GB2254185A

GB2254185A - A surface discharge corona plasma cathode assembly.

Info

Publication number: GB2254185A
Application number: GB9106352A
Authority: GB
Inventors: Simon Joseph Scott
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1991-03-26
Filing date: 1991-03-26
Publication date: 1992-09-30
Anticipated expiration: 2011-03-26
Also published as: GB9106352D0; GB2254185B

Abstract

A surface discharge corona plasma cathode assembly includes a first electrically conductive member (1), a dielectric member (2) surrounding the first member (1), a second electrically conductive member (3) partially surrounding the member (2) in a manner such as to leave an uncovered area (4) of the member (2) between two ends (3a) of the member (3). At least two electrically conductive electrodes (5) are located in the uncovered area (4) in contact the one with one end and the other with the other end of the member (3) to define between the electrodes (5) a plasma discharge region (4a). Means are provided, such as gas flow pipes (7) and/or cryogenic coolant flow through the member (1) to increase the rate of adsorption in the region (4a) of background gases and vapours by the member (2) and thereby reduce the voltage VQ to be applied across the dielectric member (2) to initiate plasma discharge in the region (4a). <IMAGE>

Description

S SURFACE DISCHARGE CORONA PLASMA CATHODE ASSEMBLY This invention relates to a surface discharge corona plasma cathode assembly particularly, but not exclusively, suitable for use in a pulsed, high repetition rate electron beam generator for the creation of X-rays for the pre-ionisation of high repetition rate gas lasers.

Electron beam generating assemblies are known for generating X-rays to pre-ionise a gas laser. Such a known assembly is a corona plasma cathode which employs an insulating dielectric tube containing an earthed metal liner. An outer conductive shield surrounds at least part of the dielectric tube and electrodes project therefrom into the unshielded region of the tube. An earthed anode is placed close to but spaced from the unshielded electrode area and the whole assembly is housed in a container under vacuum. Gas ionisations takes place at the electrode-dielectric-vacuum interface to form a plasma when a pulse high negative voltage is applied to the electrodes. The plasma spreads out on the unshielded region of the dielectric tube surface between the electrodes.

This process subjects the dielectric tube to heating from dielectric losses, voltage stress and erosion, particularly at the electrode edges. All these factors tend to reduce the working life of the dielectric tube, for example by increasing the likelihood of a crack or puncture developing in the dielectric tube material. The formation of such a crack or puncture will stop the cathode assembly functioning.

There is thus a need for an improved surface discharge corona plasma cathode assembly having a longer working life and improved performance with less likelihood of crack or puncture damage occurring in the dielectric tube material.

According to the present invention there is provided a surface discharge corona plasma cathode assembly, including a first electrically conductive member, a dielectric member surrounding the first member, a second electrically conductive member partially surrounding the dielectric member in a manner such as to leave an uncovered area of the dielectric member between two ends of the second conductive member, and at least two electrically conductive electrodes located in said uncovered area in contact the one with one end and the other with the other end of second conductive member, which electrodes define there between a plasma discharge region on the uncovered area of the dielectric member, and means for increasing the rate of adsorption, in said region, of background gases and vapours by the dielectric member and thereby reduce the voltage to be applied across the dielectric member to initiate plasma discharge in said region.

This reduction in voltage necessary to initiate plasma discharge greatly improves the working life of the dielectric member by reducing the heating of the dielectric member material from dielectric losses, reducing the voltage stress thereon and consequently reducing the erosion thereof with subsequent decrease in the likelihood of a crack or puncture developing in the dielectric member.

Preferably the first electrically conductive member is an elongated tube of metal.

Conveniently the dielectric member is an elongated tube made of an erosion resistant dielectric material such as quartz, glass, alumina or sapphire.

Advantageously the second electrically conductive member is a substantially cylindrical, longitudinal interrupted metal shield surrounding and in contact with the outer surface of the dielectric member, with the longitudinal edges of the shield defining and bounding the interruption providing said ends of the second electrically conductive member bounding said uncovered area.

Preferably the said longitudinal edges are extended to form wings projecting substantially at right angles to the outer surface of the dielectric member.

Conveniently the means for increasing the rate of adsorption in said region is in the form of means for directing a flow of gas to the region on the dielectric member.

Advantageously the means for directing gas includes at least one gas tube made of gas porous material partially coated with a gas impervious sealant in a manner such as to direct gas substantially only onto the dielectric member surface in said region.

Alternatively the means for directing gas includes at least one gas tube with spray holes formed through the walls thereof, which spray holes are so located in the tube walls to direct gas substantially only onto the dielectric member surface in said region.

Preferably the tubular first electrically conductive member has inlet and outlet means and the means for increasing the rate of adsorption includes means for the supply of a coolant through the first member interior.

Conveniently the coolant is supplied at a temperature of about -200c.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagrammatic cross sectional view through a conventional surface discharge corona plasma cathode assembly not according to the present invention, and Figure 2 is a similar cross sectional view through a surface discharge corona plasma cathode assembly according to a first embodiment of the present invention.

A conventional surface discharge corona plasma cathode assembly as shown in Figure 1 of the accompanying drawings includes a first electrically conductive member 1, a dielectric member 2 surrounding the member 1, and a second electrically conductive member 3 partially surrounding the dielectric member 2 in a manner such as to leave an uncovered area 4 of the member 2 between two ends 3a of the member 3. At least two electrically conductive electrodes 5 are located in the uncovered region 4 in contact the one with one end 3a and the other with the other end 3a of the member 3. The electrodes 5 define there between a plasma discharge region generally indicated at 4a on the uncovered area 4 of the member 2.

In the conventional assembly as shown in the accompanying Figure 1 Cc is the capacitance of the cathode assembly between the first member 1 and second member 3, C1 is an external capacitance added as shown to divide the voltage AIT T of a pulse applied to the cathode assembly at 6 so that only a fraction VQ appears across the dielectric member 2. With this known assembly V is related to VT by the relationship V =V C1 & C1 + Cc Thus by changing the value of C1 it is possible to increase or decrease the value VQ for a fixed VT. T The conventional assembly of Figure 1 operates in a vacuum which typically is in the range of from 10 S to 10 4m bar. The conventional assembly operates by the application of a voltage pulse at 6 which creates a high electric field between the edges of the electrodes 5 and the first electrically conductive member 1. Field emission of electrons takes place and as they are ejected onto the surface of the dielectric member 2 in the uncovered area 4 they cause desorption, vaporisation and ionisation of adsorbed materials.

Typically these adsorbates are air, water and oil vapour which are present at a low background pressure. A surface discharge thus spreads out from the electrodes 5 across the surface of the member 2 in the area 4 to cover the region 4a between the electrodes 5. The discharge is capacitatively coupled to the first electrically conductive member 1. An electron beam can then be extracted from the discharge in the region 4a by application of an accelerating field between the cathode assembly and an anode (not shown).

As aforesaid the material of the dielectric member 2 is subjected to heating from dielectric losses, voltage stress and erosion particularly at the junction between it and the edges of the electrodes 5, leading to an increased likelihood of a crack or puncture developing and propagating in the material of the member 2 which causes breakdown of the cathode assembly.

It has now been found that all these factors are dependent on the magnitude of the voltage VQ and a lower VQ will improve the working life of the cathode assembly. It is thereby desirable for the voltage VQ to be as low as possible, preferably just sufficient to first initiate the surface discharge in the region 4a.

The adsorption rate of background gases and vapours onto the uncovered area 4 of the member 2 is low at typical operating pressures in the range of from 10-5 to 10-4m bar and a mono layer will form in a time scale of between 0.1 and 1 second. As the time between voltage pulses decreases (i.e. the repetition rate increases), below the characteristic adsorption time, the surface density of adsorbates will fall and after a certain point the initiation of a satisfactory surface discharge from which a space charge limited electron beam can be extracted will grow progressively harder and the cathode emission will get lower and more erratic. The space charge limited current is the maximum current that can be withdrawn and is dependent only on the cathode-anode geometry and the voltage there across.It has been found experimentally that a voltage VQ of 15KV with a dielectric member in the form of a tube of quartz 2. 5mm in thickness, is sufficient to generate a good surface discharge up to a repetition rate of a few tens of Hertz. As the repetition rate is raised the voltage V must also be increased in order to maintain adequate cathode emission. A higher voltage VQ ensures a more vigorous discharge initiation and the lack of plasma from ionised adsorbates is made up for by increased field emission and erodedlionised quartz and electrode material. At a repetition rate of 1KHz the voltage V has to be 40KV for satisfactory performance.

It is thereby desirable to be able to increase the adsorption rate at the surface discharge region 4a to keep the voltage VQ as low as possible.

In the invention of the present application this is done according to one embodiment of the present invention as illustrated in Figure 2 of the accompanying drawings by creating locally a higher gas pressure in the vicinity of the surface discharge region 4a while the rest of the chamber containing the assembly and the anode is maintained at a low enough pressure to ensure adequate cathode insulation and an insignificant amount of electron beam-gas interaction. The assembly of the invention as shown in Figure 2 is basically similar to that of the conventional assembly of Figure 1 and like parts have been given like reference numerals and will not be further described in detail.In the assembly of Figure 2 means are provided for increasing the rate of adsorption in the surface region 4a, of background gases and vapours by the dielectric member 2 and thereby to reduce the voltage V to be applied across the dielectric member 2 to initiate plasma discharge in the region 4a.

In this embodiment the first electrically conductive member 1 is an elongated tube of metal and the dielectric member 2 is an elongated tube made of an erosion resistance dielectric material, preferably quartz, glass alumina or sapphire. The second electrically conductive member 3 is a substantially cylindrical, longitudinally interrupted metal shield surrounding and in contact with the outer surface of the member 2, with the longitudinal edges of 3a of the shield defining and bounding the interruption providing the ends of the member 3 bounding the uncovered area 4. The longitudinal edges 3a are extended to form wings 3b projecting substantially at right angles to the outer surface of the member 2.

In this embodiment the means for increasing the rate of adsorption in the region 4a is in the form of means for directing a flow of gas to the region 4a onto the dielectric member 2. As illustrated this means includes at least one gas tube 7, preferably two as shown, made of gas porous material partially coated with a gas impervious sealant in a manner such as to direct gas substantially only onto the dielectric member surface in the region 4a in the direction of the arrows 8. For convenience holes 9 have been shown in Figure 2 in the tubes 7 to indicate the porosity through the tube wall but it is to be appreciated that in practice such tubes would appear continuous without visible holes 9.

Alternatively the means for directing gas can include at least one gas tube 7, and preferably two such tubes, with spray holes such as 9 form through the walls thereof and with the spray holes 9 being so located in the tube walls to direct gas substantially only onto the dielectric member surface in the region 4a, such as in the direction of the arrows 8 in Figure 2.

The gas used may be of any kind either in the form of a single gas or a mixture of gases. The gas flow rate is controllable by valve means, not shown, including one or more choke valves, for controlling the rate of flow of gas to the or each gas tube 7 from a source of gas. The gas tubes 7 may be made of any suitable material such as alumina, glass or quartz.

According to a further embodiment of the present invention, not illustrated in detail, the gas tubes 7 of Figure 2 may be dispensed with and the means for increasing the rate of adsorption in the region 4a of background gases and vapours by the dielectric member 2 and thereby reducing the voltage V to Q be applied across the member 2 to initiate plasma discharge in the region 4a can take the form of inlet and outlet means on the first electrically conductive member 1 and means for the supply of a coolant through the interior of the first member 1. This coolant, which may be of any desired type, is at a cryogenic temperature for cryogenic cooling of the cathode assembly.The temperature at which coolant is supplied preferably is at about -200c which is considerably different to the present use of a water coolant at about +100 c. The use of the cryogenic coolant at about -200c reduces the dielectric losses of the member 2, which are temperature dependent. Adsorption, being an exothermic change of state, is also increased for a given background pressure with consequent reduction in voltage VQ being achievable and consequent enhancement in the working life of the dielectric member 2.

Alternatively cryogenic cooling can be used in combination with at least one gas tube 7. Thus cryogenic cooling leads to lower dielectric losses lower dielectric member temperature, increased adsorption rate and lower voltage VQ which in turn lead to enhanced dielectric member life. Gas cooling leads to increased adsorption rate, lower voltage VQ and thus to enhanced dielectric member life. With a combination of cryogenic and gas cooling the lower dielectric losses and lower dielectric material temperatures contribute to the increased adsorption rate.

Claims

1. A surface discharge corona plasma cathode assembly including a first electrically conductive member, a dielectric member surrounding the first member, a second electrically conductive member partially surrounding the dielectric member in a manner such as to leave an uncovered area of the dielectric member between two ends of the second conductive member, at least two electrically conductive electrodes located in said uncovered area in contact the one with one end and the other with the other end of the second conductive member, which electrodes define there between a plasma discharge region on the uncovered area of the dielectric member, and means for increasing the rate of adsorption in said region, of background gases and vapours by the dielectric member and thereby reduce the voltage to be applied across the dielectric member to initiate plasma discharge in said region.

2. An assembly according to claim 1, wherein the first electrically conductive member is an elongated tube of metal.

3. An assembly according to claim 1 or claim 2, wherein the dielectric member is an elongated tube made of an erosion resistant dielectric material.

4. An assembly according to claim 3, wherein the erosion resistant material is quartz, glass, alumina or sapphire.

5. An assembly according to any one of claims 1 to 4, wherein the second electrically conductive member is a substantially cylindrical, longitudinally interrupted metal shield surrounding and in contact with the outer surface of the dielectric member, with the longitudinal edges of the shield defining and bounding the interruption providing said ends of the second electrically conductive member bounding said uncovered area.

6. An assembly according to claim 5, wherein said longitudinal edges are extended to form wings projecting substantially at right angles to the outer surface of the dielectric member.

7. An assembly according to any one of claims 1 to 6, wherein the means for increasing the rate of adsorption in said region is in the form of means for directing a flow of gas to the region on the dielectric member.

8. An assembly according to claim 7, wherein the means for directing gas includes at least one gas tube made of gas porous material partially coated with a gas impervious sealant in a manner such as to direct gas substantially only on to the dielectric member surface in said region.

9. An assembly according to claim 7, wherein the means for directing gas includes at least one gas tube with spray holes formed through the walls thereof, which spray holes are so located in the tube walls to direct gas substantially only on to the dielectric member surface in said region.

10. An assembly according to claim 8 or claim 9, including two such gas tubes.

11. An assembly according to any one of claims 8 to 10, including valve means for controlling the rate of flow of gas to the or each gas tube from a source of gas.

12. An assembly according to any one of claims 2 to 11, wherein the tubular first electrically conductive member has inlet and outlet means and wherein the means for increasing the rate of adsorption includes means for the supply of a coolant through the first member interior.

13. An assembly according to claim 12, wherein the coolant is supplied at a temperature of about -200c.

14. A surface discharge corona plasma cathode assembly, substantially as hereinbefore described with reference to and as illustrated in Figure 2 of the accompanying drawings.