GB2525008A

GB2525008A - Spark-Gap Switch

Info

Publication number: GB2525008A
Application number: GB1406415.8A
Authority: GB
Inventors: Mark Owen
Original assignee: MBDA UK Ltd
Current assignee: MBDA UK Ltd
Priority date: 2014-04-09
Filing date: 2014-04-09
Publication date: 2015-10-14
Also published as: GB201406415D0

Abstract

A spark-gap switch circuit comprises a spark-gap switch 12 comprising first and second electrodes 12a, 12c defining a spark gap. A trigger electrode 12b is positioned between the first and second electrodes. A charge storing device 11a is arranged in parallel with the spark-gap switch and is arranged for discharging across the spark gap to deliver energy to a load 13. The circuit is arranged to operate in a first phase during which the charge storing device is progressively charged such that the potential difference across both the charge storing device and the spark-gap switch progressively increases. During this first phase, the potential of the trigger electrode also progressively increases so as to avoid premature triggering of a spark between the trigger electrode and either of the first and second electrodes (12a, 12c). The circuit is also arranged to operate in a second phase, after the charge storing device is sufficiently charged to be able to discharge across the spark gap when triggered, during which the potential of the trigger electrode is progressively further increased. As such, initial triggering of the spark gap switch may be achieved using smaller trigger components (e.g. trigger pulse generator 15) than previously possible.

Description

SPARK-GAP SWITCH

FIELD OF THE INVENTION

This invention relates to the field of triggered spark-gap switches. In particular, but not necessarily exclusively, the present invention relates to a spark-gap switch circuit and a method of triggering a spark gap switch.

BACKGROUND ART

An increasing number of medical, industrial and defence applications use high-voltage high-power pulses, for example in high-power laser sources. Technologies that help to reduce the size and complexity of components within these systems have significant commercial value.

Many of these pulse power systems employ triggered spark-gap switches.

Figure 1 of the accompanying drawings illustrates an example pulse power circuit. A high-voltage capacitor ta (lOnE) is charged to a required application voltage (in this example 40kV) by a high voltage l5OmA power supply lb. A mid-plane triggered gas spark-gap switch 2 is then used to switch the energy stored in I into the required application load 3 (a 10 Ohm load in this case).

In order to switch on the triggered spark gap switch 2, a high-voltage trigger pulse generator S generates a pulse (40kV for lOmicro seconds) which initiates a gas discharge between the trigger electrode 2b and the ground electrode 2c. That in turn causes gas breakdown between the high-voltage electrode 2a and the mid-plane electrode 2b which closes the circuit between the high-voltage capacitor ta and the load 3. The trigger pulse generator 5 comprises a high voltage transformer Sb and a DC blocking capacitor Sa (200pF) included so that the transformer does not short circuit resistor 4b.

The trigger electrode 2b is positioned midway between symmetrically arranged main electrodes 2a and 2c, such that the trigger electrode is exactly halfway between the main electrodes 2a and 2c.

It is important that premature firing (or a "pre-fire") of the spark gap switch 2 does not occur whilst the high-voltage capacitor Ia is being charged to the required application voltage (i.e. when the spark gap closes before the trigger pulse generator 5 is initiated). A pre-fire can for example be caused if the electric potential on the trigger electrode 2b is not kept at the electric field equi-potential between electrodes 2a and 2c.

Figure 2 is a graph showing schematically the change in potential of the trigger electrode (broken line graph in Figure 2) as the capacitor repeatedly charges and then discharges (when triggered). Figure 2 also shows the change, over time, in the potential difference between the main electrodes 2a, 2c (solid line graph in Figure 2 -also representative of course of the potential drop across the capacitor I a and therefore how charged it is). In order to reduce the risk of pre-firing, it will be seen, from Figure 2, that the trigger electrode 2b is maintained at a potential that is midway between the between the main electrodes 2a and 2c, as the high-voltage capacitor la is charged. The potential of the trigger electrode 2b is controlled in this manner by means of a resistive potential divider 4 and a compensation capacitor 6.

As the potential of the trigger electrode 2b is required to be exactly midway between the potentials of the main electrodes 2a, 2c, the resistances of the resistors 4a, 4b of the potential divider 4 shown in Figure 1 are identical. In this example, the resistors 4a, 4b each have a resistance of 106 Ohms (I DM0).

As a result of the presence of the DC blocking capacitor Sa in the trigger pulse generator 5, the voltage on the trigger electrode (broken graph line in Figure 2) might, if no measures are taken, lag behind the load capacitor voltage (solid graph line in Figure 2), due to the time it takes to charge the blocking capacitor 5a through the potential divider 4. A compensation capacitor 6 is therefore also added, having the same capacitance as the blocking capacitor Sa, which, by symmetry, ensures that the potential on the trigger electrode 2b is maintained at half the potential difference across the spark gap 2, In this example, the capacitances of the two capacitors Sa, 6, are therefore each 200pF.

The high voltage trigger pulse used to switch on the triggered spark gap can be created by a trigger pulse generator utilising a transformer. A low voltage pulse Sc, typically 600 to 900v and duration typically 1 to lOmicro seconds, is stepped up, typically to 40kv, by a high voltage transformer Sb. This high voltage pulse is sufficient to cause electric gas breakdown between the (mid plane) trigger electrode 2b and the ground electrode 2c, This in turn causes gas breakdown between the high voltage electrode 2a to the (mid plane) trigger electrode 2b which completes a closed circuit between the high voltage capacitor Ia and the load 3. The transformer needs to be large enough to generate a 40kV pulse in order to trigger and switch the spark gap. Such a large size of transformer can be limiting and/or disadvantageous in certain applications of spark gap switches, A typical 40kV transformer can have a volume of the order of 1.5 litres.

US 5,153,460 and US 5,465,030 disclose spark gap switches of the prior art It would be advantageous to provide a spark-gap switch circuit in which one or more of the aforementioned disadvantages is eliminated or at least reduced.

Alternatively or additionally, it would be advantageous to provide an improved spark-gap switch circuit and/or method of triggering a spark gap switch.

DISCLOSURE OF THE INVENTION

A first aspect of the invention provides a spark-gap switch circuit comprising a spark-gap switch including a trigger electrode, and a charge storing device arranged to discharge across the spark gap when the spark-gap switch is triggered, wherein the circuit is arranged so that, during charging of the charge storing device, the magnitude of the potential (the absolute potential could be positive or negative) of the trigger electrode is increased in a manner which avoids premature triggering of a spark across the spark gap yet is subsequently progressively further increased to a higher level at which activation of the trigger electrode would cause the charge in the charge storing device to be discharged across the spark gap. (It will be understood that when charge discharges across the spark gap, current will flow between the first and second electrodes via the spark gap,) In contrast to the arrangement shown in Figure 1 of the accompanying drawings, in embodiment spark gap switch circuits of the present invention a mid-plane trigger electrode is raised (at least during initial activation of the spark gap switch) to a potential greater than halfway between the main electrodes of the spark gap switch, thus reducing the voltage requirements of the trigger-pulse-generating transformer, but without any additional significant expense or complexity in the rest of the circuit (compared to the arrangement shown in Figure 1). It will be appreciated that the operation of the switch gap circuit may such that positive potentials exist on the trigger electrode and one or both of the first and second electrodes or such that negative potentials exist on the trigger electrode and one or both of the first and second electrodes. As such it will be appreciated that it will be the relative levels of potential as between the first, second and trigger electrodes that are typically of relevance. Thus reference to the potential on one electrode rising at a particular time will be understood to be equivalent to the potential on that electrode reducing (to become more negative) if the circuit operates at negative potentials. When reference is made to the difference in potential between two electrodes it will typically be the magnitude of the difference that is of relevance.

In embodiments of the present invention, the spark-gap switch typically comprises first and second electrodes between which the spark gap is defined. The trigger electrode is typically positioned between, optionally midway between, the first and second electrodes, The charge storing device is typically arranged in parallel with the spark-gap switch, but there may be applications in which the charge storing device may be arranged in series with the spark-gap switch. The circuit is typically so arranged that when the charge storing device has sufficient charge, activation of the trigger electrode triggers the switch so that the charge in the charge storing device is discharged across the spark gap. The charge storing device is typically arranged to discharge across the spark gap such that in use, when a load is attached to the circuit, energy is delivered to the load. The charge storing device may typically be in the form of one or more capacitors.

In embodiments of the present invention, the circuit is arranged to operate in a first phase during which the charge storing device is progressively charged such that the potential difference across both the charge storing device and the spark-gap switch progressively increases up to a level at which the charge storing device is sufficiently charged to be able to discharge across the spark gap when triggered, The circuit is also arranged so that during the first phase, the potential of the trigger electrode also progressively increases so that (a) the magnitude of the potential difference between (i) the trigger electrode and (ii) the first electrode and (b) the magnitude of the potential difference between (i) the trigger electrode aiid (ii) the second electrode are each sufficiently low to avoid premature triggering of a spark between the trigger electrode and either of the first and second electrodes. The circuit is yet further arranged to operate in a second phase, after the first phase, during which second phase the potential of the trigger electrode is progressively further increased to a level from which triggering of the spark gap switch is practicable, particularly (but not necessarily exclusively) during initial activation! triggering of the spark gap switch. (When a spark gap switch has not been triggered for a significant period of time, or at all, the next triggering of the switch may be considered as the "initial triggering". A subsequent triggering of the switch during a period of steady state and repeated successive activations of the switch would not be considered of course as an initial trigger of the switch.) The trigger may be activated by a trigger pulse. It is preferred that the progressive iticrease of the potential of the trigger electrode is separate from the rapid increase of potential caused by the trigger pulse. The circuit may include a pulse generator for generating a pulse that activates the trigger electrode and thus triggers the spark gap switch. Such a pulse generator may be an integral part of the circuit or may be provided separately. The pulse generator may include a transformer for stepping up a low voltage pulse and transforming it into a high voltage pulse.

It has been found that the initial triggering of the spark gap switch can require a greater potential at the trigger electrode than required for subsequent triggering of the spark gap switch. A circuit with the ability to progressively increase the potential of the trigger electrode so that it may reach a potential at which triggering of the switch is less demanding on the trigger pulse generator, or other aspects of the circuit or the spark gap switch, has significant commercial advantage. Embodiments of the invention may for example be able to reduce the specification and therefore size of the trigger pulse generator used to trigger the spark gap switch by up to approximately 50%, without any additional complexity. Alternatively or additionally, embodiments of the invention may for example be able to keep a similar specification and size of the trigger pulse generator, but make other changes in the circuit design or operation that would otherwise require an even larger transformer. For example, such changes might include allowing more tolerance in gas pressure or spark gap dimensions, providing better performance over a given temperature range, better repeatability and/or reduced timing jitter.

The charge storing device is typically arranged to be charged by a high voltage current source at a first voltage so that a potential difference is created across the first and second electrodes sufficient, when triggered, to discharge across the spark gap. The trigger electrode may be arranged so that as a result of the second phase of operation, immediately before the start of initially triggering successfully the switch, the trigger electrode potential is at a second voltage. The spark-gap stch circuit may be so arranged that initial triggering of the spark requires a potential difference between the trigger electrode and one of the first and second electrodes greater than half of the first voltage, and is preferably arranged such that said potential difference is reached in advance of any trigger pulse being received by the trigger electrode, for example from a trigger pulse generator. The spark-gap switch circuit may be so arranged that initial triggering of the spark requires the potential difference between the trigger electrode and one of the first and second electrodes to be greater than two-thirds (preferably greater than three-quarters, and possibly greater than 90%) of the first voltage (preferably in advance of a trigger pulse being received). The second voltage may be more than two-thirds (preferably greater than three-quarters, and possibly greater than 90?4) of the first voltage, Recognising that there does not always need to be complete correlation between the biasing of the trigger electrode and the physical position of the trigger electrode, particularly after the charge storing device is frilly charged, has advantageously enabled the provision of a mid-plane triggered spark gap switch having a trigger electrode that is arranged to be biased at significantly greater than 50% of the potential difference between the main electrodes. This, in mm, facilitates a reduction in the specification (e.g. size, rating, etc.) of the trigger pulse generator employed. By way of example, triggering may be effected by means of a +20kv pulse applied to the trigger electrode, the charge storing device may charge to about +40kv (a first voltage), initial triggering of the spark may require a potential difference between the trigger electrode and the second electrode of at least 55kv, and thus (with the use of a +20kv trigger pulse) the potential difference between the trigger electrode and the second electrode at the end of the second phase of operation and before the trigger pulse is received needs to be at least +35kv (the second voltage), and is therefore arranged to be 36kvwhich is 90% of the first voltage. Had the trigger electrode remained at the equipotential voltage of +20kv (half the first voltage) then the trigger pulse would need to be at least +35kv, requiring a bulkier trigger pulse generator. ). The trigger pulse generator may have a volume of less than 1,000cm3, and preferably less than 750cm3, which is achievable in relation to a requirement of 20kV pulses being generated by the trigger pulse generator.

The spark-gap switch circuit may be so arranged that subsequent triggering is activated when the trigger electrode is at a lower potential difference between the trigger electrode and the relevant one of the first and second electrodes than the potential difference between the trigger electrode and the relevant one of the first and second electrodes when initially triggered (and preferably at a lower potential difference when compared at the instant before the trigger pulse that successfully closes the switch is received by the trigger electrode, for example at a lower DC bias).

The circuit may be provided together with, or separately from, the power source for charging the charge storing device, The circuit may be provided together with, or separately from, the power source for triggering, for example by means of generating a pulse, the trigger electrode. The power source for triggering the trigger electrode may be in the form of an AC or wave form generating power source coupled with a transformer, The power source for triggering the trigger electrode is, in embodiments of the present invention, conveniently provided separately from the power / current source for charging the charge storing device, The power / current source for charging the charge storing device may operate at a voltage exceeding 10kv, and possibly greater than 25kv. The power / current source for charging the charge storing device preferably is able to pump current to increase the voltage of the charge storing device to greater than 10kv, and possibly greater than 25kv. The power source for triggering the trigger electrode may generate a trigger waveform having a peak voltage exceeding 10kv, and, possibly greater than 15kV (but preferably less than 3OkVThe circuit may be in the form ot or comprise, a Marx Generator. In such a case, the charge storing device may comprise a capacitor, Such a Marx Generator may comprise multiple charge storing devices and multiple spark-gap switches, for example arranged as a cascade which in use act as a voltage multiplier circuit.

There may be a potential divider arranged in parallel with the spark-gap switch(and in at least some embodiments, additionally or alternatively, in parallel with the charge storing device), the potential divider being connected with the trigger electrode such the potential of the trigger is maintained between the potential of the first and second electrodes. In the system shown in Figure I, the trigger electrode is held at a potential that matches the potential of the electric field at the location of the trigger electrode, were the trigger electrode not provided. As such the trigger electrode has a minimum (negligible or very low) effect on the electric field between the first and second electrodes. The potential divider of Figure 1 thus divides the potential accordingly. In embodiments of the present invention, the potential divider preferably divides the potential at a different ratio. The potential divider of embodiments of the present invention preferably divides the potential at a ratio different from (preferably more than 25% different from) the ratio of (a) the potential difference in the electric field between (i) the location of the trigger electrode (were the trigger electrode not present) and (ii) the first electrode to (b) the potential difference in the electric field between (i) the location of the trigger electrode (were the trigger electrode not present) and (ii) the second electrode.

In the case of the circuit of Figure 1, the first and second electrodes are symmetrically configured and the potential of the electric field varies approximately linearly from one of the first and second electrodes to the other. Thus, given that the trigger electrode is positioned midway between the first and second electrodes, the potential at the location of the trigger electrode is midway between the potential of the first and second electrodes. In embodiments of the present invention, the potential divider preferably divides the potential at a ratio different from the ratio of (a) the separation, in the direction of the electric field, of the trigger electrode from the first electrode to (b) the separation, in the direction of the electric field, of the trigger electrode from the second electrode, For example, the trigger electrode may be positioned midway between the first electrode and the second electrode; in such a case, the potential divider may be arranged to divide the potential at a ratio greater than 2:1, preferably greater than 4:1), the potential of the trigger thus being maintained between the potential of the first and second electrodes but, at least at certain times, much closer to the potential of one of the first and second electrodes than the other.

It will be appreciated that the present invention has particular, although not exclusive, application to mid-plane triggered spark-gap switches, for example those in respect of which the trigger electrode is provided at a position at which (absent the trigger electrode) the potential would be midway between the potential of the first and second electrodes. Certain embodiments of the invention may have the trigger electrode arranged non-symmetrically with the first and second electrodes. Certain embodiments of the invention may have the trigger electrode positioned closer to one of the first and second electrodes than the other of the first and second electrodes.

The first and second electrodes may be generally in the fomi of parallel arranged plates. The first electrode is preferably arranged so as to be opposite to and facing the second electrode. Other configurations are of course envisaged, including at least one of the electrodes being circumferentially arranged about the other.

There may be at least one capacitor that is separate from the charge storing device which determines the rate at which the potential of the trigger electrode increases when the charge storing device is being charged. There may be two or more such capacitors. At least one of such capacitors may be arranged to protect against DC discharge to ground, for example when the spark gap switch is triggered. At least one of such capacitors may be arranged to isolate (and preferably protect) a further component of the circuit (for example a transformer and/or trigger pulse generator). A further capacitor may be provided to counterbalance the capacitance of a capacitor arranged to protect against DC discharge to ground. In such a case, there may be capacitors that are arranged in series with each other, and in parallel with a resistive potential divider (comprising two or more resistors in series). There may be a junction between the series capacitors connected to a junction between the series resistors, either via a short or via another component. It may be that the arrangement of such series capacitors and such series resistors essentially determine the way in which the circuit progressively increases the potential of the trigger electrode during both the first and second phases. It is preferred that such series capacitors and such series resistors are provided as fixed value components.

The first and second electrodes may be fixed position electrodes. The first and second electrodes may be configured such that during a cycle of charging the charge storing device and subsequently triggering of the trigger electrode the first second and trigger electrodes remain in positions relative to each other. The trigger electrode may be a fixed position electrode, The first, second and trigger electrodes may be configured such that during a cycle of charging the charge storing device and subsequently triggering of the trigger electrode, all such electrodes remain in positions relative to each other. There may be further electrodes in addition to the first, second and trigger electrodes. There may be more than one trigger electrode per pair of first and second electrodes.

Low jitter in the frequency of triggering may be beneficial in certain applications.

The spark-gap switch circuit may be arranged to trigger repeatedly with a fixed cycle time subsequent to the initial triggering. Having a fixed cycle time may allow the arrangement of the spark gap switch circuit to be relatively simple, and may for example facilitate the use of simple fixed value components. The spark-gap switch circuit may be arranged to trigger by means of a trigger pulse having a pulse width less than 100 microseconds. The spark-gap switch circuit may be arranged to repeatedly perform a complete cycle of charging, triggering and discharging across the spark gap at a fixed frequency. The fixed frequency may be at a rate higher than 100Hz and preferably higher than 1kHz (i.e. more than 1000 complete cycles per second). The fixed frequency may be at a rate lower than 10kHz and preferably lower than 5kHz The spark-gap switch circuit may be arranged that for a given period of use, the frequency at which the spark-gap switch circuit is arranged to trigger repeatedly is fixed but that for subsequent uses the frequency at which the spark-gap switch circuit is arranged to trigger repeatedly may be fixed at a different frequency (but for example no more than 20% different in frequency). The spark gap switch may be housed in a housing. The housing may, or may not, be sealed. The housing may therefore accommodate gas, for example, air at atmospheric pressure.

The spark gap switch circuit may form part of a pulse power circuit, for example of a kind suitable for driving a laser. The spark gap switch circuit may be installed on a vehicle, for example an airborne vehicle.

The present invention provides according to a second aspect a method of triggering a spark gap switch, for example utilising a spark gap switch circuit according to the first aspect of the invention. The method may include a step of charging a charge storing device so as to create a potential difference between first and second electrodes between which a spark gap is defined. A trigger electrode may be positioned between the first and second electrodes. The method may include a step of initially maintaining the potential of the trigger electrode between the potentials of the first and second electrodes, preferably so as to reduce the risk of premature triggering whilst the charge storing device is charging. Thereafter, when the charge storing device is sufficiently charged to discharge across the spark gap, there may be a step of further raising the potential of the trigger electrode to a higher level. The method may include a step of then triggering the trigger electrode to cause the charge in the charge storing device to be discharged across the spark gap between the first and second electrodes, The step of triggering may be effected by means of quickly changing the potential of the trigger electrode relative to one or both of the first and second electrodes, for example by means of an electric pulse. The step of triggering the trigger electrode may be caused by means separate from that used to charge the charge storing device, The trigger electrode may be required to reach a potential such that a potential difference of greater than 50kV exists between the trigger electrode and the first or second electrodes. With the benefit of the present invention, this may be achieved with a transformer having a rating of about 40kV or lower.

The method may include a step of initially triggering the switch by means of raising the trigger electrode potential, preferably with a DC bias and a trigger pulse, to a first potential, and then subsequently triggering the switch by raising the trigger electrode potential to a second potential, less than 90% and preferably less than 75%, of the first potential. Several trigger pulses may be received at the trigger electrode during a period over which the DC bias is progressively raised before the switch is successfully closed, particularly for example during initial triggering. In the steady state each successive trigger pulse may successfully trigger and therefore close the switch. The trigger pulse rate is preferably constant during performance of the method, The trigger pulse peak potential is preferably constant during performance of the method.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention.

For example, the method of the invention may incorporate any of the features described with reference to the system of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 1 is an electrical schematic of an example triggered pulse power circuit (of a type not in accordance with the present invention) in a symmetric mid-plane trigger gap configuration; Figure 2 is a schematic plot against time of voltage across the spark gap (solid line) and across the trigger electrode (broken line) of the circuit of Fig. t,; Figure 3 is a plot against time of mid-plane trigger voltage required at high pulse repetition frequencies in the circuit of Fig. 1; Figure 4 is a schematic plot against time of voltage across the spark gap of a spark gap circuit according to a first example embodiment of the present invention; and Figure 5 is a schematic circuit diagram of the spark gap circuit according to the first example embodiment of the present invention.

DETAILED DESCRIPTION

An example embodiment of the invention in the form of a spark-gap switch circuit is now described with reference to the circuit diagram shown in Figure 5. The spark-gap switch circuit comprises a spark-gap switch 12 comprising first and second electrodes 12a, 12c defining therebetween a spark gap. A trigger electrode 12b is positioned midway between the first arid second electrodes 12a, t2c. A tOnE capacitor lla for storing charge to be discharged to a load 13 via the spark gap switch 12 is arranged in parallel with the switch 12.

The capacitor I Ia is charged by means of a high voltage power supply in the form of a current pump generating an output current of about I 5OmA, and able to charge the capacitor I Ia to 40kv. The trigger electrode 12b is biased by means of the potential divider 14 arranged with the power supply 1 lb. A transformer 15 is provided which generates a trigger pulse for delivery to the trigger electrode 12b.

The spark-gap switch circuit of the illustrated embodiment is of particular application for spark gap designs where the voltage on the trigger electrode needed to close the spark gap is considerably higher for the first few discharges. Figure 3 is a graph showing the trigger voltages required to trigger the spark gap switch (i.e. "close" the spark gap) on successive cycles. The potential required for the first trigger is approximately 60kv, If the DC bias on the trigger electrode is 20kv (i.e. about half the voltage across capacitor I I a) then the trigger pulse from the trigger pulse generator 5 needs to be approximately 40kv. The second trigger pulse is required to be approximately 50kv. The fourth and subsequent trigger pulses can be under 40kv. It will be seen from Figure 3 that the voltage required for the initial trigger pulse can be more than 25% higher than required in the steady state, This can be due to a number of factors which include: (I) Chemical reactions at the electrode surface during long periods of storage, which can increase the breakdown strength of the spark gap for the first few discharges, This is particularly applicable to spark gap switches which are not hermetically sealed.

(2) At pulse repetition frequencies close to the plasma recombination times of the gas used within the spark gap, after the first discharge has occurred, the insulation strength of the gas is degraded and therefore the trigger voltage required to breakdown the gas on subsequent discharges is reduced.

In the example circuit shown in Figure 1 (not being in accordance with the invention), the trigger electrode 2b is held at a potential midway between the first and second &ectrodes 2a, 2c and thus is biased to 20kv. In order to generate sufficient potential on the trigger electrode when initially switching the spark gap switch, a trigger pulse generator of sufficient output is required, in this case 40kv, However for the discharges of the switch in the steady state (i.e. after the first four cycles) a trigger pu'se of only 20kV output would be required, 40kV transformer is bulky.

In the circuit of FigureS, a 20kv transformer 15 is provided but the DC bias of the trigger electrode I 2b is increased progressively beyond the potential at the position midway between the first and second electrodes 12a, 12c, so that if a first trigger pulse has been insufficient to close the spark gap, the DC bias of the trigger electrode 12b is further increased before the next trigger pulse, The DC bias on the trigger electrode i2b continues to increase until a trigger pulse closes the spark gap. The longer the spark gap remains un-triggered, the more unstable it becomes (as the DC bias progressively increases on the trigger electrode). In this example, the effect can be achieved with no increase in complexity of the circuit (compare Figure 1 with Figure 5) as a result of realising that it is not necessary to have a symmetrically arranged potential divider.

Further explanation on how this is achieved will now be provided.

If the charge rate of the power supply I lb for the application is known and can be fixed, by choosing the appropriate asymmetric bias components the requirement of the voltage rating, and therefore size, of the trigger pulse generator 15 can be dramatically reduced. In this example the trigger pulse generator requirement is be reduced by half from 40kv to 20kv and from a volume of about,SOOcm3 to a volume of less than 750cm3.

Consider the first spark gap discharge where a trigger pulse generator of 40kV would normally be required, as indicated in Figure 3. With reference to the bias circuit of Figure 1, if a charge current of l5OmA is delivered by the application power supply 1 a, this results in a charge rate of 1 5kV/ms across the spark gap 2. The values of the resistors and the capacitors of the circuit are in accordance with the present embodiment adjusted so that (with reference to Figure 5) the value of resistor 14b is SOOMOhms (resistor 14a remaining at 10 MOhms), and the value of the capacitor ISa is 7OpF (capacitor 16 remaining at 200pF), By doing this, the voltage of the trigger electrode 12b can be maintained at, or sufficiently close to, the correct electric equi-potential to prevent pre-fires during the charging phase of the capacitor 1 la.

The graph shown in Figure 4 shows how the target line (broken line) for the potential of the trigger electrode 12b varies over time, as compared to the charging of the capacitor (which relates to the potential at the first electrode 12a -the solid line of the graph). The actual change of potential of the trigger electrode will follow a curve close to, but not identical to, the broken line shown in Figure 4, The values of the components are chosen to ensure that the peak potential is reached quickly whilst sticking close to the line of equipotential whilst the capacitor ha is charging. At the moment at which the capacitor Ia reaches its target application voltage (40kv), the trigger electrode is at a potential of about 20kV (the DC bias). At this moment a 20kV voltage pulse would not sufficient to take the mid-plime trigger electrode 12b over the (initial) breakdown threshold of 60kV (it would reach only 40kV -see arrow 7 in Figure 4). However, after this moment, owing to the asymmetric arrangement of the bias components, the DC bias voltage on the trigger electrode I 2b rises towards the potential of the trigger electrode 12a of 40kv, whilst the storage capacitor I a is held at 40kv. As shown in Figure 4, at a later time, when the DC bias on the trigger electrode reaches just below 40kv, a subsequent trigger pulse of 20kv is sufficient to cause discharge between electrodes 2b and I 2c and therefore close the spark gap -see arrow 8 in Figure 4, There may be one, two or more trigger pulses that fall to close the switch, over which time the DC bias on the trigger electrode ramps up. This however is, in many applications, not a disadvantage if, in the steady state, the pulse repetition rate can be fast, the successful triggering of the switch can be reliable and there is low jitter, Once the initial discharge has occurred the spark gap has been effectively kick-started, and the next and subsequent trigger pulses need not be at such a high voltage (see graph of Figure 3). After three of four cycles of successfully triggering the switch, a DC bias on the trigger electrode of about 20kV and a trigger pulse of about 20kV are together sufficient to trigger the spark gap. The kick-starting of the spark gap switch has been achieved with no increase in complexity.

Thus by using a combination of circuit components that are able to progressively further destabilise the spark gap if it does not trigger (by progressively increasing the DC bias), the following advantages have been achieved simultaneously: (1) Allowed the triggered spark gap to be fired with a smaller trigger pulse generator than would normally be required to start discharge.

(2) Does not interfere with the normal operation of the spark gap on subsequent charge/discharge cycles. Once the first discharge has occurred, the bias circuit maintains the voltage of the mid plane at the correct equi-potential, so that the spark gap does not pre-fire.

The aforementioned embodiment thus provides a circuit which operates in a first phase during which the capacitor I I a is progressively charged and at the same time the potential of the trigger electrode 12b is controlled to be safely between the potential of the first electrode 12a and the potential of the second electrode 12c so as to avoid premature triggering of the spark switch 12. During the initial triggering of the spark gap switch, the circuit also operates in a second phase, after the capacitor 1 la is fully charged, during which phase the potential (and DC bias) of the trigger electrode 12b is progressively further increased. Thus, the spark gap can always be triggered with a lower trigger pulse than would otherwise have been required.

In this example, only a 20kV trigger pulse would be needed to kick start the spark gap in the circuit of Figure 5, compared with a 40kv trigger pulse in the configuration shown in Figure L This results in a considerable reduction in volume, cost and complexity of the trigger generator 15 and the overall system. The size of the trigger pulse generator may be reduced by approximately 50%, without any additional complexity. This is of significant commercial value.

It will be appreciated that whilst the physical changes between the circuits shown in Figures 1 and 5 are subtle, such changes are significant. Owing to the overarching design requirement to keep the electric field potential of the mid plane trigger 12b in the equi-potential of the main electrodes 12a and 12c, there is a strong assumption that symmetric bias components should be used. Using asymmetric biasing components has however provided surprising benefits, despite the assumption that the skilled person might make that such a concept would be counterproductive.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

One advantage of the illustrated embodiment is that the transformer may be smaller, Alternatively, the transformer size can be kept the same whilst increasing the operating window in other ways. For example, there may be embodiments of the present invention in which there is more tolerance in gas pressure, spark gap dimensions, better performance over temperature, and repeatability in timing jitter, etc. The embodiments relate to the case where there is breakdown initially between the trigger electrode and the most negative electrode, so that electrons are emitted such that the second gap is readily energised, causing the second gap to breakdown more quickly (yielding lower jitter). This might be described as a "Positive Load Capacitor Voltage, Positive Trigger" arrangement. In some applications, other combinations, which form part of the claimed invention, might be possible or desirable, such as the following: * Positive Load Capacitor Voltage, Negative Trigger * Negative Load Capacitor Voltage, Positive Trigger * Negative Load Capacitor Voltage, Positive Trigger Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims, Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may be absent in other embodiments.

Claims

A spark-gap switch circuit comprising: a spark-gap switch comprising first and second electrodes defining therebetween a spark gap, a trigger electrode positioned between the first and second electrodes, and a charge storing device arranged with the spark-gap switch and arranged to discharge across the spark gap when the spark-gap switch is triggered, such that in use when a load is attached to the circuit energy is delivered to the load, wherein the circuit is arranged to operate in a first phase during which the charge storing device is progressively charged such that the potential difference across both the charge storing device and the spark-gap switch progressively increases, the circuit is arranged so that during the first phase the magnitude of the potential of the trigger electrode also progressively increases so that (a) the magnitude of the potential difference between the trigger electrode and the first electrode and (b) the magnitude of the potential difference between the trigger electrode and the second electrode are each sufficiently low to avoid premature triggering of a spark between the trigger electrode and either of the first and second electrodes, the circuit is arranged so that when the charge storing device has sufficient charge, triggering of the trigger electrode causes the charge in the charge storing device to be discharged across the spark gap, and further wherein the circuit is arranged to operate in a second phase, after the charge storing device is sufficiently charged to be able to discharge across the spark gap when triggered, during which the magnitude of the potential of the trigger electrode is progressively further increased,
2. A spark-gap switch circuit according to claim 1, wherein the charge storing device is arranged to be charged by a high voltage current source so that a potential difference, equal to a first voltage, is created across the first and second electrodes sufficient, when triggered, to discharge across the spark gap, the trigger electrode is arranged so that as a result of the second phase of operation, when in the process of initially triggering the trigger electrode, the greater of (a) the magnitude of the potential difference between the trigger electrode and the first electrode and (b) the magnitude of the potential difference between the trigger electrode and the second electrode, is equal to a second voltage, and the spark-gap switch circuit is so arranged that immediately before the successful initial triggering of the spark, the magnitude of the second voltage is greater than two thirds of the magnitude of the first voltage.

3, A spark-gap switch circuit according to claim I or claim 2, further including a potential divider arranged in parallel with the spark gap and dividing the potential applied to the trigger electrode, wherein the ratio of the resistive elements of the potential divider either side of the connection with the trigger electrode is different from the ratio of (a) the magnitude of the potential difference across the electric field from the trigger electrode to the first electrode to (b) the magnitude of the potential difference across the electric field from the trigger electrode to the second electrode - 4, A spark-gap switch circuit according to any preceding claim, wherein the trigger electrode is positioned midway between the first electrode and the second electrode and the circuit includes a potential divider arranged in parallel with the spark gap, the dividing point of the potential divider being connected with the trigger electrode such the potential of the trigger electrode is maintained between the potential of the first and second electrodes and wherein the elements of the potential dividers are arranged to divide potential at a ratio greater than 2:1.5. A spark-gap switch circuit according to claim 3 or claim 4, wherein the resistive elements of the potential divider have a fixed resistance.6. A spark-gap switch circuit according to any preceding claim, further including at least one capacitor that is separate from the charge storing device, which capacitor determines the rate at which the magnitude of the potential of the trigger electrode increases when the charge storing device is being charged. -20 -7, A spark-gap switch circuit according to any preceding claim, further including at least two or more capacitors that are separate from the charge storing device which capacitors determine the rate at which the magnitude of the potential of the trigger electrode increases when the charge storing device is being charged.8. A spark-gap switch circuit according to claim 6 or claim 7, wherein the or each capacitor has a fixed capacitance.9. A spark-gap switch circuit according to any preceding claim, wherein the first, second and trigger electrodes remain in fixed positions relative to each other during a cycle of charging the charge storing device and subsequently triggering of the trigger electrode.10. A spark-gap switch circuit according to any preceding claim, wherein the circuit is arranged to repeatedly perform a complete cycle of charging, triggering and discharging across the spark gap at a fixed frequency.11 A spark-gap switch circuit according to any preceding claim, connected to a high voltage trigger pulse generator.12. A spark-gap switch circuit according to any preceding claim, wherein the spark gap switch is configured for switching, when initially triggered by a trigger pulse generator including a transformer, when the magnitude of potential difference between the first and second electrodes is at a value greater than 20kV and wherein the trigger pulse generator has a volume of less than 1,000cm3.13 A method of triggering a spark gap switch, wherein the method includes the following steps: -charging a charge storing device so as to create a potential difference between first and second electrodes between which a spark gap is defined, there being a trigger electrode positioned between the first and second electrodes, -21 - -initially maintaining the potential of the trigger electrode between the potentials of the first and second &ectrodes so as to reduce the risk of premature triggering whilst the charge storing device is charging and thereafter, when the charge storing device is sufficiently charged to discharge across the spark gap, further changing the potential of the trigger electrode to a different level, and then -triggering the trigger electrode to cause the charge in the charge storing device to be discharged across the spark gap between the first and second electrodes.14. A method of triggering a spark gap switch according to claim 13 including a step of using a spark gap switch circuit according to any of claims I to U.