Bucking samarium cobalt magnets for crossed field devices
United States Patent 1191 MacM aster et al.  
 1 1 Apr. 29, 1975 1 BUCKlNG SAMARIUM COBALT MAGNETS FOR CROSSED FIELD DEVICES [75] Inventors: George H. MacMaster. Lexington:  
 Kenneth W. Dudley, Sudbury both of Mass.  
 [73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington. DC.  
 122] Filed: Nov. 1, 1973 [21] Appl. No.1 411.616  
 [52] U.S.Cl. ..315/39.71:313/1571315/3751; 335/229: 335/296 511 int. Cl. 1-10lj 25/50 [58] Field of Search 315/3951, 39.711313/157; 335/229. 232. 302, 306. 296  
 [56} References Cited UNITED STATES PATENTS 2.235.517 3/1941 Espc 313/157 3.346.766 10/1967 Fcinstein 315/3971 3.588.588 6/1971 Numata 315/3971 3.739.225 6/1973 Mims 315/3971 3.755.706 8/1973 Scott 335/296 Primary l;&#39;.\&#39;amincr.lames W. Lawrence Assistant E.\&#39;aminerSaxfield Chatmon. Jr. Attorney. Agent, or FirmR. S. Sciascia; P. Schneider [57] ABSTRACT A pair of bucking magnets in conjunction with a fieldproducing magnet inside the annular cathode of a crossed-field device. The bucking magnets may also be inside the cathode on both sides of the field magnet. or may be outside the cathode encircling the an nular anode structure on both sides. Samarium cobalt (Sm Co) magnets are used. The bucking magnets increase the flux-line density in the interaction region between cathode and anode.  
 7 Claims, 6 Drawing Figures PATENTEDAPRZ IHYS 3.881.127  
 SHEET u 0F 6 FIG. 4.  
 PATENIEUAPRZSEE 3,881 127 SHEET 5 BF 6 PATENTEBAPRZSIQYS 3,881 127 SZ&#39;iEET 6 6 FIG. 6.  
 BUCKING SAMARIUM COBALT MAGNETS FOR CROSSED FIELD DEVICES BACKGROUND OF THE INVENTION This invention relates to crossed-field devices (CFD) and especially to means for making the magnetic field more dense.  
  Certain types of electronic tubes use a magnetic field transverse to the electron discharge path between the cathode and the anode. These tubes may be termed crossed-field devices (CFD) and the magnetron is an example of this type of electron tube.  
  Because of recent developments, the internal diameter of the cathode, which is cylindrical in shape, can now be made somewhat larger in diameter. Also, a material with very high coercive force, samarium cobalt, is now being used as a permanent magnet material, making possible small-sized magnets with a strong field. Thus, the magnet for a CFD can now be put inside the cathode.  
  However, since such internal magnets are quite small and the space in the tube quite limited, it is sometimes desirable to increase the strength of the magnetic field provided by such magnets without increasing the size of the magnet.  
 BRIEF SUMMARY According to the present invention, the field of a magnet located inside the cathode of a CFD can be made denser in the interaction region between the cathode and the anode by the proper placement of one or more bucking magnets, either internally of or externally to the cathode structure.  
  An object of this invention is to increase the density of the magnetic field in a CFD having an internal magnet by the use of one or more bucking magnets.  
  Another object is to increase the density of such a field by the use of bucking magnets disposed internally of the cathode.  
  A further object is to increase the density of such a field by the use of bucking magnets disposed externally of the cathode.  
  Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.  
 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of a CFD using internally disposed bucking magnets;  
  FIG. 2 is a plot of the magnetic field around one bucking magnet and the main magnet, in the tube shown in FIG. 1;  
  FIG. 3 is a cross-sectional view of a CFD using externally disposed bucking magnets;  
  FIG. 4 is a plot of the magnetic field around one bucking magnet and the main magnet in the tube shown in FIG. 3;  
  FIG. 5 is a cross-sectional view of another embodiment of the invention; and  
  FIG. 6 is a plot of the magnetic field in the embodiment of FIG. 5.  
 DETAILED DESCRIPTION FIG. 1 shows, in schematic form, a cross-section taken thru a CFD, in particular, the QKS 1556 magnetron. The cathode l2 and the anode 14 are cylindrical; the tube axis 16 coincides with the axis of the cylinders. An annular interaction region 18,&#39;or space, is present between the cathode and anode.  
  A permanent, tubularly shaped magnet 20, preferably made of small, stacked, samarium cobalt magnets, is placed inside the cathode structure. This field magnet 20 is sandwiched between two high-permeability pole pieces 22 and 22&#39;, and have high-permeability end pieces 24 close to the cathode 12. The pole and end pieces may be made of high-permeability steel.  
  To bend the magnetic field so that it becomes more dense in the interaction region 18, a pair of fieldbucking magnets 26 and 26&#39; are placed to the right and to the left (as seen in FIG. 1) of the pole pieces. The field lines (flux lines) 30 are shown in a plot in FIG. 2. The flux lines 30 in the interaction region 18 have an outward curvature to inhibit the space charge from drifting out of the interaction region.  
  Before using the samarium cobalt internal field magnet, Alnico magnets were used. The magnetic circuit with Alnico magnets weighs about pounds for the QKS 1556 tube; with the new Sm Co internal magnet, the circuit weights about 6 pounds.  
  FIG. 3 shows an embodiment of the invention in which bucking magnets 28&#39; and 28 which are external to the cathode structure, are used. Each of these bucking magnets is in the form of a toroid having square or rectangular cross-sectional areas. The bucking magnets are placed as close to the field magnet 20 as possible. Here, they are placed to the right and left of the anode structure. A flux return path 36 of high-permeability material, such as steel, is used with these external bucking magnets to avoid leakage flux.  
  The plot of the flux lines 30 for this embodiment of the invention is shown in FIG. 4.  
  It should be noted that Alnico magnets would be degaussed if the bucking magnets were used as close to the field magnet as shown at least in the embodiment of FIG. 1.  
  With the external bucking magnets, a weight reduction from 190 to 15 lbs. was attained.  
  FIG. 5 shows an embodiment in which one bucking magnet 40 is employed more as a field-shaping magnet than as a field-intensifying magnet. Only one toroidal magnet 40 is employed here and is preferably situated within the anode vane structure 44. Its axis coincides with the tube axis 16 (see FIG. 1) and its plane intersects the main field magnet 20 and also coincides with the midplane of the anode. It may also be placed outside the evacuated section of the tube, as shown by the dotted lines 42, altho the first location inside the vacuum envelope 46 is preferable. In this location, only four tenths of a pound of Sm-Co makes the field straighter and increases the field strength by 25 percent.  
  Field-shaping magnets enable a more optimum field shape to be obtained (See FIG. 6). The outward curvature at each end of the interaction region near the pole end pieces is desirable to inhibit the space charge from drifting out of the interaction region. The straight region the magnetic field along the vanes is desirable to enable synchronous interaction of the space charge with the r. f. field over a greater length of the vanes. This results in a more equal dissipation of energy along a greater length of the vanes and reduces localized heating at the center of the vanes. The operating efficiency of the tube is increased and there is a dramatic reduction in the lower current-mode boundary (when related to performance of the device with only the main internal field magnet Obviously, many modifications and variations of the present: invention are possible in light of the above teachings. It is therefore to be understood that. within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.  
 What is claimed is:  
  1. In a crossed/field device having a hollow annular cathode and an annular anode encircling said cathode and spaced therefrom by a distance called the interaction region. the combination comprising:  
 permanent field magnet means disposed within said cathode providing a magnetic field within said interaction region; and  
 at least one bucking. permanent magnet near said field magnet for bending back the field lines thereof so that the field in the interaction region increases in strength,  
 said bucking magnet being located externally of said cathode.  
  2. A combination as in clain 1 said bucking magnet being a field-shaping magnet located externally of said cathode.  
  3. A combination as in claim 1 said bucking magnet being a field-shaping magnet in the form of a toroid, said toroid lying within the anode vane structure, the plane of said toroid and the mid-plane of said anode vane structure being coincident.  
  4. A combination as in claim 1, having two bucking magnets located so that said field magnet means is sandwiched between them.  
  5. A combination as in claim 1, having two bucking magnets external to said cathode each being toroidal in shape and a different one being located on each side of the anode structure.  
  6. A combination as in claim 1 all said magnets being fabricated from samarium cobalt.  
 7. A combination as in claim 5, all said magnets being fabricated from samarium cobalt.  
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