US3159333A

US3159333A - Permanent magnets

Info

Publication number: US3159333A
Application number: US132889A
Authority: US
Inventors: John C Helmer
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1961-08-21
Filing date: 1961-08-21
Publication date: 1964-12-01
Anticipated expiration: 1981-12-01

Description

1964 J. c. HELMER 3,159,333

PERMANENT MAGNETS Filed Aug. 21, 1961 FIG.|

INVENTOR. JOHN LC. *HELMER f 7 BY A} WEiTXUNCHES) TTORNEY United States Patent 3,159,333 PERMANENT MAGNETS l'ohn C. lielmer, Palo Alto, Calit, assignor to Varian Associates, Palo Alto, Calili, a corporation of California Eiied Aug. 21, 1961, Ser. No. 132,389 6 Claims. (Cl. 230-69) This invention relates in general to permanent magnets and, more particularly, to novel structure incorporating ceramic magnetic material.

Ceramic magnetic materials have primarily a ferrite composition and usually the chemical composition is barium-ferrite known as ferroxdure and sold under the trademark Index although other ferrite compositions are available such as, for example, cobalt, nickel, cadmium, zinc, etc. .ferrites. The magnetic characteristics of ceramic magnetic materials are ademagnetizing field of at least 1000 oersteds at maximum energy product and at 20 C. a permeability (,u) between 1.0 and 1.2 (in the gauss-oersted magnetic units) which is close to the value in air, and a magnetic remanence (B,) at 20 C. ofless than five kilogauss. Also in a ceramic magnet demagnetization sets in when the dem agnetizing field is greater than -intensity within the magnetic material, l is the length of magnetic material, H, is the magnetic fieldintensity within the air gap, and l is the length of the airgap). If there are any soft iron low reluctance paths withinthe magnetic circuit the magnetomotive drop therein is almost equal to zero, then:

m m= a a The total magnetic flux around the circuit is constant and:

B m m a. a

where B is the flux density within the magnetic material, A isthe area of the magnetic material, B is the flux density within the air gap and, A,, is the area ot the air gap. Then B H l A =B H l A and H ,=B, since the permeability of air is equal to one so: B i fl -l'l 11 Barn, or ann, (3) Maximum energy product which is B H for ceramic magnets is less than B H for present steel magnets, but is high enough for many applications. Ability to use bare ceramic poles (rather than iron) permits the design of low leakage magnets which in some applications is super-ior to structures employing steel magnets. 'A particular advantage of the ceramic is that H at the point of maximum energy product is very high, much higher than in presentsteel magnets so that short magnets can be used, according to (1),

H, la

m fi; which reduces the size of the magnetic circuit. In addition, there are many devices such as sputtenion pumps which require magnetic fields in the neighborhood of B equal to between .1000 and 3000 gauss which is close to B for ceramic magnets at the pointof maximum energy product. Then from (2) 3,159,333 Patented Dec. 1, 1964 ice so that the magnet area is about equal to the gap area and complicated and lossy iron pole pieces for transition from A to A,, need not be used.

However, present steel magnets according to Equation 1 must be long for economy and lightness. Therefore steel magnets are frequently bent into a horseshoe to make'them compact and save space, but this intro duces an additional disadvantage in that the leakage flux is also increased, especially within the horseshoe shape. Therefore, additional magnetic material is required to compensate for the leakage flux.

An object of this invention is to provide an improved, lighter, compact magnet. p

A feature of this invention is the provision of a magnet assembly comprising two blocks of magnetic material having a demagnetization field of at least 1000 oersteds at maximum energy product and being spaced apart to form an air gap with opposite magnetic poles facing each other, while the other poles on each block are connected by a soft iron armature.

Another feature of this invention is the provision of a permanentmagnet structure wherein .two blocks of magnetic material, each having parallel pole faces, are spaced apart with opposite magnetic poles facing each other to form an air gap so that the ratio of the total length of the magnetic material to the length of the air gap is less han 2.5 to one to provide optimum use of the magnetic material and space available.

Another feature of this invention is an arched lowreluctance path having a uniform magnetic potential therein in combination with a block of ceramicmagnetic material so that there is substantially no external leakage flux. 1

Another feature of this invention is a permanent magnet structure in combination with a sputter-ion pump to provide a device wherein the leakage flux is negligible and no additional magnetic shielding is required. 7

Another feature otf'this invention is the provision of soft iron field flatteners on ceramic magets.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein,

H6. 1 is the magnet in combination with an electric sputter-ion vacuum pump that is shown in section,

FIG. 2 is a graph representing the magnetic field distribution over one of the pole faces, and

FIG. 3 is another embodiment of the magnet of FIG. 1. The magnet of the present invention will be described incombination with an electrical sputter-ion vacuum pump of the ionization gettering type although the magnet has many other uses and is thus broadly ciaimed as well as claimed in combinationwith said pump. Up to now steel permanent magnets were used with such pumps, these magnets having a high energy-product and a high permeability of about two gauss-oersteds. For ccmpact- I ness the steel magnets were horseshoe shaped and thus I were inefiicient because of high leakage flux around the horseshoe. Electrical vacuum pumps are often used in airbone equipment andthe weight of such steel magnets is a disadvantage. Also components in airborne equipment utilizing such magnets are often placed very close together and require suitable shielding so that leakage vflux does not aliect themagnetic field of other components. There are other applications wherein the leakage A rectangular cellular anode 13 made of, for example, titanium is supported on the end of an electrical conductive rod 14 which extends through an aperture formed in one of the side walls and is in insulated relationship to the envelope by a suitable ceramic insulator 16. A positive voltage with respect to ground is applied to rod 14 by a suitable power supply (not shown) and the envelope is grounded during pump operation.

Cathode plates 17 made of, for example, titanium are mechanically locked into position against the top 11 and the bottom 12 by suitable spacer plates 18 disposed about the periphery of both cathode plates 17.

Another side wall of the cup member 11 is apertured to receive a conduit 19, which may be of any convenient inside diameter commensurate with the desired pumping speed of the pump The other end conduit f9 communicates with a chamber to be evacuated.

A magnetic field of the required value to produce eificient operation of the electric pump is supplied by a novel permanent magnet assembly 21'. The magnetic field is oriented normal to the cathode plates 17 andtherefore threads through the cells of the anode 13.

Permanent magnet assembly 21 has two blocks of ceramic

magnetic material

22 and 23 suchas,.for example, barium-ferrite that supply the magnetomotive force and are disposed with their

pole faces

24 and 25 adjacent the pump top 11 and bottom 12, respectivel Of course,

pole faces

24 and 25 are of opposite polarity. To increase the efliciency of the magnetic field, the other pole faces 27 and 28 of magnets 22'and'23, respectively, are connected together with a low reluctance arched path 29 in the form of a soft iron ring or armature; Gne understands that arched path 29 need not be a ring, but a horseshoe shape as described hereinafter with the embodiment of FIG. 3 will suifice. Also rectangular rings, squares, etc., will suffice. Since the armatitre'h'as'a low reluctance the magnetic potential drop therein is ZlOO'r almost zero. The total magnetic potential drop substantially occurs between

pole faces

24 and 25. Since the armature 29 is a constant potential, little or no leakage flux linking pole face '27 to pole face 28 will be found in the area 31 on the drawing enclosed within the armature. As mentioned above, if armature 29was a source of magnetic potential one wouldfind' considerable flux leakage around the armature and especially within the shaded area 31.

The combination provides a device which-has little or no leakage flux and is particularly useful when a sputterion pump is attached as an appendage to a vacuum system, the vacuum system encloses an apparatus requiring precise magnetic fields, and the vacuum system isrequired to be continuously evacuated.

To produce a magnet assembly of the present invention designed for economy of magnetic material, the total length between pole faces of the magnetic material should be approximately equal to the air gap length giving a ratio of length of air gap to length of magnetic material of approximately one to one if the demagnetizing field at the point of maximum energy product is greater than B /2 and the permeability of ceramic magnets is almost equal to one gauss-oersted unit over the working range of the magnet. Then preferably each

block

22 and 23 must be of equal length or half the gap length although a magnetic assembly 21 could'be made with only one lock of magnetic material such as block 22 so that the air gap is formed between pole face Maud-the armature 29.

1 To produce a permanent magnet assembly designed for maximum magnetic energy for unit magnetic circuit volume that includes the air gap and magnetic material then the total length between pole faces of magnetic material should be approximately twice the length of the air gap giving a ratio of length of air gap to length of magnetic material of approximately 1 to 2 as the magnetic strength in the gap and in the magnet is again assumed uniform, as before.

Referring to FIG. 2 the profile of the magnetic field between the pole faces of a ceramic magnet is graphically illustrated. The x-ordinate is parallel to parallel-polefaces on a ceramic magnet assembly. The y-ordinate represents magnetic field intensity. x and x lines represent opposite edges of the pole faces respectively. Curve 32 represents a profile of the field intensity as measured midway between the pole faces. Curve 32 is indicative of a magnetic field distribution of pole faces of most any plane surface, although curve 32 was obtained by measuring the magnetic field in gauss formed by 3%" x 2% rectangular pole faces on a ceramic magnet assembly separated by 1 The two ceramic blocks were disposed in a 6%" diameter cylindrical armature. The distance between x and x is 3%". The curve shows that the magnetic field is flat to within one gap width of the gap edge.

In some magnetic applications a field that is'fiat over a larger area is desirable. Referring to FIG. 3 wherein like numbers refer to like items of-FIG. 1', and the armature 29 in this embodiment as' mentionedbefore being arched and formed into a horeshoe, the flatness of the magnetic field produced'would be increased over alarger area by'a

magnetic field flattener

36 and 37 applied to faces 24an'd 25 of magnetic blocks 22' and 23; respectively. The field flattener is a soft iron plate preferably about inch thick having an area equal and similar to

faces

24 and 25. The magnetic field produced by the field flatteners when incorporated within the above magnet assembly is represented by curve 33 in FIG. 2 wherein now the gap is only 1 3 as A" of the previous 1 air gap is occupied by the field flatteners. Like curve 32, curve 33 was obtained for the particular shaped pole faces but the two curves 32 and 33 together indicate that for any plane pole faces an increase in field flatness could be obtained with a sacrifice in magnetic field strength. The field is flat to within 0.2 gap width of the gap edge. Similar types ofcurves'could be plotted for pole faces having different surface shapes.

Since many changes could be made in the above construction and many apparaently widely diiferent embodi ments of this invention could' be made without departing from the scope thereof, it is intended'that all matter contained in the above description or shown in the accompanying drawings shall be illustrative and not in a limiting sense.

What is claimed is: g

1. A magnetically confined glow discharge sputterdon vacuum pump apparatus including: a vacuum tight envelope adapted to be connected to' a structure it is desired to evacuate; an apertured anode electrode defining a glow discharge passageway and contained within said envelope; a pair of cathode members disposed on opposite sides of said anodeelectrode and contained within said envelope; means for applying operating potentials to said anode electrode and cathode members for initiating and maintainingthe glow discharge; means for pioducing and directing a magnetic field through said anode electrode and cathode member's normal to said cathode members including an armature forming an arched low magnetic reluctance path around said envelope and having a uniform magnetic potential therein, a ceramic magnetic material block mounted on said armature and having a pole face disposed adjacent a portion of said armature defining therebetween an air gap, said pole face and said portion of said armature positioned adjacent and parallel to a respective one of said cathode members, said block being normal to said pole face, the ratio of the length of said block normal to said pole face to the length of said air gap normal to saidpole face being less than 2.5 to l.

2. A magnetically confined glow discharge sputter-ion vacuum pump apparatus including: a vacuum tight envelope adapted to be connected to a structure it is desired to evacuate; an apertured anode electrode defining a glow discharge passageway and contained Within said envelope; a pair of cathode members disposed on opposite sides of said anode electrode and contained Within said envelope; means for applying operating potentials to said anode electrode and cathode members for initiating and maintaining the glow discharge; means for producing and directing a magnetic field through said anode electrode and cathode members normal to said cathode members including an armature forming an arched low magnetic reluctance path around said envelope and having a uniform magnetic potential therein, first and second ceramic magnetic material blocks mounted on said armature and having spaced-apart opposed parallel pole faces defining therebetween an air gap, each pole face positioned adjacent and parallel to a respective cathode member, each block being normal to a respective pole piece, the ratio of the total length of said blocks normal to said pole faces to the length of said air gap normal to said pole faces being less than 2.5 to 1.

3. The apparatus according to claim 2 wherein a low reluctance plate is disposed on each of said pole faces having an area equal and similar to said pole face.

4. A permanent magnet assembly comprising an armature forming an arched low magnetic reluctance path and having a uniform magnetic potential therein, a ceramic magnetic material block mounted on said armature and having a pole face disposed adjacent and parallel to a portion of said armature defining therebetween an air gap, said block being normal to said pole face the ratio of the total length of said block normal to said pole face to the length of said air gap normal to said pole face being less than 2.5 to l.

5. A permanent magnet assembly comprising an arma ture forming an arched low magnetic reluctance path and having a uniform magnetic potential therein, first and second ceramic magnetic material blocks mounted on said armature and having spaced-apart opposed parallel pole faces defining therebetween an air gap, each block being normal to a respective pole piece, the ratio of the total length of said blocks normal to said pole faces to the length of said air gap normal to said pole faces being less than 2.5 to 1.

6. The magnet of claim 5 wherein 10W reluctance plates having an area equal and similar to said pole faces on said blocks are disposed thereon.

References titted in the file of this patent UNITED STATES PATENTS 2,762,778 Gorter et al Sept. 11, 1956 2,832,932 Baermann Apr. 29, 1958 2,900,344 Stuyts et al Aug. 18, 1959 2,917,682 Kirchner Dec. 15, 1959 2,993,638 Hall et al. July 25, 1961 FOREIGN PATENTS 834,878 Germany Mar. 24, 1952 464,518 Great Britain Apr. 20, 1937

Claims

1. A MAGNETICALLY CONFINED GLOW DISCHARGE SPUTTER-ION VACUUM PUMP APPARATUS INCLUDING: A VACUUM TIGHT ENVELOPE ADAPTED TO BE CONNECTED TO A STRUCTURE IT IS DESIRED TO EVACUATE; AN APERTURED ANODE ELECTRODE DEFINING A GLOW DISCHARGE PASSAGEWAY AND CONTAINED WITHIN SAID ENVELOPE; A PAIR OF CATHODE MEMBERS DISPOSED ON OPPOSITE SIDES OF SAID ANODE ELECTRODE AND CONTAINED WITHIN SAID ENVELOPE; MEANS FOR APPLYING OPERATING POTENTIALS TO SAID ANODE ELECTRODE AND CATHODE MEMBERS FOR INITIATING AND MAINTAINING THE GLOW DISCHARGE; MEANS FOR PRODUCING AND DIRECTING A MAGNETIC FIELD THROUGH SAID ANODE ELECTRODE AND CATHODE MEMBERS NORMAL TO SAID CATHODE MEMBERS INCLUDING AN ARMATURE FORMING AN ARCHED LOW MAGNETIC RELUCTANCE PATH AROUND SAID ENVELOPE AND HAVING A UNIFORM MAGNETIC POTENTIAL THEREIN, A CERAMIC MAGNETIC MATERIAL BLOCK MOUNTED ON SAID ARMATURE AND HAVING A POLE FACE DISPOSED ADJACENT A PORTION OF SAID ARMATURE DEFINING THEREBETWEEN AN AIR GAP, SAID POLE FACE AND SAID PORTION OF SAID ARMATURE POSITIONED ADJACENT AND PARALLEL TO A RESPECTIVE ONE OF SAID CATHODE MEMBERS, SAID BLOCK BEING NORMAL TO SAID POLE FACE, THE RATIO OF THE LENGTH OF SAID BLOCK NORMAL TO SAID POLE FACE TO THE LENGTH OF SAID AIR GAP NORMAL TO SAID POLE FACE BEING LESS THAN 2.5 TO 1.