US2616040A - Electrical system for prolonging life of coated cathodes - Google Patents

Description

Oct. 28, 1952 c. w. HANSELL ELECTRICAL SYSTEM FOR PROLONGING LIFE OF COATED CATHODES Filed Sept. 24, 1948 2 SHEETS-SHEET 1 la; our ur rm\ +4000V.

INVENTOR 4 CLARENCE W. HANSELL AAAlA NEY Patented Oct. 28, 1952 ELECTRICAL SYSTEM FOR PROLONGING LIFE or COATED oA'rHoDEs Clarence W. Hansell, :Port Jefferson, N. Y.,-assignor to Radio Corporation oi. America, a corporation of Delaware Application September 24, 1948, Serial No. 51,035 claims. (01. 250-47") The present invention relates to a means for prolonging the life of electron discharge devices and more particularly to an arrangement of electrodes in association with potential sources for preventing appreciable evaporation of the emis- 5 proved electron discharge device having a coated sive coating on coated cathodes employed in such cathode that will have a relatively long life aldevices and subjected to conditions of operation though current is drawn therefrom only interrequiring the cathode to be instantaneously emismittently. sive but calling only for an intermittent current Another principal object is to provide operating output therefrom. conditions for the cathode such that it may be Some electron discharge devices employ the operated at higher temperatures and higher elec- Wehnelt or oxide type of cathode which'comtron emission densities without reduction in its prises a layer of mixed barium and strontium life expectancy." oxides held on a metal surface. High electron Another object is to preserve the emission charemission density is obtained at moderately high acteristics of a coated cathode under intermittent temperatures because the oxide in an activated conditionsof use. Wehnelt cathode does not have chemically An additional J' iS to p g the. life of balanced 'or-stoichiom'etric proportions ofoxygen electron discharge devices havin o ed at and metal but has an excess of bari m in the odes, sub ecte'd to intermittent operation involvoxide. The excess barium converts the oxide ing Standby periods h e Cathode a a d into a semi-conductor of a type which emits elec- 'a hi temperature a trons, when heated, relatively easily, and per- "Another obje i to p e a System P thaps causes to appear on the surface a thin bound ting a coated Cathode yp of electron discharge layer of barium which also emits electrons reladevice to be employed vantageously in pulse tively easily. However; under certain conditions emission w at relatively high o t es, hi h this emission density does'not remain constant' C t de sities and hi h D but is subject to large variations depending upon Another Object is to make it possible to operate the manner in whi h th 1 t i i i oxide coated cathodes at larger electron emission utilized. For example, it has been found that densities- I the activation or electron emission of a cathode A further Object is to provide a System r 0D- tends to diminish 'i k1y 'f 11o 1 maximum crating 1a coated .catho'de at electron emission activation nd then'gradually when t cathode temperatures without deterioration in its activais' kept'hot'and no emission current is drawn from tion during iiiteimitteniietandby p s. it. Thus, in applications of electron discharge Another Object is tQlpTOVilii'e System for p o devicesto usesin' radiotelegraph transmitters, i 'i life of'an electron discharge d v ce pulse communication systems electrical having a coated cathode, under conditions of use puters, and even in continuously operating A, C. ing high voltage and high power and oscillators and amplifiers where the operating periodic interruptions in t e drawing of useful frequency is relatively low, the intermittent charcurrent therefrom! acter of the operation causes a reduction in the 40 A further object is to provide a System for p activation of the th d venting the evaporation of the coating material This reduction in the electron emission density from Coated cathodes during conditions of use and phenomena, accompanying 1 of ba iu in which the cathode is maintained at an electron from the cathode, in intermittent operations has emitting temperature and the voltage difference heretofore discouraged the use of tubes of the between the cathode and anode is periodically decoated cathode type in high voltage, high power cayed to substantially zero. applications where intermittent use is involved, Another object is to pr v evaporation of such as for transmitting purposes, and has emissive materials from a hot coated cathode durlimited the use of this type of cathode to low ing intermittent periods of decay 'of the electrovoltage, low power tubes ordinarily employed in static field between the cathode and an associated radio receivers, or-in relatively low power transanode. mitters. 1 A still further object'is to provide a system for It is therefore a'principalobject of the invenionizing theparticles of emissive material leavtion to 'provide an improved electron discharge ingthe coating of a'hot coated cathode during" vic havin a coated cathode subjected to such intermittent periods when the potential differ 2 operating conditions that it will stand up well under conditions of use involving high voltage and high power;

A further principal object is to provide an im ence between the cathode and an associated anode .is not sufficient to produce an ionizing electron flow therebetween.

Another object is to provide an electrostatic field adjacent the surface of a coated cathode during intermitent periods when the cathode is at an emitting temperature, but no normal useful current is drawn therefrom for ionizing particles of emitting material leaving the cathode to cause such particles to return to the cathode.

High mercury vapor pressure very greatly reduces the rate of diffusion of barium molecules away from the cathode, while electron current is not flowing, so that they cannot move far in a given time, and also increases the probability that the barium molecules will be ionized and returned to the cathode when electron 'current'is flowing. Increased ionization of barium vapor, due to the presence of mercury vapor, is attributed to lengthening of electron paths in the electrical discharge by' non-ionizing encounters of electrons with mercury Vapor molecules.

Where great care has been taken to prevent operation at lower vapor pressures, and various other precautions have been taken, it has been observed that glass envelopes of mercury vapor rectifiers have remained almost free of cathode coatin material.

On the other hand maintainin high mercury vapor pressures tends to reduce the permissible operating voltages by lowering the strength of the vapor.

It is therefore another important object of the invention "to provide .a discharge device having an dielectric indirectly heated cathode and operable at relatively low vapor pressure and at relatively high voltages.

Further objectsand advantages of the inventiori will become clear "as the description proceeds.

.Referringto the drawing:

Figure 1 shows in sectional :elevation an evac uated electron discharge device incorporating the invention;

Figure '2 shows a circuit arrangement employed in association with the electron discharge device of Figure 1 to. form a system for providing a long life device under intermittent operation; and

.Figure3 shows an elevation partly in section of a mercury rectifier incorporating the invention.

.A Wehnelt or oxide cathode comprises a metal supporting base, commonly of nickel or metal alloy, over which has been applied a thin layer consisting of a finely divided'mixture of the oxides of strontium and barium. The oxide coating, so long as it is made of relatively pure oxides, is a poor electron emitter. It is made intoa'good emitter by giving it an excess of barium. One method to obtain an initial moderate excess of barium is to raise the oxide to a relatively very high temperature while the vacuum tube containing the cathode is still connected with a vacuum pump. The high'temperature breaks down a little of the barium oxide into barium and oxygen. The Qxygcnescapes faster than the barium and is removed by the vacuum'jpump' as a gas, leaving an excess of barium .in the cathode coat- An alternative process of initial activation omits raising the cathode temperature far above the normal operating temperature and instead uses a temporary gas discharge between the oathode and other electrodes so as to force an electrical current through theoxide coating. This alsogives an initial activation by breaking .down

some of the oxide electrolytically and allowing oxygen to escape, leaving an excess of barium.

Initial activation is sometimes obtained by using a base metal of such a nature that it will react with the oxide to some extent to remove oxygen, leaving an excess of barium.

If, after the initia1 activation, electron emission current is drawn from the cathode and the emission current density at the cathode is increased to a high enough value by applying sufficient potential between the cathode and other electrodes, it is found that an additional or final activation takes place with temporary or transient release or more oxygen, accompanied by an increase in electron emission. Sometimes this final activating step is performed while the tube is on the pump and the released oxygen is pumped out, after which the tube is sealed off.

If now a finished tube has its cathode heated, but no electron emissionburrent drawn from it, it will be found that barium evaporates from the cathode which is deposited on other tube elements, including the outer envelope. The rate of evaporation is not constant but starts at a maximum :rate which then diminishes with time toward nearly zero, or a very low value. The loss of barium due to evaporation lowers the cathode activation and diminishes the available electron emission.

Suppose now, after the activation has diminished, that potentials are applied to the other electrodes and electron emission current is drawn from the cathode. It then is found again that oxygen is evolved transiently which leaves in the cathode an added excess of'barium to more or less balance what has been lost due to barium evaporation.

The transient evolution of oxygen has been attributed to electrolytic separation of barium and oxygen due to electrical current flowing through the oxide coating. The accumulation of excess barium, as the electrolytic separation proceeds, lowers the volume resistivity of the oxide and electron conduction through the-oxide then shunts or short circuits the electrolytic path sufficiently to stop or very greatly reduce the electrolytic conduction. It therefore tends to stop evolution of oxygen with passag of time.

It will be noted that maintainin either zero emission current, or nearly emission limit-ed current, continuously soon results in a steady state condition such that neither oxygen nor barium continues .to be lost from the cathode in substantial amounts. On the other .handa change from one condition to the other is followed by loss of a quantity of either barium or oxygen. Thus, when the rate at which the electron emission current is started and stopped is not too rapid, the life of the cathode can be measured by the number of times .the electron emission current is started and stopped. 7

The oxygen and barium lost alternately from the cathode recombine on other internal tube surfaces and often are apparent as a dark deposit built up on the inside of glass tube envelopes. Tubes which finally fail in a normal manner, due to inadequate'cathode emission, often have been found to have lost practically all the barium-oxide from the cathode coating.

Drawing out a continuous electron emission current from the cathodes, especially if the current is made so large as to be nearly emission limited, tends to stop all loss of cathode material and to provide a great increase in tube life. At the same .time the other -electrodes, particularly the control mimosa:

electrodes, can" be kept relatively cleanand unactivated so as to have -low secondary;electron emission. The secondaryemission'is acommon cause of operating'difiiculty and "premature failures, especially in tubes 'oi thelarger-sizes',opere one megacycleper second, are sufiicient to keep,-

the cathode protected in high vacuum tubes. In gas-filled tubes the lowest permissible frequency depends largelyupon the gas density and pres sure- 'In the case of mercury vapor. 'rectifiers, the cathodes may be protected down to operating frequencies below 60 cycles, but only if great care is taken never to operate the rectifiers unless the mercury vapor pressure is' relatively high; corre-t sponding to operating temperatures above the.

minimum specified by, most manufacturers in the United States.

Referring now in more detail to the drawing,

there is shown in Figure 1 an evacuated electron discharge device capable of operation-at av relatively high potential: andpower level in a chain of amplifiers in a radio telegraph transmitter, in which the current in the device is keyed on and off with the signals. This is one kind of use in which ordinary oxide cathode tubes havefailed;

in the past. l

The devicev comprises an envelope formed of a plurality of tubular members [0, H, :2, I3, l4, I5 and I5 and containing an electrode assembly including an indirectly heated cathode l1, ener-' gized by heater I8, an anode I9, ascreen grid 20 and a control grid 2l-. Cooling fins 22 are provided for the anode.

According to the invention, there is provided between the cathode I1 and control grid 2!, an additional grid 23. The function of this lastnamed grid is to draw current from the cathode during the intervals when operation of the device requires no anode current. To enable grid 23to current'powe'r-source which may have a pressure of -volts'to'give the cathode i! an initialtem-- pe'rature high enough to provide electron emis-- sio'n} Thegrid 23 is connected to a sourceof posi-' tivedirect potential which mayhave a voltage of 50 volts. This voltage issufiicient to draw substantial electron emission current'from the cathode when no direct or average anode current isQfio'wing When anode current does flowthecurrent-"flow to grid 23 will diminish and may reach'zero' or even reverse if the grid itself has thermionic or secondary emission. During the' flow of anode current the effect of theg'rid 23 upon the anode current and upon cathode emis- ,-sion and life. may be negligible; However, during intermittent periodswhen no anode current is flowing, the effect of grid23 is to continue the drawing of electrons from the cathode.

Preferably, though not necessarily, the grid 23 and the: heater l8 are supplied from the same source of: power, which as shown, is a +50 volts:

Thus, by means of a suitable resistance arrangement in the power connections, the heater,

source.

current may be caused to vary to keep a nearly constant cathode temperature, even though the, electron current to grid 23 :and the consequent.

heating of this grid should vary over a large range in response to telegraph keying.

Thus, when the telegraph key or relay controlling the transmitter is open with consequent absence of radio frequency powerin the cir--' cuits and a zero anode current due to a negative control grid bias potential, the arrangement of Figure 2 provides for a combination of cathode heater power and of power dissipation at grid 23 dueto electron emission current resulting in a desirable operating temperature. At the same time this arrangement will prevent or reduce the loss of cathode material through evaporation due to the continuous fiow'of electrons'from the cathodeto the grid 23.

In addition, when the telegraph key is closed to cause radio frequency power to appear in the" amplifier circuit and D. C. current to flow from the anode, the arrangement according to the inperform this function, it is connected to a suitable sourceof voltage that is positive with respect to the cathode. v

Figure Zshows a circuit in which the device illustrated in Figure 1 may be used. It comp-rises an electron tube having an oxide cathode l1, cathode heater l8, anode I9, and grids 2G, 2|

and 23 interposed in succession between the anode ground have also been added, to permit carry-. ing out the present invention. The electrondis-w charge device and the circuit here shown op-- erate in a manner similar to a class -C radi frevention is also advantageous. In this situation the direct anode current will be made up of electrons'which are pulled through grid 23 during periods of positive or less negative direct poten-- tial on the control grid 2|. The average current to grid 23 will therefore be diminished. ,As

a result, the direct potential of this grid as well as the heating current, normally tend to rise wa higher value while the telegraph key is closed.

This rise in direct potential of grid 23 and cathode heating current however, is preferably adjustedin accordance with the invention by adquency'amplifier at a radio frequency assumed to be above one megacycle, Thecathode heater p,

Wis supplied with heater current from a direct justm'ent' of resistance values and power source potentials, so that there is substantially no tendency for the cathode temperature to change due to. intermittent operation of the amplifier. To this effect the movable arm 44 is adjustable to any desired position with respect to resistor 45 shown inFigure 2. Therefore, according to this aspect of the invention the electron emission can be held at all times at an optimum value, even though the operation is intermittent or the radio frequency power levels may change. 1

Since the grid 23 is required to dissipate considerable power, while no anode current is flow-' ing it may be raised to a relatively high temperature and may itself emit electrons. This generally does no harm so far as operation of the tube is concerned since emission from the grid 23 simply adds to the total emission available to with. strips or sheets of metal set in radial planeswith respect to the axis of the tube, to provide :for more effective cooling.

It is well. to note also that the impedance and resistance between grid 23 and cathode I! should be kept low enough so that reversed grid currents can, not result in excessive potentials :on grid 23.

Advantages similar to those secured in telergraph operations referred to above will also be secured when the invention is used in applications characterized by continuous operation, as in some industrial heating applications where the frequency at which the amplifier shown in Figures 1 andv 2 operates, is of a relatively low value, for example, from to 100 kilocycles per second. At such frequencies, even when the operation is continuous, the time periods between pulses of anode current will be long enough to permit escape of barium from oxide. cathodes of ordinary tubes. However, if the type of tube and circuit of the invention shown in Figures 1 and 2. are: employed, and the electron emission current is 'kepthigh enough while no anode current is flowing, there will be substantially no escape of cathode material. In this application also, therefore, the invention makes it possible to utilize oxide cathodes Where they have never beenused successfully before.

Anadditional important advantage of the in vention is that it makes possible the operation of oxide cathodes at higher temperatures and therefore at higher electron emission densities than heretofore possible, due to continuous inhibition of loss of cathod material. There is thus obtainable, according to the invention, greater useful electron currents per unit of power used to heat the cathode. In addition, it permit's' a' broadening of the frequency band of amplifiers such as may be employed for example in television broadcasting transmitters, by increasing the ratio of electron current to dielectric capacity current in the tube and circuits.

A further advantage of the invention is that it makes possible a wider choice of materials for activating the cathode. It permits use of more eifective activators than barium, such as sodium, potassium, rubidium and caesium. These better activators are too volatile to stay on and in the cathode coatings for any substantial time while large density electron currents are not flowing, and in which electron emission current cannot be drawn out at all times. However, when sufficient electron current is constantly drawn out as provided for by the invention, these better activators can be used.

While the foregoing description concerns an embodiment of the invention in an evacuated space discharge device, the invention may also be used advantageously in gas discharge tubes, such as a mercury vapor rectifier shown in Figure 3. This tube comprises an envelope 24 within which is disposed an electrode assembly comprising a cathode 25, heater 26, a keep alive electrode. '21, a shielding electrode 28 and anode 2.9.

The device illustrated in this figure is provided with a pool of mercury .30.

The cathode '25 is a 'unipotential electrodeequipped with vanes 3|. to increase its surface area. The surface area including the vanesis coated with barium and strontium oxides in the usual manner.

The keep-alive electrode 2! and shielding electrode 28 comprise a double walled electrode, serving the double function of keep-alive electrode and of a cathode heat shield and heater. Openings 32, 33 in the double Walls of the electrodes 21, 28 permit electron current to be drawn out in a gaseous discharge to the anode 29. It will be noted that the arrangement of the anode 29 and disc support 34 of the electrode 28 is such that. all possible paths through which barium vapor might escape to the relatively cool glass envelope for condensation thereon, are bafiled.

The cathode 25 is connected to a source of low voltage D. C. current 35 by leads 36, 3'7 in series with resistor 38. The cathode heater Z6 is also connected to the direct current source 35 by a lead 39 and an adjustable portion of resistor 42. The keep-alive electrode 21 is connected to the heater lead 39 by lead M and adjustable resistor 42. The shielding electrode 28 is connected to cathode lead 36. The current to be rectified is fed to the cathode 25 by way of lead 36 and resistor 38. The rectified output is taken from the anode connection 43 at the top of the device.

In operation of the tube shown in Figure 3, the cathode 25 is provided with initial heating energy by passing an electrical current of suitable value through the cathode heater 26. When electron emission from the cathode 25 is available due to its temperature, current" at low potential is drawn to the keep-alive electrode 2?. This results in additional heating of the cathode as a consequence of current flow between the cathode and the keep-alive electrode; However, in accordance with the circuit shown in Figure 3, the resistance ii'automatically serves as a compensating means by causing a drop in heater current with a consequent reduction in its heater power. Therefore, at all times when the cathode is hot, relatively large electron emission current is drawn from it to virtually stop barium evaporation even in the absence of anode current.

While, as shown, the keep alive electrode is sufficiently positive with respect to the cathode, during periods when no anode current is drawn from the device, to draw cathode current and thus prevent evaporation of the cathode material, when anode current is drawn from the device the keep-alive electrode loses its energization in accordance with the invention so that it does not use up electron emission but leaves it available to supply the anode current. This is due to the voltage drop across resistance 38 which makes the cathode potential more positive with respect to the keep-alive electrode and reduces the amount of electron current flowing to the keep-alive electrode.

It willtherefore be seen that the invention broadly concerns a novel tube structure having an electrode performing a novel function, and a nove1 circuit organization making possible the novel function referred to. Furthermore, the invention is helpful in connection with either evacuated or gas discharge devices including gas discharge tubes with other gases than mercury vapor. t permits use of oxide coated cathodes in applications and under conditions in which such cathodes have heretofore been regarded as unusable.

9 By preventing loss of cathode activating material the life expectancy of the cathodes is very greatly increased while, at the same time, internal arcing, instability due to secondary emission and high velocity ion bombardment of the cathode are all eliminated or greatly reduced.

Many departures may be made from the specific embodiments described without going beyond the spirit and scope of the invention as pointed out in the appended claims. For example, means to maintain electron emission current between anode current pulses may be applied to pulsed magnetrons and other types of oscillators used in radar and pulse type communications equipment and in almost all other kinds of thermionic devices utilizing thermionic emission intermittently.

What I claim is:

1. In an electronic system including an electron discharge device having an oxide coated cathode, means for preventing evaporation of the cathode coating, said means comprising a system for drawing emission current from said cathode during active and inactive periods of said device, said system including a grid adjacent said cathode and a direct current voltage supply more positive than said cathode connected to said grid; and a system for maintaining the temperature of the cathode constant during said active and inactive periods, said last named system comprising a heater for said cathode and a connection between said heater and said grid, said connection including a resistance element for predetermined ohmic valve for adjusting the power supply to said heater to a lower magnitude during said inactive periods than during said active periods, said grid drawing emission current during said inactive periods and being heated thereby, said cathode being heated by radiation from said grid and by said lower magnitude power supply to the temperature assumed thereby during said active periods.

2. In combination, an electron discharge device adapted for pulsed operation at a frequency below one-half megacycle, and having an oxide coated cathode and an anode energized by relatively high direct current voltage; means for continuously drawing emission current from said cathode during and between the pulses of said pulsed operation for at least partially preventing evaporation of the cathode coating, said means including an electrode adjacent said cathode, a voltage source, and a connection between said electrode and said voltage source; and means for maintaining the temperature of said cathode constant during and between said pulses, for further at least partially preventing evaporation of said cathode coating, said last named means including a heater for said cathode, and a resistance in said electrode-to-voltage-source connection, said heater being connected to an intermediate portion of said resistance, whereby said electrode dissipates more power and radiates more heat to said cathode during intervals between said pulses than during said pulses, for maintaining the temperature of said cathod during said intervals at the temperature assumed thereof during said pulses.

3. In combination, an electron discharge device adapted for operation at a frequency of less than one-half megacycle and including a cathode having an oxide coating thereon for electron emission and an anode for receiving said emission in pulses at said frequency; means for maintaining the temperature of said cathode constant during a period including a plurality of said cycles, said means including a voltage source, an electrode adjacent said cathode, a connection between said electrode and said voltage source, said connection including a resistance element of predetermined value, a heater for said cathode, said electrode receiving the emission from said cathode during intervals between said pulses and being heated thereby, whereby said cathode is heated. both by said heater and by heat radiated to it by said electrode, to a predetermined temperature, said last named connection being adjustable for increased energization of said heater during said pulses and during reduced current flow in said electrode, whereby said cathode is heated to said predetermined temperature during said pulses.

4. In an electronic system including an electron discharge device having a cathode provided with an oxide coating; means for preventing evaporation of said coating, said means including an auxiliary electrode adjacent said cathode; a heater for said cathode; a circuit arrangement including said electrode, said heater and a source of positive voltage for maintaining the temperature of said cathode constant during pulsed operation below radio frequency of said system; and a circuit arrangement including said electrode, said positive voltage source, an inductance between said source and said electrode and a condenser by-passed to ground between said inductance and said electrode, for drawing emission current from said cathode during operation of said system at a radio frequency below one-half megacycle and between the pulses of said frequency.

5. A system for protecting a coated cathode from coating evaporation comprising means for maintaining said cathode at a desired constant operating temperature during pulsed emissions therefrom at a frequency below one-half megacycle, and for collecting emission from said cathode during intervals when no useful emission is required, said means including a grid adjacent said cathode, a heater for said cathode, a connection between said heater and grid, a voltage source positive with respect to said cathode connected to said grid, and a resistanc in said connection of predetermined ohmic value for heating said cathode to a desired operating temperature that remains constant during said pulsed emissions and during the intervals therebetween.

CLARENCE W. HAN SELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,537,708 Schottky May 12, 1925 1,585,445 Warner May 18, 1926 1,737,224 Dijksterhuis Nov. 26, 1929 1,745,369 Holden Feb. 4,1930 1,936,758 Haffcke Nov. 28, 1933 2,060,506 Knowles Nov. 10, 1936 2,256,177 Stromeyer Sept. 16, 1941 2,292,382 Le Van Aug. 11, 1942

Images