US2645742A - Electric discharge device - Google Patents

Description

Filed March 11, 1948 Inventor A. H. REEVES Way-aw A Hnrnev Patented July 14, 1953 ELECTRIC DISCHARGE DEVICE Alec Harley Reeves, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 11, 1948, Serial No. 14,184 In Great Britain March 20, 1947 2 Claims. 1

The present invention relates to cold cathode electric discharge devices in which the initiation of a discharge between a pair of electrodes is conditioned by ionisation coupling from a previously ignited discharge across an adjacent discharge gap.

It is charactistic of cold cathode gas-filled discharge tubes, that the voltage required to initiate a discharge between two electrodes, depends upon the nature and pressure of the gas, its state of ionisation, the shape and material of the discharge electrodes and the distance between them the gap length. At low inter-electrode voltages negligible current will flow if the gas is initially ionised. As the voltage is increased the molecules of the gas become ionised until eventually, a discharge is set up with a rapid increase in current flow which may typically rise from microamperes to milliamperes at a critical voltage which is the striking or firing voltage for the discharge gap. The discharge is characterised by a glow which at first appears at the cathode and may extend to the anode and beyond the immediate neighbourhood of the gap, depending upon the degree of ionisation and is associated with the migration of ions and electrons. When once a discharge has been established, the interelectrode voltage tends to fall and to become, to a large extent, independent of the discharge current. This approximately constant voltage is called the maintaining voltage. Thus, in the familiar neon tubes used as voltage regulators and the like, the striking potential may be of the order of 100 volts or more, but the maintaining voltage tends to be constant in the region of 80 volts.

As mentioned above, ions and electrons tend to migrate from the immediate neighbourhood of the discharge. This phenomenon has been extensively used to lower the striking potential of another discharge gap in the same tube envelope. In one known device there is provided a main discharge gap between a main anode and a trigger gap between an auxiliary anode and the said cathode. The auxiliary anode is much closer to the cathode than the main anode so that the initial striking voltage of the trigger gap is considerably below that of the main gap. The trigger gap is used to lower the striking voltage of, or to prime the main gap, by ionisation coupling.

It is also well known that whereas, when a striking voltage is applied to a gap, ionisation takes place in a very short time, measured in microseconds, if the voltage across a discharging gap be removed, de-ionisation is not complete until after a substantial period of time which may often amount to milliseconds.

In my copending application Ser. No. 763,655, filed July 25, 1947, the phenomenon of ionisation coupling is utilised to provide a cold cathode discharge tube having an ordered array of gaps in which the starting gap is arranged to have a lower striking voltage than the remainder so that a recurrent voltage applied to each gap in turn or to all gaps together, fires first the starting gap and then the remainder in order. Due to the finite deionising time, it is not necessary that a priming gap be discharging simultaneously with the application of the recurrent voltage to a neighbouring gap. Thus, such a sequence discharge tube, as it may be called, may be operated by a train of pulses applied between a common anode in the form of a plate or wire and an array of cathodes, each mounted independently or forming the hills of a corrugated plate facing the anode or perhaps .taking the form of rods mounted upon a common plate or wire like the teeth of a comb, and constituting an array of gaps of which one gap (usually the first), has a length shorter than the length of each of the remaining gaps. Then the first or starting gap may be fired by the first pulse, while the second pulse reignites the starting gap and the next adjacent gap simultanenously. The third pulse will fire the first three gaps and so on. On the other hand, if a maintaining battery be connected across the gaps, each gap will remain discharging indefinitely when once fired. Thus, such a sequence discharge tube has been used, with suitable circuits to extract the information and to restore the initial conditions as to ionisation, as a counter and/or as an information storage device.

One of the difiiculties that has arisen'in the use of sequence discharge tubes operating with a maintaining battery is, that if the ionisation coupling between gaps becomes high, and depending upon the spacing between gaps and the current passed by them, there is a tendency for a gap to fire automatically before the arrival of the pulse which should initiate the discharge at that gap. It has been found that it is possible for ionisation to reduce the striking voltage even below the maintaining voltage. It will be evident that ionisation will spread from one discharge gap to another at a finite velocity and it has been found that this velocity is sufliciently constant and determinable to enable successive dischargegaps to be spaced at such distances that ionisation from one gap to another may proceed at definite time intervals without the application of a separate pulse for each gap. The present invention is particularly directed towards devices which utilise this time aspect of discharge phenomenon.

The present invention accordingly provides a cold cathode gas-filled electric discharge tube having a sequence of discharge gaps arranged in such manner that ionisation from any discharging gap lowers the striking potential of an adjacent gap, and, in such manner, thatif. all gaps are connected to a source of polarising potential, the gaps fire automatically in'succession after one gap has been fired.

The invention also provides an electric circuit arrangement comprising a cold cathode gas-filled electric discharge tube having a sequence of discharge gaps, means for applying a polarising potential across each of the said gaps of insufllcient magnitude by itself to fire any gap, but sufficient to maintain the discharge of a gap otherwise fired, and. means for firing one of the gaps, the arrangement being such that the remaining gaps fire automatically in succession, one at a time, after the firing. of the said gap.

One mode of operation of a device according to the present invention will now be described with reference to the drawing in which reference numeral l indicates a discharge device according to the present invention having an anode 2 consisting of. a flat plate or wire opposite a row of cathode rods 3, 4, projecting from a common plate or wire 5. Rod 3 forms with anode 2. a shorter gap than the remaining rods 4, and serves as astarting. gap. Cathode wire 5 is connectedto ground through a pulse transformer 6. Anode l isoormectedthrough resistance 1 to the positive terminal: of polarising battery 8. The anode; is also connected to a terminal 9 through blocking. condenser II.

The voltage: applied by battery 8 is arranged to be sufiicient tomaintain a discharge but to be less than: the normal striking. voltage at any gap.

If now, a positive pulse be applied between A terminal 9: and earth so that the peak potential of the pulse plusthe applied battery voltage becomes greater than the striking voltage for the first gap, but lower than the striking voltage of the remaining deionised gaps, this pulse acts to initiate a discharge: across the starting gap. After the pulse has died away the starting gap will remain discharging and, in due course, ionisation will. spread to the gap formed by the adjacent cathode 4 and the anode 2; a discharge will be set up across this second gap, and the striking voltage will be reduced until, in time, it becomes equal to the steady battery voltage. The time interval between the striking of the first gap and the striking of the second gap will depend inter alia upon the gap length, spacing between gaps and the value of the battery voltage. The discharge will then extend to the third gap and so on along the sequence.

During the discharge train which has just been described, the total discharge current passed by the device will increase in steps, each step being separated by a definite time interval from the preceding one. Consequently, since transformer S behaves as a difierentiating device, a train of pulses at discrete time intervals will appear across the output terminals I2 and I3.

I After the chainof events described above, cathode glow will extend from all of the cathode rods. In order torestore the tube to its original condition a negative pulse may be applied to terminal ll of sufficient magnitude to quench all the discharges and of sufiicient duration for deionisation to restore the gap striking potentials to a value greater than the voltage of battery 8.

It should be explained that the delay in the striking of the discharges from the cathode rods is believed t be principally due to the following circumstances:

(a) A small but definite time is necessary for the cathode glow, which starts at the tip of the cathode, to extend along the whole length of the cathode; thus, the lowering of the striking voltage. of the adjacent gap, which depends upon the increasing number of ions produced, takes time to develop.

(2)) The ions formed in the first gap take a small but finite time to drift into the neighbourhood of the second gap, so their efiect on the second. gap is delayed. The ionisation coupling which results from these two effects, therefore, has not, so to speak, an indefinitely steep leading edge, but has an edge which increases progressively from zero, and so the lowering of the striking voltage of the second gap increases with time in a finite and continuous manner.

For this reason it is possible to modulate the striking time of the second gap within certain limits by an applied signal voltage if this be desired. Thus, among other uses, a device according to the present invention will produce a train of pulses separated by discrete time intervals, which intervals may if desired be modulated according to some signal as described for example in my copending application Ser. No. 15,581, filed March '18, 1948. It will be evident that other applications of the device are numerous and that the unmodulated pulse trains which have been described may either be such that the time intervals between successive pulses are equal or vary according to a definite law depending upon the disposition ofthe gaps. The data given in the table below have been found satisfactory for the design of a tube to give a pulse train with an interval of 31-1 microseconds between pulses. The gas filling for this particular tube is chosen so as to give a reduced de-ionisation time.

Total pressure 100 mm./Hg.

2 millimetres.

180 volts. 1 milliamp.

20 microseconds.

of extinguishing 150 volts.

1 microsecond.

20 volts.

If desired, a separate discharge gap may be arranged so that a continuous priming discharge is maintained across it. Theme of such a discharge gap forms the subject matter of my co pending application Ser. No. 19,084, filed April 1948. By adjusting the current flowing through the priming discharge gap, the general ionisation level of all the other gaps may be raised or lowered at will. An increase in the general ionisation level has the following prin cipal eifects upon the discharges in other gaps:

1. A reduction of the minimum striking voltage;

2. A reduction of the difference between the striking and maintaining voltages;

3. A reduction in the minimum extinguishment time; and

4. An increase in the rate of spread of the cathode glow along each cathode when the discharge is struck.

If the device be used in a pulse time modulation system, effect No. 3 is very useful as it is desirable to reduce the extinguishment time so as to accommodate the maximum number of chan nels in a given train. Similarly, effect No. 4 can be used to regulate the depth of modulation if time modulation is produced, as previously BX-.

plained, by varying slightly the voltage applied to one of the cathodes. If desired, the additional priming may comprise a separate cathode with V a discharge taking place between it and the common anode of the device or separate cathode and anod for the priming discharge may be provided. In either case, however; it is desirable that the ionisation from the priming discharge affect all others equally and, consequently, it is preferable that the remaining gaps be arranged, say, in a circle with the priming gap at the centre.

On the question of geometrical tolerances for the gaps of discharge tubes according to the present invention, it may be stated that, roughly speaking, and provided apriming discharge be used and the priming current be raisedor, alternatively if the gas pressure be lowered-a. uniform increase of 100% in gap spacings such as those given above, would result in a pulse-time interval increase of the same order.

It should be mentioned that with any given tube, there is a time limit below which the time interval between the firing of each successive pair of gaps cannot be reduced, this limit depending on the electrode dimensions and gasfilling. With the dimensions and gas filling given above, this limit is about 5 microseconds. This time limit may be changed by suitable modifications of th design of the tube.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:

1. A circuit for generating a train of pulses in response to a single trigger pulse, comprising a cold cathode gas-filled electric discharge tube having a plurality of electrodes defining a sequence of substantially equally dimensioned discharge gaps each having an ionisation coupling with an adjacent gap, a maintaindng Voltage sourc coupled to said electrodes for establishing substantially th same potential gradient across each of said gaps of insufiicient magnitude to fire any gap but sufiicient to maintain the discharge of a gap once fired. a trigger pulse source couquence of substantially equally dimensioned discharge gaps each having an ionisation coupling with an adjacent gap, a maintaining voltage source coupled to said electrodes for establish- .ing substantially the samepotential gradient across each of said gaps of insufficient magnitude to fire any gap but, sufiicient to maintain the discharge of a gap once fired, a triggerp-ulse sourc coupled to theelectrodes defining one of a said gapsto initiate firing of said one can only and automatic sequential firing of the gaps following said one gap in said sequence," and a dif- Number Name Date 1,898,626 Healy Feb. 21, 1933' 2,007,932 Ruben July 9, 1935 2,076,335 Dallenbach Apr. 6, 1937 2,110,536 SWaIt Mar. 8, 1938 2,204,375 Morrison June 11, 1940 f 2,276,861 Penney Mar. 17, 1942 2,316,901 Thomay Apr. 20, 1943 2,427,533 Overbeck Sept. 16, 1947 2,443,407 Wales June 15, 1948 2,553,263

ferentiating transformer having a primary winding and a secondary winding, said primary winding connected in series withthe discharge paths of said gaps to develop in said secondary winding a train of pulses corresponding to'the number of discharge gaps firing. ALEC HARLEY REEVES.

References Cited in the file of this patent UNITED STATES PATENTS Loughren May 15, 1951

Images