US2724786A - Grid control gaseous discharge rectifier tube - Google Patents

Description

Nov. 22, 1955 w. P. KRUGER ,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4 Sheets-Sheet l 11 1O F|G|.1. I 75 44 "60 EM 435 46 i i R i INVENTOR.

BY WP Kruger WMMU,

H is ATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4 Sheets-Sheet 2 FIG.2.

52 ll EQQ 2626 25 I 19 E:- =f s l 24 INVENTOR. BY W P. Kruger His ATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 v 4 Sheets-Sheet 5 IN V EN TOR.

BY W. P Kruger MM m,

His ATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4 Sheets-Sheet 4 IN VEN TOR.

BY WP Kruger WM m,

H is ATTORNEY United States Patent GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Application January 26, 1952, Serial No. 268,390

20 Claims. (Cl. 313-29) This invention relates to controllable gaseous discharge tubes, and more particularly to a tube structure suitable for grid control gas tubes of the air-cooled mercury pool type.

In certain types of gaseous discharge rectifiers, the tube is normally in condition to conduct when its anode is made positive, and a control element commonly called a grid serves to hold off or permit conduction through the tube as and when desired. A tube having a mercury pool cathode may be maintained in such normal condition for conduction by a keep-alive anode and suitable starting device, as distinctive from the type of mercury pool tube, commonly known as an ignitronQin which conduction is initiated by an ignitor.

In tubes of this type, in order that the grid may effectively exercise its controlling influence for high positive anode voltages, there should be no paths for efiective ionization at such anode voltages which are not subject to the controlling influence of the grid, otherwise in spite of the control of the grid, the tube may conduct at the higher anode positive voltages, and impose an undesired limitation for its peak forward voltage rating. In this connection, the gaseous ionizable medium is present in all spaces and regions inside the tube envelope, and ionizing paths to initiate an unwanted discharge current are not limited to the surface of the anode opposite the control grid, but may extend to the back surface of the anode, its supporting lead, or any part at anode potential.

With these considerations in mind, one object of the invention is to provide a shielding structure, effectively insulated from the anode and its supporting elements, which completely shields the anode and associated parts at anode potential in such a manner that all paths for elfective ionization and initiation of conduction current to the anode are subject to the controlling influence of the grid, thereby enabling a much higher peak forward voltage rating to be obtained.

Generally speaking, and without attempting to define the nature and scope of this invention, it is proposed to provide such shielding functions by a metal shield at cathode or other appropriate potential, which is electrically isolated by a special structural arrangement of parts from the anode and its supporting lead-in connection, and which is disposed in such closely spaced relation to all surfaces of anode potential, except for a region opposite the control grid, that there is no path for electron movement as between the shield and a point of anode potential sufliciently long for effective ionization and initiation of a discharge current to the anode not under the control of the grid. More specifically, the invention involves a structure for the joint or connection between such shield and the anode lead adjacent its seal in the tube envelope, which will provide adequate electrical isolation for these parts for high anode voltages, and yet avoid any long paths for ionization between the shield and anode lead.

Other objects of the invention are to provide improved structures for the control grid, keep-alive anode, starting device and other elements to be used in connection with 2 a completely shielded anode in a tube of the mercury pool type, using an envelope structure of metal and glass in a manner to obtain a grid control mercury pool rectifier tube of acceptable shape and size and peak forward and inverse voltage ratings.

Various other objects, characteristic features, attributes and advantages of the invention will be in part apparent, and in part pointed out, as the description advances.

Although the invention may be embodied in various specific structural forms, it is convenient in describing the nature of the invention and its characteristic features to refer to a tangible physical embodiment of the invention, such as the typical tube structure illustrated in the accompanying drawings.

In these drawings,

Fig.1 is a side view, partly in section, illustrating the general structural organization of one physical embodiment of the invention.

Figs. 2, 3, 4 and 5 are transverse sections through the tube taken on the lines 2-2, 3-3, 4-4 and 55 indicated in Fig. 1.

Fig. 6 is a fragmentary view on the line 6-6 in Figs. 1 and 3, illustrating a part of the structure for providing an external lead-in connection to the control grid; and

Figs. 7 and 8 are diagrammatic views illustrating the circuit connections associated with the keep-alive anode and automatic starting device.

Considering the structural features of the specific embodiment of the invention illustrated as applied to a mercury pool type of gaseous discharge rectifier tube, the envelope comprises a glass body E of general cylindrical form, with electrode supports and lead-in connections sealed in its upper end, and a circular metal bottom or base B, which is sealed to the lower edge of the glass body E, supports the pool of mercury constituting the cathode and indicated at 2, and has attached thereto an external radiator R, in accordancev with the envelope structure disclosed and claimed in the application of Andrew Humphrey, Ser. No. 268,368, filed January 26, 1952. The purposes and advantages of an envelope of glass and metal in this form for mercury pool tubes are explained in this Humphrey application; and no claim is made herein to such an envelope and radiator structure by itself. Generally speaking, and for the purposes of explanation in this case, it may be said that the metal bottom B and radiator R serves to maintain a region at and around the mercury pool cathode inside the envelope at the appropriate temperature'by air cooling to establish the vapor pressure within the envelope for the desired voltage rating for the tube, all in an envelope of convenient size and with small over-all dimensions for the current rating of the tube.

In the tube envelope structure of this type, for effective heat dissipation the bottom or base B should have substantial area, be relatively thin, and be formed of a metal which is not affected by mercury and which is capable of forming a gas tight seal with the glass body E of the envelope. For these reasons'the circular base B is a thin sheet of metal or alloy, which is capable of forming a gas tight seal with the glass, such as the alloy commonly known as Kovar. I have found that when such a Kovar disc is sealed in the ordinary way at its periphery to the glass body of the envelope, deformation of this thin metal disc after seal has been made by atmospheric pressure when the envelope is exhausted, is likely to produce damaging stresses upon the glass-to-metal seal. Accordingly, the base B of the envelope structure of this invention is formed with a peripheral trough 3 pro+ portioned to leave a space between the inner wall of the trough and the inner edge of the glass body E of the envelope when the glass is sealed to the bottom of this trough. In such a structure, the radial stresses at the periphery of the base B as it is deformed by atmospheric pressure or uneven expansion will be absorbed, so to speak, by bending of the metal adjacent the inner wall of the trough, and will not be applied to the seal itself. This structural arrangement permits the sealing of a metal base of the desired area and thinness to the glass body E of the tube envelope in a simple and effective manner, without having excessive stresses applied to the seal itself by deformation of this metal base after the seal has been made.

The tube elements enclosed and supported in this envelope structure, together with the appropriate lead-in connections, comprise in general an anode A and its supporting lead-in rod 5, control grid G, a metal shield S around the anode and grid with an arrangement of baflie plates, keep-alive anode KA, and an automatic starting device in the form of a flipper armature to be actuated by an electromagnet EM suitably supported outside of the envelope.

Considering the structural features of these tube elements, the anode A in the particular arrangement shown comprises a disc of carbon formed with a flat bottom surface, a beveled top surface, and a screw-threaded hub 6 to receive the threaded end of an anode supporting and lead-in rod 5, preferably provided with a locking nut 7. The upper end of the anode supporting rod is suitably fastened, such as by resistance welding, to the inner surface of a flanged cup 10 of Kovar or like alloy, capable of forming at its periphery a gas-tight seal with the glass of the bodyE of the tube envelope. The external circuit connection to the anode A is conveniently made by a flexible cable or the like 11 attached to the outside of the cup 10. A thin metal tube 12 fits closely at its lower end around the hub 6 of the anode, and the upper end of this tube fits inside the flange of the cup 10, for purposes later explained.

In the tube structure of this invention, for reasons more conveniently discussed later, all surfaces of the anode A and the metal tube 12 around its supporting lead 5, except for an area at the bottom face of the anode opposite the control grid G, are completely shielded by a closely spaced imperforate sheet metal shield, designated as a whole S. The upper part of this metal shield S, conveniently termed an anode shield, is shaped to be closely spaced, say in the order of 2 mm, to the back or top surface of the anode A, to the tube 12 around its supporting rod 5, and to the outer periphery of the grid G.

A plurality of heat radiating fins 14 (seven as shown) of convenient shape, as illustrated in Figs. 1 and 2, are preferably secured by welded flanges to the top of the shield S to facilitate heat dissipation. The anode shield S as a whole is rigidly supported by a plurality of relatively stiif rods 15, at least three in number, to form a three-point support. These rods 15 are welded at their lower ends to certain of the heat radiating fins 14, and are anchored at their upper ends in conventional seals (not shown) in the end of the glass body E. One of these supporting rods 15 is preferably extended through its anchoring seal through the glass body E to constitute a lead-in connection to permit the shield S to be readily connected to the cathode, or carried at some other potential base adapted for the operating conditions. In this connection, it is not necessary in some cases to have such an external connection to establish a fixed potential for the shield S, which may be a floating electrode assuming the potential of the plasma in operation.

The control grid G comprises, as shown in Figs. 1 and 3 a plurality of fiat grid bars 18 of suitable heat resistant metal, which are disposed edgewise to the anode A, and are supported on a flanged grid ring 19, One important feature of the grid structure is that each grid bar 18 is rigidly attached at one end to the flange of the grid ring, as by Welded tabs 20 (see Fig. 3), while the other end of each grid bar fits between but is not attached to abutments or pillars in the form of L-shaped pieces 21 Welded to the flange of the grid ring. These abutments 21 retain the ends of the grid bars 18 in the desired space relation, but permit a sliding motion of these ends to permit the grid bars to expand or contract individually upon changes in temperature after the grid is assembled, without the tendency to bend the grid bars transversely and distort their space relationship that would otherwise occur if the grid bars were rigidly attached at both ends to the grid ring 19.

The grid G is supported and insulated from the inside wall of the anode shield S by a plurality of insulating supporting elements, three as shown, with the outer edge of the grid ring closely spaced to the wall of the shield S. In the construction shown, which involves the features of the insulator structure disclosed in the prior patent to E. K. Smith, No. 2,456,540, December 14, 1948, a bent rod 23 is welded at its upper end to the inside of the grid ring 19, and extends through a pair of tubular insulators 24, 25 of steatite or like heat resistant insulating material and having recessed ends in accordance with the disclosure of said Smith patent. These tubular insulators 24, 25, one disposed vertically and the other horizontally, are secured to the inside wall of the shield S by metal bands 26 welded thereto and seated in circumferential grooves in said insulators. After the parts are assembled, the grid G is effectively supported in the desired closely spaced relationship to the anode A and the inner wall of the heat shield S. The horizontally disposed insulator 25 limits up and down movement of the grid, and the vertically disposed insulator 24' limits sidewise and turning movement of the grid.

The external lead-in connection to the grid G comprises a relatively stiff wire 28 passing through a conventional seal (not shown) in the upper end of the glass body E of the tube envelope, and this wire 28 extends downward to a point where it passes through a hole in the anode shield S and is attached by a welded washer to the grid ring 19, as indicated at 29 in Fig. 6. In order to avoid ionic current to this grid lead, it is electrically insulated throughout its length by tubes 36 cf steatite, with the joints between the tubes covered with a ceramic cement. The tubular insulator 31 around the wire 23 where it passes through the shield S, as best shown in the fragmentary view in Fig. 6, is formed with a recessed inner end, in accordance with the disclosure of the Smith patent above mentioned, so that any conductive film likely to be deposited on the surface of this insulator by metal sputtered or evaporated during fabrication or operation of the tube will not be continuous and provide an unwanted conductive path between the grid wire 28 and the Shield S.

An arrangement of battle plates is included in the lower part of the anode shield S below the grid G for the purpose of preventing mercury being splashed up against the heated walls of the shield during operation, and to impede the movement of the electrons and ions in the plasma formed by the keep-a1ive anode into the region directly under the control grid to affect its controlling influence. In the particular baii'le structure illustrated, a lower baliie plate 34, with a center circular opening indicated at 35, has a peripheral flange welded to the inside Wall of the shield S (see Fig. l). A cruciform arrangement of upright strips 36 welded to this lower baffle plate 34 supports an intermediate circular baffle plate 37, which has a diameter slightly greater than the diameter of the opening 35 in the lower baffle plate. Above this intermediate baffle plate 37 is an upper baffle plate 3%, similar to the lower baffle plate 34, which has a center circular opening 39, and is secured by a welded peripheral flange to the shield S.

in the particular tube structure illustrated, the keepalive anode KA comprises an. arcuate strip 42, supported by insulating supporting elements from the lower edge of the anode shield S, as shown in Figs. 1 and 4. Each of the insulated elements, three as shown, for supporting the keep-alive anode strip 42 from the shield S, comprises a rod 43 (see Fig. 1) having its upper end welded to the shield S, and a tubular insulator 44 of steatite or like material around this rod, which is secured to the anode strip 42 by a strap 45 seated in a peripheral recess in said insulator and welded to the keep-alive anode strip. A suitable projection on the rod 43 to consittute a stop engaging the lower end of the insulator 44 and hold up the keep-alive anode strip 42 is provided by deformation or bending of this rod 43, or by an additional welded piece, as indicated at 46.

The external connection to the keep-alive anode KA comprises a relatively stiff wire or rod 50 which is sealed at its upper end in the usual seal (not shown) in the upper part of the glass body E. This wire 50 is bent to form a coil or winding of several turns around but spaced from the tubular extension of the shield S around the anode lead, for reasons later explained; and this wire 50 then extends down to a point where it is attached by a welded tab 51 to the keep-alive anode strip 42. The portion of this wire 50 adjacent the shield is preferably surrounded by a tube 52 of steatite or like heat resistant insulating material.

The automatic starting switch comprises a flipper armature 55 pivotally supported on a horizontal axis to be swung from a vertical position in contact with the mercury, as shown in Fig. l, to produce a starting are, by energization of an electromagnet EM supported in a convenient manner not specifically shown outside the tube close to the tube envelope opposite this flipper armature, as diagrammatically represented in Fig. 1. The pivotal support for this flipper armature 55 is preferably formed to involve loosely fitted parts of a metal, such as molybdenum, having a high melting point, so that the high temperature these parts may assume in operation will not cause any fusing or welding effect at the contacting surfaces to interfere with free movement of the armature. In the structure illustrated, the flipper armature 55 is pivotally supported by a pair of eyelets cooperating with a rod 56 secured in a flanged lip of a supporting plate 57. These eyelets are conveniently formed at the ends of a wire loop 58 which is seated in grooves in the sides and bottom of the flipper armature 55, and which provides in effect a cradle to support the block of magnetic material constituting the armature. The supporting plate 57 is welded to the bent lower end of a lead-in rod 60 of substantial stiffness, which is surrounded by a sleeve 61 of steatite or the like to insulate it from the shield S. The lower end of this lead-in rod 60 and its insulating sleeve 61 sets between a pair of cars 62 welded to the shield S. A pair of bumper steatite insulators 64 are secured by welded bands 65 to the supporting plate 57 in a position such that when these insulators engage glass body E of the tube envelope, the armature 55 is in the appropriate position to be oerated by the electromagnet EM to the best advantage without striking the glass wall. In this connection, the dimensions of the glass body B will vary somewhat for different tubes, and a flipper armature 55 pivoted in a fixed relation to a shield S would not necessarily be in the proper position relative to the glass wall. In the structure shown, the lead-in supporting rod 60 is bent during assembly to have a tendency to move outward radially at its lower end; and when the parts are assembled in a glass body E of a given size, the bumper insulators 64 engage the wall of this glass body (see Fig. 4) and establish the desired position for the flipper armature. For convenience in assembly the lead-in supporting rod 60 for the flipper armature is preferably made in two parts welded together, as shown in Fig. 1.

Considering the operation of the automatic starting switch, the flipper armature 55 tends to drop by its own weight into the vetrical position where its lower edge dips in the mercury pool. Referring to the circuit diagrams of Figs. 7 and 8, when the tube is put into use by closing a switch indicated at 67, uni-directional current from a suitable source of direct current or rectified alternating current of the appropriate voltage for the keep-alive arc flows through a load resistor 68, a starting resistor 70, to the flipper armature 55 and pool of mercury, and through the windings of the electromagnet EM. The resultant energization of the electromagnet EM attracts the flipper armature 55 fromthe position shown in Fig. 7 to an approximately horizontal position shown in Fig. 8. This movement of the flipper armature 55 draws a starting arc; and due to the starting resistance in series with this are, there is a higher voltage across the keep-alive anode KA and the cathode pool, so that the starting arc is transferred to the keep-alive anode KA, with the current of the keep-alive discharge also flowing through the winding of the electromagnet EM. In this connection, the coils of the electromagnet EM are preferably wound and connected to create a magnetic field in a direction to repel the starting are between the flipper armature 55 and the cathode pool toward the keep-alive anode KA, in accordance with the principles of the well known are blow-out devices. So long as an effective keep-alive discharge are exists, between the cathode pool and either the keep-alive anode KA or the flipper armature 55, the electromagnet EM is maintained energized to hold the flipper armature in its raised position. If, however, a suitable keep-alive discharge ceases to exist, the electromagnet EM is deenergized, and the flipper armature 55 drops by its own weight into contact with the mercury pool to establish a conductive circuit for energization of the electromagnet, whereupon the starting arc is again initiated. In this arrangement, the electromagnet EM operating to draw the initial starting are also serves to detect the existence of a keep-alive discharge, thereby avoiding the need of an additional relay for this purpose.

Considering now the functional significance of the features of tube structure illustrated and described, and the characteristic features of this invention, one important feature of the invention relates to the anode shielding to permit the grid G to exercise its desired controlling function at high anode voltages. In a grid control tube, as distinctive from an igniter type of tube, the cathode is normally active, so to speak, and the tube will conduct whenever its anode is positive unless the grid is capable of preventing initiation of cumulative ionization and an arc discharge. In general, a control grid may influence the ionizing effect of lines of force of the electric field of a positive anode that pass through the grid, but has little effect upon ionization by the anode electric field along other paths. For example, lines of force from the side or back surfaces of the anode, or from its supporting lead, may initiate an arc discharge in spite of the controlling effect of the grid. Such uncontrolled paths of ionization may be relatively long; and for a given set of conditions in the way of the concentration and state of the gas molecules, electrode surface condition and the like, positive anode voltages above a certain level can render the tube conductive in spite of the grid control, imposing undesirable limitations upon the peak forward voltage rating for the tube. In the case of a grid control mercury pool tube with a keepalive anode, the vapor subject to the ionizing influence of anode potentials beyond the control of the grid is likely to be partially ionized from the plasma of the keep-alive anode, and materially limit the peak forward voltage for the tube.

In accordance with this invention, an acceptable peak forward voltage and effective grid control are obtained by completely shielding all surfaces of the anode and associated parts at anode potential, except for an area directly opposite the grid, from the cathode and the plasma of the keep-alive anode, by an anode shield S constructed as shown and described, which is maintained at cathode or other appropriate potential. Since breakdown along any ionizing path, no matter how long or restricted its cross section may be, is likely to cause an unwanted conduction through the tube, complete shielding of surfaces at anode potential is important. Various types of anode shields have been previously proposed, but all of the structures with which I am familiar fail to provide the necessary complete shielding for satisfactory results.

In the tube structure of this invention, the anode shield S is shaped to shield the edge and back of the anode A, the entire length of the tube 12 around the anode supporting lead 5, and is also closely spaced to the outer edge of the grid G, so that all points on surfaces at anode potential, except directly opposite the grid G, are isolated by the shield S from the cathode and ionized vapor in the tube envelope.

The addition of anode shielding to enable the grid control to be effective for higher anode voltages introduces the problem of avoiding breakdown and an arc discharge as between the anode and such shield. The anode shielding structure of this invention is closely spaced to the surfaces at anode potential, in accordance with the well known phenomenon in gaseous discharges, sometimes termed the mean free path principle, that the space or distance between electrode surfaces may be made small enough with respect to the pressure, type of gas and other factors to a degree where effective ionization and breakdown between such electrode surfaces requires extremely high voltages. The appropriate spacing effective to avoid breakdown is dependent upon various complex factors, and is diflicult to define. From one point of view, it may be said that breakdown will not occur if the spacing corresponds with the electron mean free path for the existing pressure; and the appropriate spacing is sometimes termed a mean free path spacing. However, ionization sufficient to cause a sustained are discharge of substantial current involves other factors than the probability of ionizing collisions between electrons and gas molecules; and in general, effective results may be obtained with a spacing greater than the theoretical electron mean free path for the existing gas pressure. For the purposes of this case, it is proposed to refer to the spacing for the anode shield S characteristic of this invention as a short path spacing, or a space or distance in the order of the electron mean free path for the operating vapor pressure. In this connection, by way of explanation and without limiting the invention, I have found that a spacing for the cathode shield S in the order of 2 mm. is appropriate for typical operating conditions of the type of tube illustrated.

At this point in the discussion it is convenient to refer to another structural problem presented by the use of complete anode shielding, which involves maintaining the desired short path space relationship at and around the upper end of the anode shield S adjacent the seal for the anode lead, and still obtain the necessary electrical isolation of the shield for high anode voltages. In the structure illustrated, the glass body E is formed with a re-entrant tubular extension or neck 72 adjacent the cup 10 forming the metal-to-glass seal for the anode supporting rod 5 and its surrounding tube 12. The upper tubular portion of the shield S, which is closely spaced to the tube 12 at anode potential throughout its length, extends up into this glass neck 72, where it is surrounded by a packing of quartz wool, as indicated at 73, or like heat resistant insulating material of a fibrous or finely dividing nature. The glass neck '72 is preferably tapered slightly to hold the quartz wool packing 73 in place.

Considering now the significance of this structural feature of the anode shielding of this invention, the shield S as a whole may be readily supported in an insulated space relationship to the anode A and associated parts in a suitable manner, such as by rods 15 sealed in the glass body E as shown, with suflicient rigidity to maintain the desired short path spacing and electrical isolation for most of the surfaces at anode potential. There is, however, a region adjacent the seal of the anode lead in the tube envelope where a special structure is needed to maintain the short path spacing for complete shielding, and at the same time provide adequate electrical insulation for the shield. In other words, there should be no gap between the upper edge of the shield S and the envelope wall adjacent the anode lead, otherwise there will be a long path for unwanted ionization from the anode lead through such gap to the outside surface of the shield; yet the separation of the parts is so small for the differences of potential involved that adequate electrical isolation cannot be obtained by merely extending the shield into the wall of the envelope adjacent the seal for the anode, since glass or other conventional insulating materials for the envelope wall would not have in such short dimensions the requisite dielectric strength for the differences of potential involved. In the structure of this invention, the quartz wool packing 73, around the upper edge of the shield S affords the desired electrical insulation, on account of the high dielectric strength of quartz even at high temperatures, and the interstices or spaces between fibers or pieces of the quartz are so small and so disconnected that there are no long ionizing paths for a gaseous discharge between the shield S and the tube 12 at anode potential.

As previously explained, the keep-alive anode current is conducted through the wire 50 coiled around the upper part of the shield S adjacent the anode lead. Considering the purpose of this auxiliary heating coil, the anode A and in turn the upper tubular portion of the shield S are kept hot enough while the tube is conducting to avoid any condensation of the mercury vapor, which migl t otherwise establish a short-circuiting electrical connection between the shield and anode, more particularly in the region of the quartz wool packing 73; but if the tube is idle for some time with the discharge limited to the keep-alive current, it is found that the anode and its associated tube 12 and the shield S may assume a temperature low enough for condensation of mercury vapor to a degree to set up conditions for a short-circuit between tube 12 at anode potential and shield S. The auxiliary heating coil obviates these conditions by maintaining the parts at a high enough temperature by the keep-alive anode current while the tube is in service, but not conducting to the main anode.

Referring now to the arrangement of bafiie plates 35, 37, 38 in the lower part of the anode shield S, the parts of this bathe system are arranged and proportioned to carry out physical and electrical functions. While the tube is conducting, movement of the cathode spot and other changes tend to produce spurts of mercury, which except for the baffle system might splash on the hot wall of the shield S in the region under the grid G, and cause an abrupt change in pressure by evaporation of such mercury, tending to affect the controlling influence of the grid. The bafile plates act as barriers to keep spurts of mercury from reaching the region under the grid G, any evaporation or splashed mercury occurring within the battle system, where the increase pressure may quickly stabilize with the region of low pressure in the cathode region.

Considering the electrical function of the baffle plates 35, 37, 38, while conduction through the tube is intended to be prevented by the control grid G, an arc discharge is maintained between the keep-alive anode KA and the cathode, and except for the baffle system, the plasma for the keep-alive discharge would extend into the region under the grid G, and there would be a drift of positive ions to the grid tending to reduce its negative potential, and cause unwanted conduction. The parts of the battle system are proportioned to establish a boundary condition for the plasma of the keep-alive discharge by affording sufiicient surface for deionization to keep the ion concentration in the region below the grid G at a level appropriate for aceptable grid control. In this connection, a bafiie system of this character raises the starting voltage for the tube, i. e. the anode voltage which will initiate conduction at a zero grid potential, so that a baffie system proportioned to keep the plasma of the keep-alive discharge out of the region underneath the grid is likely to require a positive grid potential to fire the tube at the desired level of anode voltage. It is contemplated that the baffle system will be proportioned to provide a compromise between the desired grid control for peak forward voltage ratings and starting voltage, and that the tube will be controlled by abruptly changing the grid from a negative potential to a relatively high positive potential.

The tube structure of this invention provides a relatively high and acceptable inverse voltage rating for the tube, since the complete shielding of the back of the anode and its supporting lead eliminates the ionizing paths existing in the ordinary tube structures and capable of causing initiation of an arc discharge through the tube in the wrong direction, when the alternating supply voltage commonly used with rectifier tubes makes the anode at a high negative potential relative to the cathode. The shielding effect of the anode shield S, and the arrangement of baffie plates as between the cathode and the lower surface of the anode, together with the interposed grid G at a negative potential with respect to the cathode, tends to avoid effective ionization and initiation of a discharge in the wrong direction between the cathode and the lower surface of the anode. In this connection, the grid G is closely spaced to this lower surface of the anode to reduce the probability of ionization in the space and the formation of an ion sheath on the grid for high differences of potential between the anode and the grid, even though these electrode surfaces may be somewhat emissive and there may be appreciable electron current. It is contemplated that this close spacing will conform with the short or mean-free path spacing for the existing pressure or concentration of gas molecules to a degree avoiding effective ionization. In addition to raising the inverse voltage rating for the tube, this close grid to anode spacing minimizes the probability of loss of grid control due to grid emission.

In general, the cooperative effect of these various structural features serves to provide a high level of forward and inverse voltage ratings for the tube, and limit to an acceptable degree the probability of backfires or arc-backs characteristic of a mercury pool tube, when the tube is operated within the limits of its voltage and current ratings.

The specific embodiment of the invention just described exemplifies the structural features and functions characteristic of the invention; but it should be understood that this particular construction and arrangement of parts is merely typical or representative, and that various adaptations, modifications and additions may be made without departing from the invention.

What I claim is:

l. A gaseous discharge tube comprising, an evacuated envelope containing an ionizable medium and 2. normally active cathode, an anode having an electron receiving area opposite said cathode, an anode supporting lead sealed in said envelope, an imperforate metal shield completely surrounding said anode and its lead except for said electron receiving area of the anode, and means rigidly supporting said shield within the envelope in a short path space relationship to all points on the opposing surfaces of said anode and its lead, said short path spacing being in the order of two millimeters to inhibit an arc discharge to said shield at high anode voltages, said supporting means including a fibrous body of refractory insulating material located at the gap between said shield and the anode lead adjacent its seal in the tube envelope, said insulating fibrous body having its 1O conductive continuity interrupted by interstices smaller than said short path spacing, whereby an arc discharge to said shield from the anode lead is inhibited in the region adjacent its seal as well as throughout its length.

2. A controllable gaseous discharge tube comprising, an evacuated envelope containing an ionizable medium, an anode having a supporting lead sealed in said envelope, a normally active cathode, a grid between said cathode and an electron receiving surface of said anode, an imperforate metal shield completely surrounding and having a short path spacing to all points on the surfaces of said anode and its supporting lead except said electron receiving surface, said short spacing being in the order of two millimeters to avoid a breakdown between said shield and the anode and its lead at relatively high anode voltages and the temperature and vapor pressure conditions existing during operation of the tube, and an insulated joint between portions of said shield and said anode lead in said short path space relationship, said joint including a fibrous body having a breakdown voltage much higher than solid insulating material of like dimensions.

3. A grid control gaseous discharge tube having a mercury pool cathode and comprising, an evacuated envelope including a metal bottom supporting the mercury pool cathode and a glass body sealed to said metal bottom, an anode having a supporting lead sealed in said glass body, a control grid between said anode and cathode, an imperforate shielding element rigidly supported in said envelope with inner surfaces having a short path spacing to the periphery of said grid and to all points on the surfaces of said anode and its lead except for a discharge receiving area opposite said grid, said short path spacing being less than three millimeters to avoid a breakdown at relatively high anode voltages between said shielding element and said anode and its lead at the temperature and vapor pressure existing with air cooling of the tube envelope, and an insulating connection between a portion of said shielding element and said anode lead, said connection including a fibrous body having a breakdown voltage much higher than a solid insulating material of like dimensions.

4. A grid control mercury pool rectifier tube comprising, an air-cooled envelope having a metal bottom and a glass body sealed together, an anode having a supporting lead sealed in said glass body of the envelope, a mercury pool cathode supported by the metal bottom of said envelope, a control grid between the cathode and anode, a keep-alive anode and starting device in said envelope and a metal shield enclosing said anode and said grid, said shield having a short path spacing to all points on the surfaces of said anode and its supporting lead except for an area opposite the grid, said shield also including baflie plates below said grid for restricting movement of ions associated with the keep-alive discharge into the region of said grid. I

5. A gaseous discharge tube of the mercury pool type comprising, an evacuated envelope having a metal bottom supporting a mercury pool cathode, an anode having a supporting lead sealed in said envelope, an anode shield having a short path spacing to the surface of said anode supporting lead, a control grid opposite said anode, a keep-alive anode in said envelope and an auxiliary heating coil around said shield adjacent said anode supporting lead and connected with said keep-alive anode, whereby the closely spaced surfaces of the anode and its shield are maintained at a temperature avoiding condensation of mercury by keep-alive anode current through said auxiliary heating coil under low load or stand-by conditions of tube operation.

6. A controllable gaseous discharge tube comprising an evacuated envelope containing an ionizable medium,

' an anode having a supporting lead sealed in said envelope, a control grid opposite an area of the anode surface, a metallic shield having a short path spacing to said grid and to all surfaces of said anode and its supporting lead except the area opposite the grid, and a joint structure 'at the end portion of said shield adjacent the seal of said anode lead in the envelope maintaining short path spacing and electrical insulation, said joint structure including a packing of fibrous heat resistant insulating material between the closely spaced surfaces of said shield and anode lead in a region adja' cent the'seal of said anode lead in the envelope.

7. A gaseous discharge tube according to claim 6, in which the end portion of the anode shield extends into a re-eiitr'ant neck adjacent the seal of the anode lead in the envelope, and is surrounded by a packing of quartz wool.

8. A controllable gaseous discharge tube of the mercury pool type comprising, an evacuated envelope having a metal bottom supporting a mercury pool cathode and a glass body sealed to said bottom, an anode having a supporting lead sealed in said glass body of the envelope,

a control grid adjacent an area of the anode opposing the mercury pool cathode, a metal shield closely spaced to the periphery of said grid and all surfaces of said anode and its supporting lead except the area of said control' grid, a keep-alive anode supported by but insulated from said shield, and an automatic starting de- \"ice comprising a flipper armature dipping into the cathode pool in a normal position, an electromagnet mounted adjacent said starting device for swinging said armature from its normal position to provide a starting arc to be transferred to the keep-alive anode, and means for energizing said 'electromagnet by keep-alive anode current, whereby the automatic operation of said starting device is dependent upon a maintained keep-alive discharge.

9. A gaseous discharge tube of the mercury pool type comprising an air-cooled evacuated envelope enclosing a control grid interposed between a mercury pool cathode and a shielded anode, said envelope including a metal base supporting the cathode pool and a glass body sealed to said base, said base having a peripheral trough to receive the glass body, the inner side wall'of said trough being spaced from the glass, whereby said side wall of the trough may yield upon distortion of the metal base to avoid stress upon the seal between the glass and the bottom of the trough.

10. A controllable gaseous discharge tube of the mercur'y pool 'type comprising, an evacuated envelope enclosing an anode and a normally active cathode, a supporting lead for said anode sealed in said envelope, a control grid between said anode and said cathode and closely spaced to an electron receiving surface of said anode, said control grid comprising a supporting ring and a plurality of spaced grid bars, each of said grid bars being anchored to the supporting ring at one end only and having a slidable fit at its other end between guiding elements'on said ring, and an imperforate metal shield having a short path spacing to the periphery of said grid and to all surfaces of said anode and its supporting lead except for the electron receiving surface oppositesaid'grid.

11. A controllable gaseous discharge tube comprising, an evacuated envelope containing an ionizable medium, an imperforate metal shield supported in said envelope, an anode and a control grid within said shield and havihg a short path spacing thereto in the order of the electron mean free path of said ionizable medium, and means including tubular insulators or refractory material disposed in different planes supporting and conductively insulating said grid from said shield at a plurality of points around the periphery of the grid, said grid being maintained by said supporting means against displacement in a closely spaced relationship 'to an electron receiving surface of said anode opposite said grid.

12. A controllable gaseous discharge tube of the mercury pool type comprising, an air-cooled envelope having a metal bottom supporting a mercury pool-cathode,

an anode, a control grid between said cathode and said anode, a keep-alive anode, and an imperforate metal shield rigidly supported in said envelope around said anode and said grid, said shield having a short path spacing to surfaces of said anode and grid in the order of the electron mean free path for the operating pressure of mercury vapor, said shield having bafiie plates establishing a boundary for the plasma of the keep-alive discharge as between said mercury pool cathode and said grid.

13. A controllable gaseous discharge tube of the mercury pool type described comprising, an air-cooled envelope having a metal bottom supporting a mercury pool cathode, a radiator attached to said metal bottom outside the envelope for dissipating heat to provide a relatively low temperature in the cathode region and maintain a low pressure of mercury vapor during operation of the tube, an anode and a control grid in said envelope, a keep-alive anode, and an imperforate metal shield and attached baffle plates isolating said anode and control grid from the plasma of the keep-alive discharge except for crooked paths through said baffle plates, said shield establishing a boundary of said plasma preventing the formation of an ion sheath on the surface of the grid when at a negative potential.

14. A gaseous discharge tube of the character described. comprising, an evacuated envelope having an aircooled metal bottom supporting a mercury pool cathode, a main anode, a control grid, a metal shield enclosing said anode and grid and preventing an arc discharge to the anode not subject to the control of said grid, a keepalive anode and starting device normally maintaining an arc discharge to the mercury pool cathode to be transferred to the main anode under the control of said grid, said starting device comprising a flipper armature biased to a normal position in contact with the mercury of the cathode, an electromagnet outside the envelope for attracting said armature to a position out of contact with the mercury to provide a starting arc, and an energizing circuit for said electromagnet including circuit paths in multiple between said keep-alive anode and said flipper armature respectively and the cathode, said circuit path through said armature including a resistance to cause the starting arc to be transferred to the keep-alive anode.

15. A controllable mercury arc rectifier tube comprising, an exacuated envelope including an air-cooled metal container for a shallow cathode pool of mercury and a glass body sealed to said container, an anode supported by a lead-in rod sealed in said glass body, a control grid, shielding means including imperforate surfaces having a short path spacing to the periphery of said grid and to all points on the surfaces of said anode and its lead-in rod except for a discharge receiving area, a keepalive anode and a starting device, and auxiliary heating means energized by the discharge current of the keepalive anode for heating said shielding means above the temperature for condensation of mercury under low load or stand-by conditions of tube operation.

16. A controllable mercury arc rectifier tube comprising, an evacuated envelope including a container of thin metal for a shallow cathode pool, a heat radiator directly attached to the outside surface of said container, an anode supported by a lead-in rod sealed in said envelope, a control grid closely spaced to a discharge receiving area of said anode, a keep-alive anode and starting device, and means including imperforate shielding surfaces and a baffle system for completely isolating said anode and said grid from the plasma of the auxiliary discharge to the keepalive anode except for crooked passages through the baffle system to a region adjacent said grid inside said shielding surface, said shielding surface being closely spaced to the periphery of said grid and to all points on the surface of said anode and its lead-in rod except for the discharge receiving area of the anode.

17. An air-cooled controllable mercury arc rectifier tube Ase;

comprising, an evacuated envelope including a body of vitreous material and a metal container sealed thereto for supporting a shallow pool of mercury, an anode and a control grid supported in said envelope with separate lead-in connections and having closely spaced opposing surfaces, imperforate shielding means having inner surfaces closely spaced to the periphery of said grid and to all points on the surfaces of said anode and its lead-in connection except for a discharge receiving area opposite said grid, a keep-alive anode and starting device for establishing an auxiliary arc discharge to be transferred to the main anode subject to the control of said grid, and bafiling means connected with said shielding means and comprising surfaces separated by crooked passages to restrict movement of the plasma of the auxiliary discharge of the keep-alive anode into the region of the grid.

18. A controllable mercury arc rectifier tube comprising, an envelope including an air-cooled metal base supporting the cathode pool and a glass body sealed to said base, an anode supported by a lead-in rod sealed in said glass body and having an essentially flat discharge receiving surface, a control grid closely spaced to said discharge receiving surface of the anode, said grid comprising spaced grid bars of refractory metal rigidly supported at one end only and capable of expanding in length without bending, and an imperforate metal shield supported in said envelope and having a short path spacing to the periphery of said grid and to all points on the surfaces of said anode and its lead-in rod except for said discharge receiving surface of the anode.

19. An air-cooled mercury arc controllable rectifier tube comprising, an envelope of glass sealed to a relatively thin metal base supporting the cathode pool, an external radiator of spaced strips having their edges directly attached to the outer surface of said base, a carbon anode having a flat discharge receiving surface and supported by a leadin rod sealed in the glass body of the tube envelope, a control grid including spaced grid bars of refractory metal having their edges closely spaced to the discharge receiving surface of said anode, said grid bars having a width greater than their spacing, and an imperforate metal shield supported in said envelope with substantial rigidity and having its inner surface closely spaced to the periphery of said grid and to all points on the surfaces of said anode and its supporting lead except the discharge receiving surface of the anode.

20. A controllable mercury arc rectifier tube comprising, an air-cooled envelope including a glass body sealed to a metal base supporting the mercury pool cathode, an anode supported by a lead-in rod sealed in said glass body, a control grid closely spaced to a discharge receiving surface of said anode, an imperforate metal shield supported in said envelope and having a short path spacing to the periphery of said grid and to all points on the surfaces of said anode and its lead-in rod except for said discharge receiving surface, a keep-alive anode, a starting device including a flipper armature movable into and out of contact with the mercury pool cathode, and means including heat resistant insulating elements supporting from said shield said grid, keep-alive anode and starting device.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,966 Klemperer Jan. 19, 1937 2,101,610 Dallenbach Dec. 7, 1937 2,221,615 Slepian Nov. 12, 1940 2,282,229 Longwell May 5, 1942 2,320,685 Bertele June 1, 1943 2,392,367 Depp Jan. 8, 1943 2,477,506 Winogard July 26, 1949 2,573,618 Smart Oct. 30, 1951 2,594,851 Bertele Apr. 29, 1952 FOREIGN PATENTS 665,469 Great Britain J an. 23, 1952

Images