US2419236A - Electrical gaseous discharge device having constant starting characteristics - Google Patents

Description

April 22, 1947. p, `W STUTSMAN 2,419,236 ELECTRICAL GAsECUs DISCHARGE DEVICE HAVING CONSTANT STARTING CHARACTERISTICS i E i P. W. STUTSMAN Apnl122, 1947. 2,419,236

.ELECTRICAL GASEOUS DISCHARGE DEVICE HAVING CONSTANT STARTING CHARACTERISTICS Filed June 8, 1943- 2 Sheets-Sheet 2 Patented Apr. 22, 1947 ELECTRICAL GASEOUS DISCHARGE DEVICE HAVING CONSTANT STARTING CHAR- ACTERISTICS a Paul W. Stutsman, Needham, Mass.. assignor, by

mesne assignments, to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application June 8, 1943, Serial No. 490,094

6 Claims. (Cl. Z50-27.5)

. This invention relates to gaseous electric discharge devices, generally, and particularly to discharge devices of the cathode glow type.

It is among the objects of the invention to provide a cathode glow device having constant electrical characteristics, and particularly such a device having a constant break-down voltage, so that the same will start at the same voltage and continue to break down at this starting voltage throughout the life of the device.

A further object of the invention is to provide a device of this type which can be manufactured in quantity in such a manner that the devices so constructed differ only slightly from each other in their electrical characteristics, and particularly with respect to the starting voltage, to the end that large circuit adjustments need not be made when one device is substituted for another in the same circuit The above and other objects and features of the invention will be made fully apparent to those skilled in the art from a consideration of the following detailed description taken in conjunction with the accompanying drawings which form a part hereof, and in which:

Fig. 1 shows in perspective, and with parts cut away to show internal structure, a glow tube constructed in accordance with the present invention;

Fig. 2 shows a section taken along the line 2-2 of Fig. 1;k

Fig. 3 shows a perspective View on an enlarged scale of thecathode structure;

Fig. 4 is a View similar to Fig. 1 of a modied form of the invention;

Fig. 5 shows in section and on an enlarged scale a portion of the electrode structure of the modification shown in Fig. 4.

Referring to the drawings, and first to the form shown in Figs. l, 2 and 3, there is shown a sealed envelope I, of glass or other suitable material, having a reentrant stem 2. Current lead-in wires 3 and 4, sealed in stem 2 by a press 5, provide connections between a suitable source of power and electrodes 6 and 1.

In this form of the invention the electrodes and 1, which constitute, respectively, the cathode an anode of the discharge device, are constructed in the form of concentric cylinders, the cathode B having somewhat less external diameter than the internal diameter of the cylindrical anode 7. The cathode 6 is formed of a ne Wire screen, which screen is supported and held in a cylindrical shape by a metal cap 8 which is welded and otherwise securely attached to one end of the screen, and by a similar cap 9 aiiixed to thev opposite end. The end caps 8 and 9 are identical except that the lower end cap 9 is provided with a central opening I li for purposes which will hereinafter be set forth. Each of the caps 8 and d is provided with diametrically opposed notched portions II and I2 for the purpose of permitting supporting rods i3 and I4 to pass through the cathode structure.

The anode 'I is formed of a nickel sleeve of substantially the same length as the overall length of the cathode 6, but of suciently greater diameter to provide a discharge space I5 therebetween. The anode 'I is also provided with supporting rods Il and I8 which are welded or otherwise rigidly xed to its external surface on diametrically opposite sides thereof.

The cathode 6 and the anode 'I are held in properly spaced concentric relation by means of mica plates I9 and 20 which close or partially close the upper and lower ends of the electrodes E and 'l and through which the rods I2, I3, I'I and li! project. The mica plates I9 and 20 are substantially identical except that the lower plate has a central opening 2l registering with the opening IIJ in the base capA 9 of the cathode 6. Tabs 22 welded to the projecting ends of the rods I2, I3, I'I and I8, above the mica plate I9 and below the lower mica plate 20, prevent relative movement of the cathode and anode in a longitudinal direction, thus providing a unitary cathode and anode structure which may be assembled as a unit prior to the mountingl of same upon the supporting structure.

Lead-in wire 3 is connected to support rod I3 of the cathode, while the lead-in wire 4 is bent to connect with the rod I8 of the anode. Thus lead-in connections 3 and d support the cathode and anode structure as well as provide connections to a suitable source of power. A pair of lead-in wires 23 and 24 also sealed in the stem 2 by the press 5 extend through the opening 2| in the mica plate 2t and provide a support for a getter 25, which getter is welded or otherwise suitably fixed to the wires 23 and 24 at its opposite ends and projects upwardly through the opening I0 in the base cap 9 into the hollow interior of the cathode 6. The getter 25 serves not only to clean up the gases remaining in the envelope I after the evacuation thereof but also serves to activate a portion of the cathode, as will be hereinafter more fully described.

In the preferred Inode of manufacture of the device the cathode 6 is constructed of nickel screen of about mesh and is oxidized in air and I4, is inserted in the cylindrical anode 1.-

'I'he mica disks I9 and 20 are each provided with two diametrically opposite holes 26 to accommodate the projecting ends of rods I3 and I4, and two similarly diametrically opposite holes 2'I spaced farther toward the circumference of the disk and preferably at 90 degrees from holes 26 to accommodate the projecting ends of the rods I'I and I8. These disks, therefore, may be placed to close the ends of the cathode and anode, at the edges thereof. This provides a combined cathode-anode assembly which forms a rigid unitary structure and in which there can be no relative movement, or displacement of the cathode relative to the anode. The cathode-anode assembly is now mounted upon the stem 2 by welding leadin wire 3 to the projecting end of the supporting rod I3 0f cathode 5 and by welding lead-in wire 4 to the supporting rod IS of the anode 7. In this position the getter 25 projects upwardly through the opening 2I in the mica plate 20 and through the opening lil in the end cap 9 of the cathode E so that the getter 25 extends into the interior of the hollow cathode 6. The assembled structure may now be inserted into the envelope I, which may have an internal diameter such that the projecting points 28 on the mica disks or plates I9 and 20l are in contact with the inner wall thereof, thereby minimizing vibration of said combined cathode-anode structure. l

The envelope I is now evacuated, and preferably the electrode structure is heated by induction to assist in driving oif residual gases; getter 25 contains an activating material, such as barium, which while aiding in cleaning up the gases remaining in the tube serves the further important function of rendering that portion of the cathode 6 which is exposed thereto more active than other portions. When the getter 25 is flashed, a material such as barium is released and is deposited upon that portion of the cathode screen 6 between the end caps 8 and 9. The end portions of the cathode beyond the flat faces of these end caps are shielded by these caps from the activating material. The electrode structure is again heated by induction to the end that the temperature of the cathode screen 6 is raised sufciently for the oxide surface thereof to combine with and fix the getter and thereby activate this portion of the screen.

By the above arrangement there is provided a discharge space I5 between the cathode 6 and the anode 'I such that all points on the cathode are equidistant from the anode.

I have found that if a relatively large cathode area is spaced so that the effective distance between any point on this area and the nearest point on the anode is constant for all such points, then variations in starting potential are substantially reduced.

The effective distance between the cathode and the anode is altered at the edges and ends of the electrodes by edge effects so that, to maintain the constant effective distance between the active cathode area and the anode, variations due to edge eiects must be prevented if a constant starting potential is to be obtained.

In the form of the invention described the active cathode area is provided by that portion of the cathode 6 between the end caps 3 and 9. The effective distance between any point on this cathode area and the nearest point on the anode 'I is constant for all such points. Variations in the effective distance between the points on the active area of the cathode and the anode at the edges thereof are prevented in this instance by providing an extension of the electrostatic field between the cathode and anode by an unactivated area of the cathode which extends beyond the activated area, but remains in the same spaced Y relation to the anode. Activation of this area is Vl0 The prevented since these end portions of the cathode are shielded from the activating material of the getter by the end caps B and 9.

The activated area of th* cathode is suiciently large to insure that the effect of ionization due to external influences at any given instant will be comparable to that during any other short period as measured in microseconds. Thus while in one portion of the space I5 there may be ionization due to some transient at one instant, the

area is su'lciently large so that at some portion of the space the transient conditions due to radiation and similar effects will be similar at any subsequent instant so that variation of the breakdown potential due to differences in these transients at dilerent instants is avoided.

I have also found that the electrical characteristics of the tube are improved and constant starting potentials are assured if the equidistant spacing between the active portion of cathode and the anode, as above described, is made approximately equal to the minimum starting potential distance.

By minimum starting potential distance, I refer to the fact that if the two electrodes in a gaseous discharge device are moved relative to each other there will be found a certain spacing at which, if all other factors remain constant, the starting voltage is at a minimum. If the electrodes are moved closer together than this distance the starting potential will be found to rise sharply. If moved farther apart the starting potential also rises, but more gradually. Thus for any given electrode material in any given gas at a given pressure there may be found a spacing for the electrodes at which the starting potential is at a minimum.

While it is preferred that the distance between any point on the cathode and the nearest point on the anode in the uniform field between' the two electrodes be substantially equal to the minimum starting potential distance, as above described, it will be understood that some of the benefits of the invention may be attained at a somewhat greater distance, in which case the tube will start at a somewhat higher potential if other factors are maintained constant. However, the distance between the cathode and anode should not be substantially of less than the mini- 'mum starting potential distance since in that case the discharge will tend to seek paths between the cathode and the anode which are greater than the uniform distance therebetween. In other words the discharge will not tend to occur between a point on the cathode and the most adjacent point on the anode, but will tend to take place between more distant points, and this willv result in variations in the starting potential voltage. The minimum starting potential distance corresponds to a path suiciently long to permit a gaseous discharge due to gas ionization, and while the distance should not be less than this it may be somewhat greater. I have found that if the spacing is too great the initial discharge may also occur between some point on the cathode and a point on the anode afueras@ which is substantiallygreater than the shortest path. Accordingly, I prefer an electrode spacing which is sufciently long to permit a gaseous discharge due to gas ionization along the shortest path therebetween, but sufliciently short to prevent the initiation of the discharges along paths substantially greater than said constant distance. In one successful embodiment of the invention the gas used was argon at a pressure of millimeters of mercury, and thedistance between the cathode screen 6 and the surface of anode 'l was forty-thousandths of an inch. The getter used in this instance was an alloy containing barium and tantalum which when flashed by applying current to the leads 23 and 24 provides a source of pure barium.

' In operation the same tube will always start at the same voltage in contradistinotion to conventional constructions in which the starting voltage may vary considerably and has not heretofore in any oase been less than one volt. This uniformity in starting characteristics is due to the geometry of the tube by which there is no variation in the effective distance between any point on the cathode'and the nearest point on the anode. Furthermore, irregularity, due to edge effects which might result in the discharge seeking to traverse a path greater than the uniform distance between the cathode and the anode, is avoided by the fact that the electrode areas beyond the end caps 8 and 9 are unactivated while this unactivated portion extends in the same geometrical relationship to the anode as does the activated portion. This provides a uniform field throughout that portion of the space I5 between the activated area of the cathode and the adjacent area of the anode. Thus initial discharges through the non-uniform field which exist, due to edge effects adjacent the upper and lower ends of the cathode and anode, are prevented, since initiation of the discharge at these unactivated ends does not tend to occur at voltages approaching the break-down voltage adjacent the activated portions. The discharge must always occur through uniform distances in uniform fields regardless of position of the point on the active surface of the cathode at which the discharge may be initiated.

The possibility of discharges being initiated through a non-uniform field at the edges of the cathode and anode is further prevented by the fact that these edges are closed or blocked by the mica plates i9 and 20.

In the embodiment of the invention shown in Figs. 4 and 5, numeral 3| indicates a sealed envelope having a reentrant stem 32. Current lead-in wires 33 and 34, sealed in stem 32 by a press 35, provide connections between a source of power and electrodes 36 and 31.

In this form of the invention the cathode screen 36 is mounted parallel to the fiat anode plate 31, the screen 36 being welded or otherwise rigidly fixed at its peripheral portion to a plate 38. The plate 38 is provided with opening 39, which opening leaves the central portion of the screen 36 uncovered.. while the edges of the screen are shielded by the peripheral portion of the plate 38. The anode 3l and the cathode supporting plate 38' are held in spaced parallel relation by means of insulating washers 49, having rivets 4| extending therethrough. The heads of the rivets 4| bear on insulating washers 42 on the outer surfaces of the platesk 3'! and 38. Lead-in wires 43 and M, which are sealed in the stem 32 by the press 35, support a getter 45 in a position adjacent the outer surface of the screen 36.

" Preferably the plates 31 and 38 are spaced so that the distance therebetween is substantially equal to the minimum starting potential distance for the kind of a gas and the pressure within the envelope 3|.

The manufacture of the device is similar to the mode of manufacture described in connection with the form shown in Figs. 1 3. The screen 36 is activated by iiashing the getter i5 to which current may be supplied by wires i3 and 44. It will be seen that only that portion of the screen 36 which is exposed by the opening 39 in the plate 33 will be activated, the remaining portions being shielded by the solid peripheral portion of the plate. Thus this structure also provides an arrangement in which the gap between any point on the active cathode area and the anode 31 is the same as that of any other point. The electrostatic field between the active area of the cathode and the anode 3l is substantially uniform land distortion of this field at the edges thereof is prevented by the extension of the cathode plate @il and the anode 3l' for some distance beyond the active area, so that although the active area terminates at the edge of the opening 39 the electrostatic field continues uniform for some distance beyond this active area and the distortion at the outer edges of the plates 31 and 38, due to edge effects, does not materially affect the eld in the active area, and discharges in the non-uniform eld at the edges of the plates are prevented since these portions are unactivated.

Although there has been herein described but two embodiments of the invention, other embodiments within the scope of the. appended claims will be obvious to those skilled in the art and may be constructed in accordance with the teachings herein from a consideration of the two embodiments shown.

What is claimed is:

l. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere, an anode, and a cathode forming with said anode agap of substantially a constant distance between any point on the active surface thereof and the nearest point on said anode, said constant distance being not less than the mini- 'mum starting potential distance, said cathode having its active area spaced from its ends or edges, providing an inactive area extending for a suflcient distance from said active area toY provide an extension of the electrostatic field beyond said active area, whereby the field adjacent said edges is the same as the field between said anode and said active surface at points remote from said edges, andgaseous discharges along paths greater than said constant distance are prevented.

2. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere, an anode, and a cathode forming with said anode a gap of substantially a constant distance between any point on the active surface of said cathode and the nearest point on said anode, said constant distance being not less than the minimum starting potential distance, said cathode having its active area intermediate its ends and an inactive area extending in the same spaced relation to said anode as said active area, said inactive area of said cathode being separated from said anode by free space, whereby the electrostatic eld between said anode and the active area of said cathode is substantially uniform and distortions due to edge effects are eliminated.

3. An electric gaseous discharge device com'- 7 prising a sealed envelope containing'a gaseous atmosphere, an anode, and a cathode, the distance between any point on the active area of said cathode and the nearest point on said anode being substantially equal to the minimum starting potential distance for the pressure and kind of gas.

within said envelope, said cathode having said active area intermediate the ends thereof and an inactive area extending in the same spaced relation to said anode as said active area, said inactive area of said cathode being separated from said anode by free space, whereby the electrostatic eld between said anode and the active area of said cathode is substantially uniform and distortions due to edge eiTects are eliminated.

4. An electric gaseous discharge device comprising a sealed envelope containing a gaseous atmosphere, an anode, and' a cathode, the dis-` tance along any line of force between the active area of said cathode and said anode through free space in said tube being substantially equal to the minimum starting potential distance for the pressure and kind of gas within said envelope, said cathode having said active area intermediate the ends thereof and an inactive area adjacent the said ends, said inactive area extending in the same spaced relation to said anode as said active area, said inactive area of said cathode being separated from said anode by free space, whereby the electrostatic eld between said anode and the active area of said cathode is substantially uniform and distortions due to edge effects are eliminated.

5. An electric gaseous discharge device including a sealed envelope containing an anode cornprising a cylindrical member, a cathode comprising a cylindrical member having an activated intermediate portion and unactivated end portions, said anode and cathode being concentrically arranged one within the other and spaced to provide a gap of constant distance between any point on the active surface of the cathode and the nearest point on the anode, said unactivated end portions of said cathode extending in the same spaced relation to said anode as said acti-- to the minimum starting potential distance for.

the pressure and kind of gas within said envelope, said cathode having an active area intermediate the ends thereof and an inactive area extending in the same spaced relation to said anode as said active area, said inactive area of said cathode being separated from said anode by free space, whereby the electrostatic field between said anode and the active area of said cathode is substantially uniform and distortions due to edge eects are eliminated.

PAUL W. STUTSMAN.

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

vUNITED STATES PATENTS Number Y Name Date 1,545,207 Smith July '7, 1925 1,731,889 Donle Oct. 15, 1929 1,803,985 Walker May 5, 1931` 2,083,545 Braselton July 27, 1937 2,141,654 Kott Dec. 27, 1938 2,195,913 Bachman Apr. 2, 1940 2,256,101 Muller Sept. 16, 1941 1,948,720 Knowles Feb. 2'7, 1934

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