US 3192436 A
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Description (OCR text may contain errors)
June 29, 1965 D. M. JEWART Y, I 3,192,436
HEATER SWITCHING CATHODE FOR MAGNETRONS Filed June 2. 1961 INVENTOR Dona/a M. Jen arr BY ATTORNEY United States Patent 3,192,436 HEATER SWITCHING CATHODE FOR MAGNETRONS Donald M. Jewart, Williamsport, Pa., assignor, by mesne assignments, to Litton Industries, Inc., Beverly Hills,
Califi, a corporation of Delaware Filed June 2, 1961, Ser. No. 114,350 4 Claims. (Cl. 315-73) This invention relates to electron discharge devices employing an indirectly heated cathode and an anode and more particularly to the control of the current through the cathode heater.
In many electron discharge devices employing an indirectly heated cathode and particularly in magnetrons, it is necessary to control the temperature of the cathode. During operation of these devices many of the electrons follow a cardioid path and back-bombard the cathode, thereby causing undesirable overheating. The life characteristics of the cathode are deleteriously affected by the occurrence of overheating.
In order to reduce the harmful effects of high operating cathode temperatures, efforts have been made to control the cathode heater current. For this purpose, circuitry external to the tube, such as manual potentiometer control or relay control, has been employed to regulate the cathode heater current. A control system of this type'is expensive and diificult to operate and monitor.
Accordingly, an object of this invention is to decrease the undesirable overheating of the cathode by auto1nati cally temporarily breaking the heater circuit of the device to regulate the heating.
Another object is to'provide an internal means for controlling heater current, thereby alleviating the need for external control circuitry.
A further object of the invention is to permit a wider range of tolerance on heater power during warm up by providing automatic cathode temperature control thereafter.
The foregoing objects are achieved in one aspect of the invention by the provision of an indirectly heated cathode structure comprising two cathode assembly structures'of dissimilar metals having different thermal coefficients of expansion. These two structures are suitably fastened together at one end. The opposite end of each structure contains an electrical contact which is part of the heater circuit. In normal operation, the contacts on the ends of the structures are closed to allow current to flow in the heater circuit. When the cathode reaches a prescribed critical temperature, the longitudinal differential thermal expansion of the two structures causes the contacts on the ends of the sleeves to temporarily sepa rate, thereby breaking the heater circuit. Thus the cathode temperature is allowed to reduce to a safe operational range whereat, by thermal longitudinal contraction, the contacts are again made to reactivate the heater circuit. In this manner, the cathode operational temperature is controlled within thermal limits compatible with desired tube operation.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings in which:
FIG. 1 portrays a fragmentary sectional view of a magnetron tube and shows the portions in the interaction space normally employed in these devices; 7
FIG. 2 shows a modification of a switch structure shown in FIG. 1;
FIG. 3 is a perspective view of a leaf spring employed in the modified form of invention shown in FIG. 2.
In FIGURE 1 the numerals 10 and 12 designate pole pieces held in the body 14 of a magnetron type tube.
The magnetron is provided with ananode 16 held between the pole pieces 10 and 12. The cathode, indicated generally as 18, passes axially through the pole pieces 10 and anode 16, and into a space in the pole piece 12 wherein it is free to expand longitudinally of its axis. The cathode sleeve is a composite structure comprised of an inner cylindrical sleeve 20 of a material such as molybdenum having a relatively low thermal co-eflicient of expansion nested in an outer sleeve 22 such as nickel having a relatively high thermal co-efficient of expansion. The outer sleeve is coated opposite the anode with an electron emissive coating 24 such as the triple alkaline earth carbonates of barium, strontium, and calcium. Circular flanges 26 may be provided about the outer sleeve to prevent stray emission from the cathode from reaching the pole pieces. The inner and outer sleeves are immovably brazed with copper or otherwise suitably joined together as indicated at 28 but are otherwise unattached. However, the inner sleeve 20 lies in close contact for excellent heat transfer and longitudinal expansion relationships with the outer sleeve 22. The cathode is supported near the end jointure of the sleeves by mechanically and electrically joining the outer sleeve 22 to a metallic cone 3%, which is suitably insulatively mounted on a fixed part of the magnetron, as on the pole piece 10. At the free end of the sleeve 20 is fixedly mounted a centrally perforated ceramic button 32 surmounted by a noncorrosive contact plate 34. A cooperating non-corrosive contact button 36 is fixed onto the free end of the outer sleeve 22. Firmly attached to the contact plate 34 is the end 37 of the cathode heater 38. A retainer ring 42 biting into the wire keeps the contact plate 34 firmly against the ceramic button 32. The opposite end 44 of heater 38 leads to a suitable cur-rent supply (not shown), the other side of the supply being connected to end lead 46 which is in turn electrically connected to the metallic cone 30, sleeve 22, and button 36.
When the cathode is unheated, the outer sleeve 22 is contracted in length relative to the inner sleeve 20 and the metallic button 36 is in contact with the plate 34. When the electric circuit to the magnetron is closed externally of the tube, the heater circuit is closed and the cathode will become active and electron emission will take place. Should the cathode 18 become too hot for some reason such as from back-bombardment of the cathode with electrons, the outer sleeve 22 will heat and elongate to a greater degree than the inner sleeve due to its greater co-efficient of thermal expansion and thereby open the circuit at the free ends of the sleeves by moving button 36 away from plate 34. The heater circuit remains open until the cathode cools sufficiently to allow sleeve 22 to retract and carry button 36 into contact with plate 34 again.
Thus, by repeated operation of the cathode sleeve contacts 34 and 36, a substantially steady state cathode temperature will be maintained. Preferably, there is sufficient yield in the metals of the sleeves 20 and 22 to allow for the small amount of contraction when the cathode structure is cold and the contacts are together. However, where there is a wide variation in temperature between a cold non-operating tube and its subsequent operational temperature, additional yield of the parts may be desired to prevent deforming or rupturing of the parts or joints. Therefore, additional yield of the parts may be effected by means such as is shown in FIG. 2. In this form of the invention, the outer sleeve designated as 22' in FIG. 2 is slightly longer than the outer sleeve 22 in FIG. 1 to allow room for inclusion of a compression member 48. Contact 36 is replaced by 36 which is of sufficient thickness to be slidable within sleeve 22 for movement toward and from contact plate 34. The substantially N- aroaeee shaped leaf spring 48 has substantially normal parallel end legs brazed to a face of the contact 36' and to the inner face of a cover plate 5% which in turn is brazed to the free end of outer cathode sleeve 22'. This spring absorbs some of the contraction stresses which may otherwise occur if sleeve 22 contracts beyond the limit where contacts 34 and 36' are pressed together.
The present invention provides an efiicient means for automatically controlling heater current and resultant cathode temperature in an electron discharge device by providing an internal make-and-break heater circuit switching mechanism.
Although several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.
Having thus described the invention, what is claimed is:
1. A thermionic indirectly heated cathode structure comprising an inner and an outer sleeve in nested relationship and fastened together near adjacent support ends of the sleeves, said sleeves being of metals having different co-eificients of heat expansion, an electrical contact carried by each of the sleeves substantially at their free ends movable between an engaging and non-engaging position relative to one another, a cathode heater within the inner sleeve having one end connected to one of the contacts and the other end extending away from the cathode, and an electrical connection to the second one of the contacts, whereby the heater current may be controlled by the relative thermal expansion of the telescoped sleeves in accordance with the relative positions of said contacts.
2. A thermionic indirectly heated cathode structure comprising an inner sleeve having a relatively low thermal co-eflicient of expansion, an outer sleeve having a greater thermal co-efiicient of expansion with the two sleeves being mounted in nesting relationship and contacting each other to promote heat transfer and enable longitudinal diilerential expansion, said sleeves being fastened together at one end and each provided with a contact at their free ends for movement between an engaging and non-engaging position, electron emissive material on the outer sleeve, a cathode heater positioned within said inner sleeve having one terminal connected to the inner sleeve contact, and an electrical connection leading to the contact carried by the outer sleeve, whereby the heater current may be controlled by the relative thermal expansion of the sleeves in accordance with the relative posi- ,tions or" said contacts.
3. A thermionic indirectly heated cathode structure comprising an inner sleeve having a relatively low thermal co-ellicient of expansion, an outer sleeve of greater thermal co-efiicient of expansion with the two sleeves being mounted in nesting relationship and contacting each other to promote heat transfer and enable longitudinal differential expansion, said sleeves being fastened together at one end and each provided with a contact at their free ends for movement between an engaging and nonengaging position, said outer sleeve contact being resiliently mounted thereon, electron emissive material on the outer sleeve, a cathode heater positioned within said inner sleeve iaving one terminal connected to the inner sleeve contact, and an electrical connection leading to the contact carried by the outer sleeve, whereby the heater current may be controlled by the relative thermal expansion of the sleeves in accordance with the relative positions of said contacts.
4. A magnetron having a pair of pole pieces and an anode supported therebetween, a cathode centrally located in the magnetron and traversing the anode, said cathode containing emissive material on a portion thereof opposite the anode and a support for the cathode at one end thereof only, saidcathode having an outer sleeve and an inner sleeve in nested relationship attached to one another at one end thereof, said sleeves being mounted to provide heat transfer and dillerential longitudinal expansion, the outer sleeve being of a metal having a higher thermal co-efficient of expansion than the inner sleeve, a contact carried by each of the sleeves substantially at the free ends thereof with the inner sleeve contact being insulated therefrom, a cathode heater Within the inner sleeve having a lead-in connected to one end of the heater, the other end of the heater terminating in the contact carried by the inner sleeve, second lead-in for the heater having electrical connection with the contact on the outer sleeve, whereby the heater current may be controlled by the relative thermal expansion of the sleeves.
References Cited by the Examiner UNITED STATES PATENTS 1,772,002 8/30 Harper. 11,969,105 8/34 Smulski ZOO-422.3 2,286,929 6/42 Pond. 2,711,457 6/55 Wise. 12,957,100 10/60 Espersen et al 313-337 X GEORGE N. WESTBY, Primary Examiner. RALPH G. NILSON, Examiner.