US3643123A

US3643123A - Plasma containment device

Info

Publication number: US3643123A
Application number: US771103A
Authority: US
Inventors: Andrew V Haeff
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1968-10-28
Filing date: 1968-10-28
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15

Abstract

A plasma confinement device having separate means for repelling positive ions and for repelling electrons from the walls of the plasma chamber. The electrons are repelled from the central portion of the chamber by a sufficiently strong magnetic field. However, the positive ions which are little affected by the magnetic field are repelled from the walls of the chamber by making the walls of an insulating material which charges positively by impact of a few of the positive ions. The chamber has two end portions, one of which includes an ion generator and accelerator which tends to repel electrons, the other end portion is provided with an electron repeller. On the other hand, the positive ions are kept out of the end portions of the chamber by the positive charge on the walls thereof.

Description

United States Patent Haeff Feb. 15, 1972 [54] PLASMA CONTAINMENT DEVICE Primary Examiner-Raymond F. Hossfeld Attorney-Daniel T. Anderson, Gerald Singer and Alfons [72] Inventor. Andrew V. Haeff, Los Angeles, Cal1f. valukonis [73] Assignee: TRW Inc., Redondo Beach, Calif.

57 ABSTRACT [22] Filed: Oct. 28, 1968 1 A plasma confinement device having separate means for [21] APPINO'Z 77lm3 repelling positive ions and for repelling electrons from the walls of the plasma chamber. The electrons are repelled from 52 0.5.01. ..3l3/162, 313/63, 313/230 the Central portion of t champ r hy a sufficiently strong [51] Int. Cl ..H0lj l/50 magnet": field- Howcveri the Posmve wh'ch are 58 Field of Search ..3l3/63, 161,207,230, 162 femd by the magnetic field are l' 9 l P chamber by making the walls of an insulating matenal which charges positively by impact of a few of the positive ions. The [56] References cued chamber has two end portions, one of which includes an ion UNITED STATES PATENTS generator and accelerator which tends to repel electrons, the other end portion is provided with an electron repeller. On the 3,l66,477 l/ 1965 Leboutet "313/ l6l X other hand, the positive ions are kept out f the end portions s ftz 3 1 of the chamber by the positive charge on the walls thereof. wa

7 Claims, 2 Drawing Figures PATENTEUFEBISISI? 3.643.123

a 8(gauss) 2 Andrew Hoeff IN VENTOR.

BY WW.

ATTORNEY PLASMA CONTAINMENT DEVICE BACKGROUND OF THE INVENTION This invention relates generally to neutral plasma devices, and particularly relates to a plasma confinement apparatus.

In recent years the confinement of a neutral plasma having a relatively high-particle density and a high temperature has been intensely investigated for scientific and other purposes. In particular it is well known that plasma confinement devices are suitable for application to fusion reactions. One of the problems encountered in past research has been the instability of a plasma, particularly at high temperatures or densities.

It has recently been found by W. Bernstein, H. S. Ogawa and .I. M; Sellen, Jr., as described in Phys. Rev. Letters, Volume 20, No. 17, Apr. 22, 19 68, pages 903 to 905, that electron penetration of a transverse magnetic field .ceases as the magnetic field intensity is increased above a critical value, while the positive ions of the initially neutral plasma beam penetrate through such a field. The publication just referred .to states as follows: a

Our preliminary experimental results indicate that electron penetrationof the magnetic field ceases as the magnetic field intensity is increased above a critical value where the electron cyclotron frequency at approaches the electron plasma frequency m The-paper then continues as follows:

In FIG. 2, the critical value of field intensity and the corresponding value of w is plotted as a function of V; (center density) and the corresponding to for both a uniform density beam and one, for which n "*IB is constant across the gap."

The publication further states:

"The absolute values of magnetic field and density for each transition shown in FIG. 2 together with the observed frequencies indicate that the transition occurs for m w The present invention is based on this recent finding which makes it possible to repel electrons by means of a. magnetic field in a radial direction having a magnitude above a certain critical value. On the other hand, this still leaves the problem how to confine or repel the positive ions of the neutral plasma.

It is accordingly an object of the present invention to provide a plasma confinement device wherein electrons are repelled from the walls of the device by a magnetic field while the positive ions are repelled by the application of an electric field.

Another object of the present invention is to provide a plasma confinement apparatus including a chamber having wallsor a coating of insulating material which tend to charge positively by the ions of a plasma and thereby to repel additional ions by the electric field normal to the charged surface.

A further object of the present invention is to provide a plasma confinement device of the type discussed wherein the electrons are kept away from the main portion of the confinement chamber by a strong magnetic field, the positive ions being repelled from the walls of the chamber by a positive charge, while the end portions of the chamber are so constructed that they tend to repel positive ions by the electric field on the walls while the electrons are repelled by a negative electrode positioned in one or both of the end portions of the chamber.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided a device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions. The device comprises a closed chamber having walls consisting of an insulating material. The chamber has a central portion and two opposed end portions which may, but need not be, of narrow cross section. Means are provided for generating a magnetic field in the central portion of the chamber. This magnetic field is sufficiently strong to prevent electrons from flowing to the walls of the chamber. There is also generated a magnetic field in the end portions of the chamber which extends along an axis interconnecting the end portions.

The positive ions may be generated by an ion source disposed in one of the end portions. There is also provided a charge neutralizer which injects electrons into the chamber. This charge neutralizer is disposed in one of the end portions and preferably in proximity to the positive ion generator. Finally, electron repeller means is disposed in the other end portion of the chamber for repelling electrons back into the central portion of the chamber.

Accordingly, positive ions initially tend to impinge on the insulating material of the walls of the chamber and to charge the. insulating material positively with respect to the neutral plasma. As a result, positive ions are thereafter repelled by the walls so that they cannot approach the walls of the chamber.

The foregoing and other objects of the present invention will become more apparent and better understood when taken in conjunction with the following description and accompanying drawings, throughout which like characters indicate like parts and which drawings form a part of this application.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic cross-sectional view of a plasma confinement chamber in accordance with the invention, and ineluding voltage sources for indicating the relative polarities of the various elements of the chamber, and FIG. 2, above referred to, is a graph illustrating the relationship between the applied magnetic field and the plasma density.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now. to the drawing, there is illustrated by way of example an embodiment of the present invention. As shown in the drawing there is provided a plasma confinement chamber 10 which is a closed chamber. The chamber 1 0 has walls 11 consisting of an insulating material such, for example, as glass or ceramic. Alternatively, the walls 11 may be provided with an insulating coating, which may, for example, be provided by supplying the wall with a powder of insulating material such as Willemite. If the insulating coating or surface consists of a powder with a rough surface, the electric charge distribution remains more stable because it permits potential gradients to exist. In other words, each individual particle of the powder may assume a potential different from that of its neighbors. The chamber 10 includes a central portion 12 and two opposed end portions l4, l5 aligned along a common axis. The particular shape of the confinement chamber 10 is not important. However, the chamber 10 may have a circular cross section and may have an enlarged central portion 12 as shown and two relatively

narrow end portions

14 and 15.

A plurality of magnetic coils l6, l7, I8, 20 and 2 1 are arranged to be adjacent to the walls of the chamber 10. Preferably they are arranged to generate a magnetic field parallel to the wall surface and which is strong enough to substantially reflect electrons from the walls 11 of the chamber. The magnetic field should exceed the critical value as found by Bernstein, Ogawa and Sellen in the publication above referred to. The

end portions

14 and 15 of the chamber 10 need not necessarily be narrow in cross section. However, as will be more fully explained hereinafter, the purpose of having relatively small diameter end portions 14 and I5 is to facilitate the repelling of the ions from the end portions.

One of the end portions such as 14 is provided with a generator or source 15 of positive ions. Such positive ion sources are well known in the art. For example, it may consist of a cesium source which conducts cesium vapor through a porous tungsten disk maintained at an elevated temperature to develop ions which are fed into the chamber 10. These ions are preferably accelerated by an ion acceleration electrode 26. There is provided a charge neutralizer 27 which is preferably disposed near the ion accelerator electrode 26. Accordingly the charge neutralizer 27 is completely immersed in the ion beam. Such a neutralizer is also well known in the art and therefore a further description is not believed to be necessary. It may consist, for example, of a wire or grid of a suitable metal heated to a temperature sufficient to generate a copious supply of electrons.

Finally there is provided an electron repeller electrode 30 which is disposed in the other end portion 15 of the chamber.

The various electrodes may be energized by suitable voltage sources such as batteries. Accordingly there may be provided a battery 31 having its positive terminal connected to the ion source 25 and its negative terminal to the ion accelerator electrode 26. The negative electrode 26 will accelerate positive ions. Preferably the neutralizer electrode 27 is maintained at a potential which is positive with respect to that of the accelerator 26. To this end a battery 32 may be provided having its positive terminal connected to the neutralizer electrode 27, while its negative terminal is connected to the accelerator electrode 26. Similarly, the electron repeller electrode 30 is maintained at a negative potential with respect to the electron source 27. This may be accomplished by a battery 33 having its negative terminal connected to the repeller electrode 30, while its positive terminal is connected to the electron source 27.

The various magnetic coils, such as 16, 17, 18, 20 and 21 generate a magnetic field having a direction shown schematically by

arrows

35, 36, 37, 38 and 40. It will be noted that the arrows 35 through 40 extend substantially along the direction of the axis of chamber 10 interconnecting the two

end portions

14 and 15.

The plasma confinement device of the invention operates as follows. Positive ions such as positive cesium ions are generated by the source 25 and are accelerated by the accelerator electrode 26. At the same time electrons are injected into the plasma by the charge neutralizer 27, which may be considered to be an electron source. Accordingly a plasma is generated which is substantially neutral, that is, which contains a substantially equal number of electrons and positive ions. Such a plasma should preferably be quiescent, that is, without oscillation as is well known.

Assuming now that the magnetic field generated by the magnetic coils 16 through 21 exceeds a certain critical value, the electrons will be kept away from the walls 1 1 of the plasma confinement chamber. The required intensity H of the magnetic field depends in part on the electrondensity n which is the number of electrons or positive ions per cubic centimeter and the effective electron temperature T, in accordance with the following formula 5 n=2.5H T I) wherein H is the magnetic field in gauss developed by the coils 16 through 21 and where T is the electron temperature in degrees Kelvin.

It will be noted from formula 1) that the maximum density of the plasma is proportional to the electron temperature and to the square of the confining magnetic field. Assuming by way of example, T equal I0 and the magnetic field H equals 1,000 gauss, the limiting density n equals approximately 10 ions per centimeter.

The plasma is confined in the following manner. As pointed out above, the electrons are repelled by the magnetic field. However, because of their low velocity, due to their relatively large mass, the ions are little affected by the magnetic field. Accordingly, initially they tend to impact the wall 11 of the chamber. Therefore they tend to form a positive charge on the wall because the wall is made or coated with insulating material. This positive charge produces an electric field which then slows down any positive ions which may approach the wall. Eventually the wall tends to acquire the potential of the ion source 25. The insulating coating in a sense is self-adjusting because it tends to assume the potential of the source of the The positive ions are accelerated from the end portion 14 into the central portion 12 by the accelerator electrode 26. This acceleratorelectrode in turn tends to repel the electron which are generated by the electron source 27 toward the central portion 12.

Similarly in the end portion 15, the positive ions tend to be repelled by the positive charge on the wall 11 of the end portion 15. However this would still permit electrons to escape toward the left as viewed in the drawing. To this end, the electron repeller electrode 30 is provided which is maintained at a negative potential which tends to repel the electrons. On the other hand, such a negative potential would tend to attract positive ions. For this reason the end portion 15 is preferably made relatively narrow and relatively long. it can be shown that the electric field due to the electrode 30 falls off exponentially along the tube axis at the rate of 32 db. (decibels) per diameter. Therefore if the neck is sufficiently long the positive ions will be repelled by the wall potential of the end portion 15 before they are attracted by the repeller electrode 30. Thus, the positive ions cannot reach the electrode 30.

The ion accelerator electrode 26 which is at a negative potential also serves to repel any electrons toward the central portion 12 of the chamber in a manner similar to that of electrode 30.

As a result, it will be seen that while the negative and positive particles of a neutral plasma are separated, they can each be separately reflected. 1n the central portion of the device the electrons are repelled from the walls by a magnetic field essentially parallel to the walls while the ions are separately repelled by an electric field, normal to the walls, which is generated by the ions initially deposited on the insulating wall. Furthermore, the geometry is chosen in such a manner that both electrons and positive ions are repelled from the neck of end portions of the confinement chamber.

What is claimed is:

1. A device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions, said device comprising:

a. a closed chamber, said chamber having walls covered with an insulating material, said chamber having a central portion and two opposed end portions;

b. first magnetic means for generating a first magnetic field in the central portion of said chamber, said first magnetic field being sufficiently strong to substantially reflect electrons from the walls of said central portion of said chamber;

c. second magnetic means for generating a second magnetic field in the end portions, said second magnetic field being sufficiently strong to substantially reflect electrons from the walls of said opposite end portions of said chamber;

d. a positive ion generator disposed in one of said end portions for generating positive ions;

e. a charge neutralizer disposed in said one of said end portions and in proximity to said positive ion generator; and

f. electron repeller means disposed in the other end portion of said chamber for repelling electrons back into the central portion of said chamber, said magnetic fields being insufficiently strong to prevent positive ions from reaching the walls of said central and end portions of said chamber.

2. A plasma containment device as claimed in claim 1 wherein said generator of positive ions generates positive ions of cesium.

3. A plasma containment device as defined in claim 1 wherein the opposed end portions of said closed chamber are sufficiently narrow so that the positive ions are repelled from the end portion of said chamber by the positive charge on said insulating material.

4. A plasma containment device as defined in claim 1 wherein said electron repeller means consists of an electrode, and means for charging it to a negative potential with respect to said charge neutralizer.

5. in a device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions;

a. a closed chamber having a central portion and two opposed end portions, said chamber having walls covered with an insulating layer capable of sustaining a high-electric field;

b. magnetic means for generating a first magnetic field in the central portion of said chamber sufficiently strong to substantially reflect electrons from the walls of said central portion of said chamber and for developing a second magnetic field within the end portions of said chamber extending along an axis interconnecting said end portions, said second magnetic field being sufficiently strong to substantially reflect electrons from the walls of said opposite end portions of said chamber, and said magnetic fields being insufficiently strong to prevent positive ions from reachingthe walls of said central and end portions of said chamber;

c. means for injecting the substantially neutral plasma into one of the end portions of said chamber; and

d. electron repeller means disposed in at least one of the end portions of said chamber for repelling electrons back into the central portion of said chamber, whereby positive ions tend to impinge on the insulating material of the walls of said chamber to charge said insulating material positively with respect to the neutral plasma and to thereafter repel positive ions which might approach the walls of said chamber.

6. A device as defined in claim 5 wherein said plasma consists of electrons and positively charged cesium ions.

7. in a device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions:

a. a closed chamber having walls having a substantially unobstructed surface consisting of an insulating material; and

b. magnetic means for generating a confining magnetic field in said chamber, said magnetic field being sufficiently strong to substantially reflect electrons from the walls of said chamber, while allowing positive ions to impinge on the insulating material of substantially the entire walls of said chamber and thereby to charge said insulating material positively with respect to the neutral plasma and to thereafter repel positive ions which might approach the walls of said chamber.

Claims

1. A device for confining a substantially neutral plasma consisting of electRons and a substantially equal number of positive ions, said device comprising: a. a closed chamber, said chamber having walls covered with an insulating material, said chamber having a central portion and two opposed end portions; b. first magnetic means for generating a first magnetic field in the central portion of said chamber, said first magnetic field being sufficiently strong to substantially reflect electrons from the walls of said central portion of said chamber; c. second magnetic means for generating a second magnetic field in the end portions, said second magnetic field being sufficiently strong to substantially reflect electrons from the walls of said opposite end portions of said chamber; d. a positive ion generator disposed in one of said end portions for generating positive ions; e. a charge neutralizer disposed in said one of said end portions and in proximity to said positive ion generator; and f. electron repeller means disposed in the other end portion of said chamber for repelling electrons back into the central portion of said chamber, said magnetic fields being insufficiently strong to prevent positive ions from reaching the walls of said central and end portions of said chamber.

5. In a device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions; a. a closed chamber having a central portion and two opposed end portions, said chamber having walls covered with an insulating layer capable of sustaining a high-electric field; b. magnetic means for generating a first magnetic field in the central portion of said chamber sufficiently strong to substantially reflect electrons from the walls of said central portion of said chamber and for developing a second magnetic field within the end portions of said chamber extending along an axis interconnecting said end portions, said second magnetic field being sufficiently strong to substantially reflect electrons from the walls of said opposite end portions of said chamber, and said magnetic fields being insufficiently strong to prevent positive ions from reaching the walls of said central and end portions of said chamber; c. means for injecting the substantially neutral plasma into one of the end portions of said chamber; and d. electron repeller means disposed in at least one of the end portions of said chamber for repelling electrons back into the central portion of said chamber, whereby positive ions tend to impinge on the insulating material of the walls of said chamber to charge said insulating material positively with respect to the neutral plasma and to thereafter repel positive ions which might approach the walls of said chamber.

7. In a device for confining a substantially neutral plasma consisting of electrons and a substantially equal number of positive ions: a. a closed chamber having walls having a substantially unobstructed surface consisting of an insulating material; and b. magnetic means for generating a confining magnetic field in said chamber, said magnetic field being sufficiently strong to substantially reflect electrons from the walls of said chamber, while allowing positive ions to impinge on the insulating material of substantially the entire walls of said chamber and thereby to charge said insulating material positively with respect to the neutral plasma and to thereafter repel positive ions which might approach the walls of said chamber.