US2604602A - Cold cathode gaseous discharge device - Google Patents

Description

July 22, 1952 e. H. HOUGH COLD CATHODE GASEOUS DISCHARGE DEVICE Filed Feb. 11, 1950 2 SHEETSSHEET 1 I a: i q r Inventor 550E6 5 H. HOUQH- A itorney July 22, 1952 e. H. HOUGH COLD CATHODE GASEOUS DISCHARGE DEVICE 2 SHEETSS!-IEET 2 Filed Feb. 11, 1950 I/IAL VA l/Al VA VI Attorney therefore obtainable.

Patented July 22, 1952 COLD CATHODE GASEOUS DISCHARGE DEVICE George Hubert Hough, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 11, 1950, Serial No. 143,774 In Great Britain February 21, 1949 Claims. 1

The present invention relates to cold cathode gas-filled electric discharge tubes of the type known as trigger or relay tubes-i. e. tubes having a main glow discharge gap and an auxiliary gap of lower breakdown voltage than the main gap, whereby a voltage applied across the auxiliary, or trigger gap may cause a discharge across the main gap.

The present invention is particularly concerned with the construction of trigger tubes suitable for high speed pulse operation-by which I mean operation at pulse repetition rates in the region of 100 kc./s.

Previously known cold cathode trigger tubes have been suitable for pulse repetion frequencies in the range 50 to 1000 cycles/sec, the latter being considered to be a fast rate of working. A typical known type of trigger tube has a single cathode of larger surface area compared to that of the anodes, the surface usually being coated to reduce the work function, and a plurality of rod-like anodes, the anodes, the anode-cathode separation for the main discharge gap being greater than that of the remaining auxiliary gap or gaps, which are positioned to one side of the main gap. Mentioned should, however, be made of a specially designed trigger tube for operation at pulse repetition frequencies of 100 kc./s.,

described in the co-pending application of G. H.

Hough, C. D. B. White and W. D. R. Rivers,

filed May 5, 1949, and bearing Serial No. 91,578,

now matured into U. S. Patent No. 2,527,552. In this tube the discharge gap assembly comprises a pair of parallel opposed electrode plates defining the main discharge gap and having a trigger electrode mounted to define an auxiliary discharge gap to either of the said plates, the trigger gap so formed providing a discharge path substantially at right angles to the path across the main gap. The main gap electrodes having parallel opposed planar surfaces, the electric field between them tends to be uniform and so assists in the removal of the ionisation products following a discharge to a greater degree than if the field were concentrated as between a point and a plane electrode; a short deionisation time is Due to the fact that the paths of the auxiliary and main discharges are close together and that the main gap electrodes tend to screen the trigger electrode so that ionisation products from the auxiliary discharge are concentrated in the neighbourhood of the main discharge path, rapid transfer of discharge from auxiliary to main gap is favoured.

According to the present invention there is.

constituents and pressure of the gas.

provided a cold cathode gas-filled electric .discharge tube having a first and a second electrode defining a main discharge gap and a third trigger electrode defining with said first electrode an auxiliary discharge gap of lower breakdown than said main gap, characterised in this,'that the discharge path across the auxiliary gapis part of the discharge path across the main gap. Improved performance at fast repetition rates is assisted bya further feature of the invention according to which in such a discharge tube the trigger electrode is positioned a distance substantially equal to, but not. less than, the length of the cathode fall of potential for normal glow discharge away from the said first electrode.

Before proceeding to describe embodiments of the present invention, some discussion of the requirements of a high speedtrigger tube in relation to the phenomena of the gaseous glow discharge may be of assistance to the understanding of the invention. It should, perhaps belemphasised that as I am concerned with glow discharge at sub-atmospheric pressures the phenomena involved are, in general radically different from those connected which are and spark discharges at atmospheric pressures and above.

In the following discussion and in the description of the invention, reference will be made to the accompanying drawings in which:

Figs. 1, 2 and 3 are curves relating to glow discharge phenomena.

Fig. 4 shows in partial section, but not to scale, an embodiment of the parent invention.

Fig. 5 is a diagrammatic drawing in perspective of the electrode system of the embodiment of Fig. 4, and

Fig. 6 shows an alternative arrangement of the electrodes to that shown in Figs. 4 and 5.

It is well known that the voltage required to establish glow discharge between two electrodes in a gaseous atmosphere depends upon the work function of the cathode electrode, the separation between anode and cathode and the chemical The relationship between gap length, gas pressure and breakdown voltage is given by Paschens law: the curve relating breakdown voltage with the product of gaseous pressure and gap length shows a marked minimum at a critical value of the product. This critical value of the product corresponds to a gap length equal to the length of the cathode dark space in th discharge column between cathode and anode. A typical Paschen curve is shown at I in Fig. 1, in which voltage V is plotted as ordinate and the product 12d of the than a certain current, the gap potential rises, as-

it does als for small currentvalues. The dis,-

charge cannot be maintained'if the current falls below a certain minimum while if the cathode current density becomes too-great, .the-glowdischarge eventually becomes transformed into an arc discharge. The maintainingvoltag asdefined above is a function of .thegas-pressure and the gap length and varies with theproduct 1nd: in

the manner shown by curve 2 in Fig. 1. The two curves l and 2 relate to the same gap and gas mixture: for a given gas pressure the breakdown and maintaining :voltagesxvary rapidly withgap 'length :for lengths less than. the. critical distance "giVi'ngthe minimum va'lueszofithese voltages; with increasing .gap length the-maintaining. .voltage rises much less rapidly. than does th breakdown "voltage: the minimum valuesof-breakdown and maintaining voltages are. very close together and .for most. practicalpurposes maybe. considered identical; Whenadischarge' is being maintained *betweenftwo electrodes. the potential V -along a line-of forceibetween cathode I (K) and anod (A') varies with-distance .(d:)' in :a. manner similarto thatzshownzin Fig; 2. It rises from. the cathode to a maximum at a distance do which isthe same as thesgaplengthforminimumbreakdbwn voltage.- Itthe gapislongerthan do, the potential fallszslightlyand then-rises:practically'linearlyto the-potential ofr'the anode. The distance-do-marks the position of cathode glow. At low pressures severalidistinct regionsof luminous glow canhe distinguished in the discharge column. Inparticular regions, known as the Aston glow (called by some writers-:the cathode glow) and the neg- :aztive'rglow, separated by-the- Crookesdark-space,

arellocatediadj acent the cathode :"at higher pres- .sure-the Aston glow is hardly observable and in all cases the light intensity of-the-negative glow is much stronger than that ofth-e- Aston -glow-: in

the present specificationthe Aston -glow-'hasbeen disregarded.- The cathode glow (Lethe negative glow) is succeeded by the-Faraday dark-space, which,v inutubes .of thekind we are considering, extends, for. all practical purposes; to: the anode glowadjacenir the anode. The region extending from: he; cathode $011.2, distancezdo from it is referred to generally herein as the cathode dark --space,- while thencriticaldistance: do is defined as the length. 0f. the; cathode: :fall of potential for normal glow discharge. The cathode fall of potential is independent of discharge current provided thecathodeisnot completely covered with glow. This defines tharegime of normal glow discharge When the cathode is completely covered with glow and the dischargecurrent is increased. the discharg enters the regimenof' abnormal glow: the cathode fall of. potential: increasesand the cathodedark space contracts.-

Thecathode glow: is due-to the emission of line spectra-.from-atomsof the gas filling-up the discharge tubes and-marks a region of positive space charge formed by an accumulation of positive ions. Duringv themaintenance of discharge, posi- .tive ionsfrom this region of space charge are accelerated towards-the cathode by the cathode fall of potentialand .theseliberateelectrons which are I may besummarised as follows.

accelerated towards the anode and ionise the gas by collision, thus continuously replenishing the positive space charge in and around the cathode glow. The actual mechanism by which the positive ions cause electron emission from the oathode surface is still the subject of investigation, but there is no doubt that the setting up of the space charge associated with the cathode glow is essential for the maintenance of a steady state discharge. When this condition of space charge hasbeen established, and not until then, the discharge gap is said to be fired. It is evident that .a finite time is needed to fire a discharge gap, and

the initiation ofdischarge must now be considered.

edge the salient features .ofthe phenomenathat occur. when a rectangular pulse of' positivrevoltage isapplied to theanode of a discharge gap Atzfirst. no current Lpasses'between cathode and. anode. until some random atomic eventliberates anuelectron within the anode-cathode field and this electron is accelerated along the potentialugradient .towards:theanode to ionise :moleculesofgas in its path. The time that elapses before .such an event: occurs is called the statistical delay'time and is dependent upon the degree of? ambient illumination, the intensity of. cosmic radiation andthe like well 1 asi'upcn thezprevious history of the tube. The positive ion orfionszproduced by this first: electron are drawn by the electric fieldxto the cathode where they liberate further electrons to' produce the requisite space charge conditions in the-gap. The time required for the gap to. he fired :after the statistical lag is called theformative delay timeand'is afunctionof the anode-cathode potential difference.

Becausezof the formative delay time, the min.- imum voltage" pulse required. to fire a gap is a functionof the pulse width. Thus, for a given pulse voltage, the pulse duration :must be at least equal to the formative delay time appropriate to the, total inter-electrode voltage: a pulse of shorter duration will not-fire the gap, the space charge, formedubeing. insuificient to produce the cathode mechanism which makes the glow self ,ma'intaining; After the gap" has been fired, a

further. time interval; whichwe call the build-up time,is needed'for the discharge current to rise to its-maximum steady value determined by the D. .0. load; The buildj-up time is determinedby the-external circuit time constant and the potential appearing on the ano d'e'.

It is .usualin operationofi a: discharge tube, to superpose the firing pulses'upona steady polarising voltage whichenables-the discharge to be maintained after firing. To extinguish the discharge, the. anode cathode. potential difference must be. made lessthan th'em'aintaining voltage; for this purpose. it is: ezqiedient to: superpose a negative-going extinguishing pulseon the steady polarising voltage. In order that" the positive ions'an'd electrons may recombine as rapidly: as

- possible; we. have. found that the negative pulse therefore, desirable when short deionisation times are required.

Turning now to the operation of a trigger tube, when an auxiliary gap has been fired, ionisation products from the auxiliary discharge prime the main gap and reduce the break-down potential thereof to a value equal to, or less than, the steady potential applied between the main gap electrodes. It is of course necessary that ions are being produced at a fast enough rate to be effec tive for this purpose; there is, therefore, for any given electrode assembly and associated circuits, a minimum value of trigger electrode current required: this minimum current is referred to as the transfer current. Analogous to the formative delay time described above, there is with trigger tubes a transfer time, which is the time required for the main gap to fire after a discharge current equal to the transfer current has been built up at the trigger gap. The transfer time is made up of the time required for charged particles to migrate into the main gap, the formative delay time of the main gap and the main gap build-up time.

In view of the foregoing, one can state the general requirements of a high speed trigger tube. With regard to the trigger gap, this should be so constructed that the formative delay time and deionisation time should be a minimum, while it is also desirable that the transfer current should be low in order to present a high impedance to the driving circuit. The circuit designer almost invariably requires the further feature that the trigger firing voltage should be as small as possible. The trigger gap should be positioned relatively to the main gap so that the transfer time may be as small as possible, while, from the point of view of minimum build-up time of the main gap, the trigger electrode should distort the field across the main gap as little as possible when the main gap is discharging.

In practice, the most important consideration from the point of view of reduction of transfer time is that the space charge and electric field configuration set up across the main gap by the auxiliary discharge should be as nearly as possible the same as that which will exist when the main gap is fired. If they can in fact be made identical, then, providing it is properly biassed, the main gap will fire as soon as the auxiliary gap has firedi. e. the transfer time is nil. It is obvious that in a tube according to the present invention, where the auxiliary discharge path lies in the ma'i n'discharge path, the re-orientation of the electric field and change in position of the space charge is reduced to a minimum. Furthermore, when the trigger electrode is placed at the critical distance do (Figs. 1 and 2) from the cathode of the main gap and is itself operated 'as an auxiliary anode, since the trigger maintaining voltage is practically the same as its breakdown voltage, the field distribution across the main gap when a discharge is being maintained across the auxiliary gap is just that described with reference to Fig. 2-assuming, of course, that the main anode potential is kept at the main gap maintaining voltage with respect to the cathode. Furthermore, if the trigger electrode is in the form of a grid or apertured plate, the cathode glow for both main and auxiliary discharges is in the same position and the main gap will fire simultaneously with the auxiliary gap. Again, if all these electrodes are planar, the deionisation times for both auxiliary and main gaps are reduced to a minimum for the gas employed.

In the above discussion the trigger electrode is assumed to be an auxiliary anode co-operating withthe cathode of the main gap. For some circuit applications it is desired to fire a relay tube by a negative-going pulse applied to the trigger electrode. In such case it is usual to use a common anode for both auxiliary and main gaps rather than to use the cathode of the main gap as the anode of the auxiliary gap-if this were done a dischargewouldprobably also be set up between the main anode and the trigger electrode. A tube according to the present invention may also be be used as a high speed trigger tube, using the trigger as an auxiliary cathode, with imperceptible change in speed of operation. The connections to the main electrodes are reversed so that the trigger electrode is now adjacent the anode of the main gap. At first sight it may appear that in a tube having the trigger electrode spaced the distance do from the anode, the transfer time would be much longer than when using an anodic trigger: the cathode glow of the auxiliary discharge would tend to coincide in position with the anode and would have to be transferred across the greater part of the length of the main discharge gap before the latter fired. In fact, We find, with suitable choice of polarising potentials, that in such tubes the transfer time is too small to be measured whether an anodic or cathodic trigger circuit is used, although time intervals of A 11. sec. could have been detected in our experiments. This apparently anomalous result may be explained with the help of the curves shown in Fig. 2 and 3.

In Fig. 2 the positions of the cathode, trigger electrode and anode for the anodic trigger action previously discussed are indicated by K, T and A respectively, and the graph shows the distribution of potential along a line of force between these electrodes when the main gap is fired. In Fig. 3 the positions of A and K have been interchanged and full line curve 3 indicates the desired field variation when the main gap is fired. In order'that the trigger electrode should not unduly distort the field, it must be biassed positively to the voltage VT. Under these conditions discharge current will pass to the trigger electrode as well as to the anode; the'trigger current may be made negligible, how- 'ever, by connecting a high valued resistance in series with the trigger electrode. In order to fire the auxiliary gap, a negative voltage pulse must be applied to the trigger electrode, say through a condenser, in opposition to its bias voltages: when the auxiliary gap is fired, the potential distribution changes to that shown by the dotted curve 4. When the negative pulse is removed, the trailing edge of the pulse may be regarded as a positive pulse applied to the trigger. The potential of the trigger electrode is immediately restored to (or momentarily even exceeds) its bias voltage, and the positive ions from the auxiliary discharge are accelerated along the steep potential gradient towards the main gap cathode thus off-setting the formative delay time for firing the gap T-K. The net effect is that the transfer of glow discharge from trigger to main gap cathode is practically instantaneous. In the case of either cathodic or anodic triggering the minimum width of pulse needed. to firetheauxiliary gap-is such that it is equal to the. formative? delay timefor that gap. If triggering pulses of the minimum-width are used, the auxiliary gap, willfire on the trailing edge of the pulse and the overall time. required tofire the maingap-is' the same in either case, within the limits of' accuracy of. present measurements. Thus a tube according tothe present invention appears to be equally suitable for high speed operation whether. the trigger is used as an auxiliary anode or asan auxiliary cathode;

Fig. 4 illustrates an embodiment of the invention. Asotherwise the features of the construction would be confused on account of the small dimensional clearances, the drawing is notto scale. The electrode assembly is mounted by means ofmica sheets and supportingrods from a glass press l'sealed inja conventional envelope 2. The electrodes are madeof nickel and the -main gap electrodes are small plates band 4 (the surfaces of which are unactivated') mounted upon respective rods and- 6: the'rods 5 and 6 are welded to eyelets I and B; clamped to the top and bottom micasheets 9:and Ii respectively. The rods 5 and 6 pass through respectivelocating micas H and I2, and-in the actual construction, the sheets I I and I2 are spaced-from the rear of the respective plates 3- and 4 at a distance therefrom less than the length of the cathode dark space to function as field control plates as describedin the copending application of G. H. Hough and L..C. Baker, filed April 29, 1949, and bearing Serial No. 90,316. Cathode glow-is thereby confined to the front faces and edges of whichever plate, is used as cathode. If desired, metal fieldcontrol plates may be'mounted in the same planes as either'or both the plates'3 and 4 indicated at l3 and I4 in Fig. 6. The trigger electrode [5 comprises an apertured plate separated from the electrode'd by a distance substantially equal to the length of the cathode dark space. In the construction shown in Fig. 4 the electrode I5 is conveniently" mounted upon' the mica sheet 52 by means of bent-over portions l6 and II rivetted to the sheet. 'I'heapertures i8 are so disposed and of such size that the plate l5 offers little obstruction to discharge between the plates 3 and 4. Theedges of the plate !5 about the apertures l8 are preferably left sharp during manufacture to assist in reducing. the

formative delay timeof the trigger. gap by concentrating the field at these edges before the trigger gap breaks down. After the trigger gap has been fired, thedischarge tendsto, spread over the surface of theplate and'asubstantially uniform field is established. The arrangement of the electrodes in Fig. 4 is also shown: in the greatly enlarged sketch of Fig; 5. Connections from plate 3 and trigger. 65 to thepins- I9 of a conventional valve base 28 may conveniently be made by means of the support rods 2| and 22 and wires 23 and 24.

When usingnickel electrodes and a gas mixture of 92% neon, 1% argon and 7% hydrogen at a pressure of 100 mm. of mercury, the length of the cathode fall of potential is 0.165 mm. In a typical-construction arranged as illustrated in Figs. 4 and 5, the plates 3 and d are separated by a' distance of 1.2 mm. to give a main gap breakdown voltage'of 290 volts unprimed) and have a surface area such as will permit a normal discharge current of 20 ma. maximum at a current density of 1 ma. perv square millimetre; The plate dis spaced 0.1 mm. above the mica sheet I 2 and the trigger electrode-0.2 mm. above the plate 4. In order that the trigger electrode l5 shall not unduly obstruct the main gap discharge we have found that the size of the apertures I8 should be chosen so that, when a discharge of the maximum rating is being passed with plate 4 as cathode, the cathode glow over the surface of plate 4 should be uniform as judged by eye. If the apertures are too small, we observe that the glow is marked by areas of greater light intensity corresponding to the apertures. Uniformity of cathode glow is taken in the present specification as the criterion of an unobstructed discharge path across the main gap. With the other dimensions and ratings quoted above we find that apertures of 1 mm. diameter provide an unobstructed discharge between plates 3 and 4.

In a tube having the electrode dimensions quoted, the maintaining voltages for trigger and main gaps are 140 and 160 volts respectively, the transfer current is 50 ,u A, the transfer time in either anodic or cathodic trigger arrangements as described is less than A; a see. the deionisation time of the trigger gap for 500 ,u A discharge is less than 5 sec. While the main gap deionisation time is 15 to 20 ,u sec. (Deionisation time is here defined as the minimum duration of an extinguishing pulse for the gaps not to fire again when the working potential is restored.) A rectangular pulse of volts on volts bias and 4 a sec. duration is required to fire the auxiliary gap and to trigger the main gap when the main gap is polarised by a steady source of 200 volts.

As stated at the commencement of this description, the present invention is directed to a trigger tube capable of operating at high pulse repetition rates. A word of warning should be added to prevent misunderstanding: in the absence of ionising radiation or some other means of shortening the statistical delay time when operating at slow or random repetition speeds, as deionisation in these tubes is so rapid and complete, the short formative delay and transfer times may be more than offset by prolonged and uncertain statistical delay times. At the fast repetitive speeds for which the tubes are intended to operate, deionisation is not completed before the arrival of the next signal: except for the initial firing at the first pulse of a train of signals the efiect of statistical delay time does not occur.

While the principles of the invention have been described in connection with specific examples and particular modifications thereto, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A cold cathode gas-filled electric discharge tube having a first electrode and a second electrode defining a main discharge gap therebetween, a third trigger electrode intermediate said first electrode and said second electrode and defining With said first electrode an auxiliary discharge gap having a lower breakdown voltage than said main gap, the discharge path of said main gap including the discharge path of said auxiliary gap, separate discharge field control means surrounding said first electrode and said second electrode and spaced from those portions thereof defining said main gap a distanceless 9 than the length of the dark space characteristic of said first electrode and said second electrode respectively.

2. A cold cathode gas-filled electric discharge tube as claimed in claim 1 wherein said third electrode is spaced from said first electrode a distance substantially equal to but not less than the length of the cathode fall of potential away from said first electrode.

3. A cold cathode gas-filled electric discharge tube as claimed in claim 1, wherein the discharge surfaces of said first, second and third electrodes are planar and parallel to one another.

4. A cold cathode gas-filled electric discharge tube as claimed in claim 1 wherein said third electrode comprises a perforated plate.

5. A cold cathode gas-filled electric discharge tube as claimed in claim 4 wherein the perforations in said third electrode have sharply defined edges to favor initiation of discharge between said first electrode and said third electrode.

6. A cold cathode gas-filled electric discharge tube comprising a first electrode and a second electrode each having planar surfaces defining a main discharge gap therebetween, a third trigger electrode comprising an apertured plate mounted intermediate said. first electrode and said second electrode and parallel thereto and defining with said first electrode an auxiliary discharge gap having a lower breakdown voltage than said main gap, said third electrode spaced from said first electrode by a distance substantially equal to but not less than the length of the cathode fall of potential for normal glow discharge, the apertures in said third electrode permitting therethrough a substantially unobstructed discharge between said first electrode and said second electrode the discharge path of said main gap including the discharge path of said auxiliary gap,

10 discharge field control means surrounding said first electrode and spaced from the portion thereof defining the said discharge gaps av distance less than the length of the cathode fall of potential for normal glow discharge for said portion of said first electrode. a

7. A cold cathode gas-filled electric discharge tube as claimed in claim 6, wherein said discharge field control means are of insulating material.

8. A cold cathode gas-filled electric discharge tube as claimed in claim 6, wherein said discharge field control means are of electrically conducting material.

9. A cold cathode gas-filled electric discharge tube as claimed in claim 6 further comprising additional discharge field control means surrounding said second electrode and spaced from the portion thereof defining the said main discharge gap a distance less than the length of the cathode fall of potential for normal glow discharge for said portion of said second electrode.

10. A cold cathode gas-filled electric discharge tube as claimed in claim 6, wherein said discharge field control means comprises a sheet of insulating material, said third electrode mounted on said sheet and insulated from said first electrode thereby.

GEORGE HUBERT HOUGH.

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

UNITED STATES PATENTS Number Name Date 1,789,626 Hendry Jan. 20, 1931 2,456,854 Arnott et al. Dec. 21, 1948

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