US3255375A

US3255375A - Electrical heating device

Info

Publication number: US3255375A
Application number: US155565A
Authority: US
Inventors: Curtis E Ward
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1961-11-29
Filing date: 1961-11-29
Publication date: 1966-06-07
Anticipated expiration: 1983-06-07
Also published as: GB982206A; DE1181825B

Description

June 7, 1966 c E. WARD ELECTRICAL HEATING DEVICE 3 Sheets-Sheet 1 Filed Nov. 29, 1961 REFLECTOR VOLTAGE In A Willi-Finn FlG.4b

FIG. l2

PRIOR ART HEATER NEW HEATER INVENTOK CURTIS E. WARD VI B O R E0 T DM WWEO I M T NMO wwK R A ESE PAP ATTORNEY June C. E. WARD ELECTRICAL HEATING DEVICE Filed Nov. 29, 1961 HORIZONTAL LINES (INCHES) INDUCED OVOILTAGE '0 3 Sheets-Sheet Z FIG.7 A 1 FIG.9

(MILLIVOLTS) 8 Q. FlG." A A I I I A i com w BUTTON CONTOUR (INCHES) INVENTOR.

CURTIS E.WARD

ATTORNEY June 7, 1966 c, WARD 3,255,375

ELECTRICAL HEATING DEVICE Filed Nov. 29. 1961 3 Sheets-Sheet 3 FIG. I5

INVENTOR.

CURTIS E. WARD A'i'TORNEY United States Patent 3,255,375 ELECTRICAL HEATING DEVICE v Curtis E. Ward, Los Altos, Califi, assignor to Varian Associates, Palo Alto, Calif., a corporation of California 7 Filed Nov. 29, 1961, Ser. No, 155,565 2 Claims. (Cl. 313 337) The present invention relates in general to heating devices for electron tubes and more particularly to a novel cathode electrode heating structure which does not subject a unipotential cathode electrode being heated to harmful magnetic fields.

Existing cathode heaters for unipotential cathodes in electron tubes such as reflex and multicavity klystrons, traveling wave tubes, and magnetrons undesirably affect the performance of the electron tube. The alternating current in the heater coil sets up alternating magnetic fields in the cathode region of the tube, and these magnetic fields directionally modulate electrons in the beam emitted from the cathode. This modulation of the beam of electrons can produce not only current modulation due to interception of electrons but also electron transit time changes due to the beam being directed over different paths through the electron tube. These effects of directional modulation of the beam result in amplitude and frequency modulation of the output signal of the filar coil-coil geometry whereby A.C. magnetic fields of one wire conductor are neutralized by the fields of an adjacent wire conductor to eliminate heater hum. Also the present invention contemplates a small turn-around loop at the end of the bifilar coil with the turn-around loop on the outside of the coil furthest from the cathode.

The object of the present invention is to provide a novel heater and method of making the same wherein the cathode is not subjected to undesired modulating magnetic fields.

One feature of the present invention is the provision of a tightly wound bifilar coil-coil heater wherein the magnetic field set up by one wire of the bifilar coil is neutralized by the magnetic field set up by adjacent wires of the bifilar coil.

Another featur of the present invention is the provision of a tightly wound bifilar coil-coil heater wherein a tightly wound bifilar coil is wound into a larger helical coil.

Another feature of the present invention is the provision of a tightly wound bifilar coil-coil heater wherein a tightlywound bifilar coil is wound into a larger flat coil.

Another featur of the present invention is the provision of a tightly wound bifilar coil-coil heater wherein at one end of the bifilar coil the ends of the separate wires of the bifilar coil are connected by a connecting loop and this loop is contained in a plane which is perpendicular to the emission surface of the cathode being heated.

Still another feature of the present invention is the provision of a tightly wound bifilar coil-coil heater wherein the two wire strands of the bifilar coil are connected in parallel with the adjacentlends of the two wire strands connected to opposite heater. lead wires.

Still another feature of the present invention is the method of winding a tightly wound bifilar coil-coil heater by beginning with the midportion of a wire and winding a bifilar coil on a mandrel, winding the bifilar coil and the mandrel onto a second mandrel to form the wire into a bifilar coil-coil and then removing the mandrels from the final bifilar coil-coil.

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

FIGS. la and lb are side and end views, respectively, of one type of prior art cathode heater coil,

FIGS. 2a and 2b are side and end views, respectively, of another type of prior art cathode heater coil,

FIGS. 3a and 3b are side and end views, respectively, of still another type of prior art heater structure,

FIGS. 4:: and 4b are side and end views, respectively, of a cathode heater structure according to the present invention,

FIG. 5 is a side view of a mandrel and heater coil showing the manner in which the heater of FIG. 4 can be made,

FIGS. 6 and 7 are graphs showing the magnetic field profile of a heater coil of the configuration shown in FIG. 2,

FIGS. 8 and 9 are graphs showing the magnetic field profile of a heater coil of the configuration shown in FIG. 3.

FIG. 10 and 11 are graphs showing the magnetic field Profile of a heater coil of the configuration shown in FIG. 4,

FIG. 12 is a graph showing the frequency deviation of a reflex klystron versus percent power output for two different cathode heaters,

FIG. 13 is a side view of a cathode and heater coil showing another embodiment of the present invention,

FIG. 14 is a plan view of a cathode partially broken away and heater coil showing another embodiment of the present invention; and

FIG. 15 is a schematic line drawing of a typical electron tube apparatus employing an electron gun using the novel heater or the present invention.

Referring now to the drawing, in a prior art heater coil 20 shown in FIG. 1, a wire conductor 21 for heating a concave unipotential thermionic emissive cathode 22 for use in evacuated electron tubes was wound in the form of a helix. As used herein the term unipotential cathode shall mean a cathode emitter body having an emissive surface operating at substantially one D.C. potential. This type of emitter is to be distinguished from a thermionic filament emitter which is heated directly by passage of electr'icalcurrent through the filamentary emitter body resulting in the emitting surface operating at different electrical potentials occasioned by the potential drop necessary to drive the electrical current through the cathode filament emitter. The coil 20 was either axially aligned with the cathode 22 as shown or positioned transversely thereof, not shown. In the latter arrangement, the heater leads were applied at the opposite ends of the coil 20, whereas in the arrangement shown, the heater lead .to the end of the coil nearest the cathode 22 was conveniently brought down the axis of the coil 20. Due to the 60 c.p. s. current therethrough, the turns of the coil heater 20 as shown in FIG. I, acted as a solenoid to create a time varying magnetic field H threading the unisit time through the tube since the beam was directed along different paths through the tube. These effects of directional modulation of the beam resulted in amplitude and frequency modulation of the output signal from the electron tube. This modulation of the output signal is called heater hum.

In the past, in order to reduce this heater hum present on the output signal of an electron tube, cathode heaters have been constructed in the manner shown in FIG. 2. A heater 23 was wound in the form of a relatively large diameter bifilar helical coil, with the current in adjacent turns of the bifilar coil flowing in opposite directions and disposed within a hollow cathode body. At the end of the coil nearest a concave cathode emissive surface or button 24 closing off one end of the hollow cathode body, adjacent turns of the bifilar coil were connected by means of a. turn around loop 25. Since the wire of the heater 23 was wound in the form of a bifilar coil, the magnetic field set up by the current flowing in one turn of the coil was largely neutralized by the magnetic field set up by the oppositely flowing current in the adjacent turn of the coil as viewed from a great distance or a position an equal distance from both turns. However, the electrons being emitted from the cathode were closer to the end turn of the coil heater 23 than to any other turn, and, therefore, the magnetic field of this end turn was not completely neutralized. Also, theturn-around loop 25, which was extremely close to the cathode surface, was not neutralized by any adjacent parallel loop. Thus, there remained certain time varying magnetic fields which directionally modulated the electron beam.

In an attempt to reduce the heater hum produced by the heater shown in FIG. 2, an improved heater 26 was constructed in the manner shown in FIG. 3. A single wire conductor 27 was wound into a tight coil, and this coil was wound in the form of a bifilar coil, thereby producing what will be referred to as a coiled bifilar coil. Again as in the structure of FIG. 2, from a great distance or from a position an equal distance from two of the turns of the bifilar coil the magnetic field of the individual turns would be neutralized, but the electrons being emitted from a concave cathode emitter surface 28 were closer to the end turn of the bifilar and close to an uncompensated turn-around loop 29.

In spite of the manner in which the coils of FIGS. 2 and 3 were wound, the amount of residual heater hum was still harmful and in many instances made electron tubes in which they were placed unacceptable. Actual tests on scaled up versions of the structures of FIGS. 2 and 3 have shown that these prior art heaters produced reasonably large magnetic fields at cathodes. This is illustrated in FIGS. 6-9. For the heater coils according to FIGS. 2 and 3, respectively, FIGS. 6 and 8 show the magnetic field profile measured by an induction coil placed at different points along several different lines in the same plane perpendicular to the heater axis and at the cathode end thereof.

FIGS. 7 and 9 show the magnetic field profile along two lines across the concave emission surface of the spectively. As can be seen from these profiles the magnetic effects of the heater coils of FIGS. 2 and 3 are quite substantial. Not only did the heater coils 23 and 26 produce large average magnetic fields, but also the un neutralized turn-around loops 24 and 29 in the structures of FIGS. 2 and 3, respectively, produced high local fields which adversely affected cathode operation.

Referring now to FIG. 4, there is shown a heater coil 30 according to the present invention. A wire conductor 31 as of, for example, tungsten is wound in the form of a relatively tightly wound bifilar helical coil, and the two

separate strands

32 and 33 of this bifilar coil are connected together at one end of the heater by a turnaround loop 34 so that electrical currentfiowing in the -wire 31 will-flow in opposite directions in adjacent turns of the

separate strands

32 and 33. The tightly wound bifilar coil is itself formed into a helical coil to produce what is herein referred to as a tightly wound bifilar coilcoil.

The tightly wound bifilar coil-coil is positioned coaxial with a concave cathode 36 which is to be heated. The turn-around loop 34 which is at the end of the bifilar coil-coil closest the cathode 36 is most advantageously positioned on the outside of the bifilar coil furthest from the cathode emitter surface and in a plane perpendicular to the emission surface of the cathode. By this construction the strongest fields set up by the loop 34 do not thread the cathode emitting surface.

The wire 31 can be wound conveniently in the form of a tightly wound bifilar coil by starting with the mid-portion of the wire 31, doubling the wire upon itself to form two

strands

32 and 33, and winding the two

strands

32 and 33 on a mandrel 35 as shown in FIG. 2. Then the tightly wound bififilar coil and the mandrel 35 are helically wound on a second larger diameter mandrel .to form the wire 31 into a bifilar coil-coil. The wire 31 is then annealed in a furnace, and subsequently-the mandrels are etched away leaving the bifilar coil-coil wire. The wire 31 in then coated with insulation to complete the heater 30. Typically, when the heater is assembled into the cathode assembly, some portion of the tightly wound bifilar coil-coil heater is in direct contact with the back surface of the cathode 36.

Advantageously, in the heater according to the present invention, the coated wire turns of the bifilar coil are within three wire diameters, of one another and are preferably less than one wire diameter from one another for good neutralization of the magnetic effects of individual turns. The term tightly wound is used herein to designate a wire coil whose adjacent turns are within three wire diameters of each other.

A heater according to the present invention does not effect the electron beam emitted from the cathode to the same degree as did prior art heaters. Since current flows in opposite directions in the adjacent turns of the

strands

32 and 33 in the bifilar coil, the magnetic effects of one turn are neutralized by those of the adjacent turns. Therefore, since the bifilar coil is itself coiled in the form of a larger diameter helical coil and all of the individual turns of the bifilar coil which make up the end turn of a larger diameter coil are neutralized, the heater 30 will not produce large magnetic fields in the cathode region. Also, since the wire is first wound in a bifilar coil, the size of the turn-around loop 34 at the end of the bifilar coil is much smaller than the loops of prior art heaters thereby reducing the effect which the heater 30 has upon the cathode. Furthermore, the magnetic effects of the loop 34 on the cathode 36 are reduced to an even greater extentv by the position of the turn-around loop 34. With the loop 34 positioned on the side of the bifilar coil furthest from the cathode emitter surface 36 and in a plane which is perpendicular to the emitting surface of the cathode 36, the strongest field set up by the loop do not thread the cathode emitting surface.

The magnetic field profile produced by the novel tightly wound bifilar coil-coil heater is illustrated in FIGS. 10 and 11. As can be seen from FIGS. 6-11, the magnetic field produced by the novel bifilar coil-coil heater, according to the present invention, is at least an order of magnitude less than the magnetic fields produced by the other structures.

By way of example, a heater according to the present invention for a 1 watt output, C band, reflex klystron tube as shown in FIG. 15 is made from a length of tungsten wire about .0045 in diameter. This wire is wound on a primary mandrel .015" in diameter, beginning with the mid-portion of the wire as the turn-around loop 34 and with the equivalent wire-to-wire spacing onthe primary mandrel being 72 turns per inch. The long bifilar coil is then coiled into a larger diameter two turn coil .250" in diameter and .125" long by using a second mandrel. After the bifilar coil-coil wire has been annealed and the mandrels removed, the wire is coated with cataphoretic aluminum Oxide (Al O to produce a coated wire .009".O11" in diameter. The reflex tube is shown in FIG. and includes a nnipotential cathode assembly as shown in FIG. 4A with an emitting surface 36 and a bifilar coil-coil heater 30. A centrally apertured anode is spaced from said cathode 36 for forming and projecting a beam of electrons into an elongated beam path. A target electrode or reflector 61 is disposed at the end of the beam path for reflecting the electrons of the beam back toward the cathode 36. A cavity resonator 62, apertured for passage of the beam, is disposed in the beam path intermediate the cathode 36 and reflector 61 for electromagnetic interaction with the beam in the well-known conventional reflex klystron manner. Portions of the beam, after being reflected back through the resonator v62, are reflected from the cathode back to and collected on the anode 60. RF. power is abstracted from the cavity resonator 62 by a coupling loop 63 for transmission to a suitable load (not shown) by a coaxial line 64. A suitable vacuum envelope as of glass or metal encloses the tube elements and is evacuated to a low pressure as of 10- millimeters of mercury. D.C. operating potentials are supplied to the tube by a suitable power supply 66. AC. current is supplied to the cathode heater via leads 31 from a suitable A.C. current supply 67 such as a 60 cycle filament transformer.

In actual tests of operating C band reflex klystron tubes, producing 1 watt of output power, initial frequency modulation heater hum on a group of tubes using the type of heater shown in FIG. 4 average 9.9 kc. peak to peak, whereas the average in tubes using the type of heater shown in FIG. 2 was 43 kc. peak to peak. After optimizing the tubes for power and beam current, final heater hum for tubes using FIG. 4 type heaters averaged 5.2 kc. peak to peak, whereas the tubes using FIG. 2 type heaters average 25 kc. peak to peak. Also, in attempting to lower heater hum on tubes with FIG. 4 type heaters by cocking the cathode with respect to the tube axis, a process normally used on prior tubes using FIG. 2 type heaters, it was found the tubes using FIG. 4 type heaters were much less sensitive to cathode alignment. This allows hysteresis in reflex klystrons and beam misalignment in electron tubes to be corrected without seriously affecting heater hum.

In these tests it was also found that for FIG. 2 type heaters there was a relation between heater hum and the percent of peak power (heater hum as a function of the position on the operating mode of the reflex klystron) whereas this elfect is minimized on tubes with FIG. 4 type heaters. This phenomenon is illustrated in FIG. 12 which is a graph showing percentage of output power plotted against frequency deviation for two reflex klystron tubes, one with a prior art FIG. 2 type heater and one with a FIG. 4 type heater. For FIG. 2 type heaters a difference of up to 56 kc. could be found, depending where on the mode the hum was read. Also, in FIG. 2 type heaters, the amount of heater hum is not symmetrical with respect to the peak of the operating mode, whereas the amount of heater hum in FIG. 4 type heaters is symmetrical. This means that the performance of tubes using heaters according to the present invention is better, since the hum is lower, and not as critical, since operation need not be at a certain position on the mode.

Referring now to FIGS. 13 and 14, there are shown alternative embodiments of the present invention. In FIG. 13 a heater 41 is wound in the form of a tightly wound bifilar coil-coil similar to that shown in FIG. 4. However, instead of a turn-around loop as shown in FIG. 4, the ends of the two

strands

42 and 43 of the bifilar coil are returned axially down the heater and are connected to the heater current leads in such a manner that 6 current flows in opposite directions in the two

strands

42 and 43 of the heater 41. In FIG. 13 the heater is shown as used to heat a cylindrical cathode 44 supported on a sleeve 45 in a magnetron.

In FIG. 14 is shown a plan view of a tightly. wound 4 bifilar coil-coil heater 51 according to the present invention wherein the tightly wound bifilar coil is wound in a larger diameter fiat coil. The flat coil can take the form of a double spiral pancake coil heater as shown on the back surface of a cathode button 52. The bifilar coil could be wound in other forms such as, for example, a zigzag (not shown) or single spiral (not shown) pancake coil. The adjacent turns of the bifilar coil neutralize the magnetic elfect of each other. The turn-around loop can be at either end of the flat heater and can be positioned away from the cathode. type shown in FIGS. 4 and 14, a flat heater as shown in FIG. 14 is not as desirable as a helical heater as shown in FIG. 4, since the cathode is exposed to the magnetic field from the entire length of a flat bifilar ooil-coil heater, whereas the cathode is only exposed to the end turn of the larger diameter coil in the helical bifilar coil-coil heater.

By providing a turn-around loop in certain of the heaters, according to the present invention described above, the heater leads for the bifilar coil-coil heater can be closely spaced for ease in fabricating electron tubes.

Under the proper circumstances filamentary wire conductor cathodes can be constructed in a bifilar coil-coil manner according to the present invention if it is possible to space the turns of the filamentary wire conductor apart by such a distance so as to prevent short circuits between adjacent turns of the bifilar coil-coil.

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. An electron gun comprising, in combination, a unipotential thermionic cathode emitter body having an electron emissive surface for emitting electrons when heated, heater means disposed in heat exchanging relationship with said cathode body for heating said emitting surface of said cathode to electron emitting temperature, said heater means including a first helical wire strand and a second helical wire strand, said first and second helical wire strands wound bifilar to form a first bifilar helical coil, said first bifilar helical coil formed int-o a second coil having a larger diameter than said first coil, and said heater means being provided with a pair of heater lead wires with said first and second wire strands of said heater means each connected in parallel and the adjacent ends of said first and second wire strands being connected to opposite heater lead wires to pass current in opposite directions in the separate strands of said bifilar heater, whereby for the electrons leaving said cathode the magnetic field set up by said current passing through said one wire strand substantially neutralizes the magnetic field set up by the current passing through the other wire strand.

2. An electron gun assembly including, a unipotential cylindrical thermionic cathode emitter body having an inwardly dished wall portion closing off one end of said cylindrical emitter body, said inwardly dished wall portion being provided with an electron emissive surface on the concave outside surface thereof for emitting a stream of electrons when heated, heater means disposed in heat exchanging relationship within said cylindrical cathode body for heating said inwardly dished cathode wall portion to electron emission temperature, said heater means including first and second helical wire strands wound bifilar and connected in series by a connecting loop to However, for cathodes of the form a first bifilar helical coil adapted to pass current in opposite directions in the separate strands' thereof, said first bifilar helical coil formed into a cylindrical coil having a larger diameter than said first bifilar coil, and said connecting loop being disposed at the end of said cylindrical coil adjacent said dished wall portion with said loop defining a plane which is substantially perpendicular to the dished emission surface of said cathode emitter body, whereby for the electrons leaving said cathode the magnetic field set up by current passing through one wire strand substantially neutralizes the magnetic field set up by the current passing through the other wire strand.

8 References Cited by the Examiner UNITED STATES PATENTS 1,797,990 3/ 1931 Lucian 313-337 1,865,442 7/1932 Parrott 313342 X 2,394,474 2/1946 Peters 3 13344 2,406,850 9/1946 Pierce 313344 X 2,922,916 1/1960 Sharkey 313-344 X 3,003,077 10/1961 Thomas 313344 X GEORGE N. WESTBY, Examiner.

Claims

1. AN ELECTRON GUN COMPRISING, IN COMBINATION, A UNIPOTENTIAL THERMIONIC CATHODE EMITTER BODY HAVING AN ELECTRON EMISSIVE SURFACE FOR EMITTING ELECTRONS WHEN HEATED, HEATER MEANS DISPOSED IN HEAT EXCHANGING RELATIONSHIP WITH SAID CATHODE BODY FOR HEATING SAID EMITTING SURFACE OF SAID CATHODE TO ELECTRON EMITTING TEMPERATURE, SAID HEATER MEANS INCLUDING A FIRST HELICAL WIRE STRAND AND A SECOND HELICAL WIRE STRAND, SAID FIRST AND SECOND HELICAL WIRE STRANDS WOUND BIFILAR TO FORM A FIRST BIFILAR HELICAL COIL, SAID FIRST BIFILAR HELICAL COIL FORMED INTO A SECOND COIL HAVING A LARGER DIAMETER THAN SAID FIRST COIL, AND SAID HEATER MEANS BEING PROVIDED WITH A PAIR OF HEATER LEAD WIRES WITH SAID FIRST AND SECOND WIRE STRANDS OF SAID HEATER MEANS EACH CONNECTED IN PARALLEL AND THE ADJACENT ENDS OF SAID FIRST AND SECOND WIRE STRANDS BEING CONNECTED TO OPPOSITE HEATER LEAD WIRES TO PASS CURRENT IN OPPOSITE DIRECTIONS IN THE SEPARATE STRANDS OF SAID BIFILAR HEATER, WHEREBY FOR THE ELECTRONS LEAVING SAID CATHODE THE MAGNETIC FIELD SET UP BY SAID CURRENT PASSING THROUGH SAID ONE WIRE STRAND SUBSTANTIALLY NEUTRALIZES THE MAGNETIC FIELD SET UP BY THE CURRENT PASSING THROUGH THE OTHER WIRE STRAND.