WO1995005060A1

WO1995005060A1 - A radiofrequency gas discharge

Info

Publication number: WO1995005060A1
Application number: PCT/GB1994/001722
Authority: WO
Inventors: Howard John Baker; Alan David Colley; Denis Roderick Hall
Original assignee: Lumonics Limited
Priority date: 1993-08-05
Filing date: 1994-08-05
Publication date: 1995-02-16
Also published as: GB9316282D0; EP0664072A1

Abstract

A radiofrequency gas discharge comprises a gas containment envelope (26), electrodes (1, 2) having a large electrode area mounted within the envelope (26), a power distribution network (5, 6, 7, 8) coupled to the electrodes (1, 2), and a radiofrequency power source (9). The power distribution network comprises a low impedance transmission line (5, 6, 7) and a plurality of coils (8). These are so combined as to form a distributed parallel resonant circuit having a quality factor Q which is greater than unity in the presence of very low discharge electrical impedance. The radiofrequency power source (9) is connected to a feed point on the distributed parallel resonant circuit.

Description

DESCRIPTION

A RADIOFREQUENCY GAS DISCHARGE

The present invention relates to transverse radio- frequency gas discharges, and particularly those involving parallel or concentric electrodes of large area, such discharges may be used, for example, for the excitation of high power gas lasers, or for applications in plasma processing of materials, where the spatial uniformity of a large area high power density discharge is important. In particular, the present invention relates to a power distribution network which facilitates the uniform excitation of a radiofrequency discharge of large electrode area.

Although, as described below, the invention may be applied to a number of applications of large area RF discharges, the background and need for the invention is described in terms of the requirements for uniform excitation of large area slab lasers, and particularly slab waveguide lasers employing one of the usual gas mixtures appropriate for operation of the carbon dioxide laser. There are already well known a number of designs for the construction of gas lasers, such as the carbon dioxide laser, where transverse discharges provide the excitation of the laser gain medium. In such designs, a transverse electromagnetic field in the radiofrequency regime (approximately lMHz to 1 GHz) is applied between parallel or concentric metal, or metal- clad electrodes, whose separation may be such as to permit either optical waveguiding between the said electrodes, or alternatively, the propagation along the optical axis of a free space Gaussian laser beam which exhibits negligible interaction with the said electrodes.

It has been pointed out that difficulties arise for certain laser discharge structures useful for example for laser excitation (such as for example those described in US Patent 4,169,251 entitled Waveguide Gas Lasers with High Frequency Transverse Discharge excitation, granted to K D Laakmann on January 16 1978), when the length of such laser discharge structures is comparable to or exceeds /4, where is the wavelength in the relevant material of the radio- frequency field produced by the generator which supplies power to the discharge. In such circum- stances, unless precautions are taken, a serious longitudinal non-uniformity in the transverse (inter- electrode) voltage occurs as a result of the interference between the forward and reflected RF waves travelling along the electrode structure. The said longitudinal voltage variations give ruse to spatial non-uniformities in the discharge which are often unaσceptably high and may even cause severe discharge instabilities. The said patent asserts the necessity of multiple RF discharge feed points or of inductive termination to reduce the scale of the deleterious discharge non-uniformities along the length of the electrode structure.

Improvements on this proposed solution have been claimed by others who propose the use of multiple shunt inductors in parallel with the electrodes, for example in US Patent 4,352,188 entitled RF Pumped Waveguide Laser with Inductive Loading for Enhancing Discharge

Uniformity, and granted to G A Griffith on September 28 1982: or by the use of single parallel inductor, selected to form a resonant circuit with the capaci¬ tance of the electrode structure, ad at a resonant frequency such that the real part of the complex impedance of the discharge chamber is matched to the output impedance of the driving power generator, as in US Patent 4,363,126 entitled Tuned-circuit RF Excited Laser, and issued to P P Chenausky et al on December 7 1982.

A further improvement in the art of producing uniform transverse radiofrequency discharges in waveguide lasers whose length exceedsλ./4 has been claimed by P P Chenausky et al in US Patent 4,443,387 entitled Uniformly excited RF Waveguide Laser, and granted on April 17 1984, wherein the use of multiple shunt _ n -

inductors in parallel with the laser discharge and selected to resonate with the capacitance of the structure at a frequency chosen to be close to the RF Power Generator frequency is claimed to produce a uniform distribution of transverse voltage along the waveguide laser for lengths in excess ofΛ-/4. A particular and compact embodiment of this parallel resonance technique is claimed in US Patent 4,787,090 entitled Compact Distributed Inductance RF-Excited Waveguide Gas Laser issued to Newman et al, which demonstrates the use of a compact structure incorporating a plurality of flat inductor coils.

The technique of US Patents 4,443,877 and 4,787,090 has become well-known in the field of RF discharge excited waveguide lasers of modest size and high capacitance, and has been applied either to laser structures employing bare metallic electrodes, or else to electrodes ballasted with dielectric material positioned between the metal electrodes and the gas discharge medium. In structures not employing dielectric ballast, the discharge is particularly susceptible to an instability process, wherein a transition may occur from the stable low current (alpha) form of the discharge to either a pure high current (gamma) mode, (which has catastrophic consequences for the operation of the laser) or to a hybrid form of the discharge in which patches of gamma discharge co-exist with a highly diffuse, reduced current alpha discharge. These hybrid discharges, containing patches of. gamma discharge and referred to as "hot spots" in US Patent 4,493,087 are also highly deleterious to efficient laser action.

Thus, the avoidance of the alpha-to-gamma transition is a matter of the highest importance in the design of transverse RF excited gas lasers. Of particular relevance are the issues involved in the choice of the frequency of the RF generator. If it were the case that very low values of drive frequency, say a few tens of MHz, could be chosen without prejudice to the efficient operation of the discharge and the laser, then the deleterious transmission line effects which cause longitudinal variations in the transverse RF discharge voltage, as discussed above, could be significantly reduced. However, the freedom to choose low RF frequencies is reduced, particularly for lasers without dielectric ballast for two important reasons. Previously published research such as for example D R Hall and H J Baker, "RF Excitation of Diffusion Cooled and fast Axial Flow Lasers", SPIE 1031, 60 (1989), or P Vitruk, H J Baker, D R Hall, "The Stability of Alpha RF Discharges in Carbon Dioxide Laser Mixtures", J Phys D: Applied Physics 25, 1767 (1992) makes clear the necessity of operating at frequencies which are high enough (for the particular value of inter-electrode separation in use), to prevent the onset of the damaging alpha-to-gamma transition. Secondly, other literature, such as in the paper entitled RF Excited C0₂ Lasers by D R Hall and C A Hill, published in Handbook of Molecular Lasers, Ed P K Cheo, Marcel Dekker, New York 1988 documents the benefits in laser power and efficiency of careful selection of the generator frequency in concert with the inter-electrode separation. Failure to do so results in significant reduction in laser performance.

Although the distributed parallel resonance technique (US Patent 4,443,877) maybe successfully applied as claimed, to discharges used for example for RF excited waveguide lasers of modest size, it has a number of serious drawbacks when applied to devices of larger electrode area, or devices where the discharge resistance is low. There is an increasing number of cases where the operation of transverse radiofrequency discharges maintained between electrodes of much larger than in the prior art have important practical applica¬ tions, such as, for example, the excitation of high power gas lasers. In such cases, the circuit Q is low, and the distributed parallel resonance technique fails (_as demonstrated below) to produce a distribution of transverse RF voltage which has adequate uniformity across the plane of the electrode. This deficiency may be particularly acute and damaging when applied to discharges whose geometry is planar in cross section, forming for example a rectangular slab of discharge, which may be used for example as a laser gain medium. This situation is in contrast to the case of square or circular cross section discharges (of smaller electrode area) which are characteristic of the devices whose transverse RF discharge voltage spatial uniformity has been the subject of the prior art inventions described above.

As examples of such large electrode area devices may be cited the lasers described by K M Abra ski, A D Colley, H J Baker and D R Hall in applied Physics Letters 54, 1833 (1989), or by P E Jackson, H J Baker, D R Hall in Applied Physics Letters 54, 1950 (1989). Further examples of discharge and laser geometries, where the inter-electrode separation is such as to allow free space Gaussian beams to propagate may be seen in the work of H J Baker, X S Zhang, D R Hall published in Journal of Physics D:Applied Physics 26, 359 (1993).

In all such cases, if RF fields are used to excite the discharge, and if the frequency is chosen for optimum excitation, then a non-uniform distribution in the transverse voltage will result when the area of the discharge is increased sufficiently, leading to a high probability of discharge instabilities, such as the alpha-to-gamma transition and/or inefficient laser excitation. As will be illustrated more specifically below, the distributed parallel resonance technique, cited above, fails in these circumstances to produce a uniform voltage distribution, because the magnitudes of the structure capacitance and discharge impedance are such as to lead to very low Q values and no effective resonance can be produced by the methods of the prior art. In these cases, such as for example those appropriate for high power large area diffusion-cooled discharge lasers such as that described by A D Colley, H J Baker, D R Hall in Applied Physics Letters 61, 136 (1992), the resultant voltage non-uniformity may lead to the onset of alpha-to-gamma transitions, or to the non-uniform deposition of electrical energy in the discharge, so producing, at the very lease, inefficient laser excitation and for severe non-uniformities, localised gas heating and the generation of RF arcs.

It is an object of the present invention to produce in power distribution network which produces a high degree of spatial uniformity of the transverse RF voltage over the whole surface of a large area transverse radio- frequency gas discharge. The said transverse voltage uniformity is such as to prevent the onset of any of the above-mentioned discharge instabilities, and more- over may be translated into a corresponding uniformity of the electrical power deposited in the discharge. Embodiments of the present invention will now be described, by way of examples, with reference to the accompanying drawings, in which:

Figure 1(a) shows a large area discharge structure in the form of a rectangular slab formed by a pair of parallel plate electrodes separated by a pair of dielectric spacers;

Figure 1(b) shows a large area discharge structure in the form of a rectangular slab formed by a pair of parallel plate electrodes with no dielectric sidewall spacers;

Figure 2 shows a large area high frequency discharge structure connected to a single source of RF power through a power distribution network in accordance with the present invention and a reactive impedance matching network;

Figure 3 illustrates an aspect of the operation of the circuit shown in Figure 2;

Figure 4 shows the normalised deviation of the transverse inter-electrode voltage from the mean value as a function of the distance from the end of the electrode structure, when the distributed parallel resonance technique of the prior art is employed to provide power distribution;

Figure 5 shows the normalised deviation of the transverse inter-electrode voltage from the mean value as a function of the distance from the end of the electrode structure when a power distribution network in accordance with the present invention is employed;

Figure 6 shows an alternative representation of the network shown in Figure 2;

Figure 7(a) shows a power destination network embodying the present invention in which the power distribution network is supplied by a plurality of power amplifiers, each with an individual input;

Figure 7(b) shows a power distribution network according to a second embodiment of the present invention in which the power distribution network is supplied by a plurality of power amplifiers which are in turn supplied by a second power distribution network similar to that shown in Figure 2 from a single low power source;

Figure 7(c) shows a power distribution network according to a third embodiment of the present invention in which positive feedback is provided by a reactive network to produce self oscillation at the resonant frequency of the power distribution network;

Figure 7(c) shows a power distribution network according to a fourth embodiment of the present invention in which self-oscillation is achieved by distributed positive feedback from multiple points along the power distribution network via multiple reactive networks to multiple distributed power sources;

Figure 8 shows a particular mechanical embodiment of the invention;

Figure 9 shows an alternative particular mechanical embodiment of the invention.

In Figure 1, two arrangements of a pair of electrodes 1.2 for the creation of a transverse radiofrequency discharge gas discharge 3 are shown. The electrodes may be provided with a means of temperature control, such as for example as water cooling, and will also normally be inside a gas-sealed enclosure. They may have surfaces prepared for efficient optical wave- guiding. The present invention is particularly appropriate for this discharge geometry in which the electrode spacing is small, and the electrode area is large, resulting in a very low electrical impedance. Whilst planar electrodes as shown in Figure 1 may be commonly used, other electrode surface shapes of matched curvature are equally applicable to the invention. Figure 1(a) shows an arrangement where dielectric material pieces 4 are used to set the spacing of the electrodes and define the area of electrode where the discharge 3 may exist. In Figure 1(b) the electrodes are supported at the desired spacing by an external means not shown, and the area of the discharge 3 is that of the whole area of the opposing faces of the electrodes 1,2. In the following description of the invention, the electrode and dis¬ charge arrangement may be of any of the types just described.

In a principle embodiment of the invention shown in

Figure 2, a pair of electrodes 1,2 is used to define a region of high frequency gas discharge 3 in the form of a large area sheet of small thickness. The properties of the discharge are such as, by way of example, to excite efficiently a gas laser gain medium, or in a second type of example to allow plasma processing of suitable surfaces. A second pair of metal plates 5,6 is separated by a sheet of insulating or dielectric material 7 so as to form a low impedance electrical transmission line which is situated in close proximity to the said discharge electrodes 1,2. One of the electrodes 2 is connected to one of the transmission line plates 5 by a plurality of metal rods or a continuous metal plate or other means, so as to form a low inductance electrical path, as illustrated by lines 101, between the electrode 2 and the transmission line plate 5. The opposing electrode 1 and transmission line plate 6 are connected electrically by a plurality of coils 8. The inductances of the said coils 8 are chosen in conjunction with the capacitance of the electrodes 1,2 in the presence of a gas discharge, the capacitance of the transmission line and stray inductances so as to create an electrical resonance at or near the desired operating frequency. In the following, the network enclosed as 11 in Figure 2 will be referred to as a power distribution network. Electrical power is introduced into this network 11 from a high frequency power generator 9 through a reactive impedance matching network 10 connected electrically to the centre of the transmission line plate 6 and to the centre of the electrode 1. The reactive impedance matching network 10 is well known in the prior art. The high frequency power generator 9 may be of the oscillator-amplifier type or of the power self-oscillator type, both well known in the prior art, or of any other general type. The purpose of the structure depicted in Figure 2 is to ensure the uniform distribution of electrical power to all points of the discharge electrode by creating a highly uniform value for the magnitude of the high frequency voltage between all matching regions of the electrodes 1,2 referred to hereafter as the inter-electrode voltage.

The operation of the network 11 in Figure 2 involves two aspects associated with the metal plates 5,6 and dielectric 7. The first aspect is illustrated in

Figure 3 which shows one section of the full network of Figure 2. This circuit consists of one of the coils 8, the capacitance and resistance of a section of the discharge region 3 between electrodes 1,2, the capaci- tance of a section of the transmission line formed by the metal plates 5,6 and dielectric material 7, and the stray inductance of the connections between the capaci¬ tances of these sections are a fraction 1/n of the total capacitance of the inter-electrode region 3 and the transmission line 5,6,7 where n is the number of coils 8 used in the network. The circuit in Figure 3 operates as a series resonant circuit with resonant frequency close to the frequency of the electrical power generator 9. The presence of the capacitance introduced by the transmission line acts to increase the Q or quality factor of this section of the full network when the impedance present between the electrodes 1,2 is the very low value encountered in discharges between large areas of electrode such as used for the excitation of high power slab or coaxial gas lasers. The full network 11 in Figure 2 is made up of sections as illustrated in Figure 3 essentially in parallel, except that the high operating frequency in combination with the large physical size requires the use of an electrical transmission line viewpoint for the description of current flow between the sections. This gives rise to the second aspect of the operation of the circuit shown in Figure 2, in that the trans¬ mission line formed by the electrodes 1,2 and their discharge region 3, is effectively connected in parallel with the second transmission line formed by metal plates 5,6 and dielectric material 7, for the transmission of high frequency current between the resonant sections of the type in Figure 3. The two transmission lines form a composite line of very low characteristic impedance which assists the attainment of a very uniform inter-electrode voltage as power is distributed from the electrical input at the centre of the network 11 in Figure 2 to the large area of dis¬ charge 3. To obtain maximum benefit of both aspects described above, the dielectric constant of the dielectric material 7 should be a low value amongst the range available for high frequency insulating materials.

In the prior art, the condition of parallel electrical resonance is created by a plurality of coils connected directly across the electrodes. For large electrode areas, composed of both a much wider and longer dis¬ charge region than envisaged in the prior art, the resistive loading of the electrical circuit by the discharge produces a low quality factor Q for the parallel resonance, which prevents the attainment of a uniform voltage distribution along the electrodes. Even with an optimum configuration of the circuit, the uniformity of the local inter-electrode voltage becomes inadequate for the maintenance of a uniform discharge. To compare the present invention with prior art, and to demonstrate the clear advantage offered, linear network analysis has been used to determine the root-mean- square value of the inter-electrode voltage as a func¬ tion of position along the electrodes. This analysis procedure has been carried out for the following example of a structure composed of planar metal electrodes of area 770 mm x 95 mm and separated by 2 mm, and a gas discharge with characteristics appropri¬ ate for the operation of a carbon dioxide laser with a total power input of lOkW and laser output power exceeding lkW. Figure 4 shows the voltage distribution along the electrode expressed as a percentage of the average inter-electrode voltage for the aforesaid planar electrodes when the prior art method of parallel resonance is used. The voltage variations are large and cannot be reduced further from the values of ÷12% to -8% shown in Figure 4 by an variations of the number of coils or their inductance value. Because of the highly non-linear properties of gas discharges, such voltage variations would be sufficient to give rise both to regions where the discharge is extinguished and other regions where it operates at too high a power density, with possible severely deleterious consequences.

The results of an analysis of the same discharge structure and discharge using the power distribution network 11 of the present invention are given in Figure 5, which shows the inter-electrode voltage uniformity obtained for the case where the total capacitance of the transmission line 4,5 in Figure 2 is 270 pF and where the dielectric material 6 is poly¬ tetrafluoroethylene (PTFE) . At the optimum point, selected either by the inductance value of the coils 8 or the frequency of the power generator 9 in Figure 2, the voltage variations are less than ±1% and the dis¬ charge can operate in the highly uniform manner required for laser operation and other similar applications.

The electrically resonant circuit depicted in Figure 2 involves the input of power at or near the central point through connections to the transmission line 6 and the electrode 1. This type of configuration is particularly suited to the use of a remote power generator 9 connected to the network in Figure 2 by the usual 50Ω coaxial cable, in that it presents an impedance to the matching network 10 which is much larger than the discharge impedance and which is of similar magnitude to that of the connecting coaxial _l cable. This provides a simplification in the design of the matching network 10. Connection of the power source may also be at other than a central point, for example at one or other end of the power distributing network 11, with some small reduction in the degree of inter-electrode voltage uniformity.

The basic network of Figure 2 can be used with other power input configurations which can be understood with reference to Figure 6. This is a simplified electrical circuit diagram which shows all the coils 8 in parallel as a single coil 17, the total capacitance of the electrodes 1,2 and discharge region 3 as single capaci- tance 16 and the total capacitance of the transmission line made up of 5,6 and 7 as a single capacitor 15. Figure 6 shows that there are 3 possible configurations for the supply of high frequency power to the network. Connection of the power source between circuit nodes 12 and 14 corresponds to that in Figure 2 and its advant¬ ages have been discussed above. Connection between nodes 12 and 13 produces a configuration of intermedi¬ ate impedance which can make use of high values of stray inductance associated with vacuum feedthroughs as _a significant fraction or all of the inductance 17.

Connection between circuit nodes 13 and 14 produces a low impedance which is suited to the use of low voltage power sources based on semiconductors.

The ability of the network exemplified by Figure 2 to distribute power uniformly to an extended discharge is used in further embodiments of the invention using multiple sources of high frequency power. Such sources may be connected in one of the three configurations just described in association with Figure 6, and with the multiple power sources connected at a range of points evenly spaced along the power distributing network 11 as in Figure 7(a), where the power sources are configured as amplifiers with individual inputs 19. Each power source 18 consists of an active device or devices in association with reactive components and DC power supply so as to form an effective high frequency power amplifier, in accordance with prior art. In the case of active devices such as triode or tetrode vacuum tubes, which naturally operate into high impedance, the distributed power sources 18 may be connected between nodes 12 and 14 or between nodes 12 and 18 for best effect. In the case of active devices based on semi¬ conductors such as bipolar or field effect transistors which naturally operate into a low impedance, the distributed power sources may be connected between nodes 14 and 18 for best effect. In Figure 7(a), an external means may be provided to adjust the phase of each signal applied to the inputs 19 to match the requirement of the power distribution network 11. A specific embodiment of the invention occurs when the multiple power sources 18 are assembled in close association with the power distribution network 11, and no coaxial cable or other transmission line method is used between the power sources 18 and the distribution network. This integrated power source is most appropriate when a large number of transistors are the active devices but many also be appropriate for a smaller number of moderate power rating vacuum tube devices. This embodiment limits high current signals to the immediate surroundings of the gas laser device and avoids the need for high power, high frequency coaxial cables. Advantages are gained from the reduced risk of electromagnetic interference with other equip- ment and improved electrical reliability and safety.

Such integrated power sources may be operated either in an amplifier mode or as a self-excited power oscillator. In the case of the multiple active devices operating as electrical amplifiers a second power distribution network 20 similar to that in Figure 2 can be configured to drive the inputs of the power sources 18 in parallel from a single source connected at 21, as in Figure 7(b). In this case the input impedances of the power sources 18 act as a distributed load on the power distribution network 20 in the same manner as the gas discharge 3 loads the network 11 in Figure 2. The network 20 ensures uniform power delivery to all active devices, and can be arranged to provide the correct relative phase of input signal to all active devices.

In the case of self-oscillator operation, positive feedback is required to cause oscillation at or close to the resonant frequency of the power distribution network 11. In Figure 7(c), the necessary positive feedback is provided by a single reactive network 22 connecting for example central points of the two distribution networks 11 and 20. More effectively, self-oscillation may be attained by distributed positive feedback from multiple points along the power distribution network 11 via multiple reactive networks 23 to the multiple distributed power sources 18 as in Figure 7(d). This configuration can preserve the correct phase relationships and avoids the use of a second power distribution network 20.

In the simplest mechanical embodiment of the invention, the electrodes 1,2 and associated power distribution network components 5,6,7,8 are assembled within a vacuum envelope which is arranged to contain the low pressure gas required by the discharge and separate it from the atmosphere. This embodiment has the advantage of requiring only one electrical feedthrough in the vacuum envelope, but requires great care to avoid unwanted gas discharges within or between components. It is appropriate for the simplest configurations of the invention such as that in Figure 2.

Figures 8 and 9 show cross-sections of particular mechanical embodiments of the invention for the excitation of a planar discharge region of the type in Figure 1(a). These embodiments place the power distri¬ bution components outside the vacuum envelope and are appropriate for all the electrical configurations previously described. In Figure 8, the electrodes 1,2 are mounted within a metallic vacuum enclosure 26 which acts to separate the low pressure gas fill from the atmosphere. Electrical connections to the electrode are made by a row of feedthroughs composed of metal rods 24 surrounded by insulating material 27. Electrical connections to electrode 1 are made by the internal connecting plates 25 to the vacuum enclosure 26. The electrical transmission line made up of plates 5,6 and dielectric 7 is mounted on the rods 24 with the upper plate 6 connected to the outer surface of the metallic vacuum enclosure 26 by the multiplicity of coils 8. In Figure 8 an alternative arrangement, which achieves the same electrical configuration, has two rows of electrical feedthroughs made up of rods 24 and insulators 27.

In one particular embodiment of the invention a large area radiofrequency discharge or a plurality of such discharges, excited in a uniform fashion according to the present invention, is used as a technique for energising a high power carbon dioxide laser. In other embodiments, a large area radiofrequency discharge or a plurality of such discharges, excited according to the present invention is used to excite a carbon monoxide xenon or other gas laser type which is suitable for excitation by the well known radiofrequency discharge technique. In yet a further embodiment, a large area radiofrequency discharge, established in a suitable gaseous medium in a uniform fashion according to the present invention may be used to provide a uniform high power source for plasma processing of a suitably constituted material or surface.

Claims

- 2-\ -

1) A radiofrequency gas discharge comprising a gas containment envelope (26) electrodes (1,2) having a large electrode area mounted within the envelope (26), a power distribution network (5,6,7,8) coupled to the electrodes (1,2) , and a radiofrequency power source

(9), wherein the power distribution network comprises a low impedance transmission line (5,6,7) and a plurality of coils (8) which are so combined as to form a distributed parallel resonant circuit having a quality factor Q which is greater than unity in the presence of very low discharge electrical impedance, and the radio- frequency power source (9) is connected to a feed point on the distributed parallel resonant circuit.

2) A radiofrequency gas discharge according to Claim 1 comprising a plurality of radiofrequency power sources (18), each of which is separately connected to the distributed parallel resonant circuit (11) which acts to combine their power output and provide uniform power distribution throughout a discharge.

3) A radiofrequency gas discharge according to Claim 2, wherein the said plurality of radiofrequency power sources (18) comprises an array of active devices which are configured as electrical amplifiers and which are connected directly to the power distribution network. 4) A radiofrequency gas discharge according to Claim 3, wherein the said array of the active devices (18) have their input signals provided by a further distributed parallel resonant circuit (20) connected to a single external radiofrequency power source (21).

5) A radiofrequency gas discharge according to Claim 3 or 4, wherein each of the said active devices (18) is configured to operate as an oscillator by the provision of positive feedback (22).

6) A radiofrequency discharge according to Claim 5, wherein separate positive feedback (23) is provided for each of the said active devices (18).

7) A radiofrequency gas discharge according to any preceding Claim, wherein the power distribution network (5,6,7,8) is mounted within the gas containment envelope.

8) A radiofrequency gas discharge according to any of Claims 1 to 6, wherein the power distribution network (5,6,7,8) is mounted externally of the gas containment envelope (26), and the low impedance transmission line (5,6,7) is coupled to the electrodes (1,2) through the gas containment envelope (26) by mean of an array of vacuum electrical feedthroughs (24,27). 9) A radiofrequency gas discharge according to any preceding Claim wherein the discharge region defined by the electrodes is annular or some other geometrical form and is maintained between closely and evenly spaced metallic or dielectric electrodes such that the lateral and longitudinal extents of the discharge are very much greater than the spacing of the electrodes and where the gas discharge is cooled by conduction to the said electrodes.

10) A radiofrequency gas discharge according to any one of Claims 1 to 8 wherein the discharge region defined by the electrodes is comprised of multiple narrow sections which are excited electrically by common metallic or dielectric electrodes such that the lateral and longitudinal extents of the electrodes is much greater than the cross-section of the individual narrow discharge sections so as to provide an array of parallel gas laser elements and where the gas discharges are cooled by conduction to the said electrodes.

11) A radiofrequency gas discharge according to any one of Claims 1 to 10, wherein the electrode surfaces in contact with the discharge or discharges are smooth, polished and of such a material as to create conditions of optical waveguiding of the laser light through the discharge region in the operation of a gas laser.