GB2180094A

GB2180094A - Discharge tube arrangements

Info

Publication number: GB2180094A
Application number: GB08620350A
Authority: GB
Inventors: Dr Arthur Maitland; Hugh Menown
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1985-08-22
Filing date: 1986-08-21
Publication date: 1987-03-18
Anticipated expiration: 2006-08-21
Also published as: GB8521065D0; GB2180094B; GB8620350D0

Abstract

A discharge tube arrangement includes a gas-filled discharge tube 15, coaxially surrounded by a capacitor 20 and by a switch which is a toroidal discharge tube 16. Initially, the toroidal discharge tube 16 is non-conducting, and a potential is applied at 26 to charge the capacitor 20. When it is desired to trigger the first discharge tube 15, a pulse of high frequency energy is applied to a coil 23 wound around the outer circumference of the toroidal discharge tube 16. This initiates a glow discharge within the toroidal tube 16, thus lowering its breakdown potential. Because of the potential difference applied between two circular ring electrodes 17 and 18 within the toroidal tube, breakdown of the gas filling occurs to give an annular discharge, and it becomes conducting. The stored charge of the capacitor 20 is discharged principally through the discharge tube 15 causing breakdown of its gas filling and establishment of a discharge. The coaxial symmetry of the arrangement gives a low inductance circuit and an arrangement which is physically compact. In another embodiment of the invention an annular discharge is arranged to cause excitation of a laser active medium. <IMAGE>

Description

SPECIFICATION Discharge tube arrangements This invention relates to discharge tube arrangements.

A discharge tube is a device which comprises an envelope containing a gas andtwo electrodeswhich are spaced apart from one another. When a suitably large potential difference is applied acrosstheelectrodes, the gas tends to break down. When ionisation occurs, due to the value of said potential difference alone, orotherwise,thetube becomes conducting.

In one application, a discharge tube is employed in apparatus for excitation of a laser active medium.

The laser active medium is excited when it absorbs incident radiation from a discharge within the discharge tube.

Absorption by the active medium occurs when radiation incident thereon has a spectrum at least part of which coincides with the absorption spectrum of particles, which may befor example atoms, ions or molecules, ofthe active medium. Atoms in a lower energy level absorb photons and are excited into a higher energy level. Irradiating the active medium with radiation to be absorbed to achieve this is known as "pumping".

In a spontaneous transition, particles, such as atoms, in a higher energy level reverttoaloweren- ergy level, possibly with the emission of a photon.

Stimulated emission of radiation occurs when an incident photon interacts with a particle in an excited state causing itto return to a lowerenergylevelac- companied by the emission of a photon which is identical to the stimulating photon and in the same direction as it. When many particles are able to undergo stimulated emission, amplification ofthe stimulating radiation occurs and laser action is initiated.

A conventional method of pumping the laser active medium to increase the number of particles in higher energy levels is illustrated in Figure 1. A laser active medium 1 in the form of a rod having mirrored ends 2 and 3 is surrounded by a coiled discharge tube or flash tube 4. Theflash tube 4 comprises an envelope 5 containing a gas such as xenon or krypton and two electrodes 6 and 7 spaced apart. When it is desired to initiate a discharge within the flash tube 4 a large potential difference is applied between the electrodes 6 and 7 by discharging a bank of capacitors and breakdown of the gas filling occurs to produceadischarge which emits incoherent optical radiation which is absorbed by the laser active medium.

The present invention seeks to provide an improved discharge tube arrangement.

According to a first aspect of th is invention, there is provided a dischargetube arrangement comprising a discharge tube, an electrical conductor, and means fortransmitting high frequency electromagnetic energy along the conductor, the arrangement being such that, when high frequency electromagnetic energy istransmitted along theconductor, an annular discharge is produced within the discharge tu be.

According to a second aspect of this invention, there is provided a discharge tube arrangementcomprising a discharge tube having two electrodes and means for transmitting high frequency electro magneticenergyalong an electrical conductor,the arrangement being such that when the energy is transmitted along the conductor, a glow discharge is established within the tube, whereby when a suitable potential difference is applied between said electrodes, an annular discharge is produced within the tube.

An annular discharge permits currents to be uniformly conducted across the tube and an arrangement in accordance with the invention is particularly suitable where a low inductance circuit is required.

The high frequency electromagnetic energy could be, for example, radio frequency or microwave energy.

The coil may be arranged about or inside the tube, whichever is most convenientfor a particular pur- pose. Preferably the electrical conductor is a coil, having a numberofturnswound aboutthe discharge tube.

Preferably, the discharge tube is substantiallytor- oidal. In this specification the term "toroidal" should be taken to mean a tube having a transverse section which istopologically equivalentto an annulus, i.e., that it is defined by two non-intersecting boundaries.

It may be advantageous that, where the discharge tube is toroidal, the conductor is a coil wound coaxially around the outer circumference ofthe discharge tube.

A load may be included in the arrangement and the discharge tube arranged to act as a switch for triggering a currentwithin the load. This is particularly useful where the load is a second discharge tube, and preferably a laser discharge tube. Preferably, where the first-mentioned discharge tube is substantially toroidal, it coaxially surrounds the second discharge tube. This is a particularly advantageous arrangement since, because of the coaxial symmetry, the in ductance of the circuit containing the discharge tube is low. The conductor may be located within the cen tral aperture ofthe first mentioned tube, and the wall of the second tube comprise metallic material arranged to substantially shield the tube from a field produced by passing energy along the conductor.

The discharge tube may be arranged such that an annular discharge within the tube produces excitation of a laser active medium, and it is preferred that where the discharge tube is substantially toroidal, it is arranged aboutthe laser active medium, which may be, for example, a crystal or a dye-stream.

According to a third aspect of this invention,there is provided apparatus for excitation of a laser active medium comprising a discharge tube arranged such that a discharge therein produces excitation of the laser active medium; an electrical conductor; and means fortransmitting high frequency electromagnetic energy along the conductor, the arrangement being such that, when the energy is transmitted along the conductor, a discharge is initiated within the dischargetube.The dischargetube may include two electrodes and transmission of the electromagnetic energy along the conductor arranged to produce ionisation within the discharge tube such that when a suitable potential difference is applied between the electrodes a discharge is initiated within the dischargetube, i.e., the high frequency electromagnetic energy is used to trigger the discharge which produces excitation of the laser active medium.

By employing apparatus in accordance with the invention it is possible to achieve a more uniform and rapidly established discharge within the tube, and hence improved excitation ofthe active medium, than would be possible using conventional apparatus.

The invention is nowfurtherdescribed byway of example with reference to the accompanying drawings in which: Figures 2a, 2b, 2c and 2dare explanatory diagrams; Figures 3 and3a iliustrate schematically an arran- gement in accordance with the invention; Figure 4schematically illustrates another arrangement in accordance with the invention; Figure 5schematically illustrates yet another arrangement in accordance with the invention; and Figure 6shows schematically a further arrangement in accordance with the invention.

Figure 2a illustrates the electricfieldEwhich results when microwave energy is conducted along a coil of radiusR represented ata, withrbeing in the dir- ection ofthe diameter ofthe coil. Above a threshold level for a particular discharge tube, the electricfield produced results in breakdown of the gas filiing.

Figure 2b shows the ionisation produced within a toroidal discharge tube when the coil is located outside, Figure 2cthe ionisation when the coil is located in its central aperture, and Figure 2d when the coil is loc atedwithinthe discharge tube itself.

With reference to Figures 3 and 3a, a non-toroidal discharge tube 8 has an electrical conductor 9 wound circumferentially about it and two ring electrodes 10 and 11 spaced apartwithin it. Awaveguide 12 sur roundsthedischargetube8and has an inputsection 13 and an output section 14. Figure 3a is a transverse section ofthe discharge tube 8, which is of internal radius.

During operation, a microwave pulse istransmit- ted along the input waveguide section 13 and coupled out along the electrical conductor 9. There- sulting eiectricfield is at a maximum at the wall ofthe tube 8 and at a minimum atthe centre. The results in an annular distribution of ionisation, as shown in Figure 3a, where the shading represents the ionisation. When ionisation is produced within the tube, a suitably large potential difference between the ring electrodes 10 and 11 causes an annular discharge to be established between them.

With reference to Figure 4, a gas-filled laser discharge tube 15 is coaxially surrounded by a toroidal discharge tube 16, also containing a gas, which acts as a switch for triggering a discharge in the laser discharge tube 15. The toroidal discharge tube 16 includes two circular ring electrodes 17 and 18 spaced apartfrom one another. One ring electrode 17 is con nected to an electrode 19 ofthe laser dischargetube 15 via an annular capacitor 20, which is also arranged to coaxially surround the discharge tube 15. The other ring electrode 18 is connected, with another el ectrode 21 ofthe laser discharge tube 15, to earth. A resistor 22 is connected in parallel with the first dischargetube 15.

Acoil 23iswound aroundthe outside ofthetor- oidal dischargetube 16 and is coupled to input and output radio frequency or microwave waveguide sections 24 and 25 respectively.

During operation, a large potential is applied at26 to the plate of the capacitor 20 which is connected to the ring electrode 17 of the toroidal discharge tube 16, energy being stored in the capacitor 20. Whilstthe capacitor 20 is being charged, the gas filling ofthe toroidal tube 16 remains non-ionised and no conduction occurs between its ring electrodes 17 and 18.

When itis desired totriggerthe discharge ofthe laser dischargetube 15, a high frequency electromagnetic pulse of energy is applied to the input section 24 of the coil 23. The magnetic and electrostatic field produced by the coil 23 causes a giow discharge to be produced within the toroidal discharge tube 16. Thus, the breakdown potential ofthetoroidal discharge tube 16 is effectively lowered and, if the potential dif ference applied between the two ring electrodes 17 and 18 is large enough, further ionisation occurs and an annulardischargeisproducedwithinthetoroidal tube 16. The rise time ofthe main discharge current is relatively small since conduction occurs between substantially the whole circumference of the two ring electrodes 17 and 18.

When the annular discharge has been established within the toroidal discharge tube 16, it permits a large voltage difference to be applied across the laser discharge tube 15, resulting in breakdown ofthegas, and rapid discharge of the capacitor 20. Thus a large discharge current is produced through the laserdischargetube 15.

The coaxial symmetry of this arrangement results in a particularly low inductance circuit.

With reference to Figure 5, a laser includes a crystal laser active medium 27 which is cylindrical in form.

The active medium 27 is coaxially surrounded by a discharge tube 28 which is substantiallytoroidal, being of a hollow cylinder configuration. The discharge tube 28 includes an envelope 29 which contains a low pressure gas and two ring electrodes 30 and 31 ateach end locatedateachendofthedischargetube28.

An electrical conductor 32 in the form of a coil is coaxially located aboutthe discharge tube 28 and is coupled to input and output microwave waveguide sections 33 and 34 respectively.

When it is desired to initiate a discharge within the discharge tube 28, and thereby excite the laser active medium, a large potential difference is applied between the electrodes 30 and 31. A pulse of microwave energy is transmitted along the input microwave waveguide section 33 and applied to the coil 34.

The magnetic and electrostatic fields produced by the coil 32 cause ionisation to be produced within the discharge tube 28. The ionisation produced has an annularconfigurationand,becauseoftheannular character of the discharge tube 28, is produced substantially uniformly throughoutthe volume enclosed by the envelope 29 ofthe dischargetube 28. The ionisation produced causes the breakdown potential ofthe discharge tube 28 to be lowered and, since a large potential difference is present between the el ectrodes 30 and 31, breakdown occurs and a dis charge is established within the discharge tube 28.

The optical radiation which results irradiates the laser active medium 27 and enables a population inversionto be achieved, and hence initiates laseraction.

With reference to Figure 6, another laser arrange ment is similartothatshown in Figure 4, but the coil 35 is located within the central aperture 36 of the toroidal discharge tube 37 which surrounds a laser dischargetube 38. A layerofmetal 39 is laid down on the laser discharge tube wall to preventthe gas con tained therein being ionized by the electric field produced when microwave energy is transmitted along the coil 35.

It may not be necessary to include a metal shielding layer on the wall ofthe inner laser discharge tube 38 ifit is located in aweakfield region ofthe arrangement, where its threshold level is not exceeded.

Claims

1. Adischargetubearrangementcomprising a discharge tube, an electrical conductor, and means fortransmitting high frequency electromagnetic energy along the conductor, the arrangement being such thatwhen high frequency electromagnetic en ergyistransmitted along the conductor, an annular discharge is produced within the discharge tube.

2. A discharge tube arrangement comprising a discharge tube having two electrodes and means for transmitting high frequency electromagnetic energy along an electrical conductor, the arrangement being such that when the energy is transmitted along the conductor, a glow discharge is established within the tube, whereby, when a suitable potential difference is applied between said electrodes, an annular dis charge is produced within the tube.

3. An arrangement as claimed in claim 1 or 2 and wherein the high frequency electromagnetic energy is radio frequency or microwave energy.

4. An arrangement as claimed in claim 1,2 or3 and wherein the conductor is arranged about the tube.

5. An arrangement as claimed in claim 1,2 or3 and wherein the conductor is arranged within the tube.

6. An arrangement as claimed in any preceding claim and wherein the electrical conductor is a coil.

7. An arrangement as claimed in any preceding claim, andwherein the dischargetube is substantiallytoroidal.

8. An arrangement as claimed in claim 7 and wherein the conductor is a coil wound coaxially around the outer circumference of the discharge tube.

9. An arrangement as claimed in any preceding claim, and including a load, the discharge tube being arranged to act as a switch fortriggering a current within the load.

10. An arrangement as claimed in claim 9and wherein the load is a second discharge tube.

11. An arrangement as claimed in claim 10 and wherein the second dischargetube is a laserdischarge tube.

12. An arrangement as claimed in claim 10 or 11 and wherein an electrode of the first-mentioned discharge tube is electrically connected to an electrode ofthe second discharge tube via a capacitor.

13. An arrangement as claimed in claim 10,11 or 12 and wherein the first-mentioned dischargetube is substantially toroidal and coaxially surrounds the second-mentioned discharge tube.

14. An arrangement as claimed in claim 13 and wherein the conductor is located within the central apertureofthefirst-mentionedtube,andthewall of the second tube comprises metallic material arrangedto substantially shield thetubefrom a field produced by passing energy along the conductor.

15. An arrangement as claimed in any of claims 1 to 8 and wherein the discharge tube is arranged such that an annular discharge within the tube produces excitation of a laser active medium.

16. An arrangement as claimed in claim 15 and wherein, where the discharge tube is substantially toroidal, it is arranged about the laser active medium.

17. Apparatus for excitation of a laser active medium comprising a discharge tube arranged such that a discharge therein produces excitation ofthe laser active medium; an electrical conductor; and means fortransmitting high frequency electromagnetic energy along the conductorthe arrangement being such that, when the energy is transmitted along the conductor, a discharge is initiated within the discharge tube.

18. Apparatus as claimed in claim 17 and wherein the dischargetube includes two electrodes and transmission ofthe electromagnetic energy along the conductor is arranged to produce ionisation withinthedischargetubesuchthatwhenasuitable potential difference is applied between the electrodes a discharge is initiated within the discharge tube.

19. Adischargetubearrangementsubstantially as illustrated in and described with reference to Fig ures 4 or 6 of the accompanying drawings.

20. Laser apparatus substantially as illustrated in and described with reference to Figure 5 of the accompanying drawings.