US2103159A - Electric discharge device - Google Patents

Description

Dec. V21, 1937. K. H. KINGDON ELECTRIC DISCHARGE DEVICE Filed Feb. 12, 1936 l (su Inventor: Kenneth H. Kingdon,

H l5 Attonnqg.

the operation of the rectifier.

Patented Dec. 2l, 1937 PATENT OFFICE ELECTRIC DISCHARGE DEVICE Kenneth H. Kingdon, Schenectady, N. Y., assignor to General Electric Company, a. corporation of New York Application February 12, 1936, Serial No. 63,533 2 Claims. (VCI. 2250-275) My invention relates to electric discharge devices adapted to operate in the presence lof an ionizing medium, and more particularly to such devices as are intended for use with extremely l high voltages.

In high voltage rectiflers which utilize a vapor or a gas to supply ionization for eliminating space charge there is' a strong tendency during the non-conductive period of the rectifier for lo quantities of solid material to be thrown 01T the surface of the anode and collected on the envelope walls. This phenomenon, generally knownas sputtering, is due to impingement of residual ions driven at high velocity by the relatively in- 16 tense electric eld created in the neighborhood of the anode immediately after the cessation of the conducting period. 'Ihe degree of such sputtering is roughly proportional to the voltages employed and becomesvery considerable at--voltages m of the order of 80 kilovolts, the walls o f the envelope being noticeably darkened after operation lof an hour or so. This darkening is due to the gradual development of a more or less continuous coating of conducting material on the inner surg face of the envelope. Since in the ordinary oase this coating extends substantially along the entire length of the discharge path, it has distinctly harmful effects on the breakdown resistance and In the rst instance it forms a partial short circuit extending fromthe positive to the negative electrode and tends to` concentrate the voltage stress over a much shorter space than that existing between the electrodes themselves. In actuality the full inverse voltage may be impressed on the relatively short gaps existing between the main electrodes and that portion of the Aenvelope wall which is covered with the conducting deposit. It' will be obvious that the eect of this condition is to de- 40 crease seriously the breakdown strength of the device.

Furthermore, the existence of a large metal surface at iloating potential in close proximity to the discharge path favors the accumulation of electrostatic charges of such polarity as to exercise an inhibiting effect on the normal rectier discharge. The result of this pseudo grid eiect may be to postpone the initial conductivity of the device considerably beyond the proper point.

It is an object of the present invention to prevent the formation of continuous sputtered coatings capable of producing the harmful effects mentioned above. For attaining this end I provide shielding means interposed between the discharge path and the envelope walls for effectively intercepting sputtered particles of material and to subdivide the resultant deposit into a series of spaced and insulated sections.

The novel features which I believe to be characteristic of my invention are set forth with 5 particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with other objects and advantages' thereof, will best be understood by reference to the following specll0 cation taken in connection with the accompanying drawing in which Fig. 1 illustrates an elevation in partial section of a discharge device utilizing a preferred embodiment of my invention; Fig. 2 illustrates in partial section an alternative i5 modication of which my invention is capable; Fig. 3 illustrates a second alternative modification of my invention, and Fig. 4 is a partially sectionalized view showing the application of my invention to a discharge device adapted for use 2o with extremely high voltage circuits.

Referring particularly to Fig. 1, I have shown a two-electrode rectifier comprising a sealed envelope of insulating material enclosing an ionizable medium such as a small quantity of argon 25 0r mercury. The envelope includes a pair of cylindrical sections I and I and is provided at one end with a reentrant stem 2 terminating in a press 3 adapted to be hermetically sealed to an' electrode lead-in conductor. Such a con- 30 ductor 4 is shown as being connected directly to an anode 5 which may comprise a cup-like member of thoroughly degassed nickel or graphite. On that portion of the lead-in conductor between the anode and the press may be provided a plu- 35 rality of metallic disks or ballles 6 adapted to trap random electrons, such as secondary electrons generated from the surface of the anode, and to prevent their deposit on the portions of the stem 2 adjacent the lead-in conductor. In 40 this Way the accumulation of electrostatic charges suilcient to rupture the envelope wall adjacent the lead-in seal is substantially avoided- At the other end of the envelope is mounted a second reentrant stem terminating in a press 3' 45 which supports a number of lead-in conductors 1 adapted to supply heating and load current to a thermionic cathode 8. Although the nature of the cathode is not essential to an understanding of my invention, it will be understood that it 50 may take the form of a hollow cylinder having therein a number of vane surfaces of extended area adapted to be coated with an electron emissive substance, such as an alkaline earth metal. Cathodes of this type are conventionally heated 66 by means of a coaxial lament 9 shown in dotted outline. Such a cathode is fully described, for example, in Patent No. 1,924,318, A. W. Hull, issued August 29, 1933.

It will be noted that the cylindrical envelope sections I and I nt rather closely around the discharge electrodes. It is apparent that if such an envelope were extended along the entire length of the discharge path, any high voltage operation of the tube would very quickly result in the production of a continuous coating of sputtered material extending between the electrodes. As previously explained, such a coating, because of its close proximity to the electrode tips, would seriously weaken the resistance oi.' the tube to undesirable breakdown occurrences.

In order to avoid this danger in an inexpensive and practical manner, I provide at the central portion of the envelope a third cylindrical section II of greater diameter than the end portions. This is in partially telescoped relation with the smaller cylinders I and I but is adapted to be joined to them in such a manner as to provide a continuous completely closed envelope surface. For this purpose the overlapping edges of the telescoped cylinders are connected and maintained in radially spaced relation by means of serpentine portions I2 and I2 having a substantial portion of their length parallel with the main axis of the envelope.

By means of the arrangement described I insure that any deposit of sputtered material shall be broken up into a numberof spaced sections consisting of portions I3 and I3' collected on the inner extremities of the cylinders I and I', and a further segregated portion Il on the center of the cylinder Il. Consequently the eective insulating gap between the electrodes 5 and cannot be less than twice the distance from such electrodes to the inner surface o! the cylinders I and I plus twice the distance from such cylinders to the inner wall or the cylinder II. Furthermore adequate creepage distance between the conducting coatings deposited on the concentric cylinders is provided by the relatively extended dimensions oi.' the serpentine portions I2 and I2'.

In Fig. 2 I have shown an alternative mode of application of my invention In this iigure parts such as the electrodes and lead-in connections are identically numbered with corresponding parts in Fig. 1. In this embodiment the shielding elements take the form of a series of rings 20 of insulating material such as glass or quartz having a circular cross section and spaced axially along the central portion of the enclosing envelope. This latter is provided with an enlarged section 2l to receive the shielding rings and to avoid the possibility of their inner surfaces approaching too closely to the discharge electrodes. With the arrangement illustrated sputtered material particles proceeding linearly from the region of the discharge path toward the envelope wall will necessarily fall either on the inwardly exposed surfaces of the rings 20 or on those portions or the surface of the cylinder 2| not subtended by a projection of the rings. Those por tions of the cylinder 2l which are shielded by the rings 20 will be maintained substantially free of conducting particles as will those portions of the rings themselves which might be visible from outside the envelope. The ultimate result will be that the conducting 'deposit is broken up into a series of non-contiguous rings suiiiciently insulated from one another to reduce substantially the danger of inter-electrode breakdown.r

It is also possible, and lmy invention contemplates, that the shielding means may comprise two or more radially spaced concentric elements. As illustrating such a modication I have shown in Fig. 3 a discharge device resembling those already described in comprising a cylindrical envelope 25 enclosing an anode 2B and a cathode 28. In addition an electrostatic control electrode 21 having a lead-in connection 21 is also provided. In this case, however, the shielding means consists of a plurality oi radially spaced concentric cylinders 29 and 30 and 29', 30', and 3|', respectively, extending along a portion oi the discharge path. It will be understood that each of these is open at its ends to permit passage of the discharge current. The lengths of the various cylinders are so related that the projections of adjacent cylinders on the envelope wall are of progressively increased area as one proceeds in theoutward direction. By this arrangement material removed from the anode by the impact of ions of the gas or vapor within the envelope is collected and sectionalized by the various shielding cylinders, only a small portion proceeding directly to the outer wall of the envelope.

Due to the longitudinally staggered relation of the individual shields the deposited coatings are positioned to distribute the total potential drop as evenly as possible throughout the space between the electrodes. The distribution o! electrostatic stress may be still further improved, however, by providing externally of the envelope one or more metallic rings such as that shown at 32. Such rings should be somewhat spaced from the envelope wall to avoid any danger of puncture and may be connected to a suitable potential to cause a desired redistribution of tbe electrostatic eld.

While I have shown the potential grading ring 32 as being applied in a particular location, it should be understood that similar rings may be arranged at any region where experience indicates the existence of a dangerous iield distortion. If the ring is positioned to overlap slightly the edge of a deposited coating and is maintained at a xed potential with respect to the cathode, it will serve to cause a suiilcient redistribution of the electrostatic eld to avoid the possibility of a disruptive breakdown to the sharply terminating edge of the coating. A further advantage accruing from the use of externally mounted potential grading rings in connection with external shields of the type dealt with by my invention lies in their tendency to prevent the accumulation of positive charges on the cbated surfaces and consequent interference with the normal tube operation.

In Fig. 4 I have shown the application of my invention to a controllable discharge device embodying a supplementary electrode for governing the distribution of the discharge potential along the tube. As illustrated, this comprises an enclosing envelope 35 adapted to contain an ionizable medium and having at one end thereof an anode 36 consisting or a closed cylinder ofA spun nickel, or similar material. This is mounted on a reentrant stem 31 and is provided with a suitable lead-in conductor 38. At the other end of the envelope is provided a thermionic cathode 3l which in this instance is shown as consisting ot a simple lament supplied with heating current through lead-in conductors 40. Surrounding the cathode 29 and connected with a suitable external source of controlling potential is arranged a 7l4 tions M grid member Il. This may conveniently comprise an enlarged cylinder of spun metal having an opening 'at the inner end thereof closed by means ofa mesh screen I2, forexample, of molybdenum.

Since discharge devices of the type with which my invention is concerned are intended `to be used in connection with a source of very high potential, I have in the present instance provided an intermediate electrode 43 adapted to be maintained at a potential between that of the anode 36 and that of the cathode 3 9 for the purpose of properly distributing the electric eld along the length of the discharge device. In accordance with my invention as described in connection with Fig. 1 the creation ofa continuous conducting coating on the walls of the envelope between any two adjacent discharge electrodes is prevented by the provision of telescoped cylindrical secand 45 whose outer circumferential portions considerably overlap the edges of the smaller cylinders which make up the main envelope.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto since many modiiications in the structure may be made, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device comprising a sealed envelope of insulating material enclosing an ionizable medium and spaced electrodes, at least one of which is subject to sputtering, said envelope comprising a plurality of continuously joined cylinders in partially telescoping relation, the overlapping edges of said cylinders being radially spaced and positioned to prevent deposit of a continuous coating of sputtered material.

2. In an electric discharge device, a sealed envelope of insulating material enclosing an ionizable medium and spaced electrodes, at least one of which is subject to sputtering, said envelope comprising a continuous outer wall so formed that portions of the Wall directly opposite the interelectrode space are shielded from the discharge by other portions of the Wall, thereby to prevent deposit of a continuous coating of sputtered material.

KENNETH H. KINGDON.

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