US2400457A - Spark gap electrical apparatus - Google Patents

Description

M. a HAINE 2,400,457 SPARK GAP ELECTRICAL APPARATUS Filed Dec. 18, 1943' m L 5 W 2 46 Negative 0.0 Sup 0g Oscillator Load (r Ne gaiire [24' 6upp/y Oscillator I 2 Load a- 1mm? MICHAEL EDWARD HAINEJ I y a; la, 1 3

' MT cams Patented May 14, 1946 SPAR-K GAP ELECTRICAL APPARATUS Michael Edward Haine, Sale, England, assignor to Metropolitan-Vickers Electrical Company gland, a company of Great Limited, London, En Britain Application December 18, 1943, Serial No. 514,864 In Great Britain December 19, 1941 Claims.

This invention relates to spark gap electrical apparatus adapted under predeterminable control to be tripped as nearly as possible at a predetermined instant, but more particularly at a plurality of accurately though not necessarily equally spaced time intervals at recurrence frequencies which may be of the order of several thousands per second.

The present application is closely related to the copending United States application Serial No. 514,863, invented by Michael Edward I-Iaine and John Millar Meek and copending United States application Serial No. 514,865, invented by Michael Edward Haine, John Millar Meek and John Drummond Craggs, both applications filed December 18, 1943.

The primary object of the present invention is to provide a trippable spark gap device of the kind disclosed in the aforesaid applications which shall be reliable in operation as stated hereinafter, and will reduce erosion of the tip of the triggering electrode. v

One of the important uses of the invention requires that the spark gap shall on breakdown pass a relatively large discharge current of about 50 amperes, with a low voltage drop across the gap of the order of 200 volts, that is, low in relation to the applied gap potential which may be of the order of 10 to kv each discharge being required to take place during very brief period, for example one or a few micro-seconds, and at very accurately spaced recurrence intervals up to relatively great frequencies of, for example, 3,000 or even 5,000 per second, the instant of breakdown being always accurately determined by the tripping voltage pulse. Considerations which then arise are, inter alia, (l) the reliability of operation throughout a reasonable life of the device, that is in respect of erosion and corrosion of the electrodes, and changes which may be made in the ambient gas, (2) avoidance of a tendency to arc with relation to the requisite recovery properties of the device after each main discharge, this beinga factor otherwise limiting the recurrence frequency. The main use of the invention, more specifically, is for the modulation of a generator or oscillator producing ultra high frequency oscillations in pulses of very short duration and considerable peak power. The results achieved have been of successful importance. The generator or oscillator may thus be considered as a resistive load circuit into which the energy stored in the charged condenser or network passes through the trigger spark gap of the present invention and the electrical connection of the oscillation generator is preferably such that it produces the oscillation pulses only during the instant or instants during which the spark gap breaks down. I

In the accompanying drawing:

Figs. 1, 2 and 3 are diagrams of different cir- -cuit arrangements with which the improved spark gap device is adapted to be used according to the invention.

Fig. 4 is a purely diagrammatic view of a spark gap per se in accordance with the invention.

Fig. 5 is a detail diagrammatic view of a spark gap having associated with it the means in accordance with the present application for reduc ing erosion of the tripping electrode.

Figs. 1, 2and 3 show circuits like those shown in the aforesaid application Serial No. 514,863 to which thepresent invention is applicable.

The cathode la of the spark gap device I has associated with it and the feeding and current limiting ch'oke coil L an artificial transmission line 5 in series with the gap and load 2 as shown in Fig. 1 of the accompanying drawing, in which case the breakdown of the spark gap device I would cause a negative voltage pulse to appear across the valve oscillator, shown as the load 2, and it would be necessary to connect'the valve cathode to the anode 3 of the spark gap device and the valve anode to earth. It is found in practice that difiiculty is experienced due to the anode of the spark gap device and the valve cathode rising to the high negative pulse potential. This difficulty arises mainly by reason of provid ing requisite insulation in connection with the means providing the recurrent tripping voltage pulse to the trigger electrode 4.

Therefore it has been found expedient to make use of the circuit as shown in Fi 2. It will be readily seen therein that the anode of the said spark gap device is earthed, as is one side of the The artificial line 5 is, however, fully insulated from earth. It will be seen that the breakdown of the spark gap device I in this circuit will produce a voltage pulse of positive polarity across the oscillator load, so that the cathode of the said oscillator valve will be connected to earth. This is found to be of advantage when a triode valve oscillator as is per se known is required to be modulated.

In the case of the oscillator being a magnetron,

such as is being used in connection with the invention, it. is very desirable to earth the anode of the magnetron, and in this case it is expedient to make use of a phase reversing pulse transformer 2a as shown in Fig. 3. A pulse transformer is one capable of transmitting short pulses of voltages from primary to secondary with no appreciable distortion of wave shape.

It should be noted that the use of the modified circuits of Figs. 2 and 3 in no Way affects the per formance of the spark gap device. But owing to the oscillator load being by nature a unidirectional conductor it is found necessary to connect across it an impedance indicated at 6 which is high with respect to the oscillator impedance but low with respect to the charging choke-impedance. ihe purpose of this impedance, which might be in the nature of a pure inductance, is to pass the charging current to the artificial line and limit the follow-up current.

It will be well understood by those skilled in the art that by a point electrode there is not necessarily herein involved one having a very sharp point, the requirement being a slim trigger electrode of such shape in relation to the relatively large spherical or like shape of the main electrode for giving the requisite main field, as to cause a sufficient distortion and concentration of this field whereby the depression of the sparkover voltage is of the order of thirty per cent and upwards, according to varying conditions of use. With regard to the life and stability of the device as determined by electrode wear, I have found that superior results are obtained when the electrodes or parts thereof are formed of molybdenum ortungsten.

Referring next to Fig. 4, the spark gap device according to the invention comprises a cathode l a which is conveniently a, sphere at least on that part thereof which faces the anode 3 which as shown comprises a tubular member but having a substantially spherical left-hand end wherein there is an aperture 3a through which extends the rounded end of the slim trigger electrode 4 which in the arrangement diagrammatically shown in Fig. 4 is held within and spaced from thetubular anode 3 by the insulating sleeve 4a which serves the further purpose hereinbefore stated. The cathode la has a terminal or connector la the anode 3 has a terminal or connector 3 and the trigger 4 has its terminal or connector i. The insulator 4a may assume various other forms, conveniently that shown at ib in Fig. 5 of the aforesaid application Serial No. 514,863. As hereinbefore indicated the presence of the insulator 4a causes the potential gradient to be augmented at the surface of the trigger electrode 4 for a given trigger voltage pulse so that the corona is suddenly produced along with the arrival of the trigger voltage pulse. The

radial width or the surface spark-over distance of the insulator 4a must be large enough to obviate surface spark-over on the application of the trigger pulse. At 3" is shown a pipe going to the interior of the electrode 3 and 3" is a pump for causing gas fiow through the aperture So, as in the aforesaid application Serial No. 514,863.

With further reference to the theory of operation of spark gap devices in accordance with the present invention, it may be explained that, as is per se known, the voltage gradient required to cause breakdown of the gap with uniform field is that which causes an electron avalanche, which crosses the gap to the anode and is of such intensity that the radial field produced by the positive-ion space-charge in the avalanche system is of the order of the interelectrode field since the space-charge forms an effective point on the anode surface. A positivestreamerthen develops from anode to cathode to form a conducting filament bridging the gap.

The voltage required to cause the propagation of the positive streamer across the gap is much lower than that required to cause the electron avalanche which leads to the initiation of the streamer, and corresponds roughly to that for breakdown between a positive point and an earthed plane. The field distortion produced by the positive space-charge in the avalanche is simulated in the spark gap device of the invention by the field distortion produced by the trip pulse on the trigger electrode, and the breakdown voltage of the gap is thereby reduced by an amount approximately given by the difference in breakdown voltage for a sphere-to-sphere gap and a gap of the same length between a positive point and an earthed sphere. Thus for a gap of 1 cm. between sphere each of 1.3 cm. diameter, the breakdown voltage in air at normal atmospheric pressure is 29 kv., whereas for the positive pointto-sphere gap of the same length the breakdown voltage is 12 kv. approximately, which is a pression of sixty per cent.

I have found that the magnitude of the potential pulse applied to the point electrode also affects the depression of the breakdown voltage of the main gap. It not only creates a radial field at the point and initiates a positive streamer, but it also augments. the main field across the gap. Thus the higher the applied pulse voltage the greater is the depression, other conditions'being unchanged.

Fig. 5 is a detail View of the arrangement adapted to be employed in the hereinbefore de-. scribed circuits, and which has for its object the reduction of the erosion of the tip of the slim trigger electrode 4. In preliminary explanation of this, it will be appreciated that in the case of spark discharges it is the cathodic electrode which sufiers the more from erosion: in the case of the electrode la (Figs. 1-4) since this is of relatively large size the erosion is not of importance compared with that arising at the tip of the slim trigger electrode Asince it is the secondary spark from the latter to the anode 3 which causes the erosion, this secondary spark carrying. the whole current of that of the main and all important spark between the cathode la and the electrode 4. In. the arrangement shown in Fig. 5 there is on the one hand connected between the anode 3 and earth a resistance 30 or inductance which is relatively low in value but is say ten times higher than the impedance of the load 2 (Figs. 1-3) through which the current of the sparks passes on tripping. On the other hand there-is connected between the tripping electrode 4 and earth the auxiliary spark gap 40 which is adjusted to sparkover at a voltage slightly in excess of the trip voltage. It will beappreciated that the operation of the arrangement shown in Fig. 5 depends on the relative impedances of the various electrodes. The electrode 4 normally has a very high impedance to earth, which may even be infinite: the impedances of the sparks-are relatively very low. On tripping a spark takes place between the cathode la and the tip of the trippin electrode 4, immediately followed by the secondary spark from' the latter to the edge of the perforation in the anode 3, so that approximately the voltage of cathode la appears across the relatively low resistanceBc. whereby the current in the spark from the electrode 4 to the anode 3 is limited while nearly the voltage of the cathode I a remains on the electrodes 3 and 4. Immediately following the secondary spark, therefore, is a spark acrossthe gap 40, the current in which tertiary spark is many times that of the secondary spark; thus the erosion of the tip of the tripping electrode 4 may be reduced for example a thousandfold. It will be appreciated that the spark gap 40 may be made robust.

In general for any given electrode spacing it is, of course, the uniform field gap which requires the highest voltage for breakdown. The spark gap arrangement according to the invention will operate with voltage pulses of either polarity in conjunction with either polarity of the high voltage electrode of the main gap. However, the best operation for any given polarity of high voltage electrode is obtained when a pulse of opposite polarity is applied to the poin electrode, and the greatest depression of breakdown voltage occurs for a positive voltage pulse when the high voltage electrode is of negative polarity. The fact that the depression is not so great for a negative pulse as for a positive pulsemay be explained, since the negative streamer does not propagate as readily as a positive streamer, and the voltage required to cause breakdown between a negative point and an earthed plane is about twice that for a positive point, although it is still considerably below that for the uniform field.

It will be appreciated that in the arrangements of spark gap illustrated in the drawing with the active end of the trigger electrode 4 approximately coincident with the active surface of the main electrode 3, the potential at which the electrode 4 is normally biased is the same, or nearly the same, as the normal potential of the electrode 3.

I claim:

1. A tripping spark gap electric system comprising a circuit having capacity and impedance therein, means for supplying said circuit with a source of current for charging said capacity to a high potential, a load connected to said circuit for discharging said capacity at predetermined instants of time, a, spark gap device with which said load is connected in series, said device comprising input and output main electrodes forming a gap between them, and a trigger electrode associated with one of said main electrodes and forming a gap therewith, means for applying a triggering voltage pulse to said trigger electrode to cause a spark to pass between one of said main electrodes and said trigger electrode and between the latter and said main electrode with which it is associated, and spark gap means adapted to ischarge at a voltage slightly in excess of the triggering voltage for discharging voltage from said trigger electrode.

2. A tripping spark gap electrode system comprising a circuit having capacity and impedance therein, means for supplying said circuit with a source of current for charging said capacity to a high potential, 9, load connected to said circuit for discharging said capacity at predetermined instants of time, a spark gap device with which said load is connected in series, said device comprising input and output main electrodes forming a gap between them, and a trigger electrode associated with one of said main electrodes and forming a gap therewith, means for applying a triggering voltage pulse to said trigger electrode.

to cause a spark to pass between the latter and said main electrodes, said trigger electrode having a high impedance to earth, an impedance of low value relatively to that of said trigger electrode between the main electrode with which the trigger electrode is associated and earth for limiting the current in the gap between said electrodes, and an auxiliary spark gap connected to said trigger electrode and adapted to spark over at a voltage silghtly in excess ofthe triggering voltage.

3. A tripping spark gap electric system comprising in combination, a circuit having capacity therein and means for connecting it to a source of current, an impedance connected in said circuit for charging said capacity to a high potential, a load connected across said circuit through which the charged capacity is dischargeable at predetermined instants of time, .a spark gap device with which said load is connected in series, saiddevice comprising input and output main electrodes fixed apart appreciably beyond their normal sparkover spacing, and a fixed slim trigger electrode associated with one of said main electrodes, means for applying triggering voltage pulses to said trigger electrode, means for applying a normal potential bias to said trigger electrode, said trigger electrode being formed with small radius of curvature and located sufiiciently nearto one of the main electrodes that when normally appropriately biased thereis so little disturbanceof the substantially uniform field between said main electrodes that'breakdown between'them is avoided with appreciable tolerance, and when a triggering pulse of suflicient magnitude is applied to said trigger electrode breakdown is caused due to the appreciable distortion of said field, and an auxiliary spark gap bridging the gap between said trigger electrode and said main electrode with which it is associated, and adapted to "spark over at a voltage slightly in excess of the triggering voltage.

4. A tripping spark gap electric system comprising in combination, a circuit having capacity therein and means for connecting it to a source of current, an impedance connected in said circuit for charging said capacity to a high potential, a load connected across said circuit and to earth and through which the charged capacity is dischargeable at predetermined instants of time, a spark gap device with which said load is connected in series, said device comprising input and output main electrodes fixed apart appreciably beyond their norma1 sparkover spacing, and a fixed trigger electrode associated with one of said main electrodes, means for applying triggering voltage pulses to said trigger electrode, means for applying a normal potential bias to said trigger electrode, said trigger electrode being formed with small radius of curvature and located sufficiently close to the main electrode with which it is associated that when normally appropriately biased there is so little disturbance of the substantially uniform field between said main electrodes that breakdown between them is avoided with appreciable tolerance, and when a triggering pulse of sufiicient magnitude is applied to said trigger electrode breakdown is caused due to the appreciable distortion of said field, and an impedance connected between said main electrode with which said trigger electrode is associated and earth and having a value substantially higher than that of said load.

5. A tripping spark gap electric system according to claim 4, wherein said last-mentioned impedance is a resistance of low value relatively to that of said trigger electrode.

6. A tripping spark gap electric system accord- '7. A tripping spark gap electric system according to claim 4, wherein said last-mentioned impedance has a relatively low value and connects to earth said main electrode With which said trigger electrode is associated, and said trigger electrode has a relatively high irnpedance'to earth.

8; A tripping spark gap electric system com prising in combination, a circuit having capacity and impedance therein, means for supplying said circuit with a source of current for charging said capacity to a high potential, a load connected to said circuit and to earth for discharging said capacity at predetermined instants of time, a spark gap device with which said load i connected in series, said device comprising input and output main electrodes forming a gap between them, and a trigger electrode associated with one of said main electrodes and forming a gap therewith, means for applying triggering voltage pulses to said trigger electrode to cause a spark to pass between one of said main electrodes and'said trigger electrode and between the latter and said main electrode with Which it is associated, an impedance having a substantially higher value than that of said load andconnecting to earth said main. electrode with which said trigger electrode is associated, and an auxiliary spark gap between said trigger electrode and earth and adapted to spark over at a voltage slightly in eX- cess of the triggering voltage.

9.- A tripping spark gap electric system comprising a current supply, discharge electrodes connected to said supply and spaced to provide a main spark gap, and an auxiliary electrode adapted to trip the main spark gap when supplied With suitable impulses, and an auxiliary spark gap in parallel with the tripping electrode and one side of the supply, which auxiliary gap discharges at a higher voltage than the applied tripping voltage for the purpose of minimizing wear on said tripping electrode.

10. A tripping spark gap electric system having a current supply and comprising a capacitative network, means for charging said network from the supply, a discharge circuit including a spark gap and a load, a tripping electrode associated with said spark gap, and an auxiliary spark gap in parallel with said tripping electrode and one side of the supply, which auxiliary gap discharges at a higher voltage than the applied tripping voltage.

MICHAEL EDWARD I-IAINE.

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