US2834906A - Gas-filled electric discharge tubes - Google Patents

Description

May 13, 1958 G. KLEPP ET AL GAS-FILLED ELECTRIC DISCHARGE TUBES F iled'April 2a. 1954 Inventom G.F KLEPP- D.A.BEARD A Horn e y United States GAS-FILLED ELECTRIC DISCHARGE TUBES George Francis Klepp and Douglas Albert Beard, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

The present invent-ion relates to cold cathode gas-filled electric glow discharge tubes and is particularly concerned with such tubes designed as stabilisers.

Glow discharge tubes are used as voltage stabilisers, generally, in two distinct ways. In one mode of application a stabiliser tube is used to supply current to a load connected directly across the tube so that the current passed through the tube varies as the load current is altered, the voltage across the tube and load remaining nearly constant. In the other application the tube is used as a source of reference voltage to control the voltage of a power supply providing a current larger than would normally be passed through the tube. Although there is, for most purposes, no fundamental reason why the same tube should not fulfill both functions, it is usual to employ tubes of different construction for the two kinds of circuit. In the case where the load is fed directly from the stabiliser tube, which thus acts as a low impedance source, the cathode geometry in commercial tubes has heretofore been determined largely by the current handling requirements-generally calling for a cathode with as large an area as possible-and the convenience of manufacture of the cathode. In general, these considerations have led to the choice of a cylindrical cathode, the diameter of which is such that it just fits the tube envelope, the inner surface of the cathode cylinder being used.as the active discharge surface; the anode, forming therewith the necessary discharge gap, takes the form either of a concentric inner cylinder or a rod or wire along the cathode axis. In tubes for use, on the other hand, as sources of reference voltage, large current handling capacity is not generally required, the current passed by the tube being maintained at some conveniently small value. For such tubes small flat cathodes vhave been used, although, as stated above, for many purposes a stabiliser tube through which a comparatively large current could be drawn can also be used for voltage reference purposes, the discharge current being then considerably less than the mean current rating of the tube.

For-the stabiliser tube directly feeding a load circuit it is necessary that the change in voltage drop across the tube with change of discharge current should be a minimum, and, if not entirely constant, should vary as smoothly as possible. With the source of reference voltage, on the other hand, a principal difliculty is the fluctuation of voltage drop across the tube when the current is maintained constant. This is due to wandering of the cathode glow about the cathode surface. With the cylindrical tube construction for direct connection to the load, we have found, on the other hand, that a cause of trouble is that there is too great a tendency for the cathode glow at small discharge currents to remain concentrated on one area. As the discharge current is increased, the glow does not spread smoothly and uniformly across the oathode surface but tends to form into a cylindrical ring, around the cathode, not necessarily so constant in position that there is no voltage fluctuation across ,the tube,

atent O but such that when the discharge current is increased, instead of the glow spreading out axially and smoothly along the cathode surface, it will prefer the ring formation. This causes an undesirable rise in the voltage across the tube. Our investigation into this particular trouble shows that the phenomenon giving rise to it may be utilised to prevent fluctuation of voltage due to wandering of the cathode glow in reference voltage arrangements, and that, at comparatively slight cost to optimum performance of the tube in either of the two fields with which we are concerned, enables us, in large measure, to overcome the principal defects of wandering of the cathode glow in the one case and excessive localization of the glow in the other.

The present invention is based upon our observation that cathode glow tends to concentrate on surfaces which are concave towards the anode and to avoid convex surfaces, other conditions such as the discharge gap lengths and anode geometry being the same. This is not unexpected when it is borne in mind that the cathode glow is due to excitation of the atoms of the gas-filling of the tube in the region of space charge associated with the cathode glow. This glow is situated at a distance from the cathode surface equal to the depth through which the cathode fall of potential takes place, i. e. approximately the minimum sparking distance. The space charge density thus tends to be less at a convex surface, from which the charge can more readily diffuse, than at a concave one; hence, other factors being equal, it is to be expected that the maintaining potential for glow discharge between an anode and a cathode will tend to be less if the cathode has a concave surface than if the surface be convex. In a tube with, say, a cylindrical cathode and an axial rod-shaped anode, for small discharge currents, in which the glow does not cover the whole of the cathode surface, the occurrence of glow at any particular spot is quite fortuitous.

Due to such phenomena as sputtering, contamination, clean-up and re-deposition, the nature of the cathode surface tends not to remain constant, and, according as the properties of an area of the cathode surfaces change slightly, so will the glow tend to move about onto that portion which, at the moment, favours the lowest maintaining voltage. If, however, the surface is not of uniform curvature, but has a region more concave than others, then the glow will tend to concentrate in this more concave region.

In the cylindrical cathode type of tube the cathode has a negative curvature in a plane normal to the axis of the cylinder, but has zero curvature in a plane containing this axis. In the present specification the convention is used that a surface which is concave when viewed from the anode has a negative curvature, while a convex surface has positive curvature.

According to the present invention, there is provided a cold cathode gas-filled electric glow discharge tube comprising an anode and a cathode having acommon axis of symmetry and defining a discharge gap between them, the cathode being shaped so that the radius of curvature of the intersection of the cathode discharge surface with any plane containing the said axis is not positive over the region at the minimum distance from the anode and is finite over at least a portion of said intersect-ion.

In the embodiments of the invention the excessive localization, at low current values, of glow discharge in a ring around a cylindrical cathode is avoided by providing curvature of the cathode surface in the plane containing the cathode axis, and, where it is desired to localize the cathodeglow to a given area of the cathode, the curvature of the cathode surface in this region is made more negacurrent handling range, the embodiments-of the invention utilise a generally barrel-shapedcathodesurrounding the anode, while in tubes designed more specifically for. use

as sources of reference voltage and having smaller cur rent handling capacity, the anode and cathode are situated at opposite ends of the axis of symmetry, the cathode being, for example, in the form of a disc having a region of negative curvature in the middle, or being a modification of the known type of construction inwhich the cathode is in the form of a spherical'cap covering the anode.

With some small sacrifice of regulation, dilierent curve.- tures in the plane of the axis of a barrel shaped cathode may be provided, so that the curvature is a maximum at the middle of the cathode and decreases towards the ends, thus adapting a tube for use down to lower currentlevels than would otherwise be the case, thus extending its operational current range. Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 shows a barrel-shaped cathode in section according to the invention,

Figs. 2 to 6 show diagrammatically various means of shaping a cathode according to the present invention, and

Fig. 7 shows a stabiliser tube having a cathode according to the invention.

In Figs. 1 to 6 an anode 1 is indicated merely to show its general location, without regard to the anode geometry or to the anode-cathode clearance. In each case the axis of symmetry of the anode and cathode is indicated by the dash-dotted line 2.

In Fig. 1 the cross-section of the cathode by a plane through the axis of symmetry is indicated at 3. The cathode surface is provided with a negative curvature in the plane of the axis, a single radius vector 4, originating from the centre of curvature 5, indicating that the curvature is constant.

In Figs. 2 to the anode and cathode are situated at opposite ends of the axis of symmetry. In Fig. 2. the cathode 6 has maximum negative curvature at the middle, the curvature decreasing uniformly towards the ends. This is indicated by the short radius vector 7, originating from a centre of curvature 8 on the axis of symmetry, and the longer radii 9 and 10 originating from respective centres of curvature 11 and 12 to either side of the axis of symmetry. In Fig. 2 it will be seen that the curvature of the cathode is always negative.

In Fig. 3, as indicated by the radius vector 13, originating from the centre of curvature 14 on the axis 2, the curvature in the middle of the cathode is negative, and increases positively towards the edge to become zero at the periphery 15, where the cathode surface is substan- In Fig. 4 the curvature in the middle of the anode is similar to that in Fig. 2, but at the cathode edge the curvature has become positive, as indicated by the radius vectors 16 and/17, originating from respective centres of curvature 18 and 19 on the opposite side of the cathode to the centre of curvature 14. In Fig. 5 the middle of the cathode is fiat, i. e. it has zero curvature, while at the edges the curvature, as indicated by the radius vectors 20 and 21, originating from the respective centres of curvature 22 and 23 on the side of the cathode remote from the anode, the curvature is positive.

In Figs. 2 to 5 the cathode shape is such that at small discharge currents the glow will tend to be localized at the middle of the cathode. The uniformly curved cathode of Fig. 1, however, does not have this property, but insures an even axial spread of glow and consequent minimum steepness of the regulation curve. If desired, at some slight worsening of the regulation curve as compared with the cathode of Fig. 1, the principles of Figs. 2 to 5 can be applied tothe barrel-shaped cathode as shown in Fig. 6, in which a cathode 24 has a minimum radius of curvature in the middle, as indicated by the radius vector 25, originating from the centre of curvature 26 in the median plane of the cathode, and has reduced curvature nearer the ends of the cathode, as indicated by the longer radius vectors 2'7 and 27, originating respectively from centres of curvature 28 and 29 to either side of the median plane.

A cold ca hode stabiliser tube embodying the invention is illustrated in Fig. 7. The tube comprises an envelope 3% having a base 31, provided with a conventionallocating spigot 32 and connecting pins 33. Within the. envelope a barrel-shaped cathode 34, generally similar to the cathode 3 of Fig. 1, shown partially cut-away. The cathode is supported between insulators 35 and 36. A pair of rods 3'7 act as the anode of the tube and support, above a further insulator 38, a getter assembly-39-.' Near their mid-points a pellet 4th of cerium mischmetal joins' the two rods 37. A third longitudinal rod 41 forms-, with a closely spaced wire 42 projecting from the cathode, an auxiliary discharge gap for starting purposesr During processing of the tube a discharge is passed between the rods 37 and the cathode to vapourise the cerium from=the pellet 40 onto the surrounding cathode so to reduce the work function of the cathode surface.

In an experimental tube constructed as describedwith reference to Fig. 7, the cathode was 1 /8 inches long, having a radius of curvature in the plane of the drawing of 2 /2 inches. The greatest cathode diameter was ym inch. The regulation of this tube was such that, over. a

current range from S to 60 milliamperes, the voltage drop across the tube varied substantially linearly from mini mum to maximum discharge current by about 5 volts,- corresponding to an internal source impedance of ohms. The mean voltage drop was 75 volts.

While the principles of the invention have been'de scribed above in connection with specific embodiments, and particular modifications thereof, it is to -be:clearly understood that this description is made only by'way of example and not as a limitation on the scope 0f-=the invention.

What we claim is:

l. A cold cathode gas-filled electric glow-discharge tube comprising an anode and a cathode having a common axis of symmetry and defining a discharge gap between them, the cathode being shaped so that the radiusof curvature of the intersection of' the cathode discharge surface with any plane containing the said'axis'is'not positive as viewed from the anode over the region at' the minimum distance from the anode and is finite over'at least a portion of said intersection, whereby-the cathode glow is spread uniformly across the cathode surface.

2. A cold cathode tube as claimed in claim 1 inwhich a barrel shaped cathode surrounds an elongated anode.

3. A cold cathode tube as claimed in claim 2 in=which the said cathode has the same curvature throughout.

4. A cold cathode tube as claimed in claim 2 in which the said cathode has a minimum radius of curvature in the middle and has reduced curvature near the ends.

5. A cold cathode tube as claimed in claim 1 in which the anode and cathode are situated'at opposite ends of the axis of symmetry.

6. A cold cathode tube as claimed in claim 5 in which the said cathode is in the form of a disc having a negative curvature (as viewed from the anode).

7. A cold cathode tube as claimed in claim 6 in which the curvature of said cathode is a maximum on said axis of symmetry and decreases away from said axis.

8. A cold cathode tube as claimed in claim 5 in which the said cathode is in'the form of'a disc concave to.the

centre of the anode in its central portion havinga flat-pen tion surrounding said peripheral portion.

9. A cold cathode tube as claimed in claim 8 in which 5 6 said cathode has a portion convex to said anode surround- References Cited in the file of this patent ing said flat portion.

10. A cold cathode tube as claimed in claim 5 in which UNITED STATES PATENTS the said cathode is in the form of a disc flat in its central 2,523,287 Friedman Sept. 26, 1950 portion and having a surrounding portion convex to said 5 2,548,225 Linder Apr. 10, 1951 anode. 2,668,254 Stutsman Feb. 2, 1954

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