GB1559755A - Electronic discharge devices - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRONIC DISCHARGE DEVICES (71) We, ENGLISH ELECTRIC VALVE COMPANY LIMITED, a British Company, of 106, Waterhouse Lane, Chelmsford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns electronic discharge devices incorporating discharge confinement structures, and relates in particular to lasers.

In certain kind of lasers, notably the argon ion laser, it is necessary to generate an electrical discharge having an extremely high current density. The discharge must be sufficiently great to generate and maintain an ionised gas plasma in which laser action can take place. In order to produce a sufficiently high current density it has been proposed to confine the current, and hence the plasma, within a narrow bore of a confinement structure which provides a discharge path from a cathode to an anode.

Unfortunately, the action of the discharge can cause severe damage to the confinement structure, the degree of damage being dependent both on the nature of the material used to fabricate the structure and on its particular design.

One object of the present invention is to provide an improved electronic discharge device incorporating a discharge confinement structure, in which the latter is relatively resistant to the effects of the discharge.

According to this invention, an electronic device is provided having a discharge confinement structure comprising a stack of plates contained within an evacuated envelope which is in contact with the plates and serves to locate their positions, each plate having a central aperture which aligns with the corresponding apertures in the other plates to form a passage for a discharge, and each plate being composed of pyrolytic graphite having its plane of high thermal and electrical conductivity aligned with the plane of the plate.

Pyrolytic graphite is a form of molecularly-ordered carbon which is produced by vapour deposition resulting from the decomposition of a hot carbonaceous gas.

Although the material is referred to as pyrolytic graphite, it is not a true graphite in the crystallographic sense. The properties of pyrolytic graphite are described in the article "Pyrolytic Graphite" by W.H. Smith and D.H. Leeds published in Modern Materials, Volume 7 at page 139 et seq., Academic Press Inc. New York and London, 1970; briefly, pyrolytic graphite is a highly anisotropic material, and exhibits a high thermal conductivity and mechanical strength in a direction parallel to the plane in which the material is built up by a vapour deposition, while, conversely, it exhibits relatively poor thermal and electrical conductivity in the direction transverse to this. It has been realised that these properties make pyrolytic graphite particularly suitable for use in a discharge confinement structure.Furthermore, it has been found, rather unexpectedly, that pyrolytic graphite appears to be significantly less susceptible to erosion in the presence of high discharge currents than the pure graphite, which in any event would not be a satisfactory material for the purpose.

Each plate may be insulated from the other plates (as described in the Provisional Specification filed with our Application No.

38,015/77), but preferably, in accordance with this invention the plates are assembled one upon the other in electrical contact therewith.

Preferably the device is a laser.

Each plate is advantageously circular, with the central aperture lying on the axis of the circumference of the circular plate, each of the further apertures being significantly smaller in size than the central aperture; these smaller apertures allow non-ionised gas to return from the anode end of the structure to the cathode end to equalise the gas pressure at both ends.

Both the central aperture and the further apertures are conveniently circular in shape.

If desired, the stack of plates may be so arranged that the size of the central aperture increases progressively towards each end of the stack in such a manner that the central bore of the discharge confinement structure, which is constituted by the central apertures in the plates, is in the form of a relatively long narrow central portion and a relatively short flare at each end. This profile serves to increase the current density of the discharge current, and may be desirable for certain applications.

Preferably, each plate is of uniform thickness, and, where each plate is insulated from the next, is formed with a strengthening circular rib (this provides a useful degree of rigidity to the plates which, in practice, are rather thin).

Additionally, means for producing an axial magnetic field may be provided so as to confine the discharge current to the region of the central axis. Where such a magnetic field is provided, preferably the means for producing it is located outside of, and in close proximity to, the evacuated envelope.

The invention is further described, though only by way of illustration, with reference to Figures 1, 2 and 3 of the drawing accompanying the aforementioned Provisional Specification of our Application No. 38015/77, in which: Figure I illustrates a section view through a discharge confinement structure forming part of an argon ion laser; Figure 2 shows one of the apertured plates forming part of the discharge confinement structure; and Figure 3 illustrates a section of such a plate; and also with reference to Figures 1 and 2 of the drawings accompanying the Provisional Specification of our Application No. 2719/78 (which have been renumbered hereinafter as 'Figure 4' and "Figure 5" for ease of reference), in which: Figure 4 shows a section view through a discharge confinement structure forming part of an argon ion laser; and Figure 5 shows one of the apertured plates forming part of the discharge confinement structure.

Referring to Figures 1 to 3, the discharge confinement structure comprises a number of disc-shaped plates 1, composed of pyrolytic graphite, each of which is provided with a central circular aperture 2, a plurality of smaller circular apertures 3 (not shown in Figure 1) disposed around its circumference, and a circular strengthening rib (as in Figure 3). The discs 1 are spaced apart by short lengths of tubular sleeving 4 which, in this case, is composed of silica. The whole structure is contained within an evacuated envelope 5 the cylindrical walls of which are in contact with the outer edges of the plates 1 and serve to locate the plates in their correct positions. The discharge confinement structure shown forms part of an argon ion laser, and the evacuated envelope 5 contains argon gas.A cathode 6 located at one end of the evacuated envelope generates a very large discharge current which is attracted to an anode 7 at the other end.

The discharge current is confined by the plates 1, and for practical purposes flows along a bore defined by the central apertures 2. Within this region, the argon gas is ionised to form a high temperature plasma in which laser action takes place. The light generated within the plasma is extracted from the envelope 5 via holes 8 and 9 in the anode and cathode respectively, and emerges via Brewster windows 10 and 11.

Typically, each disc 1 is about an inch in diameter, and the central aperture 2 is about 22 mm in diameter. The cathode 6 produces a total current of about 30 amps, which corresponds to a current density within the bore of about 600 amps per square centimetre. The potential difference between anode and cathode is typically about 400 volts. If desired, the central apertures 2 may be progressively increased over a few plates towards each end of the stack so as to provide a lead-in and lead-out for the discharge current. At the cathode end this serves to progressively concentrate the current to the current density needed for effective laser action to take place.

It will be realised that such a laser generates a great deal of heat, and, in practice, the outside of the envelope 5 is water-cooled. The plates 4 provide an exceilent radial heat path from the axis of the structure to the outer walls by virtue of the anisotropic properties of pyrolytic graphite which, in this case, provides a fairly good thermal path radiaily from the axis of the structure to the envelope. In addition, the relatively poor electrical conduction of pyrolytic graphite in a direction along the axis of the structure means that a potential drop can appear across the thickness of each plate, thereby improving the electrical properties of the plasma in this region. If desired, an external magnetic focusing system can be employed to produce an axial magnetic field which, additionally, serves to confine the plasma to the axis of the structure.

Referring to Figures 4 and 5 (and using the same reference numerals where possible), the discharge confinement structure comprises a stack 1 of disc-shaped plates (as 1') composed of pyrolytic graphite, each of which is provided with a central circular aperture 2 and a plurality of smaller circular apertures 3 disposed around its circumference. The smaller apertures 3 are not represented in Figure 4, and only the first and last five of the individual plates 1' forming the stack 1 are shown in detail; the remainder are represented by dashed lines.

The discs 1' are assembled one upon another so as to be in electrical contact.

Each of the plates 1' forming the stack 1 has its plane of high thermal and electrical conductivity aligned with the plane of the plate, so that the conductivity of the stack 1 is relatively high in a radial direction but relatively low in an axial direction. The whole structure is contained within an evacuated envelope 5 the cylindrical walls of which are in contact with the outer edges of the plates 1' and serve to locate the plates in their correct positions.

The discharge confinement structure shown forms part of an argon ion laser, otherwise as known per se, and is effectively the same as the corresponding structure in figure 1, so needing no further explanation.

WHAT WE CLAIM IS: 1. An electronic device having a discharge confinement structure comprising a stack of plates contained within an evacuated envelope which is in contact with the plates and serves to locate their positions, each plate having a central aperture which aligns with the corresponding apertures in the other plates to form a passage for a discharge, and each plate being composed of pyrolytic graphite having its plane of high thermal and electrical conductivity aligned with the plane of the plate.

2. A device as claimed in claim 1, wherein each plate is insulated from the other plates.

3. A device as claimed in claim 1, wherein the plates are assembled one upon another in electrical contact therewith.

4. A device as claimed in any of the above claims and embodied as a laser.

5. A device as claimed in any of the above claims, wherein each plate is circular, with the central aperture lying on the axis of the circle.

6. A device as claimed in claim 5, wherein further apertures are placed adjacent the circumference of the circular plate, each of the further apertures being significantly smaller in size than the central aperture.

7. A device as claimed in claim 6, wherein both the central aperture and the further apertures are circular in shape.

8. A device as claimed in any of the above claims, wherein the stack of plates is so arranged that the size of the central aperture increases progressively towards each end of the stack so that the central bore of the discharge confinement structure, which is constituted by the central apertures in the plates, is in the form of a relatively long narrow central portion and a relatively short flare at each end.

9. A device as claimed in any of claims 2 and 3 to 8 as dependent upon claim 2, wherein each plate is formed with a strengthening circular rib.

10. A device as claimed in any of the above claims, wherein means for producing an axial magnetic field is provided so as to confine the discharge current to the region of the central axis.

11. A device as claimed in claim 10, wherein the means for producing said axial magnetic field is located outside of, and in close proximity to, the evacuated envelope.

12. An electronic device substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings accompanying the Provisional Specification of our Application No. 38,015/77.

13. An electronic device substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings accompanying the Provisional Specification of our Application No. 2719/78.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. axis of the structure means that a potential drop can appear across the thickness of each plate, thereby improving the electrical properties of the plasma in this region. If desired, an external magnetic focusing system can be employed to produce an axial magnetic field which, additionally, serves to confine the plasma to the axis of the structure. Referring to Figures 4 and 5 (and using the same reference numerals where possible), the discharge confinement structure comprises a stack 1 of disc-shaped plates (as 1') composed of pyrolytic graphite, each of which is provided with a central circular aperture 2 and a plurality of smaller circular apertures 3 disposed around its circumference. The smaller apertures 3 are not represented in Figure 4, and only the first and last five of the individual plates 1' forming the stack 1 are shown in detail; the remainder are represented by dashed lines. The discs 1' are assembled one upon another so as to be in electrical contact. Each of the plates 1' forming the stack 1 has its plane of high thermal and electrical conductivity aligned with the plane of the plate, so that the conductivity of the stack 1 is relatively high in a radial direction but relatively low in an axial direction. The whole structure is contained within an evacuated envelope 5 the cylindrical walls of which are in contact with the outer edges of the plates 1' and serve to locate the plates in their correct positions. The discharge confinement structure shown forms part of an argon ion laser, otherwise as known per se, and is effectively the same as the corresponding structure in figure 1, so needing no further explanation. WHAT WE CLAIM IS:

1. An electronic device having a discharge confinement structure comprising a stack of plates contained within an evacuated envelope which is in contact with the plates and serves to locate their positions, each plate having a central aperture which aligns with the corresponding apertures in the other plates to form a passage for a discharge, and each plate being composed of pyrolytic graphite having its plane of high thermal and electrical conductivity aligned with the plane of the plate.

2. A device as claimed in claim 1, wherein each plate is insulated from the other plates.

3. A device as claimed in claim 1, wherein the plates are assembled one upon another in electrical contact therewith.

4. A device as claimed in any of the above claims and embodied as a laser.

5. A device as claimed in any of the above claims, wherein each plate is circular, with the central aperture lying on the axis of the circle.

6. A device as claimed in claim 5, wherein further apertures are placed adjacent the circumference of the circular plate, each of the further apertures being significantly smaller in size than the central aperture.

7. A device as claimed in claim 6, wherein both the central aperture and the further apertures are circular in shape.

8. A device as claimed in any of the above claims, wherein the stack of plates is so arranged that the size of the central aperture increases progressively towards each end of the stack so that the central bore of the discharge confinement structure, which is constituted by the central apertures in the plates, is in the form of a relatively long narrow central portion and a relatively short flare at each end.

9. A device as claimed in any of claims 2 and 3 to 8 as dependent upon claim 2, wherein each plate is formed with a strengthening circular rib.

10. A device as claimed in any of the above claims, wherein means for producing an axial magnetic field is provided so as to confine the discharge current to the region of the central axis.

11. A device as claimed in claim 10, wherein the means for producing said axial magnetic field is located outside of, and in close proximity to, the evacuated envelope.

12. An electronic device substantially as hereinbefore described with reference to Figures 1 to 3 of the drawings accompanying the Provisional Specification of our Application No. 38,015/77.

13. An electronic device substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings accompanying the Provisional Specification of our Application No. 2719/78.