US2961557A

US2961557A - Apparatus for creating by induction an electric discharge in a gas at low pressure

Info

Publication number: US2961557A
Application number: US740644A
Authority: US
Inventors: Hubert Pierre
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1957-06-12
Filing date: 1958-06-09
Publication date: 1960-11-22
Anticipated expiration: 1977-11-22
Also published as: CH350726A; GB857686A; FR1180695A; BE568261A; LU36148A1

Description

Nov. 22, 1960 P HUBERT 2,961,557 APPARATUS FOR CREATING BY mnuc'rxon AN ELECTRIC mscmncs IN A ens AT LOW PRESSURE Filed June 9, 1958 2 Sheets-Sheet -1 Nov. 22, 1960 P. HUBERT 61,557

APPARATUS FOR CRE NG BY INDUCTION AN ELECTRI DISCHARGE A GAS AT LOW PRESSURE Filed June 9. 1958 2 Sheets-Sheet 2 2,961,557 'Patented Nov. 22, 196i) APPARATUS FOR CREATING BY INDUCTION AN ELECTRIC DISCHARGE IN A GAS AT LOW PRESSURE Pierre Hubert, Sceaux, France, assignor to Commissariat a lEnergie Atomique, Paris, France, a state administration of France Filed June 9, 1958, Ser. No. 740,644

Claims priority, application France June 12, 1957 11 Claims. (Cl. 313-62) The invention, relates to apparatus for creating by electromagnetic induction a high discharge current in a gas contained under low pressure in a tubular chamber closed upon itself, that is to say of doughnut shape, and preferably of toroidal shape.

The object of the present invention is to improve the stabilizing of this discharge in such a manner as to prevent any contact of the ionized gas or plasma with the walls of the chamber, which contact would result in a soiling of the plasma by evaporation of the material of which said walls are made.

Such a stabilizing permits of reducing the causes of cooling of the plasma and consequently of increasing the temperature obtained therein. It is known that the production of such a discharge constitutes one of the means for obtaining a controlled thermonuclear fusion in order to supply a considerable amount of energy.

Various apparatus have already been imagined to create a discharge such as above mentioned and to confine it at a distance from the walls of the annular chamber.

Among these apparatus, may be cited the following ones:

The Smith apparatus (Physical Review, 1947, 71, 135), which makes use of the stabilizing properties of a metal lic tore surrounding the discharge, said tore being provided with a radial insulating cut to prevent shortcircuiting the driving electro-motive force.

The Bostick et al. apparatus experimented at Tufts College (Nucleonics, vol. 14, No. 2, 1956, page 44) in which an auxiliary winding wound about a tore creates therein a homogeneous magnetic field, but for other purposes than stabilization of a discharge, for instance to study the oscillations of a plasma (Physical Review, 97, No. L, 1955, page 13).

The apparatus known under the name of Perhapsatron (Nucleonics, vol. 15, 1957, page 108) in which the inductor winding is made of insulating turns disposed in parallel along planes parallel to the center line of the tore.

The Cousins et al. apparatus (Proceeding Physical Society, 1951, 64 B, page 159), in which the inductor winding is constituted by a metallic tore surrounding the discharge.

Each of these apparatus has drawback such as the following ones:

Imperfect stabilization of the discharge,

Defective efficiency of the discharge due to the high distance between the inductor circuit and the induced circuit (Smith, Bostick or to a defective adaptation to the impedance between the source and the inductor circuit (Cousins),

Lack of regularity of the magnetic field in the vincinity of the zone where the current lead-in conductors are connected to the inductor winding (devices such as the perhapsatron in which the casing of the chamber is not metallic),

Lack of gastightness of the chamber (in the case of apparatus in which only the metallic envelope is provided to ensure said gastightness).

In order to obviate these drawbacks, according to the present invention, use is made of an annular discharge tube the wall of which is constituted by, or preferably lined with, a sheath of a conducting material provided with at least one insulating cut, along which wall is wound a stabilizing winding connected with a source of direct current, the turns of said winding being disposed substantially in planes perpendicular to the central line of said chamber.

Furthermore, the inductor winding is advantageously constituted by a conductor connected with a source capable of supplying within a very short time a high electrical energy, the turns made by said conductor being disposed substantially in planes parallel to the center line of the annular chamber.

The shape of the tubular chamber in which the discharge is produced is not critical. It is sufficient to have this chamber constituted by a tube closed upon itself and which does not include portions that are too sharply curved. In the following description, this shape will be called a doughnut shape.

Advantageously, this tube is of toroidal shape.

This tube is preferably made of a material which is gastight, is a good insulator, can be perfectly degassed and is as refractory as possible. In this respect the glass known under the trade-name of Pyrex, porcelain and vitrified silica are particularly suitable.

In addition to the gastightness it ensures, this chamber permits of distributing the potentials in the vicinity of the insulating cut provided in the conductor sheath and therefore of preventing arcs between these lips.

Said sheath, disposed on the outside of the insulating wall of the tube (when such a tube is provided) conforms to the shape of this wall as closely as possible.

In order to distribute in a more uniform manner the field in the vicinity of the cut in the sheath, said out is preferably covered by a local annular strip of conducting material, for instance in the shape of a portion of a metallic tore, electrically insulated from the conductor sheath.

Said conductor sheath may be made, for instance, by winding about the annular chamber a copper braid so as to form two crossed layers connected by welds. It is also possible merely to use a suitable shaped metal sheet, the metal that is chosen being then as good an electrical conductor as possible (aluminium, copper).

The whole of the annular chamber and of the conductor sheath may also be made in the form of a metal tore the inner wall of which is enamelled.

The advantage of the conductor sheath of shield is that any deformation of the plasma filament causes the formation in said sheath of induced currents which create a magnetic field exerting on the plasma a force which opposes said deformation.

The stabilizing winding is constituted by insulated conductor wires the turns of which are wound regularly about the annular tube in planes perpendicular to the center line thereof.

Such a stabilizing winding creates a longitudinal magnetic field parallel at any point to the discharge, that is to say to the plasma filament, which is confined along the central line of the chamber, due to the pinch effect created by the electromagnetic attraction between parallel current lines.

If there is a deformation of this discharge, said field exerts thereon forces which oppose said deformation.

Theory teaches that the action of the magnetic field produced by the currents induced in the metal envelope chiefly opposes the deformations the wave length of which is higher than the length of the central line of the chamber whereas the action of the magnetic field produced by the stabilizing winding chiefly opposes the deformations the wave length of which is lower than the length of said central line.

Concerningth'e inductor winding, it is preferably constituted by insulated conductor wires disposed regularly along the external surface of the-conductor sheath, the turns of this inductor winding being located substantially in" planes'parallel to the plane of the central line of the chamber and being advantageously disposed between the sheath and the stabilizing winding.

1 Said inductor winding consists of N elementary windingsconnected in parallel, each of these elementary windi'n'gs being constituted by a conductor forming it turns each adjoining the next one and covering a continuous annular band'o'f the external surface of the conductor sheath.

'The number of turns it of each of said elementary windings is' chosen in such'm-anner as to adapt the source of current in the best conditions to the impedance of the inductor winding. The'use of a plurality of elementary windings connected in'parallel permits of preventing deformations of the discharge having a wave length close to the length of the central line of the chamber (for instance contraction or expansion of the whole of the discharge).

This result is due to the fact that if the discharge gets closer to one of the elementary windings, there is produced, according to the Lenz law, an increase of the current flowing through said elementary winding, this increase having for its effect to oppose the displacement of the discharge.

In these conditions, stabilization of the discharge is ensured by the combined actions of the currents circulating respectively in the conductor sheath, in the stabilizing Winding and in the inductor winding.

Theuse of a plurality of such elementary windings, as above described, is not quite necessary since the conductor sheath also serves to oppose this kind of deformation. But it is useful because it reduces the importance of the part to be played by the sheath and therefore permits of reducing its thickness.

The location of the inductor winding on the surface of the conductor sheath is preferred to thearrangement according to which the coupling of the discharge is ensured through a ferromagnetic core as in an ordinary transformer.

r This is due to the following causes:

The natural self inductance of the discharge is reduced to aminimum, which permits of obtaining high current discharges without having to handle too high a reactive energy.

The mutual inductance between the discharge and the inductor Winding is maximum so that it is then possible to have a suflicient coupling even in the absence of a magnetic circuit; one might thus consider the possibility of working with a saturable magnetic circuit which would act only during a fraction of the cycle of operation.

This arrangement permits of easily dealing with the considerable electromagnetic forces which exist between conductors through which high currents are flowing: in the case of an annular chamber of toroidal shape, it can be demonstrated that the electromagnetic forces exerted on the turns of the inductor winding by the field created during a discharge correspond to a pressure equal to the pressure existing in the discharge divided by the ratio by the inductor winding. It is therefore possible to concentrate the plasma under very high pressure without 4, having to deal with mechanical problems which are practically impossible to solve.

The operation of the device is as follows:

When the voltage V of the source connected with the inductor winding is suddenly applied to the terminals of said winding, there is produced, in the gas under low pressure contained in the chamber, an electromotive force which is substantially uniform and equal to When this electromotive force becomes suflicient, it produces a disruptive discharge in the gas, through which a very high current may flow if'the value of V is sufiicient; the gas is then heated by the Houle effect to a very high temperature, which may permit of obtaining nuclear fusions in this gas. The device works as a step-down transformer the primary of which is the inductor winding and the secondary of which includes only one turn constituted bythe discharge itself.

' It is reminded that the'-pin'ch effect? may be suflicient by itself to keep the discharge at a distance from the wall if the following condition is complied with:

where I is the current in amperes in the discharge,

P is the total number of electrically charged particles per centimeter of length of the discharge,

K is the Boltzmann constant, equal to l.380 l0- ergs/ C., and

T is the absolute temperature of the plasma in centigrade degrees."

The arrangements according to the present invention complete the stabilizing effect,'in particular for insufficient values of I.

Thus the longitudinal field created by the stabilizing winding and also the fields due to the current circulating through the metal casing both have, in addition to their stabilizing action, the property of slowing down the diffusion of plasma toward the wall of the chamber when the electromotive force which'produces the pinch effect is reduced to Zero.

The presence of these fields therefore makes possible a continuous operation of the apparatus in which the inductor winding would be fed from a source of reciprocating current provided that the frequency of this current is suificiently high, that is to say that the portion of every cycle during which the current in the discharge is zero or practically so (duning which portion of the cycle there is no confinement due to'the pinch effect) is sufficiently short.

With such a reciprocating current feed, the pinch effect confinement is possible permanently if condition (I) is complied with as an average during every cycle, that is to say if:

100/2 PK T (11) 1e being the effective current developed during the discharge.

It should be noted that the total longitudinal field existing between the plasma and the wall of the chamber may in some cases have a direction opposed to that created separately by the stabilizing winding, since the field due to the current induced in the metallic casing may have in some conditions a preponderating value: only the sum of these two effects is to be considered.

In some cases, a magnetic circuit increasing the mutual inductance between the inductor winding and the dischargemay have afavorable action. This circuit might for instance be constituted by a'soft iron core surrounding at least a portion of the chamber, in a manner "analogous to the cores on which are wound the conventional inductor windings (Bostick et al.).

The plasma, maintained by a system of electromagnetic forces, may be the seat of oscillations, either transient or not, giving rise to alternative compressions and expansions, for instance as a consequence of a tendency to instability. These oscillations induce in the inductor winding (or in another suitable winding) an electro- Emotive force which may be used to contribute in the heating of the plasma or to extract energy therefrom.

The apparatus according to the present invention has many advantages over those used up to this time to heat very rarefied gases to very high temperatures, and in particular:

A stabilizing of the deformations for all wavelengths of the discharge;

An excellent efficiency due to the fact that the inductor winding and the discharge are close to each other and also to the possibility of adaptation between the impedances of this winding and of the inductor source;

A homogeneous distribution of the field in the vicinity of the current inlet and of the cut;

A good gastightness of the chamber;

The possibility of obtaining a very high current discharge while producing the inductor current with a conventional machine, since the current in the discharge may be n times higher than that supplied by the source (as it is known in transformers) which further reduces the losses in the feed conductors;

The possibility of a continuous operation.

It shouid be noted that the fact of making the inductor winding in the form of a plurality of elementary windings as above indicated, may play, due to the stabilizing efiect it involves, a very important part in the case of discharges of very long duration (for instance .averaging of a second) for which it would not be possible to give the metal envelope a thickness corresponding to the skin thickness. This case might occur when :a temperature higher than thirty millions of degrees is obtained, for which the conductivity of the plasma will be higher than that of all known metals and in particular than that of the metal of which said envelope is made. It is known that, in this case, the feed of the stabilizing winding should be cut a short time t before the beginning :of the discharge, the value of this time t, which depends upon the speed of diffusion of the magnetic field through the metal envelope, being at most equal to In this formula, e is the thickness of the metal sheath and p the resistivity of the metal thereof in ohms per centimeter.

A preferred embodiment of the present invention will be hereinafter described with reference to the accompanying drawing given merely by way of example and in which:

Fig. 1 diagrammatically shows a discharge according to the invention, and

Fig. 2 is a diametral vertical section of this apparatus with parts in cross section.

On this drawing, only portions of the winding, respectively designated by reference numerals 1 (inductor winding) and 2 (stabilizing winding) have been shown.

The annular chamber 3 is constituted by a toroidal envelope of Pyrex glass having a mean diameter of 78 cuts, the diameter of the circle the rotation of which forms the tore being equal to 8 cms.

The inside of this chamber is connected with a pumping conduit 4 of a diameter equal to 1.5 cm. itself connected with a pump capable of producing a vacuum ranging from 3 10- to 10" mm. of mercury.

This chamber 3 is covered with a conductor shield 5 made of a copper braid provided with a cut 6 the width (about 1 centimeter) of which corresponds substantially to one tenth of the diameter of the diameter of the above mentioned circle.

This cut is covered by a metal strip 7 insulated from shield 5.

The inductor winding 1 includes six conductors eadh constituted by a copper wire of 2 mm. diameter enclosed in a sheath of polythene of a diameter of about 1 cm. connected in parallel with the current source 8 and each of which makes four turns about the tore. The four turns of every conductor are adjacent to each other and occupy the portion of the external surface of the tore corresponding to an angle of 60.

The stabilizing winding 2 is constituted by four superimposed layers of enamelled copper wire of a diameter of 2 mm. These layers are wound in such manner that the respective magnetic fields they produce are added to one another but that the sum of the mutual inductance of one layer with the inductor winding and of the mutual inductance of the next layer with said inductor winding is practically zero. For this purpose, it suffices, in the example shown to wind wire 2 always in the same direction; for instance, supposing that the center line of chamber 3 (Le. the circle on which are located the centers of all the circular cross sections of said chamber) is located in a horizontal plane, each turn of each layer is wound so that it runs upwardly, from below said plane to above it, on the outer portion of said turn and downwardly, from above said plane to below it, on the inner portion of said turn.

The envelope constituted by the stabilizing winding 2 is provided with a cut analogous to that provided in shield 5. This cut must constitute an electrical insulation capable of supporting between its edges a potential difference equal (in this case 1L 4 induced by winding 1.

The source of current 8 includes a bank of condensers of 50 kv. and 60 microfarads fed by a high voltage generator. Its discharge is controlled by spark gap 11 having three electrodes and it is transmitted to the elementary inductor winding 1 through six coaxial cables 12 mounted in parallel, which ensures a better resistance to mechanical efforts and reduces the importance of the parasitic self-inductances due to the mounting.

A direct current source 13, constituted by a rotary machine of 600 v. and A. of usual type supplies current to the stabilizing Winding 2.

An observation window is constituted by a mere orifice of 5X20 mm. provided in shield 7 opposite the cuts, this window extending between the wires of winding 1 which are slightly spaced apart for this purpose.

In order to facilitate starting, there is provided an induction coil 14 capable of exciting an electrode 15 surrounding on the outside the pumping conduit 4, thus ionizing some particles of the gas circulating through this conduit.

For this purpose, use may be made of an auxiliary discharge established between electrodes passing through the wall of chamber 3, or also of electron-guns.

A pumping system 16 connected with conduit 4 permits of evacuating chamber 3, of degassing it and of introducing the desired gas thereinto.

In order to measure the discharge current, it is advantageous to make use of two current transformers one of which 17 surrounds the coaxial cables 12 and is therefore sensitive only to the intensity of the current in the inductor winding, Whereas the other one 18 surrounds a portion of the tore and is sensitive both to this intensity and to that of the current flowing through the discharge. This transformer cooperates with a suitable measurement device 19.

In a general manner, while I have, in the above de scription, disclosed what I deem to be a practical and efiicient embodiment of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form the parts without departing from the principle of the present invention as comprehended Within the scope of the accompanying claims.

What I claim is:

1. An apparatus which comprises, in combination, a closed doughnut-shaped discharge tube containing a low pressure atmosphere of an ionizable gas, means for inducing a high discharge current to flow in said tube, a sheath of a conducting material closely surrounding the space inside said tube, said sheath being doughnut-shaped except for a transverse out between the ends thereof located opposite each other, a band of an insulating material filling said cut, a stabilizing winding surrounding said tube, each turn of said stabilizing winding being located in a plane substantially at right angles to the portion of the center line of said tube surrounded by said turn, and means for passing direct current through said'stabilizing winding, said current inducing means being independent of said sheath of conducting material.

2. An apparatus which comprises, in combination, a closed doughnut-shaped discharge tube containing a low pressure atmosphere of an ionizable gas, a sheath of a conducting material closely surrounding the space inside said tube, said sheath being doughnut-shaped except for a transverse cut between the ends thereof located opposite each other, a band of an insulating material filling said cut, at least one inductor winding wound on said sheath, the turns of said winding being located in respective planes parallel to the center line of said tube, a source capable "of releasing a high electrical energy within a short time, the terminals of said source being connected with those of said winding respectively, a stabilizing winding surrounding said sheath, each turn of said stabilizing winding being located in a plane substantially at right angles to the portion of the center line of said tube surrounded by said turn, and means for passing direct current through said stabilizing winding.

3. An apparatus according to claim 2 including a plurality of such inductor windings disposed side by side on said sheath, said inductor windings being connected in parallel with said source, each of said inductor windings including a conductor forming a plurality of turns located side by side, all of said respective inductor windings having the same number of turns, the whole of the turns of said plurality of inductor windings covering the whole external surface of said sheath.

4. An apparatus according to claim 1, in which said discharge tube is made of a gas-tight insulating material and said sheath is mounted on the outer wall of said tube.

5. An apparatus according to claim 1, further including an annular strip of a conducting material mounted around the ends of said sheath so as to surroundfsaid band of insulating material," said' strip being insulated from said sheath.

6. An apparatus according to claim 2, in which said stabilizing winding includes an even number of superirnposer layers of turns of a conductor, said layers being wound so that the respective magnetic fields they produce are added to one another but that the sum of the mutual inductance of one of said layer with said inductor winding and of the mutual inductance of the next of said layers with said inductor winding is practically zero, said layers stopping short of said cut in said sheath, on both sides of said cut.

7. An apparatus according to claim 2, further including means for measuring the discharge current, said means including a current transformer surrounding one of the terminals of said source and a current transformer surrounding a portion of said discharge tube.

8. An apparatus according to claim 2 in which said source is a high frequency alternating current source.

9. An apparatus which comprises, in combination, a closed doughnut-shaped discharge tube containing a low pressure atmosphere of an ionizable gas, means for inducing a high discharge current to flow in said tube, a sheath of a conducting material closely surrounding the space inside said tube, said sheath 'being continuous except for a transverse cut between the ends thereof, a band of an insulating material filling said cut, a stabilizing winding surrounding said tube, each turn of said stabilizing winding being located in a plane substantially at right angles to the portion of the center line of said tube surrounded by said turn, and means for passing direct current through said stabilizing winding, said current inducing means being independent of said sheath of conducting material.

10. An apparatus according to claim 9 having inductor windings parallel to the axis of said tube and on said sheath, said windings disposed side by side on said sheath and connected in parallel with said source.

11. The apparatus according to claim 10 wherein said inductor windings comprise a conductor forming a plurality of turns located side by side, all of said inductor windings having the same number of turns, the whole of the turns of said plurality of inductor windings covering the whole external surface of said sheath.

References Cited in the file of this patent UNITED STATES PATENTS 2,627,552 Gurewitsch Feb. 3, 1953 2,640,923 Pollock June 2, 1953 2,658,999 Farley Nov. 10, 1953 2,683,216 Wideroe July 6, 1954 2,790,902 Wright Apr. 30, 1957