US2409717A - Field emission arc discharge tube - Google Patents

Description

Oct. 22, .1946'.v c.-M. sLAcK ETAL y 2,409,717

I FIELD EMISSION ARC DISCHARGE TUBE Filed sept. 2s, 194;'v

INVENTORS C2 M S2/76K A?. PFE/F'FE B c'. E; twwf/L EY ATTORNEY Patented Oct. 22, 1946 FIELD EMISSION ARC DISCHARGE TUBE Charles M. Slack, Glen Ridge, and Andrew Pfeiifer and Clarence E. Dawley, Bloomfield, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 7 Claims.

The `present invention relates to discharge devices and more particularly to such devices wherein operation is initiated by a field emission arc as an electron source, and constitutes an improvement over the structure such as shown in copending application, Serial No. 412,566, led Sept. 27, 1941, in the name of -Charles M Slack, 4Louis F. Ehrke and Clarence E. Dawle and assigned to the same assignee as the present invention.

In discharge devices of this type and as pointed out in the above-identiiied copending application, held emission of electrons occurs due to the high potential gradient at the cathode. The resulting electrostatic field in turn causes a minute arc to forni between the starting electrode and the cathode, apparently due to evolved metallic particles and the positive ion bombardment, resulting from ionization of the evolved metal vapor caused by the arc, forming a cathode spot and a reduction in the space charge. This causes the impedance oi the device to be so reduced that an electron discharge is almost instantaneously initiated between the cathode and the anode of the device.

We have found that for extended useful life and reliability of operation of discharge devices of this type certain considerations become of paramount importance. For example, it is desirable that loss of theevolved metallic particles from one of the electrodes in no way adects the characteristics of the deviceand that a uniform electrode spacing be maintained throughout life.

It is accordingly an object of the present invention to provide a discharge device operating by iield emission of electrons and wherein the same has a` relatively long commercially useful liie in comparison with such devices previously known to the art.

Another object of the present invention is the provision of a discharge device operating by eld emission of electrons and wherein the operating characteristics of the device remain substantially constant during` a relatively long commercially useful life.

Another object of the present invention is the provision of a discharge device operating by iield emission of electrons and wherein the electrode spacing is maintained substantially uniform throughout a relatively long commercially useful life of the device.

A still further object of the present invention is the provision of a discharge device operating by" field emission of electronsl and wherein the electrodes ar'ri'gidly supportedso" asmto prevent 26, 1942, Serial No. 459,774

the electrostatic pull of the voltage from varying the electrode spacing and causing erratic performance.

Still iur-ther objects of the present invention 5 will become obvious to those skilled in the art by reference to the accompanying drawing wherein:

Fig. l is a perspective View partly in section and with parts thereof broken away to better illustrate the device, and schematically shown in an `operating circuit;

Fig. 2 is a sectional view taken on the line II-II of Fig. 1;

Fig. 3 is a sectional View similar to Fig. 2 and showing a slight modification which the electrode structure may take;

Fig. 4 is a sectiona1 View similar to Fig. 2 and showing another modification which the electtrode structure may take;

Fig. 5 is a sectional View similar to Fig. 2 and showing a still further modification which the electrode structure may take, and

Fig. 6 is a side fragmentary view showing some of the electrodes after a period of operation oi the device.

Referring now to the drawing in detail, the discharge device 4 of the present invention as shown in Fig. 1 comprises a vitreous envelope -5 provided with a pair of electrodes 6 and 'l shown as of rectangular pieces of metal of substantially uniform cross-section, such as copper, gold, silver, uranium, or the like, and riveted or otherwise aixe'd to very rigid leading-in conductors 8 and 9 so that the free ends of the rectangular elecv trodes 6 and 'i are disposed in a veritcal plane parallel to each other with a small spacing of the order of a .0l in. therebetween depending `upon the voltage to be applied.

y" The leading-in conductors 8 and I9 are each in 40 turn welded, soldered, or otherwise secured to metallic terminals l0 and Il. These terminals may be of tantalum, molybdenum, or the like, but preferably are of copper and as shown, these terminals l0 and li are provided with an enlarged anged portion, the periphery of which is feather-edged and forms an hermetical seal with an annular projection l2 provided at the bottom of the envelope.

An anode electrode I3 of a refractory metal having a higher melting point than that of the electrodes t and l, such as tungsten, molybdenum or the like, is also provided interiorly of the envelope 5. As shown in Fig. 1, this anode is of substantially U-shaped configuration and supportedin an inverted postiori'bya rigid leadingin conductor I4 so that the sides of the anode I3 are equi-distant on both sides of the electrodes 6 and 'I and overlap the gap between the latter, as shown in Fig. 2, which allows an increase in the thickness of the electrodes 6 and 'I without reducing the accessiblity of the current ow tothe anode. The ano-de leading-in conductor I4 is secured to the top of the envelope 5 by a terminal I5 hermetically sealed to a projection I6 in the same manner as the leadingin conductors 8 and 9.

After assembly of the device, it is exhausted to a high degree of evacuation through an exhaust tip II with the various metallic parts being thoroughly degasified as is customary in the art so that satisfactory operation is entirely independent of vapor or gas present in the device. If desired, however, a metal vapor such as mercury may be introduced into the envelope 5 prior to tipping-off at Il which also results in satisfactory operation so long as its pressure is such that the spacing between the electrodes is less (and preferably many times less) than the mean free path of electrons in the vapor so that ignition and operation of the device is entirely independent of the mercury vapor present.

As illustrated in Fig. l, the electrode 6 constitutes the starting electrode while the electrode l is the cathode. In order to initiate a discharge between the Starting electrode 6 and the cathode 1, a high tension transformer I8 is shown, the primary winding I9 of which may be oonnected to the customary commercial source of supply of 115-230 volts. The secondary winding 20 has one end grounded at 22 while the opposite end is connected to the terminal Iil through one blade of a reversing switch 2I and hence to the starting electrode 6. Inasmuch as the device of the present invention is particularly operable as a control tube, a source of supply such as a transformer 23 is shown which may likewise have its primary winding 24 connected to the customary commercial source of 115-220 volts.

The secondary winding 25 of this transformer 23 has one end thereof connected to ground at 25 and to the terminal Il through the remaining blade of the reversing switch 2|, thus connecting to the cathode l, while the remaining end of the secondary winding and a connection from the anode terminal I5 extends to the load, thus positioning the discharge device between the source and the load in order that energization of the latter may be controlled by the device 4.

When it is desired to operate the discharge device 4, a high voltage impulse is applied between the starting electrode 6 and the cathode 'i from the transformer i8, such as by momentarily energizing the primary winding I9. This causes eld emission of electrons from the cathode 'I due to the high potential gradient atthe cathode with the resulting electrostatic field causing a minute arc to form between the starting electrode 6 and cathode 1 because of evolved metal vapor. The positive ion bombardment resulting from ionization of the evolved metal particles caused by the arc forms a cathode spot on the electrode 1, and the impedance of the device is so reduced that an electron discharge is almost instantaneously initiated or transferred between the closely spaced cathode I and anode I3, resulting in the load being energized from the high tension transformer 23- Contrary to what might be expected, the net loss of evolved material does not always occur from the cathode 'I but, as we have found, from the starting electrode 6 and, in fact, the cathode 'I may accumulate material so that after considerable operation the appearance of the electrodes 6 and l may be somewhat as shown in Fig. 6 with the starting electrode 6 eaten away and a deposit built up on the cathode l. For this reason it is desirable to reverse the connections to the electrodes 6 and 'I so that the transfer of material will be in the opposite direction, Thus since the electrodes 6 and 'I are of uniform crosssection and of similar material, it is a simple matter to operate rst one and then the other of the electrodes 6 and 'I as cathode, by changing the reversing switch 2| from its full line position to that shown in dotted lines in Fig. 1, and so extend the useful life of the device.

It is also to be noted that while most of the electrode material is transferred from one of the electrodes 6 and l to the other, there is some of this material vaporized away. This accumulates on the anode I3 and in the absence of provisions to the contrary, since in most instances it is desirable to have the anode fairly closely spaced, a further reduction in the spacing would result from the accumulated material on the anode, thus changing the operating characteristics of the tube. For this reason it is preferable to employ a refractory material which operates continuously or intermittently at a temperature above the melting point of the material of which the electrodes 6 and 'I are composed. Moreover, by the anode I3 overlapping the gap between the electrodes 6 and allowance is thereby made for an increase in thickness of the electrodes 6 and l without reducing the accessibility of the current flow to the anode.

It will thus be seen that although the spacing between the electrodes is rather minute, their i mounting is not as critical as might be supposed tween the electrodes at all points.

since any irregularity in the spacing will be eliminated after operation of the tube. Also throughout tube life a uniform spacing is maintained be- This follows because any point that tends to become nearer than another will cause the discharge to transfei to such point with resultant vaporization of material and the discharge thus moves around constantly maintaining an equalized spacing.

For exceptionally long tube life electrodes in parallel may be employed such as shown in Figs. 4 and 5. In these gures the starting electrode 6 and cathode 'I may be formed of a plurality of parallel metallic plates and the anode I3 formed in the same manner as shown in Fig. l except that it is provided with a plurality of tines disposed between the parallel plates of the starting electrode 6 and cathode 'I and also overlapping the space therebetween.

A still further requirement for stable operation is rigidity of the electrode structure for if the electrode supports, and particularly the supports for the starting electrode 6 and cathode 1, are not sufficiently rigid, the electrostatic pull of the voltage tends to affect the electrode spacing and produce erratic performance. Although it is not possible to state the exact amount of rigidity required for all applications, it is believed preferable to have rigidity greater than that obtainable with mil diameter molybdenum three inches long. Also to increase rigidity the leading-in and supporting conductors 8, 9, and I4 may be formed as channels such as shown in Fig. 3 with the electrodes and 1 welded or riveted thereto in the same manner as when rods are employed.

It thus becomes obvious to those skilled in the art that a discharge device operable by field emission of electrons is herein provided in which a substantially uniform electrode spacing and hence constant operating characteristics are maintained throughout a relatively long commercially useful life. Moreover, the device is of exceptionally rigid construction so that electrostatic pull of the voltage has no material affect on the device, which still further contributes to the elimination of erratic perfomance during operation.

Although several embodiments of the present invention have been shown and described, it is to be understood that still further modifications of the same may be made without departing from the spirit and scope of the appended claims.

We claim:

l. A discharge device wherein operation is initiated by field emission of electrons comprising a pair of oppositely disposed spaced electrodes between which a. field emission arc discharge occurs upon the application of a potential thereto, and an anode positioned on each side of said oppositely disposed electrodes and overlapping the space therebetween for supporting an electron discharge with one of the electrodes of said pair immediately following initiation of the eld emission arc discharge between said pair of electrodes.

2. A discharge device wherein operation is initiated by eld emission of electrons comprising a pair of oppositely disposed spaced metallic electrodes between which a field emission arc discharge accompanied by evolved metallic particles occurs upon the application of a potential thereto, and a refractory metal anode positioned on each side of said oppositely disposed electrodes and overlapping the space therebetween for supporting an electron discharge with one of the electrodes of said pair immediately following initiation of the eld emission arc discharge between said pair of electrodes and said anode being operable at a temperature above the melting point of the metal of said pair of electrodes to prevent accumulation of evolved metallic particles from the latter on said anode.

3. A discharge device wherein operation is initiated by field emission of electrons comprising a pair of oppositely disposed spaced metallic electrodes, means for applying a potential to said electrodes of sufficient magnitude to cause the initiation of a eld emission arc discharge therebetween with attendant evolution of metallic particles from one of said electrodes which build up on the adjacent surface of the other of said electrodes, an anode positioned on each side of said oppositely disposed electrodes and overlapping the space therebetween for supporting an electron discharge with one of the electrodes of said pair as cathode immediately following initiation of the eld emission arc discharge between said pair of electrodes, and means for reversing current flow between said pair of electrodes to cause the evolved metallic particles accumulated on one of the electrodes of said pair to be re-transferred to the other of said electrodes from which such particles originated.

4. A discharge device wherein operation is initiated by field emission of electrons comprising a pair of oppositely disposed spaced metallic electrodes, means for applying a potential to said electrodes of sufficient magnitude to cause the initiation of a field emission arc discharge therebetween with attendant evolution of metallic particles from one of said electrodes which build up on the adjacent surface of the other of said electrodes, a refractory metal anode positioned on each side of said oppositely disposed electrodes and overlapping the space therebetween for supporting an electron discharge with one of the electrodes of said pair immediately following the eld emission arc discharge between said pair of electrodes and said anode being operable at a temperature above the melting point of the metal of said pair of electrodes to prevent accumulation of evolved metallic particles from the latter on said anode, and means for reversing current flow between said pair of electrodes to cause the evolved metallic particles accumulated on one of the electrodes of said pair to be returned to the other of said electrodes from which such particles originated.

5. A discharge device wherein operation is initiated by eld emission of electrons comprising a pair of oppositely disposed spaced metallic electrodes each consisting of a plurality of parallel plates and between which electrodes a i'leld emission arc discharge occurs upon the application of a potential thereto, and an anode provided with a plurality of portions and positioned to dispose a portion thereof on each side of the parallel plates of said pair of electrodes and overlapping the space therebetween for supporting an electron discharge with one of the electrodes of said pair immediately following initiation of the eld emission arc discharge between said pair of electrodes.

6. A discharge device wherein operation is initiated by eld emission of electrons comprising a pair of electrodes including metallic plates disposed in a common plane with their edges in spaced parallel relation to each other and between which a field emission arc discharge occurs upon the application of a potential thereto, and a substantially U-shaped refractory metal anode positioned to dispose its parallel portions substantially equi-distant on each side of said pair of metallic plate electrodes and being of a width greater than the spacing between said pair of electrodes so as to overlap the latter and operable to support an electron discharge with one of the electrodes of said pair immediately following initiation of the field emission arc discharge between said pair of electrodes.

7. A discharge device operable by field emission of electrons comprising a pair of oppositely disposed spaced metallic electrodes and between which a field emission arc discharge occurs upon the application of a potentia1 thereto, leadingin and supporting conductors for said electrodes having a rigidity greater than that attainable with molybdenum of mil diameter to prevent the electrostatic pull of the voltage from changing the electrode spacing and causing erratic performance of the device during operation, and a refractory metal anode supported by a leading-in conductor of equal rigidity as the leading-in conductors for said pair of electrodes and provided. with a portion thereof on each side of said pair of electrodes which portions overlap the spacing therebetween and said anode being operable to support an electron discharge with one of the electrodes of said pair immediately following the initiation of the field emission arc discharge between said pair of electrodes.

CHARLES M. SLACK. ANDREW PFEIFFER. CLARENCE E. DAWLEY.

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