US3400349A - U-shaped magnetic circuit including three permanent magnets separated by pole pieces - Google Patents

Description

Sept. 3, 1968 o. JQHARRA 3,400,349

U-SIIAPED MAGNETIC CIRCUIT INCLUDING THREE PERMANENT MAGNETS SEPARATED BY POLE PIECES Filed Jan. 14, 1966 FIG I FIG.2

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' DAVD J. HARR'A BY M AIR GAP POSITION NEY United States Patent 01 fice 3,400,349. Patented Sept. 3, 1968 3,400,349 U-SHAPED MAGNETIC CIRCUIT INCLUDING THREE PERMANENT MAGNETS SEPARATED BY POLE PIECES David J. Harra, Menlo Park, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 14, 1966, Ser. No. 520,606 6 Claims. (Cl. 335-210) ABSTRACT OF THE DISCLOSURE The web or connecting portion of the U is for-med by a single bar magnet while each of the arms of the U, perpendicular to the web, includes a bar magnet poled in magnetically aiding relationship to the magnet in the web. A pair of pole pieces magnetically interconnect the web and arms of the U, while another pair of pole pieces at the ends of the arms are tapered to provide a region of uniform flux density between the arms.

The present invention relates generally to U-shaped magnetic circuits. More specifically, it appertains to a magnetic circuit including a plurality of straight permanent magnets separated by pole pieces arranged in a U- shaped configuration which is particularly suited for use with ionic vacuum pumps.

U-shaped magnetic circuits fall into two general categories; those having curved or C-type magnets, and those having bar-type magnets forming straight segments of a rectangular U-shaped magnetic circuit. Both known C- type magnets and rectangular U-shaped magnetic circuits are characterized by various limitations and disadvantgaes. For example, C-type magnets have varying magnetic lengths measured along the direction of magnetization. As is well known, the maximum magnetic field produced in the air gap of a given magnet occurs when the magnet material is operated at the knee of the demagnetization curve. For a given air gap, the point of operation on the demagnetization curve will vary as the magnetic length. Hence, in order for all of the magnetic material to be operating at the knee of the demagnetization curve, the length of the magnetic material along the direction of magnetization must be uniform. Since the C-type magnets have varying magnetic lengths, its operating efliciency, i.e., maximum air gap field intensity throughout a given air gap size for a given magnet size, is inferior in comparison to a magnet characterized by a uniform magnetic length. Furthermore, the manufacturing process of such C-type or other irregularly shaped permanent magnets is exceedingly more complex than the process employed in manufacturing bar-type magnets.

To overcome the problems characterizing the C-type magnets, various U-shaped magnetic circuit configurations employing straight bar-type magnets and pole pieces have been used. One such type employs a single bar magnet having a pole piece extending perpendicularly away from each magnetic pole. Such magnetic circuits are capable of generating air gap magnetic field intensities comparable to the coercive force coefficient of the magnets employed. Hence, such magnetic circuits can optimally produce magnetic fields of, essentially, only a given intensity, therefore being quite inflexible.

To be able to provide magnetic fields of various intensities with a magnetic circuit employing magnets of a given type material while also preventing large flux leakages, magnets are placed close to the air gap, generally, in the arms of the U-shaped configuration. Such a magnetic circuit includes a pole piece having a bar magnet extending perpendicularly away from each end of the pole piece. The latter type is described in the United States Patent 3,182,234 to Paul Meyerer, entitled Permanent Magnet System for Focusing an Electron Beam in a Travelling Wave Tube, issued May 4, 1965, particularly, the embodiment of FIGS. 1 and 2. However, in each of these cases, the structure of the magnetic circuit required to generate a given air gap magnetic field intensity is large, and in the case of the Meyerer type structure, the profile of the magnetic field intensity established in the air gap is poor, i.e., far from being uniform.

Considerable advantage is therefore to be gained by the provision of a magnetic circuit configuration which overcomes those limitations and disadvantages characterizing the prior art configurations. Other advantages will be realized Where a compact and efiicient magnetic circuit characterized by a more uniform air gap magnetic field intensity profile is employed to guide the ionizing electrons of a sputter-ion vacuum pump having a cellular anode structure.

The present invention provides an improved U-shaped magnetic circuit configuration which is characterized by being compact and efficient, having a uniform magnetic length, being capable of generating with a given magnetic material an air gap magnetic field of any desired magnitude of intensity, and having an enhanced air gap magnetic field intensity profile by utilizing what is commonly considered lost flux leakage to increase the air gap magnetic field intensity. More specifically, the magnetic circuit configuration of the present invention includes a first straight permanent magnet having a pole piece at each of its poles. Second and third straight permanent magnets are disposed in magnetic potential adding relation with the first magnet. The second magnet is disposed to extend from the pole piece at one pole of the first magnet in a direction perpendicular to the direction of magnetization of the first magnet. Similarly, the third magnet is disposed to extend from the other pole piece in direction perpendicular to the direction of magnetization of the first magnet. Additional pole pieces as required are disposed to extend from and in the same direction as the pole ends of the second and third magnets distal the first magnet to complete the ,U-shaped magnetic circuit configuration.

By constructing the magnetic circuit with straight magnet sections which are separated by pole pieces so that magnetic paths are straight, the magnetic length of the circuit is made uniform, hence providing a more efficient magnetic circuit. Furthermore, by utilizing a magnet in each arm and at the web section of the U-shaped configuration, an enhanced air gap magnetic field intensity profile is provided as a result of the normally considered lost flux leakage of the web disposed magnet permeating the air gap adding with the field generated by the arm disposed magnets. Also, such magnet placement facilitates varying the magnitude of the air gap magnetic field intensity. Moreover, the three piece magnet structure allows for the construction of a magnetic circuit requiring less material than prior art circuit configurations for a given air gap and magnetic field intensity. Consequently, the efiiciency of the magnetic circuit of the present invention will be improved considerably over those of the prior art.

Accordingly, it is an object of the present invention to provide a flexible U-shaped magnetic circuit having an enhanced efiiciency characteristic.

More particularly, it is an object of the present invention to provide a U-shaped magnetic circuit having a uniform magnetic length and which generates an enhanced air gap magnetic field intensity profile.

Another object of the present invention is to provide a compact U-shaped magnetic circuit including magnets constructed of a given material whose air gap magnetic field intensity magnitude may be varied as desired.

A further object of the present invention is the provision of a compact U-shaped magnetic circuit configuraof FIG. 1.

FIG. 3 is a top view of a two element unit sputter-ion vacuum pump and magnets.

FIG. 4 is a graph representing the air gap magnetic field profile of the magnetic circuit of the present invention as compared with the prior art systems with curve (a) depicting the profile of the magnetic circuit of the present invention, curve (b) depicting the profile of a Meyerer type magnetic circuit, and curve (c) depicting the profile of a U-shaped magnetic circuit employing a single magnet disposed at the web of the U.

Referring to FIGS. 1 and 2 the magnetic circuit 11 comprises three straight magnets 12, 13 and 14 arranged in a magnetic potential adding U-shapw configuration to define an air gap 15. The magnets are disposed with magnet 12 defining the web and each of the magnets 13 and 14 defining a segment of one of the arms of the U- shaped configuration. Although the magnets may be of any cross sectional configuration, for sake of simplicity, a rectangular or square configuration is recommended.

To provide a magnetic circuit 11 having a uniform magnetic length, the north pole 16 of magnet 12 is coupled to the south pole 17 of side magnet 13 by a rectangular corner pole piece 18 interposed therebetween. Since pole pieces are constructed of low reluctance material and hence do not provide any magnetization, any pole piece configuration can be employed without affecting the mag netic length characteristics of the magnetic circuit 11. However, a more efiicient magnetic circuit which is considerably easier to manufacture is obtained by employing rectangular block pole pieces whose sides adjacent the .poles of the magnets are of the same cross sectional area and geometric configuration. Hence, in the preferred arrangement of the magnetic circuit 11, perpendicularly extending sides 19 and 21 of pole piece 18 are mated respectively with the north pole 16 of magnet 11 and south pole 17 of magnet 13. Similarly, the south pole 22 of magnet -12 is coupled by a second rectangular corner pole piece 23 to the north pole 24 of a second side magnet 14.

In many magnetic circuit applications, e.g., for establishing a magnetic field to trap and guide the ionizing electrons in ionic vacuum pumps, it is desirable to provide a nearly uniform magnetic field profile in the region where the pump elements are disposed in order to minimize the establishment of pressure gradients due to an uneven concentration of electrons. Furthermore, it is desirable to provide such a nearly uniform magnetic field profile over a large area. In such cases, it is contemplated that the magnetic circuit 11 of the present invention will further include additional pole pieces extending from the rectangular pole pieces 18 and 23. More specifically, a rectangular shaped straight side pole piece 26 is mated to extend from the north pole 27 in the same direction as magnet 13. Similarly, another rectangular shaped straight side pole piece 28 is mated to extend from and in the same direction as the south pole 29 of magnet 14. The resultant seven-piece magnetic circuit is assembled by bolts 30 extending between pole pieces on each side of the magnets to threadingly engage the pole piece opposite the pole piece at the bolt head. Other suitable means can be employed to facilitate holding the magnetic circuit structure together, e.g. brackets secured between pole pieces on opposite sides of each magnet or by welding the magnets and pole pieces together.

Any of the permanently magnetic materials can be employed in constructing the magnetic circuit of the present invention. However, for a given size air gap 15 and de sired air gap magnetic field intensity, some materials are more desirable than others. from the standpoint of magnetic circuit size and efficiency. The selection of magnet materials, generally, is based upon the peak energy product characteristic of the material, i.e., the maximum external energy that can be maintained by a unit volume of the material. Furthermore, the peak energy product is governed, to some extent, by the residual induction and coercive force characteristics of the material. In the case of the present invention, it has been found particularly advantageous to utilize magnets constructed from materials having a peak energy product of at least 5.0)(10 gauss-oersteds. A particular material having a peak energy product falling in this range is heat treated cast alnico, consisting of iron, nickel, aluminum and cobalt, cooled in a magnetic field and whose magnetic orientation is accomplished during heat treatment. Such a material is manufactured by Indiana General Corporation of Valparaiso, Indiana under the designation of Alnico V and has a residual inductance coefficient of 12.5 kilogauss, a coercive force coefficient of 600 oersteds and a peak energy product of 5.25 X 10 gauss-oersteds.

For highest possible operating efficiency, i.e., maximum air gap field with the minimum magnet material, the magnet structure is constructed such that the magnet material operates a the knee of the demagnetization curve of the material. For a given magnetization length, air gap size, air gap field, and magnet material, the required magnet structure can be determined by standard techniques. A particular magnetic circuit 11 constructed to provide a 1000 gauss air gap magnetic field for a sputter-type ionic vacuum pump 31 having two pump element units 32 is shown in FIGS. 1-3. As shown therein, the vacuum pump 31 comprises a vacuum envelope 33 having a main chamber 34 and two spaced apart elongated rectangular parallelepiped appendage chambers 36 extending from one side of the main chamber 34 defining three interconnected volumes. The main chamber 34 is communicated to a chamber to be evacuated (not shown) through a port 37 terminating at a flange 38 adapted to be mounted in gas tight relation to the chamber to be evacuated. A flange suitable for such gas tight coupling is the ConF-lat flange described in United States Patent 3,208,758 to Maurice A. Carlson and William R. Wheeler entitled Metal Vacuum Joint, issued Sept. 28, 1965. It is noted that other vacuum pump container configurations may be employed with the magnetic circuit of the present invention. For example, container configurations of only one or more than two appendage chambers 56 could be employed, with appendage chambers 36 extending from any or all sides of main chamber 34. Also, appendage cham ber configurations other than rectangular, that is, for example, circular, could be employed. However, the rectangular appendage chambers 36 facilitate most eflicient use of the magnet material since the configuration of the rectangular U-shaped magnetic circuit can be conformed more exactly to that of the appendage chambers 36.

A pump element unit 32 is inserted in each appendage chamber 36. A typical pump element unit 32 includes a multi-cell anode comprised of, for example, a plurality of parallelly disposed anode cells 39, preferably circular cylinders, held together in a rectangular-like bunch by spot welds at their tangent points 40. Additional support is provided by a strap 41 tightly wrapped about the circumference of the rectangular-like bunch. The anode cells 39 are mounted spaced apart and between cathode plates 42 and 43 with the principal axes of the cells 39 perpendicular thereto, The cathode plates 42 and 43 are constructed from reactive material that is disintegratable upon ion bombardment, e.g., titanium. Alternatively, a coating of such material on a support member could serve as cathode plates. Other multi-cell anode configurations other than that formed by the individual circular cylindrical anode cells 39 could be employed, such as, a rectangular cylindrical cell or any partitioning means defining a plurality of open ended cellular compartments. In all cases, the anode cells 39 must be open enderd cellular compartments mounted with a cathode structure covering the open ends of the cellular structure.

Mounting of the anode cells 39 is accomplished by insulators 44 secured between strap 41 and conductive plate 46 secured to cathode plates 42 and 43. The required electrostatic field is established by connecting a high volt age source, e.g., 3000 v. DC, (not shown) through a high voltage feedthrough 47 connecting to the anodes 39 by conductors 48 electrically connected to strap 41.

Each appendage chamber 36 is arranged to be received in nested relation by the air gap of a magnetic circuit structure 11 including elongated rectangular plate magnets and pole pieces. Magnetic circuit 11 may also be mounted within the appendage chamber 36 to receive in its air gap 15 pump element unit 32. The adjacent magnetic circuits 11 are mounted between platforms 49 (top platform shown) to have like poles proximate each other. With like poles arranged proximate, the magnetic fields generated by each magnetic circuit 11 do not interlink to interfere with or cancel one another in the pump element unit regions. In fact, the leakage flux of each magnetic circuit 11 links with the air gap magnetic field of its adjacent magnetic circuit to produce an enhanced air gap magnetic field. Furthermore, the magnetic fields generated by each magnetic circuit 11 in the space 51 therebetween are in opposition thereby tending to cancel one another therein. The etfect of such cancellation is a reduction of the stray magnetic field in the vicinity of the sputter-ion vacuum pump 31.

A particular magnetic circuit structure constructed to provide a 1000 gauss air gap magnetic field for the sputter-ion vacuum pump 31 had the following specifications. The overall dimensions of magnetic circuit 11 measured 4.50 inches wide, 7.375 inches long, 14.55 inches high, and defined an air gap 15 of 2.5 inches wide by 6.25 inches long. Magnet 12 measured 2.5" x 1.126" x 14.55". Each corner pole piece 18 and 23 measured 1.0" x 1.25 x 14.55". Side magnets 13 and 14 measured 1.0" x 1.875" x 14.55. The side pole pieces 26 and 28 measured 4.25 inches long by 14.55 inches high. More efficient use of the side pole pieces 26 and 28 was obtained by tapering the .pole pieces from a 1.0 inch maximum width proximate the side magnets to a inch width at their extremities. The magnets 12, 13 and 14 were constructed from the hereinbefore described alnico material. The pole pieces 18, 23, 26 and 28 were constructed from soft iron or low reluctance mild steel. The magnitude of the air gap magnet field can be varied by varying the length of magnets 12, 13 and 14.

Referring to curve (a) of FIGURE 4, the air gap magnet field intensity profile of the above identified magnetic circuit 11 taken in the direction of the extending arms of the U-shaped configuration along the center line of air gap 15 was found to be 500 gauss along the surface of magnet 12 facing air gap 15. The air gap magnetic field intensity increased to 1000 gauss at the junction of side pole pieces 26 and 28 and side magnets 13 and 14. The air gap magnetic field intensity remained substantially at 1000 gauss in the air gap region between the pole pieces 26 and 28 and decreased towards zero in the region beyond the air gap 15.

Curve (b) of FIGURE 4 depicts the air gap magnetic field intensity of a magnetic circuit of the type described in the hereinbefore identified Meyerers Patent 3,182,234. As shown by the curve, the air gap magnetic field intensity goes to zero at the web of the U-shaped configuration. This is because the magnetic potential along the surface of the pole piece located at the web of Meyerers magnetic circuit is zero. Hence, it is seen that the air gap magnetic field-intensity profile of the magnetic circuit of the present invention is considerably enhanced over that of Meyerers. Furthermore, for a given maximum air gap magnetic field intensity, a Meyerers type magnetic circuit configuration would be much larger than one constructed in accordance with the present invention. This is seen when it is considered that to generate a magnetic field of a given intensity, a given length of magnetization is required. Hence, to generate a 1000 gauss magnet field, the length of the side magnets in Meyerers magnetic circuit would be longer than those of the magnetic circuit of the present invention by one-half the length of the Web section of the U-shaped configuration.

Curve (c) of 'FIG. 4 depicts the air gap magnetic field intensity profile of a U-shaped magnetic circuit configuration having a single magnet disposed at the web of the U and having the same overall dimensions as the magnetic circuit configuration of the present invention. Although the air gap magnetic field profile is improved, it is seen that the intensity is only one-half that of the magnetic circuit of the present invention. To generate a magnetic field intensity comparable to that generated by the magnetic circuit of the present invention, a considerably larger single magnet type magnetic circuit structure would be required employing a permanent magnet constructed of material whose coercive force coefficient at least equaled the desired air gap magnetic field.

As noted hereinbefore, many advantages accrue to the magnetic circuit of the present invention as a result of its compactness, increased efiiciency, and enhanced air gap magnetic field intensity profile. Most importantly is the ability to construct a more compact and efficient sputterion vacuum pump.

While the present invention has been hereinbefore described with respect to a single embodiment, many modifications and variations are possible within the scope of the invention. For example, a U-shaped magnetic circuit configuration of more than three magnets and four pole pieces could be constructed by adding additional pole pieces and magnets in alternation in each leg of the U- shaped configuration. Also, more than one magnet could be positioned in the web section of the U-shaped configuration. Furthermore, the pole pieces could be curved sections instead of straight since the magnetic length is unaffected.

Therefore, the scope of the magnetic circuit of the present invention is not intended to be limited except by the terms of the following claims.

What is claimed is:

1. An improved U-shaped magnetic circuit comprising first, second and third straight permanent magnets arranged in a generally U-shaped magnetic potential adding configuration with said first magnet disposed at its web and said second and third magnets at its arms, and first and second pole pieces, said first pole piece secured at the north pole of said first magnet, said second pole piece secured at the south pole of said first magnet, said second magnet secured at said first pole piece with its south pole adjacent thereto, said third magnet secured at said second pole piece with its north pole adjacent thereto.

2. The magnetic circuit according to claim 1 further comprising a first straight pole piece secured at the north pole of said second magnet to extend therefrom in the same direction as said second magnet, and a second straight pole piece secured at the south pole of said third magnet to extend therefrom in the same direction as said third magnet.

3. The magnetic circuit according to claim 2 wherein said pole pieces and magnets are rectangular blocks.

4. The magnetic circuit according to claim 3 wherein said first and second straight pole pieces each have a side which is tapered from a large end to small end thereof, the tapered side of each pole piece defining a surface facing away from the other straight pole piece with the large end of said first straight pole piece proximate said second magnet and the large end of said second straight pole piece proximate said third magnet. V

5. The magnetic circuit according to claim 1 wherein said magnets are constructed from materials having a peak energy product of at least 5.00 10 gauss-oersteds.

-6. The magnet circuit according to claim 5 wherein said magnets are constructed from alnico having a residual induction coefficient of 12.5 kilogauss, a coercive force coeificient of 600 oersteds and a peak energy product of 5.25 10 gaus s-oersteds. 7

References Cited I UNITED STATES PATENTS Lloyd et a1 23069 King et a1. 335306 Adler 335-304 Holland 313-7 X

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