US3194739A

US3194739A - Fusion research apparatus

Info

Publication number: US3194739A
Application number: US217959A
Authority: US
Inventors: Donald W Kerst; Ohkawa Tihiro
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1962-08-20
Filing date: 1962-08-20
Publication date: 1965-07-13
Anticipated expiration: 1982-07-13
Also published as: DE1251879B; GB967430A

Description

July 13, 1965 I p. w. KERST ETAL. v 3,1

FUSION RESEARCH APPARATUS paged Aug. 20, 1962 w 2 Sheets-Sheet "-1 July 13, 1965 D. w. KERST ETAL FUSION RESEARCH APPARATUS 2 Sheets-Sheet 2.

Filed Aug. 20, 1962 United States Patent 3,194,739 FUSION RESEARCH APPARATUS Donald W. Kerst, Madison, Wis, and Tihiro Ohkawa,

San Diego, Calif., assignors to General Dynamics Gorporation, New York, N.Y., a corporation of Delaware Filed Aug. 2-0, 1962, Ser. No. 217,959 9 Claims. (Cl. 176-1) The present invention relates to fusion research apparatus and more particularly to an improved apparatus for heating and confining particles for such research.

In prior artfusion research apparatus, magnetic fields have been employed to confine plasma. The magnetic field has been set up by passing a high current through a plasma disposed in a discharge tube, which current produces a concentric magnetic field which confines and compresses the plasma. Plasma has been found to be highly unstable in such apparatus. Attempts have been made to stabilize the plasma by adding a magnetic field which extends parallel to the discharge in the tube. This additional stabilizing field has proven to be fairly effective and in some cases the plasma may be stable. However, because of the concavity of the fields with respect to the plasma, the plasma has been difficult to stabilize. The stability of the plasma is dependent critically on the influence of the plasma on the magnetic field distribution. Moreover, the confined plasma has been extremely sensitive to magnetic field errors.

An alternate method for confining hot plasma is the so-called cusp configuration of magnetic fields. In such a geometry, plasma is injected into a central volume defined by at least a pair of loops of wires or coils. Oppositely directed currents are passed through adjacent coils to thereby provide a series of convex magnetic fields (i.e., convex toward the plasma).

Such a geometry provides a magnetic field which increases with radius from the axis of symmetry. In other words, the equilibrium position of plasma is at the center of the cusped geometry. Perturbations of the plasma in this equilibrium position protrude. into regions of stronger magnetic fields which force the perturbations back to the equilibrium position. Even individual particles escaping from the main body of plasma find themselves in a stronger magnetic field from which they are ejected back into the body of the plasma.

The most serious drawback of the cusp confinement geometry has been the high particle loss rate at the ring of the cusp or at the point of the cusp. The lines of force leading out of the confining region carry particles outwith their ion thermal speed, and thus the leakage at the ring cusp and point cusp is at a very high rate. It has been suggested that the leakage at the ring cusp may be eliminated by connecting adjacent ring cusps together. In one such proposed apparatus current carrying rings are disposed inside a solenoid having a longitudinal magnetic field. Such a geometry may eliminate the losses in the ring cusps but the proposed apparatus is unstable and complicated.

An object of the present invention is the provision of an improved plasma research apparatus. Another object plasma research apparatus employing various features of the present invention, and with portions thereof being broken away to show certain features thereof;

FIGURE 2 is an enlarged fragmentary perspective view as viewed along line 22 of FIGURE 1;

FIGURE 3 is an enlarged diagrammatic view of a portion of FIGURE 2; and

FIGURE 4 is an enlarged diagrammatic view of a portion of FIGURE 3 showing how plasma may be trapped in the apparatus.

Generally, as shown in the drawings, a plasma research apparatus includes means 10 for providing alternate interconnected convex and concave magnetic confining fields about an area 12 Where plasma is to be confined. Additional means 14 is included in the apparatus in each concave field for providingan additional convex magnetic field between each pair of the first mentioned convex fields. The total magnetic field strength in the space between the additional means and the portion of said first mentioned means which provides the concave field is made substantially greater than that of the first mentioned convex magnetic field to thereby stabilize the plasma.

More specifically, in the illustrated embodiment, a toroidal construction is shown for confining plasma. In the structure illustrated in FIGURE 1, convex fields are provided around the minor circumference of the toroid by a plurality of endless rods or members 14 of conductive material, such as copper. A current is induced in each of the rods by a power source 15, as hereinafter described in detail. The current establishes a magnetic field about each rod 14, the fields being convex relative to the minor axis of the toroid.

The alternate convex and concave magnetic confining fields are provided by a toroidal jacket 10 of conductive material such as copper. The jacket 16 is internally fiuted or corrugated top rovide a plurality of alternate longitudinally extending ridges 16 and grooves 18. One each of the rods 14 is received in each groove in spaced relation to the wall thereof. When a current passes longitudinally through each rod, current is induced in the jacket which provides concave fields between the rods and the wall of the grooves 18 and convex fields at the ridges 16.

In the illustrated embodiment, the rods 14- are supported centrally within the grooves 18 by a plurality of small supports 24 in the form of retractable, pointed pins exof the invention is the provision of a plasma research tending through the jacket 10. The pins 24 are disposed so as to maintain the rods 14 in position against the force of gravity when the magnetic fields are not present in the torus. The pins 24, by means such as solenoids 2d, are quickly retracted at the same time as the magnetic fields are established and prior to the injection of the plasma.

Once the magnetic fields are established the rods 14 are maintained in the proper position by the magnetic field, the rods 14 and the grooves 18, in the illustrated embodiment, being shaped to maintain the rods 14 in an equilibrium position. In this connection, ,as shown particularly in FIGURE 3, each rod 1 is of a generally ovalcrosssection. The smaller arcuate end portion 28 of the oval is faced inward or toward the minor axis of the toroid and the larger arcuate end portion, 30 of the rod 14 is faced outward or toward the wall of the groove 18. The oval end portions 28 and 3d of each rod 14 areintegrally connected by an intermediate portion 32 having generally fiat sides.

The jacket liladjacent the rod 14 is formed so as to generally conform to the shape of the rod 14. The gap 34 between the side portion 32 of the rod 14 and the jacket 10 is made slightly narrower than the gap .36 between the enlarged end portion 39 of the rod 14 and the jacket 19. In this way, magnetic pressure at the fiat sides of the rod 14 is directed partially back into the groove 18 to thereby compensate for the magnetic pressure applied by the field in the region between the enlarged end portion 39 of the rod 14 and the wall of the groove 13, which pressure tends to force the rod 14 out of the groove To obtain an equilibrium position, the dimensions of the rod 14 and groove 18 are preferably selected so as to satisfy the following:

B22 X D ZBGZZ where, as shown in FIGURE 3 l is the projection of the straight sides of the rod 14 on the diameter of the large end 3 of the rod 14,

B is the strength of the magnetic field in region between the rod 14 and the wall of the groove 13.

D is the diameter of the large end 30 of the rod, and

B is the strength of the magnetic field in the region between the flat side of the rod 14 and the adjacent jacket wall It).

Other means may be employed to initially position the rods 14 in the grooves 18 before the magnetic fields have een applied. For example, with no current passing through the rods 14 and no support, the earths gravitational field causes each rod 14 to rest on the wall of the grooves 1% immediately below the rod 14. The rods 14 may be then initially positioned in the proper position in the groove 18 by a swift downward motion of the whole jacket for a short distance. The rods 14, because of inertia, move away from the downwardly moving jacket 14? and, at the same time, the magnetic fields are applied which then easily maintain the rods 14 in position against the force of gravity.

As previously indicated, current is induced in each rod 14 and, in turn, in the jack t 10 by a power source 15. As shown in FIGURE 1, the power source includes a torodial core transformer 38 which is disposed so that the rods 14 pass through the center hole or major axis thereof, and thereby act as short circuited secondary windings of the transformer. The transformer 38 includes a core 40 of iron laminations, and a primary winding or coil 42 of wire wrapped about the core 40.

As shown in FIGURE 1, the transformer is provided with a toroidal housing 44 of conductive material, such as copper, which is disposed about the primary winding 42. An annular gap 46 is provided in the housing adjacent the major axis thereof. The housing 44 is connected to a transverse gap 47 in the jacket 10 by a pair of spaced apart flanges 48 of conductive material, such as copper, which extend between the ends of the jacket 1d and the ends of the housing 44. The primary winding 42 of the transformer 38 is connected to a source of pulsatory power (not shown) which may be of the conventional type, such as a mercury tube and a capacitor bank.

Because of the toroidal shape, the rods 14 have different lengths or major circumferences. When a current is passed through the primary winding 42, each rod 14, because it is a shorted turn, generates the same magnetic flux. Therefore, the current density required in the jacket wall It around the short rods 14 is higher than the current density required in the jacket wall it) around the longer rods. If the proper current density is not available to locally match the current density required, a transverse magnetic field will be set up at the gap 47 between the ends of the jacket 10. Therefore, the turns of the primary winding 42 are preferably distributed on the core 46 so that the flow pattern of the flange surface currents goes generally directly in toward the grooves 13 and does not cross the whole flange 48 to the grooves 18 surrounding the shorter rods 14.

Preferably, to further insure proper distribution of the current, the gap between the flanges 48 is tapered toward the major axis of the torus It) so that the Width of the gap is inversely proportional to the distance of the point on the flange 43 from the major axis of the torus. Moreover, perforations 50 are preferably provided in the flanges 48 to additionally insure the proper distribution of current flow lines on the surface of the flanges 48.

So that the trapped plasma does not exchange its charge with neutral gas, and thereby cause hot ions to be neutralized and to escape across the magnetic confining fields, the plasma region 12 is maintained at as high a vacuum as possible (i.e., as free as possible from neutral atoms and molecules). In the illustrated embodiment means 52 are provided for rapidly evacuating the plasma region. in this connection, a substantial number of apertures 54 are provided in the jacket it and a manifold 56 in the form of a generally rectangular toroidal can of conductive material is disposed around the jacket 10, the jacket being suitably supported and insulated therefrom. The manifold 56 is also suitably insulated from the housing 44. A suitable vacuum pump (not shown) is connected to the manifold 55.

As shown particularly in FIGURE 2, the plasma is injected into the jacket 10 by a source 53 of plasma, such as a conventional plasma gun, extending through the manifold 5t; and the wall of the jacket 16. Preferably, a plasma gun is employed in which the neutral gases are trapped and not permitted to enter the plasma region. \Vhile only one gun is shown, a sufficient number of guns are arranged about the jacket to provide the desired plasma density.

The plasma which is injected into the magnetic confining field adjacent one of the rods 14 is trapped by discharging the polarization thereof. In this connection, as shown particularly in FIGURE 4, the injected plasma crosses a magnetic line of force established by the rod 14 at M and progresses to the same line of force looped back at N. For a plasma volume to cross a line of force, a polarization charge must be created near the surface of the plasma volume. The surface charge of the plasma volume must reverse in polarity or else the plasma volume cannot cross the line of force looped back in the opposite direction. This reverse polarity cannot be established on the looped back line of force and hence the plasma comes to rest and is trapped in the system. More specifically, the reverse polarity cannot be established because the electrons are free to travel along the looped back line of force and thereby short-circuit the polarities called for at M with the reverse polarities called for at N. Rather than short-circuiting the plasma by employing a line of force looped back into the plasma path, an oppositely directed plasma may be employed to short-circuit the polarization fields.

Other methods may be employed to trap the particles within the plasma region. For example, the plasma may be injected at one of the ridges. While the injected plasma is pushing on the concave side of the ridge field, an unstable configuration exists and the injected plasma punches through into the plasma region. However, once inside, plasma attempting to reverse its direction and to emerge through the now convex lines of force, encounters a stable configuration, thereby finding escape impossible.

In order for the plasma to be stable in the apparatus described above, the magnetic field strength B in the region between the rod 14 and its groove wall (groove region) is made substantially greater than the magnetic field strength B established in the region adjacent the ridge 16. In order to avoid the worst perturbations that could cause trouble, the dimensions of the structure are selected so that:

where, as shown in FIGURE 3,

d=half the gap between the jacket wall and the rod,

r is the radius of the groove, less half the gap (d),

and

(p is the angle around the rod in which the lines of force are curving around the rod and not away from the same.

In high order multipoles (i.e., sixpole fields or greater) 5 is approximately equal to 1r, and hence the square of the ratio of the magnetic field strength in the ridge region to that in the groove is made less than the ratio of the half gap to the radius of the groove 18.

In one embodiment of the plasma research apparatus, a copper jacket having a 3 cm. wall, is formed so as to provide six ridges and six grooves. The wall is perforated so that approximately 25 percent of its area is open and the jacket is disposed within a copper manifold. The ridges are made with a radius of .15 cm., and the grooves are made with a radius of 28 cm. Six oval shaped rods are disposed in the grooves. The radius of the larger end of each rod is 22 cm., and the radius of the smaller end of each rod is 15 cm. The projection of the fiat side of the rods on the diameter is 7 cm., and a cm. gap is provided between the flat side of the rod and the jacket. The rods are supported in position by retractable pins which are retracted in approximately 0.1

sec.

The capacitor bank is charged to a voltage such that a flux density of 15,000 gausses is in the core of the transformer, and the length of the current pulse is 0.1 second. The plasma region is maintained at a vacuum of mm. of Hg. A sufiicient number of plasma guns are connected to the jacket to provide a plasma density of 10 particles per cm.

It should be realized that while the structure shown and described above is a toroidal construction, certain features of this invention may be employed in a linear type of plasma research device. Moreover, instead of having the current induced in the rods, the rods may be separately excited by parallel leads extending through a wall of the jacket. Local magnetic fields generated about .the parallel leads guard or protect the leads from the hot plasma. 7 7

Various other changes and modifications may be made in the above described plasma research apparatus without deviating from the spirit or scope of the present invention. 7

Various features of the present invention are set forth in the accompanying claims.

We claim:

1. Apparatus for confining plasma comprising means for providing alternate interconnected convex and concave magnetic confining fields about an area where the plasma is to be confined, and additional means in each of the concave fields for providing an additional convex magnetic field between each pair of the first mentioned convex fields whereby the plasma area is encircled by a series of convex fields, the total magnetic field strength in the space defined between the additional means and the portion of said first mentioned means which provides the concave field being made substantially greater than that of the first mentioned convex magnetic field to thereby stabilize the plasma.

2. Apparatus for confining plasma comprising means for providing alternate interconnected convex and concave magnetic confining fields about an area where the plasma is to be confined, and additional means in each of the concave fields for providing an additional convex magnetic field between each pair of the first mentioned convex fields whereby the area is encircled by a series of convex fields, the square of the ratio of the total magnetic field strength in the region between the additional means and the concave field providing portion of said first mentioned means to the first mentioned convex magnetic field strength being less than the ratio of half the thickness of the region to the radius of the concave field providing portion.

3. Apparatus for confining plasma comprising an elongated, generally tubular jacket of conductive material, said jacket being internally fluted to provide a plurality of generally longitudinally extending, generally arcuate ridges and grooves, an elongated conductive member disposed in each groove in spaced relation to the wall thereof, the portion of said conductive member adjacent the wall of said groove being generally arcuate in shape, and means causing a current to pass through said members and said jacket thereby establishing a first magnetic field in an area adjacent said ridges and a second field in an area defined by the wall of each groove and the adjacent arcuate portion of the conducting member, the strength of the second magnetic field being substantially stronger than that of the first magnetic field.

4. Apparatus for confining plasma comprising an elongated, generally tubular jacket of conductive material, said jacket being internally fiuted to provide a plurality of generally longitudinally extending, generally arcuate grooves andridges, an elongated conductive member disposed in each groove in spaced relation to the wall thereof, the portion of said conductive member adjacent the Wall of said groove being generally arcuate in shape, and means causing a current to pass through said members and said jacket thereby establishing a first magnetic field in an area adjacent said ridges and a second field in an area defined by the wall of each groove and the adjacent arcuate portion of the conducting member, the square of the ratio of strength of the first magnetic field to that of the second magnetic field being less than the ratio of half the spacing between the arcuate portion and the groove wall to the radius of the groove wall.

5. Apparatus for confining plasma comprising an elongated, generally tubular jacket of conductive material, said jacket being internally fluted to provide a plurality of generally longitudinally extending, generally arcuate grooves and ridges, an elongated conductive member disposed in each groove at a distance 2d from the wall of the groove, the portion of said conductive member adjacent the groove being generally arcuate in shape, and means for causing currents to pass through said members and said jacket thereby establishing a first magnetic field having a field strength B in an area adjacent said ridges and a second field in which the lines of force curve around the arcuate portion in an area defined by the groove wall and the surface of the arcuate portion of the conductive member and which has a field strength E the field strengths being defined by the relation r =the radius of the groove wall minus d, and

=the angle around the arcuate portion in which the lines of force are curving around the conducting member and not away from the same.

6. Apparatus for confining plasma comprising an elongated, generally tubular jacket of conductive material, said jacket being internally fiuted to provide a plurality of generally longitudinally extending, generally arcuate grooves and ridges, an elongated conductive member disposed in each groove at a distance 2d from the wall of the groove, the portion of said conductive member adjacent the groove wall being generally arcuate in shape, removable means on said jacket for supporting said con ductive members in position when current is not passing therethrough, and means for causing currents to pass through said members and said jacket thereby establishing a first magnetic field having a field strength B in an area adjacent said ridges and a second field in which the lines of force curve around the arcuate portion in an area defined by the groove wall and the surface of the arcuate portion of the conductive member and which has a field strength B said grooves and associated conductive members being shaped so that the magnetic fields maintain the members in an equilibrium portion in the grooves, the field strengths being defined by the relation 1 z) 2) 2) where r =the radius of the groove wall minus d, and

7. Apparatus for confining plasma comprising a toroidal jacket of conductive material, said jacket being internally fluted to provide-a plurality of arcuate grooves and ridges which extend parallel to the minor axis of the jacket, a conductive member formed into a ring dis posed in each groove at a distance 2d from the wall of the groove, the portion of said conductive member adjacent the groove wall being arcuate in shape, said jacket being provided with a transverse gap, and means coupled to said jacket at said gap for inducing a pulsatory current in said member whereby a magnetic field of strength B in which the lines of force curve around the arcuate portion is established in an area defined by the groove wall and the surface of the arcuate portion of the conductive member, said field inducing a current to flow through said jacket thereby establishing a magnetic field having a field strength B in an area adjacent said ridges, the field strengths being defined by the relation r =the radius of the groove wall minus a', and

=the angle around the arcuate portion in which the lines of force are curving around the conducting me: her and not away from the same.

8. Apparatus for confining plasma comprising a toroidal air tight chamber, means connected to said chamber for evacuating the same, a toroidal, perforated jacket of conductive material disposed in said chamber, said jacket being internally fluted to provide a plurality of arcuate grooves and ridges which extend parallel to the minor axis of the jacket, a conductive member formed into a ring disposed in each groove at a distance 2d from the wall of the groove, the portion of said conductive member adjacent the groove being arcuate in shape, said jacket being provided with a transverse gap, and means coupled to said jacket at said gap for inducing current to flow in said members whereby a magnetic field of strength B in which the lines or" force curve around the arcuate portion is established in an area defined by the groove wall and the surface of the arcuate portion of the conductive member, said field inducing a current in said jacket thereby establishing a magnetic field having a field strength B in an area adjacent said ridges, the field strengths being defined by the relation where r the radius of the groove wall minus d, and

9. Apparatus for confining plasma comprising an elongated, generally tubular jacket of conductive material, said jacket being internally fiuted to provide a plurality of generally longitudinally extending oval grooves and arcuate ridges, the larger end of the oval being outermost, an elongated conductive oval shaped rod disposed in each channel with the larger cross section outermost, the rod being spaced at a distance 2d from the outermost wall portion of the core and at a slightly smaller distance at the sides thereof, and means for causing currents to pass through said rods and said jacket thereby establishing a first magnetic field having a field strength B in an area adjacent said ridges and a second field having a field strength B in a region defined by the wall of each groove and the rod, the field strengths being defined by the relation 1 z) 2) M where r =the radius of the groove wall minus d, and

References Cited by the Examiner UNITED STATES PATENTS 2,961,559 11/60 Marshall 1'/68 3,009,080 11/61 Loos 1768 3,012,955 12/61 Kulsrud et al. 1769 3,038,099 6/62 Baker et al. 315111 X 3,085,173 4/63 Gibson et al 315111 X FOREIGN PATENTS 859,459 1/ 61 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

Claims

1. APPARATUS FOR CONFINING PLASMA COMPRISING MEANS FOR PROVIDING ALTERNATE INERCONNECTED CONVEX AND CONCAVE MAGNETIC CONFINING FIELDS ABOUT AN AREA WHERE THE PLASMA IS TO BE CONFINED, AND ADDITIONAL MEANS IN EACH OF THE CONCAVE FIELDS FOR PROVIDING AND ADDITIONAL CONVEX MAGNETIC FIELD BETWEEN EACH PAIR OF THE FIRST MENTIONED CONVEX FIELDS WHEREBY THE PLASMA AREA IS ENCIRCLED BY A SERIES OF CONVEX FIELDS, THE TOTAL MAGNETIC FIELD STRENGTH IN THE SPACE DEFINED BETWEEN THE ADDITIONAL MEANS AND