US3223876A

US3223876A - Cathode heater assembly for use in strong d.c. magnetic fields

Info

Publication number: US3223876A
Application number: US221796A
Authority: US
Inventors: Maurice F Liscio
Original assignee: SFD LAB Inc
Current assignee: SFD LAB Inc; S-F-D LABORATORIES Inc
Priority date: 1962-09-06
Filing date: 1962-09-06
Publication date: 1965-12-14
Anticipated expiration: 1982-12-14

Description

M. F. LISCIO CATHODE HEATER ASSEMBLY FOR USE IN STRONG D.C. MAGNETIC FIELDS 2 Sheets-Sheet 1 Filed Sept. 6, 1962 INVENTOR.

MAURlCE F. LISCIO 1% i f/%;

ATTORNEY ec. 14, 1965 M. F. LISCIO 3,223,876

CATHODE HEATER ASSEMBLY FOR USE IN STRONG D.C. MAGNETIC FIELDS Filed Sept. 6, 1962 2 Sheets-Sheet 2 INVENTOR. MAURICE F. LISCIO ATTORNEY United States Patent M Jersey Filed Sept. 6, 1962, Ser. No. 221,796 4 Claims. (Cl. 313-276) The present invention relates in general to electron discharge devices of the crossed electric and magnetic field type and more specifically to a reverse magnetron useful for generating high power microwave energy at extremely high frequencies such as required in high power, high resolution radars.

A reverse magnetron tube typically comprises a circular electric mode cavity or circular electric mode wave propagating structure surrounded by a circumferal array of outwardly directed vane or cavity resonators coupled to the excited circular electric structure via a circular array of axial slots communicating with alternate anode resonators. The array of anode resonators are surrounded by a magnetron interaction region formed by an annular cathode emitter emitting radially inwardly into the anode, in the presence of a strong axial magnetic field. Rotating spokes of electron space charge interact with the 71' mode fields of the anode resonators to excite the circular electric mode in the circular electric mode cavity. Since the stored energy of the circular electric mode cavity is much higher than that of the vane resonator circuit the anode vane resonator system is locked in the 1r mode to the circular electric cavity mode thereby stabilizing the magnetron. The reverse magnetron structure may be used as an oscillator or as an amplifier and microwave energy is extracted from the circular electric mode wave propagating structure of cavity and fed to a suitable load.

Heretofore a reverse magnetron of the above described type has been built operating at approximately 35 gigacycles and generating a peak power of approximately 150 kilowatts with an average RF. power of 75 watts. When an attempt is made to achieve long life and reliability it is found that the prior art design of the heater and heater support was one of the major factors affecting life.

It was found that the prior art ceramic coated helical cathode A.C. heater element experienced motoring in the strond D.C. axial magnetic fringe fields causing the ceramic heater insulating coating to chafe off the filament at elevated temperatures producing shorting of the heater filament thereby reducing tube life to something in the order of five minutes.

The reverse magnetron tube of the present invention solves the aforementioned ditficulties associated with the prior art tube and provides a 32-35 gigacycle magnetron having a peak power output in the order of 290 kilowatts with average power output of approximately 50 watts while yielding overall efficiencies of approximately 30% and a turnable bandwidth of 12%. This tube represents more than an order of magnitude increase in peak power output with approximately double the previously obtained eificiency while having a long operating life in excess of 2,400 hours.

The principal object of the present invention is to provide an improved high power reverse magnetron tube yielding substantially enhanced operating life.

One feature of the present invention is the provision of an insulating support structure of a helical A.C. filamentary cathode heater wherein the helical A.C. heater element is tangentially supported at spaced points about its periphery via refractory longitudinally directed members, whereby relative motion between the heater helix 3,223,876 Patented Dec. 14, 1965 and the cathode structure is permitted without shortening the AC. filament to the cathode structure.

Other features and advantages of the present invention will become apparent upon a perusal of the specification taken in connection with the accompanying drawings wherein:

FIG. 1 is an outside perspective view of the reverse magnetron tube of the present invention,

FIG. 2 is an enlarged fragmentary view partly broken away and partly in section of the structure of FIG. 1 taken along the line 22 in the direction of the arrows,

FIG. 3 is a fragmentary view partly in cross-section and partly broken away of the portion of the structure of FIG. 2 taken along the line 33 in the direction of the arrows,

FIG. 4 is an enlarged fragmentary cross-sectional view of a portion of the structure of FIG. 2 delineated by line 44 and rotated in the counter clockwise direction,

FIG. 5 is an enlarged perspective view of a portion of the structure of FIG. 4 delineated by line 5-5,

FIG. 6 is an enlarged perspective view of an alternative structure to that of FIG. 5, and

FIG. 7 is an enlarged cross-sectional view of an alternative structure to that portion of the structure of FIG. 4 delineated by line 7-7.

Referring now to FIGS. 1, 2 and 3, character 1 represents the hollow tubular supporting body of the reverse magnetron, as of copper, to which other parts are brazed or otherwise suitably fastened to form a structure capable of being evacuated. On opposite sides of the body 1 in axial alignment there are brazed to the body 1 a tubular output waveguide assembly 2 and tuner assembly 3. Cathode lead-in insulator structure 4 extends outwardly from the main body of section 1 in quadrature with the axially aligned output waveguide and

tuner structures

2 and 3 respectively.

The term circular electric mode cavity as used herein is defined to mean a cavity formed, dimensioned, and excited in such a manner as to support at its certain pre selected operating frequency a certain circular electric mode, of the general form TE to the exclusion of other modes. A circular electric mode cavity typically includes an outer cylindrical side wall and may or may not have an axially directed center conductor.

A circular electric mode cavity 5 is disposed centrally of the anode body 1 on the axis of the tube. In amplifier embodiments of the present invention the circular electric mode cavity 5 is replaced by a circular electric wave propagating wave structure such as, for example, a hollow cylindrical pipe having an input port as well as an output port. A circumferal array of outwardly directed vanes 6 surround the circular electric mode cavity 5 and from an array of anode resonators by the spaces between adjacent vanes 6. Alternate anode resonators are electromagnetically coupled to the circular electric mode cavity 5 via an array of axially directed slots 7 communicating through the common wall between the anode resonators and the circular electric mode cavity 5. A magnetron interaction region 8 surrounds the outer tips of the vanes 6 and is defined by the space in between the vanes 6 and a surrounding cathode emitter ring 9.

A strong axial magnetic field of 12,000 to 15,000 gauss for the magnetron interaction region 8 is provided by a magnet 11, only partially shown in FIG. 2, enveloping the anode body portion 1 and having a re-entrant internal magnetic gap extending in the axial direction through the magnetron interaction region 8 between the magnetic pole pieces 12 disposed on opposite sides of the anode vanes 6.

Tuning of the tube over its approximate 12% tuning band, centered at approximately 34 gigacycles, is obtained by means of axial translation of a combined cavity end wall and output coupling plate 13 carried upon the end of an axially directed and positioned rod 14 which is axially translatable via the intermediary of a captured nut 15 and bellows assembly 16, partially shown. The coupling plate arrangement forms the subject matter of copending application 216,228 filed August 10, 1962, and assigned to the same assignee as the present invention.

The negative cathode potential of approximately 23 kv. is applied to the cathode emitter 9 via high voltage lead-in insulator assembly 4. The cathode 9 uses a low voltage A.C. filament heater and therefore a dual wire cathode lead-in 10 is used.

In operation the 11' mode of the magnetron interation region is locked to the circular electric mode resonator via the intermediary of the coupling slots 7 serving to drive the resonator 5. An annular slot mode absorber 24 juxtapositioned the coupling plate end of the slots 7 suppresses the undesired slot mode. The slot mode absorber and anode wall arrangement forms the subject matter of copending application 223,499 filed September 13, 1962, and assigned to the same assignee as the present invention. Output energy from the resonator 5 is extracted via the coupling plate 13 and transmitted to the load, not shown, via the intermediary of the circular electric mode output waveguide structure 2 and output wave permeable window 17.

The tube structure and mode of operation will now be described in greater detail as it specifically relates to each of the before mentioned features of the present invention.

The novel A.C. cathode heater filament insulating support structure feature of the present invention is more clearly seen by reference to FIGS. 4, 5, 6 and 7. The A.C. filament support structure is characterized by the provision of a plurality of curved refractory longitudinally directed members being disposed about the periphery of the bare A.C. heating filament helix serving to tangentially support the helix while allowing relative motion between the helix and the support Without producing excessive chafing of the insulating members.

More specifically, the cathode emitter ring 9 as of molybdenum is provided with an enlarged hollow body portion 61 near the outer periphery of the cathode emitter 9. The hollow interior of the emitter 9 forms an annular cathode heater filament nest 62 which in a preferred embodiment has a generally square cross-section. The bare wire heating filament 63, as of tungsten wire, is wound into a helix longitudinally directed of the annular nest 62.

A plurality of annular insulating members or segments 64 as of a refractory insulating material as of, for example, alumina ceramic are disposed about the periphery of the bare helix 63 at spaced apart positions, i.e., the corners of the rectangular nest 62, to tangentially support the helix 63. The insulating members 64 are disposed with their longitudinal axes directed around the annulus of the nest 62 and serve to prevent the A.C. heating filament 63 from shorting to the conductive walls of the filament nest 62.

The strong static axial magnetic field, as of 12,000 gauss, in the magnetron interaction region 8 produces a very strong fringing field component which threads generally in the axial direction of the tube through the filament nest 62. The A.C. heater current supplied to the helical cathode filament 63 causes the helix to produce relative movement in accordance with the changing A.C. current, between the filament 63 and the walls of the nest 62. This movement of the filament will hereinafter he referred to as motoring. Previous attempts to utilize a heater wire insulated with a ceramic coating resulted in the motoring causing the insulation to chafe off, through to the filament, in about five minutes of use due to the extremely high operating temperature of approximately 1200 C. Once the insulating coating has worn off the A.C. heater filament, the filament shorts to the walls of the cathode heater nest 62 rendering the tube inoperative.

By utilizing the A.C. heater support structure of FIG. 4 the operating life of the cathode has been extended from five minutes to an excess of 2400 hours.

An alternative A.C. heater filament support structure is shown in FIGS. 5 and 6 wherein the insulating members 64 have been replaced by a ceramic coated wire 65. This embodiment has the advantage that the ceramic coated insulated wire 65 is more economical to procure than the specially ground insulating rods 64.

The cathode emitter 9 is surrounded by a combined cathode end hat and heat shield assembly 67. More specifically, the heat shield 67 is composed of a plurality of annular overlapping plates closely spaced to the molybdenum cathode body 9. The two cathode end hat annular members are spaced apart from the thin neck portion of the cathode emitter ring 9 via a plurality of detents pressed into the cathode end hat plates. The detents providing a 0.003 spacing between the end hats and the cathode emitter ring 9. The annular heat shield ring segments are all held together by the provision of a plurality of peripherally spaced axially directed stainless steel pins 68 passing through the cathode emitter ring and being provided with localized spacing washers 69 serving to provide a separation between the overlapping heat shield plates.

Since many changes could be made in the above construction and many apparently widely different embodiments 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 interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A cathode heater assembly for use in strong D.C. magnetic fields including, means for producing a strong magnetic field, a bare helical A.C. filament cathode heater element having spaced apart turns immersed in the strong magnetic field with the longitudinal axis of the helix being directed transversely of the direction of the magnetic field, a plurality of refractory insulating members peripherally spaced apart about the circumference of the cross-section of the helix and being longitudinally directed of the helix for tangentially supporting the bare A.C. heater filament within the cathode structure whereby motoring movement between the helix and said tangential support is permitted without shorting of the A.C. filament to the cathode structure.

2. In a crossed field tube apparatus employing a strong axially directed D.C. magnetic field, an anode structure having a circumferally directed wave supporting structure formed therein, a circumferally directed cathode emitter disposed adjacent said anode wave supporting structure and defining a magnetron interaction region therebetween permeated by the strong axially directed D.C. magnetic field, a circumferally directed heater chamber formed in said emitter, a bare filament wire wound into a helical form with axially spaced turns and disposed within said heater chamber, said helix being directed within said heater chamber about the circumference of said cathode emitter and generally transversely directed to the direction of the strong D.C. magnetic field, a plurality of refractory insulating members peripherally spaced apart about the circumference of the cross-section of said helix and being generally longitudinally directed in the direction of the helix for tangentially supporting said helical filament within said heater chamber of said cathode emitter, whereby a motoring motion between the helix and the tangential support produced by an A.C. current flow transverse to the direction of the strong magnetic field is permitted without shorting said A.C. filament to said cathode structure.

3. The apparatus according to claim 2 Wherein said refractory insulating members are curved rods having a radius of curvature approximately equal to the radius of curvature of said filament helix.

4. The apparatus according to claim 2 wherein said refractory insulating members are curved ceramic coated Wires, said wires having a curvature substantially approximating the radius of curvature of said filament helix.

References Cited by the Examiner UNITED STATES PATENTS 2,448,573 9/1948 Blazier et al. 313-340 2,482,495 9/1949 Laidig 31339 3,027,481 3 1962 Baber ct al. 313-275 FOREIGN PATENTS 787,458 12/ 1957 Great Britain.

10 JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, Examiner.