US3210602A - Traveling wave crossed-field electron tube with specific grid construction - Google Patents

Description

1965 J. E. ORR 3,210, 02

TRAVELING WAVE CROSSED-FIELD ELECTRON TUBE WITH SPECIFIC GRID CONSTRUCTION Filed Dec. 21, 1960 2 Sheets-$heet l Oct. 5, 1965 J. E. ORR 3,210,602

TRAVELING WAVE CROSSED-FIELD ELECTRON TUBE WITH SPECIFIC GRID CONSTRUCTION Filed Dec. 21. 1960 2 Sheets-Sheet 2 Friar 4H i 150 1 g f J00 v i 74 i 7 1 l 0 l I I 4000 -1800 -100 4400 11200 -1fld0 400 60a 4 400 0 Jamar 6". Orr

% QT/M'W" United States Patent 3,210,602 TRAVELING WAVE CROSSED-FIELD ELECTRON TUBE WITH @PECIFIC GRED CONSTRUCTION James E. Orr, Redwood City, Calif., assignor to Litton Precision Products, Inc., a corporation of Delaware Filed Dec. 21, 1960, Ser. No. 77,403 1 Claim. (Cl. 31539.3)

This invention relates to crossed-field devices employing a special grid for providing a high amplification factor, or mu, and more particularly to a high-mu grid for crossed-field beam-type microwave frequency oscillators and amplifiers to control the beam current thereof while concomitantly providing an arrangement for shaping the beam.

In modern high frequency radar applications, as well as in numerous other applications, there has long been a need for crossed electric and magnetic field oscillators and amplifiers with arrangements for shaping an electron beam and especially for controlling the beam current with an unusually small grid voltage swing. In accordance with the teachings of the prior art, oscillator and amplifier devices of the crossed field type have employed cylindrical grid structures or equivalent rectangular structures for controlling the beam current; however, the grid devices available have been severely restricted in their ability to meet this long standing need since they require a comparatively large voltage swing to effectively control the beam current.

In the prior art, typical crossed-field oscillator or amplifier devices have been constructed to include several basic elements, namely, a slow-wave structure, such as an interdigital line, an associated sole electrode, a cathode, a grid electrode, an accelerating electrode, and apparatus for supplying a static magnetic field. Generally, the construction of these devices is such that the sole and the slowwave structure are both circular and concentric with one another, forming an electron and electromagnetic wave interaction space therebetween. Moreover, there is also applied between the sole and slow-wave structure a potential difierence so that there is provided an electrostatic field within the interaction space of the devices which is perpendicular to the static magnetic field. These crossed electrostatic and magnetic fields cause an electron beam generated by the cathode, grid and accelerator to flow through the interaction space in energy coupled relationship with the slow-wave structure and the traveling wave energy propagating in the slow-wave structure to provide for the generation of oscillations or the amplification of an input signal.

Heretofore, there has been a long standing belief by those versed in the cross-field tube art, that it was virtually impossible to provide a crossed-field oscillator or amplifier with a grid having a portion of its structure disposed across the path of the electron beam while being adapted for use as an effective high current beam control requiring only a small voltage swing for such control. The apparent reasons for such a concept were, firstly, the belief that a grid which substantially enclosed the cathode would intercept a large percent of the electron beam thereby generating large currents in the grid circuit or possibly burning up the grid itself. The generation of large grid currents would in turn require higher power voltage supplies which are costly and generally large in physical size than would be desirable for airborne applications, while the generation of high temperatures would require suitable cooling means for the grid structure. Secondly, it was considered impossible to place a grid structure, such as a mesh screen for example, adjacent the cathode across the path of the electron beam without seriously distorting or destroying the electron optics of the device. This was 3,210,602 Patented Oct. 5, 1965 considered to represent a special problem in view of the crossed electric and magnetic fields which normally produce complex curved electron trajectories. The reasoning was that any slight distortion of the electron optics would cause the beam to spread appreciably or cause the beam to enter the interaction space in an adverse manner. With an appreciable spreading of the beam, many of the electrons may assume the undesired cycloidal trajectories characteristic of electrons in crossed electric and mag netic fields. It was then concluded that either or both of these effects would cause poor interaction with an electromagnetic wave propagating in the slow-wave circuit of the device, thereby resulting in poor efficiency and other undesirable eftects, such as high sole current for example.

Finally, it was considered impractical if not impossible to produce a grid structure which could be reproduced in a substantially identical form from grid to grid. The difliculty in reproducing grids of the same configuration arose to a great degree from the fact that the major eifort in the field to provide a satisfactory grid was: centered about using inexpensive conventional grid-making techniques, with apparent disregard for the possible use of more so phisticated techniques, such as photo-etching for example. As a consequence, attempts to utilize conventional grid structures and construction techniques, such as the techniques employed in the receiving tube gridmaking art wherein a plurality of small wires are combined to form a basket weave mesh and the like, for example, failed to produce a reproducible grid having the extremely high precision dimensions which are required. In summation, it may be stated that the apparent inertia in the thinking of those versed in the art, predicated upon the views set forth hereinabove, created an unfortunate hindrance to a practical and effective solution to the problem.

The present invention overcomes the foregoing enumerated and other limitations of the prior art grid controlled beam current microwave frequency devices by providing an improved high mu grid control device which can be mechanized and utilized in a simplified and economic manner in comparison with prior art devices. More particularly, in the device of the present invention, a high mu grid is utilized whereby a relatively high current electron beam is turned off and on in response to a small signal imposed upon the grid electrode. Furthermore, the grid electrode is disposed across the path of the electron beam in such a manner to avoid disturbing the electron optics of the oscillator or amplifier in which it may be employed.

In accordance with an illustrative embodiment of the present invention, the grid control device comprises a substantially cylindrical body having a plurality of openings formed in the wall of the body along the length of the body. The cylindrical body concentrically surrounds an associated cathode, with the aforesaid openings confronting an associated accelerator electrode which is adjacent the openings in the grid body. The size and shape of the openings formed in the grid body and the relative space relationship of the body with respect to the cathode, accelerator and associated slow-wave structure is chosen to provide optimum electron optics for the oscillator or amplifier tubes in which the grid is utilized, notwithstanding the fact that part of the grid structure lies across the path of the electron beam. Specifically, the grid structure is not of the conventional basket-weave form, but is formed of .a single element of metal in which the grid wires all lie in the surface of the metal, thus avoiding electric field irregularities in the vicinity of the grid. Moreover, the configuration of the grid and its space relationship with respect to the other elements of the tube, enable the grid device to effectively control the electron beam during the operation of the tube when a signal of unusually low voltage swing is imposed upon the grid.

The phrase high mu grid may be used in the specification and claims, and as so used is intended to cover the element or grid structure comprising a continuous surface, such as a modified cylinder for example, having a plurality of openings formed in the wall thereof, or any structure of equivalent scope which is capable of increasing the amplification factor of a device in which it is employed.

In order to more fully appreciate the function of a high mu grid a short review of the theory relating to the factors responsible for the amplification capabilities of a grid will be given, with reference to the theory of triodes employing grids and as applied to crossed field devices, in the absence of a magnetic field and ignoring the initial velocity of electrons of the beam. In a conventional low frequency triode the grip control characteristics may be analyzed by considering the following equation, which results when the initial velocity of the electrons and the presence of a magnetic field are neglected:

3/2 raw...)

where l zbeam current V zgrid to cathode potential V zaccelerator to cathode potential C zgrid to cathode capacitance C zaccelerator to cathode capacitance Pzconstant which depends upon the geometry of the device It can be seen from the above equation that it is desirable to have the coefficient C /C as large as possible so that a given incremental change in V will have a substantially greater effect upon the value of I than would occur in prior art devices. Stated differently, if the ratio of C /C is for example .35 for the conventional prior art device and is 3.5 for one representative embodiment of the present invention, a given incremental change in V will have a ten times greater effect on I thereby providing a device in which much more control of the beam current may be obtained by changing V Consequently, it may be said that the amplification factor commonly known as mu (a) associated with the grid must be large in a device if the device is to be used as a grid controllable device. From the discussion hereinabove it will be obvious to those versed in the electron tube art that the following equations may be derived as applicable characteristics of a grid employed in a tube:

where is a function of the geometry of the electron gun.

In accordance with the present invention, it has been determined that the effect of the magnetic field may be neglected provided the grid-to-cathode spacing is small in comparison with the grid-to-accelerator spacing. Under this arrangement the electrons will arrive at the grid and pass therethrough along a path which is substantially perpendicular to the surface of the grid, and therefore is equivalent to the condition which occurs in the conventional triode tube. The mu of the vacuum tube construction of the present invention may therefore be validly predicted by triode equations. And this is true despite the fact that the trajectory of the electrons after they pass the surface of the grid in flight toward the interaction space is greatly affected by the magnetic field, the electron optics, the electric fields in gun region and other factors which are considered in the design of crossed field devices of the prior art.

It is therefore, an object of the invention to provide an improved grid electrode adapted to operate in crossedfield traveling wave microwave frequency devices.

Another object of the invention is to increase the amplification factor of grid-controlled crossed-field microwave frequency amplifier or oscillator devices, over a wide beam current range.

A further object of the invention is to provide an improved grid electrode adapted to operate in crossed-field traveling wave devices without adversely affecting the electron optics and efficiency of the device.

Still another object of the invention is to provide an improved grid electrode adapted to provide a cathode to grid capacitance which is substantially larger than the cathode to accelerator capacitance, without adversely affecting the electron optics and efficiency of the device.

The novel features which are believed to be characteristic of the invention both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings are for purpose of illustrating and description only, and are not intended as a definition of the limits of the invention.

FIGURE 1 is a schematic diagram illustrating the manner in which a high mu grid may be employed with crossedfield oscillators, in accordance with the invention;

FIGURE 2 is a fragmentary isometric view of the high mu grid shown in FIGURE 1, illustrating its configuration and spaced relationship with respect to an associated cathode and accelerator electrode;

FIGURE 3 is an isometric view of another embodiment of the high mu grid shown in FIGURE 1, illustrating the configuration of a mesh grid structure;

FIGURE 4 is a fragmentary isometric view of a conventional grid employed with a well-known type of prior art crossed-field traveling wave device; and

FIGURE 5 is a typical plot of beam current versus grid voltage characteristics of a prior art crossed-field traveling wave oscillator employing a conventional grid electrode and a crossed-field traveling wave oscillator employing a high mu grid in accordance with the present invention, illustrating a comparison in the performance of the two devices.

With reference now to the drawings, wherein the same reference characters designate like or corresponding parts throughout the several views, there is shown in FIGURE 1 a schematic diagram of an M-type backward wave oscillator, generally designated 10, which has associated therewith a steady biasing magnetic field descriptively illustrated by the encircled crosses showing that the field is perpendicular to plane of the paper, and a number of associated potential sources which are applied to the various elements of the oscillator. As shown in FIGURE 1, the oscillator includes a cathode 12, a beam forming and control electrode 14, hereinafter referred to as a high mu grid electrode, circumjacent to the cathode, an accelerator electrode 16 adjacent the grid and cathode and forming therewith an electron gun for the oscillator, a sole electrode 18 adjacent the cathode and grid, and a slow wave structure 20 positioned adjacent the accelerator at the output end of the device and parallel to the sole and forming an electron and electromagnetic wave interaction space 22 therebetween. In addition, there is also provided an output means 24, which may have any design suitable for transferring or extracting microwave energy generated within the oscillating system to an external load circuit. The magnetic source is further indicated by the dashed lines 30 which represent one of the: two pole pieces of a permanent magnet which provide magnetic flux lines within the electron gun region and the interaction space of the device normal to the plane of the drawing. The oscillator is completed by an attenuator 26 which is disposed at one end of the delay line for absorbing high frequency reflected signals on the slow;

wave line, and a collector 28 disposed at the remote end of the interaction space for collecting residuary electrons which do not impinge upon the slow wave structure or the sole electrode. For purposes of simplicity the evacuated tube envelope is also indicated by the dashed lines 30.

It should be noted at this point that the oscillator shown in the accompanying drawings is in the form of a linear device for clarity of understanding. However, in practice these devices are constructed preferably in a circular configuration to provide a device of smaller physical size and of more rugged construction. When a circular configuration is employed the magnetic and electric fields are adjusted in accordance with known electron optics principles to provide a generally circular trajectory for the electron beam.

Turning now to the details of the elements of the electron gun shown in FIGURE 1, as shown drawn to a larger scale, in FIGURE 2, it can be seen that the electron gun comprises three basic elements, the cathode, grid and accelerator. The gun has a unique feature which distinguishes it from that of the prior art gun illustrated in FIGURE 4, namely, the use of high mu grid electrode 14. As shown in FIGURE 4, a grid electrode 14' of the prior art superficially resembles grid 14 of FIGURE 2, but is radically different in its capabilities. The primary distinction between the two elements is the addition of a perforated or photo-etched wall portion 32 adjacent and between cathode 12 and accelerator 16.

Consider now the techniques employed through which grid structure 14 is formed such that the wire size and configuration will be substantially the same from grid to grid. First, a thin sheet of grid material, such as .003 inch thick molybdenum for example, has a series of openings formed therein by photo-etching, each having a predetermined width and separation therebetween. The slots formed in the sheet may be in the order of .070 inch long and .030 inch wide being separated by wirelike sections .003 inch wide. Secondly, the sheet, with the slots formed therein, is wrapped around a high precision forming mandrel and heat treated to permanently set the peripheral configuration of the grid. Thirdly, the two parallel free ends of the sheet formed along the length of grid body are fixedly connected as by brazing to provide a mechanically strong and electrically continuous conductive structure. Finally, the grid body formed is removed from the forming mandrel having a cylindrical configuration, such as that shown in FIGURE 2. It has been found that the formation of grid structures in this manner insures uniformity in the electrical characteristics from grid to grid.

It should be noted at this point that the grid structure 14 may have an arrangement of openings formed therein which is different from that shown in FIGURE 2. Referring to FIGURE 3, there is shown an alternative form of grid having a plurality of openings 38 therein which constitute a mesh-like arrangement. The mesh-like configuration shown in FIGURE 3, is especially useful in low frequency devices wherein the elements, including the grid, are larger in size. The disadvantage of the form shown in FIGURE 2 for low frequency operation arises from the fact that wires 34 would have to span too great a distance to insure mechanical strength, and the slot-like openings would be too great to provide effective shielding of the cathode from the accelerator. In contrast, the structure in FIGURE 3 provides both the required mechanical strength and effective cathode to accelerator shielding.

criteria for a given design is known, known mathematical techniques and experimental data are employed to design jigs and fixtures which will enable the cathode, grid and accelerator to be placed in the correct space relationship with one another. It should be noted that grid wires 34 lie between the emitting surface of cathode 12 and accelerator 16, and tends to shield the cathode from the accelerator. The degree of shielding provided by the grid wires is determined by the size of the grid wires and the relative spacing therebetween. Thus by suitable selection of size and spacing of wires 34 it is possible to provide any predetermined shielding desired and thereby control the capacitance between the gun elements.

The capability for obtaining preselected cathodetogrid and cathode-to-accelerator capacitances to provide a ratio C /C greater than unity according to the teachings of the present invention is best illustrated by way of an example. It has been found, for example, that a crossed field oscillator operable in the C-band frequency range which has a cathode of 0.140 inch diameter, a grid of 0.156 inch inside diameter, a cathode-to-accelerator separation of 0.057 inch, a grid-to-accelerator separation of 0.052 inch, and openings in the grid structure which are 0.030 inch wide by 0.070 inch long and having wires 0.003 inch wide, is capable of providing a ratio of C /C greater than unity, that is, where mu is about 4.5. A mu of 4.5 for the present invention is considerably larger than the mu of 0.35 for prior art cathode-grid-accelerator arrangements of the type shown in FIGURE 4. The size of cathorde 12' and grid 14' of FIGURE 4- is the same as cathode 12 and grid 14 of the present invention, shown in FIGURE 2. However, there is a difference in struc tural configuration which arises from the fact that cathode 12' is under-cut by 0.004 inch to provide a resultant chordal surface 38 which is disposed 0.061 inch from accelerator 16; and grid 14 does not have a plurality of openings formed in the body of the structure, and therefore is incapable of shielding the cathode from the accelerator as is.required to provide a low cathode-toaccelerator capacitance or which does not have the added structural elements of the grid required to bring the cathode closer to the grid to provide a high cathode-to-grid capacitance.

As noted above, the grid is spaced from the cathode by a distance of 0.008 inch and is concentric with the cathode. Furthermore, the elemental portions of the grid in front of the cathode lie in a single smooth surface, unlike the irregular basket-weave grid structures, which are often employed in conventional tubes. The factors of the close spacing of the grid to the cathode, and the absence of local variations in the geometry of the grid are important in avoiding dispersion of the beam. For normal levels of magnetic field strength and conventional crossed'field tube geometries, the grid is advantageously within 0.030 inch and preferably within 0.015 inch of the cathode, to control the beam before the electrons are appreciably deflected from their initial paths, perpendicular to the cathode surface. With weaker magnetic fields or different tube geometries, a greater spacing may be employed.

Continuing with the description of the elements of the electron gun and the characteristics thereof, it will be appreciated that the capacitance between cathode 12 and grid 14 (C and between cathode 12 and accelerator 16 (C are determined and controlled by the spacing therebetween. Thus, as discussed hereinabove, the amplification factor (,u) of the device may be increased simply by changing the size and spacing of the grid wires and the spacing between the accelerator and grid. Stated differently, by making the grid to cathode spacing and the grid wire spacing small (C can be made large, and by making the grid to accelerator spacing large (C can be made small, so that one can obtain greater grid control of the beam when the ratio of C /C is substantially greater than unity.

It should be noted at this point that the electron gun in accordance with the present invention is designed in two parts, firstly, the elements thereof are arranged to provide an optimum amplification factor ignoring the magnetic effects; then the effects of the magnetic field are taken into account and the electron optics of the device are optimized. In order to optimize the electron optics, it is assumed that the high speed electrons pass through the grid openings in substantially a straight line. This assumption is valid, since the spacing between the cathode and grid wires is very small in comparison with the path of curvature of the beam beyond the plane of the grid openings. Thus, the second part of the gun design is predicated upon the electrons leaving the grid with a predetermined velocity and traveling in a straight line, and the periphery of the grid structure is used as the plane of reference for initial emission. It has been found that once these two aspects of the design are completed through the use of known mathematical techniques and experimentation they may be combined to produce a device providing amplification characteristics consistent with the present invention.

Consider now the electrical operation of a crossed-field backward wave oscillator device in which there is provided a substantial increase in the amplification factor of the device as taught in the present invention. Operation of the tube is based upon the interaction which will occur between the electron beam and the wave energy propagating in the slow-wave structure in the backward direction. It is expressly understood that the invention is not limited in its application to backward Wave oscillators, but is also applicable to other crossed-field electron tubes.

In operation, cathode i2 is activated to provide a stream of electrons which are drawn toward the accelerator under the influence of a positive potential applied to accelerator 16. As the electrons leave cathode 12, grid 14 functions as an electrode which helps to form an electron beam which is under the influence of the crossed electric and magnetic fields in the electron gun region. The electron beam initially follows a substantially straight path until it passes grid 14 and then follows a cycloidal trajectory. At the peak of the cycloid, the electrons are injected in the interaction space 22 between sole electrode 18 and slow-wave structure 20. For a given set of electric fields in the gun region and in the interaction space these electrons are injected into the interaction space in tight coupling relationship with the wave energy propagating along the slow-wave structure.

During continuous wave (CW) operation the power output of the device may be amplitude modulated very effectively by modulating the beam current, which may be accomplished by imposing a modulating potential between the grid and cathode. With reference to FIGURE 5, curves D, E, and F are plots of operating data for beam current (I versus grid voltage (E for typical devices according to the invention.

It can be seen by referring to curve E, for example, that it is possible to control the beam current from the start of oscillation at a point corresponding to about 80 milliamperes, to its normal operating current of 250 milliamperes, by varying the grid voltage from about 325 volts to 100 volts. In contrast, curve B of the same drawing illustrates the performance of a similar device employing a prior art arrangement such as that shown in FIGURE 4, and shows that the voltage swing required to control the beam current over the same current range as the present invention is about 1500 volts, or about 7 times as much as that required by the present invention.

It will be apparent to those versed in the amplifier and oscillator tube art, that the grid of the present invention may be employed in such devices when pulse operation is required. From the discussion above, regarding CW operation and curve E, it becomes obvious that one could impose a pulse signal of 400 volts for example, between the grid and cathode, and stop the tube from oscillating and swing the grid voltage back to voltage for full pulse power output. In a similar manner it would require more than a l500-volt swing to pulse a prior art grid device to obtain similar pulse-type operation.

Consider now the mechanical and electrical advantages which are derived through the use of the novel form of grid control electrode in accordance with the teachings of the invention. Firstly, from a mechanical point of view, the photo-etched grid structure has the advantage of a structure which is capable of being reproduced in large quantities economically, while having substantially identical physical configuration from grid to grid thereby providing uniformity of electrical characteristics in the grids when they are employed in an oscillator or amplifier. In addition, the closed continuous surface of the present grid, as opposed to the open structure of the prior art, is physically more rugged than the prior art grids thereby providing a structure which will not readily be distorted by handling during the fabrication of a device which may employ it, nor will it be distorted or deranged by the heat generated in the oscillator or amplifier devices during their operation.

Secondly, from an electrical point of view, the configuration and space relationship of the grid with respect to the other elements in the gun region enables the amplification factor of the device in which the grid is employed to be established at a preselected value to provide optimum beam control, with a minimum grid voltage swing for such control. Moreover, minimizing the grid voltage swing eliminates the necessity for large power supplies required to handle large voltage swings, such as that required in the prior art for example. For airborne applications where size and weight are important, the reduction in the required power supply is particularly advantageous. In addition, a reduction in the size of a power supply usually means a saving in cost, since the large power supplies are generally more expensive than the smaller ones.

Still another electrical advantage arises from the fact that the grid, notwithstanding the fact that it is disposed across the path of the electron beam, does not significantly affect the electron optics of the tube, contrary to the general belief of those skilled in the art. Furthermore, the device of the present invention is as efiicient, if not more so than similar prior art devices. Finally, since the grid generally is biased negative or slightly positively with re spect to the cathode, there is provided the advantage of generating only minimum a amount of grid current, creating very little heat which could damage the grid wires by overheating.

While the high mu grid electrode of the invention has ben described with reference to only two particular configurations is a crossed field backward wave oscillator,

it is to be expressly understood, of course, that further alterations and modifications may be made in the grid structures shown and that they also may be utilized in any type of beam-type crossed field device, such as amplifiers for example, without departing from the spirit and scope of the invention. More particularly, it has been found that the high mu grid of the present invention is especially adapted for use in high power crossed-field amplifiers for pulsed operation. In such pulse-type operation the grid is normally biased to cut-0E, that is, no significant amount of the beam is allowed to flow, and the device is triggered into operation by applying a pulse signal which drives the grip positive to permit the required beam current to flow.

Accordingly, it is to be expressly understood that the foregoing description shall be interpreted only as illustrative of the invention and that the spirit and scope of the invention is limited only by the appended claim when accorded the broadest interpretation consistent with the basic concepts taught herein.

What is claimed as new is:

A vacuum tube comprising a cathode, a slow-wave transmission structure, electrode means for accelerating electrons from said cathode, the capacitance between said accelerating electrode and said cathode having a predetermined value, a collector, means for providing a magnetic field generally parallel to the surface of said cathode for directing electrons along a curved trajectory adjacent said slow-wave circuit and toward said collector, and a control grid structure closely spaced to said cathode between said cathode and said accelerating electrode, said control grid structure comprising a generally cylindrical integral hollow metal body having a plurality of substantially uniformly spaced openings formed in the body along the length thereof providing grid Wires being transverse to the direction of said magnetic field wherein the width of each wire is smaller than the opening between adjacent Wires, the capacitance between said grid and said cathode being significantly greater than the said predetermined capacitance.

References Cited by the Examiner UNITED STATES PATENTS Sheer 3l5--39.3 X Chick et a1. 2925.14 Labin 315--39.3 X Pierce 315-39.3

Charles 315-393 Iskenderian 3325 X Willner 29-25.14 Smith.

Dench 31539.3 X Haase 313-348 X Dench 315-39.3 X

15 GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, Examiners.

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