GB2255438A

GB2255438A - Discharge apparatus with pre-ionising electron cloud sustained by voltage

Info

Publication number: GB2255438A
Application number: GB9109625A
Authority: GB
Inventors: Simon Joseph Scott
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1991-05-03
Filing date: 1991-05-03
Publication date: 1992-11-04
Anticipated expiration: 2011-05-03
Also published as: GB2255438B; GB9109625D0

Abstract

In discharge apparatus (1) a high voltage field is created across electrodes during and after a pre-ionizing electron cloud formation, which high voltage field is enough to reduce the rate of electron loss from the cloud, but not high enough to cause premature avalanching. The apparatus may be high pressure, pulsed electric, glow discharge apparatus (1) including a discharge vessel (2) for containing a gas or gas mixture incorporating at least one species whose electron attachment rate decreases with increasing electric field and electron energy, such as one or more of F2, SF6, CCl4, Cl2 and NF3. A pair of spaced apart electrodes (3) is contained in the vessel (2) between which electrodes (3) the pre-ionising cloud of electrons (4) is formed by a pulse of ultraviolet or X-ray radiation (5), and a glow discharge is formed by the application of a high voltage pulse. A voltage supply (6) and isolating element (7) provide the high voltage field that is high enough to reduce the rate of electron loss. The apparatus can incorporate a laser gas or can pump a laser. The apparatus relates to high pressure transversely excited gas lasers. <IMAGE>

Description

A HIGH PRESSURE, PULSED ELECTRIC, GLOW DISCHARGE APPARATUS This invention relates to a high pressure, pulsed electric, glow discharge apparatus for providing a pulsed electric discharge in gases that require to be pre-ionised before the discharge is fired. Such apparatus relates specifically, but not exclusively, to high pressure, transversely excited gas lasers.

High pressure, pulsed electric, glow discharges in a gas or gas mixture need to be "seeded" with a low density (typically equal to or greater than 106cam 3) cloud of electrons between the main electrodes of the discharge apparatus, prior to the application of a high, fast rising voltage pulse across the electrodes. The low density electron cloud typically is generated by a pulse of ionising radiation which can, for example, be ultraviolet radiation created by a separate discharge close to the main discharge region and within a discharge vessel of the apparatus, or X-rays created externally of the discharge vessel and propagated through the vessel walls into the main discharge region.Once the pre-ionisation electron cloud has been formed, or even during formation, it is liable to lose electrons by attachment to electro negative species in the gas or gas mixture, for example according to the relationship:

Where X2 is the electro negative species, m is a third body which carries away the energy of ionisation and e is the electron. This electron loss can be a particular problem with some gases or gas mixtures, particularly those containing or including F2,SF6, CC14, C12 or NF3. A high rate of electron loss can lead to a reduction in efficacy of the pre-ionisation and quality of the main discharge and is normally countered by increasing the energy input to the pre-ioniser with consequent increase in expense.

There is thus a need for a generally improved high pressure, pulsed electric, glow discharge apparatus in which the rate of loss of electrons from the pre-ionising electron cloud is minimised or at least reduced.

According to a first embodiment of the present invention there is provided a high pressure, pulsed electric, glow discharge apparatus, including a discharge vessel for containing a gas or gas mixture incorporating at least one species whose electron attachment rate decreases with increasing electric field and electron energy, a pair of spaced apart electrodes contained in the discharge vessel, between which electrodes a pre-ionising cloud of electrons is formed, by a pulse of ultraviolet or X-ray radiation, in the gas or gas mixture and a glow discharge then formed in the pre-ionised gas or gas mixture by the application of a high voltage pulse across the electrodes, and means for creating a high voltage field across the electrodes during and after said pre-ionising electron cloud formation, which high voltage field is high enough to reduce the rate of electron loss from the cloud but not high enough to cause premature avalanching.

Advantageously the species utilised is one or more of F2, SF6, CC14, C12 and NF3.

Preferably the high voltage field creation means includes a high voltage power supply.

Conveniently the high voltage power supply is a Direct Current (DC) power supply operable to generate a DC field across the electrodes.

Advantageously the high voltage field creation means includes an isolating element.

Preferably the isolating element is a high resistance resistor or a high value inductor.

Conveniently the pulse of Ultra-Violet radiation is caused by spark or surface discharge.

Advantageously the apparatus includes means for creating said high voltage pulse across the electrodes, which includes a peaking capacitor connected in parallel with the electrodes, a main energy storage capacitor connected in parallel with the peaking capacitor, a switch connected in series with the main energy storage capacitor between said main energy storage capacitor and earth, and a capacitor charging input connected between the main energy storage capacitor and the switch.

Preferably the high voltage power supply is connected in series with the isolating element and the high voltage power supply is connected in parallel with the electrodes, so that, in operation, the isolating element protects the high voltage power supply from discharge of the main energy storage capacitor and acts to limit the amount of current drawn from the high voltage power supply.

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 single figure drawing in which: Figure 1 is a schematic diagram of a high pressure, pulse electric, glow discharge apparatus, according to a first embodiment of the present invention.

As shown in Figure 1 of the accompanying drawings, a high pressure, pulsed electric, glow discharge apparatus of the present invention generally referenced 1 includes a discharge vessel 2 for containing a gas or gas mixture incorporating at least one species whose electron attachment rate decreases with increasing electric field and electron energy, such as one or more of F2, SF6, CC14, C12 and NF3. The background thermal electron energy is normally about 0.025ev, and if this energy is raised by applying an accelerating electric field then the rate of electron attachment will fall, for example by approximately an order of magnitude for F2, SF6, C12 and CC14, when the electron energy is lev.

The apparatus 1 also includes a pair of spaced apart electrodes 3 contained in the vessel 2, between which electrodes 3 a pre-ionising cloud of electrons 4 is formed by a pulse of ultraviolet or X-ray radiation 5, in the gas or gas mixture in the vessel 2. A glow discharge is then formed in the pre-ionised gas or gas mixture between the electrodes 3 by the application of a high voltage pulse across the electrodes 3. The apparatus 1 also includes means for creating a high voltage field across the electrodes 3 during and after the formation of the pre-ionising electron cloud 4, which high voltage field is high enough to reduce the rate of electron loss from the cloud, that is the rate of electron attachment, but not high enough to cause premature avalanching.

The high voltage field creation means includes a high voltage power supply 6 which preferably is a Direct Current (DC) power supply operable to generate a DC field across the electrodes 3. The field need not be DC provided that it is applied during and after pre-ionisation. The high voltage field creation means also includes an isolating element 7 preferably in the form of a high resistance resistor or a high value inductor.

The pulse of ultraviolet radiation 5 may be provided in any convenient manner such as by spark or surface discharge. The apparatus 1 also includes means for creating the high voltage pulse across the electrodes 3 and this means includes a peaking capacitor 8 connected in parallel with the electrodes 3, a main energy storage capacitor 9 connected in parallel with the peaking capacitor 8, a switch 10, connected in series with the main energy storage capacitor 9 between the capacitor 9 and earth 11, and a capacitor charging input 12 connected between the capacitor 9 and the switch 10.

The high voltage power supply 6 is connected in series with the isolating element 7 and in parallel with the electrodes 3, so that, in operation, the element 7 protects the high voltage power supply 6 from discharge of the main storage capacitor 9 and acts to limit the amount of current drawn from the power supply 6.

The value of the high voltage power supply 6 required depends on the spacing of the electrodes 3 and the pressure and composition of the gas or gas mixture utilised in the discharge vessel 2.

With the apparatus of the invention by applying a high voltage field across the electrodes 3 during and after formation of the pre-ionising electron cloud 4, the rate of electron loss from attachment to the aforementioned species in the cloud 4 is decreased and the pre-ionisation is effectively enhanced. This enhancement is economic in that a doubling of the pre-ionisation electrons available for glow discharge initiation can be achieved by raising the field across the electrodes 3 by a few miilljoules to give a result equivalent to that achievable in the absence of such a field by approximately doubling the energy input to the apparatus which would be an increase of at least several joules.

Claims

1. A high pressure, pulsed electric, glow discharge apparatus, including a discharge vessel for containing a gas or gas mixture incorporating at least one species whose electron attachment rate decreases with increasing electric field and electron energy, a pair of spaced apart electrodes contained in the discharge vessel, between which electrodes a pre-ionising cloud of electrons is formed, by a pulse of ultraviolet or X-ray radiation, in the gas or gas mixture, and a glow discharge then formed in the pre-ionised gas or gas mixture by the application of a high voltage pulse across the electrodes, and means for creating a high voltage field across the electrodes during and after said pre-ionising electron cloud formation, which high voltage field is high enough to reduce the rate of electron loss from the cloud but not high enough to cause premature avalanching.

2. Apparatus according to claim 1, wherein the species utilised is one or more of F2, SF6, CC14, C12 and NF3.

3. Apparatus according to claim 1 or claim 2, wherein the high voltage field creation means includes a high voltage power supply.

4. Apparatus according to claim 3, wherein the high voltage power supply is a Direct Current (DC) power supply operable to generate a DC field across the electrodes.

5. Apparatus according to any one of claims 1 to 4, wherein the high voltage field creation means includes an isolating element.

6. Apparatus according to claim 5, wherein the isolating element is a high resistance resistor or a high value inductor.

7. Apparatus according to any one of claims 1 to 6, wherein the pulse of ultraviolet radiation is caused by spark or surface discharge.

8. Apparatus according to claim 6, including means for creating said high voltage pulse across the electrodes, which includes a peaking capacitor connected in parallel with the electrodes, a main energy storage capacitor connected in parallel with the peaking capacitor, a switch connected in series with the main energy storage capacitor between said main energy storage capacitor and earth, and a capacitor charging input connected between the main energy storage capacitor and the switch.

9. Apparatus according to claim 8, wherein the high voltage power supply is connected in series with the isolating element and wherein the high voltage power supply is connected in parallel with the electrodes so that, in operation, the isolating element protects the high voltage power supply from discharge of the main energy storage capacitor and acts to limit the amount of current drawn from the high voltage power supply.

10. A high pressure, pulsed electric, glow discharge apparatus, substantially as hereinbefore described with reference to the accompanying drawing.