  • the cathode temperature determines the voltage gradient.
  • the cathode temperature is varied to control and stabilize the discharge.
  • the total cathode-anode voltage automatically adjusts itself, through the constant-current supply to the value determined by the gradient established and the discharge geometry.
  • the relatively large changes in voltage with small changes in cathode temperature give a powerful negative feedback with which to hold the average voltage substantially constant.
  • the voltage gradient near the cathode approaches the total voltage applied across the discharge gap divided by the distance between the cathode and the virtual anode. It will be apparent from the curves of 3 that a change in cathode temperature can compensate for a change in position of the virtual anode to maintain the voltage (and therefore the power) constant, since the current is substantially constant by postulate.
  • the position of the virtual anode depends (among other factors) on the rate at which gaseous matter is evolved from the anode, either from evaporation of the treated material or liberation of impurities. It is therefore a function of the power dissipated in the discharge (which mostly goes into electron bombardment of the anode) among several other factors, including the structure of the furnace and the composition of the raw iaterial, its melting point, vapor pressure, and purity.
  • the nominal current and voltage ratings are design parameters of the particular furnace to be controlled.
  • the cathode temperature may first be set to give a higher emission than required to carry the value of current delivered by the constant-current network 17. Initially, the discharge will be space-charge limited, the resistance of the discharge path will be high, and the oathode-anode voltage will be correspondingly high. As the bombarded and heated anode releases gaseous matter, ions form which lower the resistance of the discharge path, and the voltage drops. The cathode-heating current can then be gradually reduced to keep the voltage and power at the desired values.
  • the cathode temperature should be lowered to increase the voltage gradient and the resistance of the discharge; conversely, whenever the voltage rises above the desired value, the cathode temperature should be raised.
  • the operation can now be switched over to automatic control, which will maintain the voltage and power at substantially constant value.
  • the quantities that aifect the discharge areso interrelated that operating parameters cannot be set arbitrarily.
  • the density and pressure of gaseous matter at the anode depends on the temperature of the melt, the material melted, and particularly on its purity.
  • the rate at which the cathode loses energy by radiation also depends, in part, on the anode temperature, and this, in turn, affects the direct heating energy that must be supplied to it to maintain its temperature. Too much power in the discharge heats the melt too fast, speeds up the evolution of gaseous matter, increases the volume of the ionic plasma, and decreases the spacing between the ionic plasma and the cathode, whereby the discharge becomes unstable and forms a low voltage arc.
  • the characteristics of the melt, the rate of evolution of gaseous matter, and the size of the ionic plasma may change during the course of a melting operation.
  • the average resistance of the discharge path must be kept substantially constant.
  • the resistance of the discharge is regulated by controlling the cathode temperature responsive to the cathode-anode voltage. When the voltage drops, the heating current to the cathode is reduced; and when the voltage rises, the heating current is increased.
  • the heating current is not proportional to the voltage; but may be proportional to the algebraic sum of the voltage and a negative constant of approximately the same magnitude.
  • I represents the cathode heating or filament current
  • V represents the voltage between the cathode and the melt
  • stable operation may be achieved by controlling the filament current so that where S and K are experimentally determined constants which have different values for difierent materials, but have never been found to be zero.
  • S is usually only slightly smaller--say, lessthan the average value of V.
  • the cathode In addition to the above factors, there is a threshold temperature below which the cathode will not emit any considerable number of electrons, but secondary effects may be sufficient to maintain the cathode at full emission even though the heating current supplied to it may be insufiicient of itself to raise it above the threshold temperature; the resistance of the cathode varies with temperature, and the heating current supplied to the cathode may vary non-linearly with the control current supplied to the saturable reactor 15.
  • FIG. 4 wherein filament current is plotted against cathodeanode voltage.
  • Curves D and E of this figure show typical characteristics for stable operation in treating two different materials. The important facts to observe about these curves are their different slopes and their different intercepts on the zero current axis; these intercepts are never at the origin if stable operation is to be maintained. Stated otherwise, neither the cathode heating power, current nor voltage is directly proportional to the discharge voltage.
  • FIG. 2 Equipment whereby all of the necessary adjustments may initially be made manually and then be transferred to automatic control is illustrated in FIG. 2. It is more convenient, however, to describe first the automatic control equipment, as the manual control elements are, in
  • FIG. 1 Such parts of the apparatus illustrated in FIG. 1 as are necessary to the complete description of the second figure are designated by the same reference characters as in FIG. 1.
  • the equipment comprised within the block 47 of FIG. 1 is shown enclosed within broken lines 47 of FIG. 2; similarly the equipment within block 51 of FIG. 1 is enclosed within broken lines 51 in FIG. 2.
  • the resistors 41 and 42 forming a voltage divider for providing a convenient voltage of about-200 volts proportional to the much higher cathode-anode voltage, are shown at the extreme right of FIG. 2, connected between ground and the lead 35 that connects the high-voltage power supply to the center of the secondary of filament transformer 13 which feeds the cathode 7.
  • Lead 53 extending from the circuit junction between resistors 41 and 42, connects through one pair of contacts 55 of a ganged, automatic-manual changeover switch, to one end of a p0 tentiometer 43.
  • the lower end of this potentiometer connects to the moving contact of a second potentiometer 57, one end of which goes to ground and the other to a source of negative potential, preferably a negative tap on a conventional power-pack.
  • the setting of potentiometer 57 determines the bias setting of the amplifier, and thus determines the approximate average voltage between cathode 7 and the melt during automatic operation.
  • This bias may be set at any value between ground potential and 275 volts; for the particular apparatus illustrated it will usually be somewhere in the neighborhood of volts, so that the grid of the first amplifier tube is about ten volts more negative than its cathode with normal voltage across the discharge in the furnace and lead 53 at about 2OO volts.
  • the setting of potentiometer 57 effectively determines the average voltage across the discharge during automatically controlled operation.
  • the setting of the potentiometer contact 45 determines the slope of the curve, i.e., the rate at which the filament current is increased with increasing voltage across the discharge, during automatic operation.
  • potentiometer 57 connects directly to one cathode 53 of a dual triode 59, while contact 45 connects directly to the grid 60 for controlling the current in this triode.
  • The. anode of the same tube section connects through a load resistor 61 to the adjustable tap of a potentiometer 63 connected from a +250 volt tap on the power supply to ground. This arrangement makes it possible to adjust the average anode voltage of the vacuum tubes, which is usually done during factory calibration of the amplifier.
  • the drop across resistor 61 is applied directly to the grid 69 of the second section of tube 5% through the usual protective resistor 65.
  • the cathode 58 of the second section of the tube is connected back to potentiometer 57 through a cathode resistor 67, whereby this section of the tube 59 acts as a cathode follower.
  • the voltage developed between the cathode 58 and ground is applied to the four control grids of two dual tubes 65 in parallel.
  • the cathodes of these tubes are grounded directly; their anodes connect, also in parallel, through small protective resistors 71 to the control winding 49 of the saturable reactor 15.
  • the current controlled by these tubes is supplied from a +200 volt tap on the same power supply as is used to provide the other operating voltages for the amplifier circuit.
  • the voltage at the cathode 58 of the output section of tube 59 is also applied through a second set of contacts 55 on the automatic-manual changeover switch to a lead 73, connected through a variable resistor 75 to the cathode of a thyratron 77 and also, through a condenser 79, to ground.
  • the control electrode of the thyratron is connected through a current-limiting resistor 81, to the movable tap of a potentiometer 33, incorporated in a voltage- -divider-string connected between the -275 volt tap of the power pack to ground.
  • the anode of thyratron 77 is connected directly to ground.
  • potentiometer 33 can adjust the voltage on the'control electrode of thyratron 77 through a range of 50 or 60 volts, from approximately 110 volts negative to somewhere in the neighborhood of 160 volts negative.
  • the positive voltage drop across cathode resistor 67 and the negative voltage of approximately l90 volts from potentiometer 57 add algebraically to make the cathode of thyratron 77 sufficiently positive relative to its control grid that the thyratron remains nonconductive.
  • Striking of the thyratron 77 also reduces the resistance effective across the condenser 79 substantially to zero, discharges tlie condenser, brings cathode and anode of the thyratron substantially to the same potential, and thus breaks the discharge in the thyratron. If the arc in the furnace has not broken by this time, the condenser recharges through resistor 75, the thyratron strikes again, and continues to make and break the circuit across condenser 79 as long as arcing in the furnace continues. Tube 77 therefore acts as an approximately sawtooth oscillator. The frequency and breakdown point can be varied, respectively, by shifting the contacts on the resistor 75 and the potentiometer 83.
  • the time during which reactor is saturated, and hence its average impedance can be adjusted so as to pass enough current to maintain the temperature of cathode 7 in the emitting range.
  • the thyratron circuit does not operate responsive to localized discharges of such short duration that its action is not needed.
  • the core of the saturable reactor 15 is never fully saturated while thyratron 77 is oscillating as described above, and control-winding 49 has a considerable inductance; hence the current passed by it is an inverse function of frequency. Therefore the degree of saturation of the reactor core can be controlled by adjusting the frequency of the thyratron oscillations by adjusting resistor 75.
  • a potentiometer tap is taken off from the resistor 42 at the low potential end of the voltage divider string, which connects to the control grid of a thyratron 87.
  • the cathode of this thyratron connects to ground through a variable resistor 88; its anode connects to the winding of a relay 89 and thence, through a condenser 91, to the +250 volt tap of the amplifier power supply.
  • Condenser 91 is bridged by a high resistance 92, of a value such that the time constant of the condenser-resistor combination is in the order of one second.
  • Potentiometer contact 85 is set to a point that will hold thyratron 87 nonconductive until the voltage across the cathode-anode gap of the furnace drops to the low value indicative of an arc.
  • thyratron 87 fires and charges condenser 91 through the winding of relay 89, tube 87, and resistor 83, in series.
  • the last-mentioned resistance is much smaller than resistor 92, and it is adjusted so that the time constant of the series circuit which includes the relay winding and condenser 91 is of the order of one-sixtieth second.
  • relay 89 closes and actuates the magnetic contactor 93. The latter is supplied by the main AC.
  • the contacts 95 carry only the current normally supplied to transformer primary 27, because of the characteristics of the constant-current network, but the voltage across the rectifiers 31 immediately drops substantially to zero and causes the are within the furnace to break. This usually happens within a half-cycle of the 60-cycle input power.
  • the time during which relay 89 re mains closed can be adjusted so that it is long enough to insure the breaking of the are within the furnace, but no longer.
  • the capacity of condenser 91 must be large enough to store sufiicient energy to hold relay 89 closed for the required interval. This will, of course, depend on the sensitivity of the relay.
  • the values of resistors 88 and 92 are chosen accordingly.
  • Resistor 97 is preferably calibrated in terms of the gap voltage indicated by voltmeter 39, so that the current through potentiometer 43 at a given setting is the same as at the indicated voltage when on automatic control.
  • the current through the cathode filament 7 can be read on an ammeter 99 in the primary circuit of transformer 13, and when proper operating conditions have been established this current can be matched by the automatic setting.
  • an entirely separate control could be used for regulating the cathode heating current, but although such separate control mechanism would be much simpler per se than operating through the amplifier 47 it would actually add to the complexity of the system, and make transfer from manual to automatic operation more difiicult.
  • the biasing point for tube 59 which determines the average voltage across the discharge in the furnace during automatic control, is set by potentiometer 57 and will usually difier least of all of the operating parameters as between different materials treated. This potentiometer therefore seldom needs readjustment. Much more critical is the slope of the filament current vs. discharge voltage curve. This is set by means of the movable tap 45 on potentiometer 43. In the present case this is done by means of a small, reversible, electric motor 1M, which may be operated to adjust the position of contact 45 by operating one or the other of push buttons 103.
  • one of the major advantages provided by this invention lies in the fact that the various interrelated factors that maintain stability of the discharge can be adjusted manually during experimental operations until an optimum operating point is found for each specific parameter, and the control of the various operating parameters can be transferred to the automatic control equipment, one-by-one, as these points are determined. Furthermore, because of the order in which the steps are taken, the danger of destructive voltages or currents that would normally be inherent in supplying large amounts of power to a load of unpredictably varying resistance from a constant current source, or from a constant voltage source as in common prior practice, is avoided.
  • the power dissipated 1d at the molten surface of the material treated can be maintained constant, to within +5 percent or less, over long periods of operation where the materials melted are reasonably pure or are of constant composition. This constancy is to be expected where the discharge is used to remelt and further purify metals that have previously been melted and cast in a vacuum.
  • the term maintaining the power in the discharge constant is applicable only in connection with average, rather than instantaneous, power. It is frequently a characteristic of such materials that they contain gas inclusions which are released into the discharge in sudden, violent bursts. When such a burst occurs the volume of gaseous material that is released may be so great as to raise the pressure within the vacuum chamber to a point where the entire volume is filled momentarily with a luminous discharge, even though the pumps used to evacuate the chamber may have capacity suflicient to keep the pressure outside of the discharge path down to a fraction of a micron of mercury were the gas liberated at a constant rate.
  • the low-resistance discharge following a burst of gas emission may persist for time intervals varying from a small fraction of a second to several seconds, and in spite of the thermal capacity of the cathode, its temperature in comparison to its immediate surroundings is so high that it is quite possible for it to drop below minimum emission temperature in less than a second.
  • the cathode either non-emit ting or emitting fewer electrons than required to carry the constant current, the result would be a voltage of a value that could be even more destructive than the shortcircuit currents that would develop were the supply from the more usual prior-art constant-voltage system.
  • the present invention operates to prevent instantaneous interruptions of the desired process from developing into complete breakdown and permits immediate re-establishment of substantially the desired operating conditions, and by so doing permits large-scale commercial melting and casting operations at higher power levels than were practicable heretofore.
  • the method of controlling an electric discharge in a vacuum between a hot cathode and a bombardmentheated anode which comprises supplying heating current to said cathode su'fficient to produce electron emission and maintain the discharge between said cathode and said anode, the electrons so emitted bombarding and heating the anode thereby causing the evolution of gaseous matter therefrom and producing an ionic plasma between the anode and the cathode, supplying a substantially constant direct current to the discharge between said cathode and said anode, and continually adjusting said heating current as an inverse function of the voltage across said cathode and anode and thereby maintaining a substantially constant average. voltage between said cathode and said anode.
  • heating current is continually adjusted to a value proportional to the difference between the voltage across the discharge and a constant in the order of smaller than said voltage.
  • Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma comprising a thermionically emissive, filamentary cathode, a bombardment-heated anode, a constanturrent D.C.
  • control means is a saturable reactor having a variable-impedance winding connected between said filamenucurrent supply and said filamenatry cathode, and having a control winding connected to said amplifying means for receiving said control signal.
  • Apparatus for controlling an electric discharge in vacuo wherein the path of said discharge includes a lowresistance ionized zone and a high-resistance Zone, comprising a thermionically emissive cathode, an anode heated by said discharge, means connected across said anode and cathode for supplying thereto a substantially constant direct current at a voltage substantially proportional to the resistance of the discharge, cathode-heating means for supplying electric heating power to heat said cathode to electronemitting temperature, a saturable reactor connected to said cathode effectively in series with said cathode-heating means, means connected across said anode and cathode for deriving a voltage proportional to the voltage across said discharge, and a direct current amplifier connected to respond to the so-derived voltage and to provide to said saturable reactor a control current varying with the so-derived voltage from a maximum producing substantial saturation of said reactor to a minimum at cutoff of said amplifier.
  • Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma comprising a thermionically emissive, fila mentary cathode, a bombardment-heated anode, a constant-current DC.
  • Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma comprising a thermionically emissive cathode, a bombardment-heated anode, a constant-current DC. power supply connected between said cathode and said anode for supplying a substantially constant direct current to the discharge, means for automatically controlling the temperature of said cathode to keep the average voltage between said cathode and said anode substantially constant, and means for automatically shorting said power supply upon a decrease of the voltage between said cathode and said anode to a low value relative to said average voltage.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Plasma & Fusion (AREA)
  • Electromagnetism (AREA)
  • Power Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Automation & Control Theory (AREA)
  • Discharge Heating (AREA)
  • Plasma Technology (AREA)
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US769927A 1958-10-27 1958-10-27 Method and apparatus for controlling electrical discharges Expired - Lifetime US3078388A (en)

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Application Number Priority Date Filing Date Title
LU37811D LU37811A1 (de) 1958-10-27
NL244036D NL244036A (de) 1958-10-27
US769927A US3078388A (en) 1958-10-27 1958-10-27 Method and apparatus for controlling electrical discharges
GB33608/59A GB929831A (en) 1958-10-27 1959-10-05 Improvements in or relating to apparatus for controlling electrical discharges
FR807725A FR1239527A (fr) 1958-10-27 1959-10-16 Procédé et appareil pour contrôler des décharges électriques
CH7960059A CH386020A (de) 1958-10-27 1959-10-19 Verfahren und Vorrichtung zur Regelung des Spannungsabfalles über einer Gleichstrom-Gasentladung
DEST15697A DE1208428B (de) 1958-10-27 1959-10-19 Verfahren und Vorrichtung zur Regelung einer elektrischen Entladung

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US769927A US3078388A (en) 1958-10-27 1958-10-27 Method and apparatus for controlling electrical discharges

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DE (1) DE1208428B (de)
FR (1) FR1239527A (de)
GB (1) GB929831A (de)
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NL (1) NL244036A (de)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3346769A (en) * 1965-10-07 1967-10-10 Nat Res Corp Orbiting vacuum pump power supply with a filament current regulator
US3413517A (en) * 1967-01-13 1968-11-26 Ibm Filament current control by a superposed dithering voltage
US3429501A (en) * 1965-08-30 1969-02-25 Bendix Corp Ion pump
US3442252A (en) * 1965-07-22 1969-05-06 Varian Associates High voltage d.c. converter cathode supply circuit having means for controlling the voltage to the cathode
US4267487A (en) * 1980-04-02 1981-05-12 Rca Corporation Regulated filament supply for high-power tubes
US6476340B1 (en) 1999-04-14 2002-11-05 The Boc Group, Inc. Electron beam gun with grounded shield to prevent arc-down and gas bleed to protect the filament

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2159767A (en) * 1935-08-19 1939-05-23 Telefunken Gmbh Electron discharge device
US2310286A (en) * 1941-06-25 1943-02-09 Rca Corp Voltage regulating system
US2408091A (en) * 1944-05-19 1946-09-24 Cons Eng Corp Electrical regulating system
US2850676A (en) * 1954-11-05 1958-09-02 Hewlett Packard Co Regulated filament supply

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2554902A (en) * 1948-03-25 1951-05-29 Nat Res Corp Thermionic discharge device control
US2792500A (en) * 1954-02-26 1957-05-14 Phillips Petroleum Co Ion source

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2159767A (en) * 1935-08-19 1939-05-23 Telefunken Gmbh Electron discharge device
US2310286A (en) * 1941-06-25 1943-02-09 Rca Corp Voltage regulating system
US2408091A (en) * 1944-05-19 1946-09-24 Cons Eng Corp Electrical regulating system
US2850676A (en) * 1954-11-05 1958-09-02 Hewlett Packard Co Regulated filament supply

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3442252A (en) * 1965-07-22 1969-05-06 Varian Associates High voltage d.c. converter cathode supply circuit having means for controlling the voltage to the cathode
US3429501A (en) * 1965-08-30 1969-02-25 Bendix Corp Ion pump
US3346769A (en) * 1965-10-07 1967-10-10 Nat Res Corp Orbiting vacuum pump power supply with a filament current regulator
US3413517A (en) * 1967-01-13 1968-11-26 Ibm Filament current control by a superposed dithering voltage
US4267487A (en) * 1980-04-02 1981-05-12 Rca Corporation Regulated filament supply for high-power tubes
US6476340B1 (en) 1999-04-14 2002-11-05 The Boc Group, Inc. Electron beam gun with grounded shield to prevent arc-down and gas bleed to protect the filament

Also Published As

Publication number Publication date
GB929831A (en) 1963-06-26
CH386020A (de) 1964-12-31
DE1208428B (de) 1966-01-05
FR1239527A (fr) 1960-08-26
LU37811A1 (de)
NL244036A (de)

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