US2793315A - Resistive-inductive wall amplifier tube - Google Patents

Description

BY ya 2'@ A. V. HAEFF ET AL.

RESISTIVE-INDUCTIVE WALL AMPLIFIER TUBE Filed Oct. 1, 1952 May 21, 1957 United States Patent O 2,793,315 REsrsTrvE-INDUCTIVE WALL AMPLIFIER TUBE Andrew V. Haeff, Pacific Palisades, and Charles K. Birdsall, Los Angeles, Calif., assignors, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application October 1, 1952, Serial No. 312,568 13 Claims. (Cl. S15-3.6)

This invention relates to microwave amplifier tubes, and more particularly, to an improved electron stream amplifier tube.

The present invention is directed to an electron stream amplifier tube of the type which is capable of amplifying microwave energy by virtue of the interaction of the electrons of a modulated electron stream with electromagnetic fields produced by currents induced by the modulated electron stream in an impedance wall disposed contiguous to the stream.

The present electron stream amplifier tube may be considered an improvement over that disclosed in a copending application for patent entitled, Electron Stream Amplifier Tube, by Andrew V. Haeff, filed April 12, 1952, Serial No. 282,000, now Patent No. 2,740,917, dated April 3, 1956. In the copending Haeff application, an electron stream amplifier tube is described having three sections in addition to the usual electron gun and collector electrodes. The first section is a relatively short input structure whose function is to transform signal energy into modulations of the electron stream. The second section, referred to as an impedance member, is a structure surrounding the stream and having walls of resistive or inductive material. By way of example, the impedance member may consist of a long piece of glass tubing having a resistive coating on its inner surface, the electron stream being projected through the hollow portion of this structure. The original modulations of the stream are then amplified through interaction between the modulated electron stream and the electric fields produced by the currents induced in the resistive or inductive wall material. The third section of the device is an output structure where the amplified signal energy of the electron stream is converted into a useful output signal.

As emphasized in the Hae application, the resistive or inductive wall, even in its simplest form, does not present a pure resistive impedance to the electron stream, but inherently includes distributed capacitance that is undesirable since it acts as a low impedance path for the modulations of the stream, thus decreasing the available gain.

The present invention discloses a novel electron stream amplifier tube for amplifying microwave signals having substantially no distributed capacitance effects in the resistive wall. Alternatively and, if desired, the tube may be designed to have a resistive-inductive wall, or an inductive wall. This is accomplished by positioning a highly conducting surface, such as a metallic wall, at a distance between an even and an odd multiple of an electrical one-quarter wavelength at the signal frequency behind the resistive surface exposed to the electron stream; that is, the spacing between the conductive and resistive surfaces should be less than one-quarter, or between twoquarters and three-quarters of a wavelength, and so on. In order to decrease the distributed capacitance of the wall structure, or to obtain an inductive impedance effeet, it is necessary that a wave be able to proceed from 2,793,315 Patented May 21, 1957 ICC the resistive surface to the metallic wall or conductive surface and be reflected back to the resistive surface at a velocity slightly less than that of the electron stream without being unduly attenuated.

This is made possible by inserting a slow-wave material between the resistive wall and the reflecting or conductive wall which allows a wave excited by the electron stream to be propagated without appreciable attenuation transversely to the direction of the electron flow. A suitable slow-wave material for use in the tube of the present invention is defined as one wherein electromagnetic waves are propagated through an unbounded sample of the material at a phase velocity equal to or less than the electron stream velocity. In the event such a material is not used, there will be reactive attenuation transverse to the electron flow, similar to propagation in a waveguide below its cut-off frequency. Further, a suitable slow-wave material may be obtained artificially by the choice of a material having suitable dielectric and magnetic characteristics. The effect of this transverse electromagnetic wave traversing such a material, as previously mentioned, is to decrease the effect of the distributed capacitance of the resistive wall or to place an inductive impedance electrically in parallel with the resistive wall; this transverse electromagnetic wave is not to be confused with an axial slow-wave having an axial velocity synchronized with the velocity of the electrons of the stream for the purpose of obtaining amplification.

It is, therefore, an object of this invention to increase the gain of an electron stream amplifier tube by decreasing the distributed capacitance inherent in the resistive wall of the impedance member of the tube.

Another object of this invention is to provide a member for presenting a purely resistive impedance to the electron stream of an electron stream amplifier tube.

An additional object of this invention is to provide a member for presenting a resistive-inductive impedance to the electron stream of an electron stream amplifier tube.

A further object of this invention is to provide a member for presenting a substantially pure inductive impedance to the electron stream of an electron stream amplifier tube.

A still further object of this invention is to provide an electron stream amplifier tube capable of amplifying microwave signal energy of brood bandwidth wherein the admittance presented to the electron stream by the impedance member of the tube is capable of producing optimum amplification with tubes of this type.

The novel features which are believed to be characteristie 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 drawing in which an embodiment of the invention is illustrated, by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a vertical cross sectional view of the tube with associated circuits; and

Figs. 2 and 3 are equivalent circuit diagrams.

Referring to Fig. 1, the amplifier tube comprises an evacuated cylindrical glass envelope 2 with an enlarged portion at the left extremity as viewed in the drawing, which houses an electron gun 3 for producing an electron stream. The electron gun 3 has a cathode 4 with a heater 6, a focusing electrode 8, and an anode 10. Heater 6 is connected across a source of potential, such as a battery 16, the negative terminal of which may be connected to cathode 4, as shown. Cathode 4 and focusing electrode 8 are connected together and are, in turn,

connected to ground. Anode is connected to the movable arm of a potentiometer 24, which is connected across a source of potential 26. the negative terminal of which is connected to ground. Potentiometer 24 is used to adjust the potential applied to anode 10 which functions as a control element for determining the current of the electron stream. A potential of 500 volts with respect to ground is representative of the potential normally applied to anode 10.

Positioned axially about the electron stream in the direction of the electron ow, are a matching ferrule 12 connected by a lead 14 to a conducting input helix 30, an impedance member 38, a conducting output helix 44 connected by a lead to a matching ferrule 46 and a collector 47.

Matching ferrule 12 and input helix 30 are maintained at an appropriate positive potential with respect to ground by connecting it over a conductor 39 and a potentiometer 24 to the positive terminal of a battery 26. A potential of 1000 volts with respect to ground is representative of the voltage normally impressed on ferrule 12 and input helix 30.

Under normal operation, a growing electromagnetic wave employed to modulate the electron stream is propagated on the helix 30 along with the flow of stream electrons. A coating of resistive material 31, such as Aquadag. is applied on the outside of envelope 2 about the last few turns of the helix 30 to terminate this wave. Termination of the wave is effected by the resistive coat ing 31 being inductively coupled to the wave in such a manner that currents induced by the wave flow within the resistive material 31 thereby dissipating the energy of the wave. ln addition, helix 30. being axially aligned with the electron stream emanating from electron gun 3, generally has an inner diameter that is substantially equal to the inner diameter of the ferrule 12 so that the stream electrons pass as close to the helix as possible without being intercepted by the latter. A material, such as tungsten, is suitable for making the helix, the main prerequisite being that the helix retains its form, especially with respect to its pitch and diameter.

An input waveguide 34 is mounted so that lead 14, connecting input helix 30 to electrode 12, is located approximately one-quarter wavelength from termination 36 which comprises a shorting surface adjustable in position by a matching element 35. Lead 14 is also disposed so as to be parallel to the electric field in waveguide 34 to allow maximum transfer of energy from the waveguide to lead 14. The matching element 35 provides a means for adjusting the distance from lead 14 to termination 36 so that the voltage induced in lead 14 is at an optimum value.

Cylindrical collar 18 is concentric with ferrule 12 and extends for a distance of roughly one-quarter Wavelength. Since collar 18 is open-circuited at the farthest extremity with respect to its associated waveguide, an apparent shorting plane is produced at the inner surface of waveguide 34.

A preferred embodiment of impedance member 38 is tubular in form. Impedance member 38 is axially aligned with helix 30, the inside diameters of the two being approximately equal. Impedance member 38 comprises a tubular element 60, a resistive coating 61 deposited on the inner surface of element 6l), and a highly conductive coating 62 deposited on the outer surface of element 60.

Tubular element 60 may be fabricated of several types of materials having suitable electromagnetic wave propagating characteristics. ln general, if VKv represents the voltage (in kilovolts) by which the stream electrons have been accelerated, then the necessary prerequisite for the slow-wave material of tubular element 60 is that the product RER be greater than or equal to wherein y, is the permeability relative to that of free space, and fR is the relative dielectric constant, as measured for propagation transverse to the electron stream direction. In addition to the foregoing, it is desirable that the material used have relatively low conductive losses, particularly in the case of materials having a high dielectric constant. Materials satisfying the above requirements may be divided into three groups, namely, materials having a high dielectric and a low permeability constant; materials having a high permeability constant; and materials having artificially produced dielectric and magnetic characteristics, for example, nonhomogeneous materials.

Included in the class of materials having a high dielectric constant and a low permeability constant are the titanate bodies which include titanium dioxide, calcium titanate, strontium titanate, and barium titanate, the relative dielectric constant of these materials being, respectively, of the order of 100, 150, 250 and 1000. Titanate bodies may be processed so as to have physical characteristics of a ceramic material which may be readily produced in the tubular shape as required for element 60.

Secondly, materials having a high permeability constant at microwave frequencies include ferrites, the general chemical formula for ferrites being R'FezOs, wherein R represents a metal such as magnesium or copper. The permeability of ferrite materials exhibits a magnetic resonance at frequencies which are in the microwave range, thus providing suticiently high values of permeability to make their use practical as a slow-wave material in the present invention. It is preferred to use a material having a high permeability constant to decrease the velocity of an electromagnetic wave in that higher losses can be tolerated than would be in the case with high dielectric constant materials without adversely affecting the impedance.

The third type of slow-wave material is produced artiiically by distributing small pieces of resonant metallic particles throughout a dielectric or magnetic material. As is commonly known, metallic particles are said to be resonant at a particular frequency when one dimension `approximates one-half wavelength at the particular frequency. At frequencies just less than the particular frequency at which resonance occurs, the metallic particles exhibit inductive reactance. Materials of this class together with the techniques for making them are well known in the art and need no further description. See, for example, an article entitied, Metallic delay lenses" by W. E. Koch which appears on pages 58-82 of vol. 27, No. l of the Bell Systems Technical Journal for January 1948, published in New York, New York.

Impedance member 38 may bc manufactured. for example, by using a material such as strontium titanate in u ceramic form shaped as tubular elements, the thickness of the walls being made equal to the length of an electrical one-quarter wavelength at the signal frequency. When the thickness of element 60 is equal to one-quarter wavelength, it presents a purely resistive impedance to the electron stream. As stated previously, the wall thickness may also be made any odd multiple of one-quarter wavelength. Alternatively, in case a resistive-inductive or pure inductive impedance is desired, the wall thickness may be made less than an odd multiple of one-quarter wavelength and greater than an even multiple of oncquarter wavelength. Highly conductive coating 62 may then be provided by plating or evaporating a metal, such as silver, on the outer surface of element 60.

The resistive coating 61 is then deposited on the inner surface of tubular element 60. Resistive coating 6l may consist of a layer of stannous oxide formed by reacting stannous chloride with a suitable agent on the surface material. Resistive coatings of this type are much thinner than the skin depth which is representative of the penetration of microwave energy and, hence, exhibits a arsenic` radio-frequency resistance that approximates that for direct currents. For obtaining maximum gain, the surface resistivity must be chosen to have a value depending upon the desired operating frequency, the velocity of the electron stream and the dielectric constant of the material. By surface resistivity is means the actual resistance per unit surface area to an electrical wave. For stannous oxide on a dielectric surface, the direct current resistance per unit area approximates the value of surface resistance presented to the wave, since its thickness is much less than the usual skin depth encountered at the frequencies used.

The choice of surface resistivity of the wall required for obtaining maximum amplification for unit length of the tube depends upon the operating frequency, the separation between the electron stream and resistance wall, the electron velocity, and the dielectric constant of the Wall. Furthermore, the surface resistivity may be very high when a resistive-inductive wall is desired, the value of resistivity in this case being many times that used with a purely resistive wall. For a purely inductive wall, the resistivity must be as high as is compatible with collecting stray electrons and maintaining a uniform static potential over the entire length of the wall. Obviously, this structure provides a purely inductive wall only when infinite resistivity is approached, which limit can never be attained due to the above static requirements. In normal practice, values of surface resistivity will be found to range from 500 to 10,000 ohms per square centimeter, or even as high as, for example, 100,000 ohms per square centimeter for the inductive wall. The resistive coating 61 is normally maintained at a potential approximately equal to that of the matching ferrule 12. For convenience, the same battery 26 is used to apply a potential of the order of 1000 volts through leads 40 and 42 to both extremities of resistive coating 61.

The output portion of the amplifier tube comprises the output helix 44 connected by lead 20 to matching ferrule 46 and collector electrode 47, all maintained at a potential equal to the potential of matching ferrule 12. This is accomplished by means of wires 48 and 50 and source 26, the latter being of the order of 1000 volts.

The construction of helix 44 is identical to that of helix 30, and it is terminated in a manner similar to helix 30 by applying a coating of resistive material 43, such as Aquadag, to the outside of envelope 2.

The construction of the output waveguide 52 is identical to that of the input waveguide 34. It has a matching element 53 for positioning a termination 54, similar to matching element 35 in the input waveguide 34. A cylindrical collar 45 extends concentric with ferrule 46 for a distance of approximately one-quarter wavelength producing an apparent shorting plane on the inner surface of output waveguide 52. Although a rather specitic means has been disclosed for coupling signal energy to and from the disclosed impedance wall amplifier tube, other expedients, such as replacing the input and output waveguides with coaxial lines may be used. Also, the input and output helices may be replaced by resonant cavities, or .a grid may be used to directly modulate the stream of electrons at the input, and a screen and anode similar to those of a tetrode can be used to extract the signal energy from the electron stream at the output end. In addition, the disclosed invention need not be restricted to the particular geometrical configuration described as the electrons can be made, for example, to ilow along any desired curved path merely by the utilization of appropriate electrostatic and magnetic fields.

A solenoid 56 is axially positioned symmetrically about the complete length of the glass envelope 2. An appropriate direct current is maintained by a battery 57 in solenoid 56 to produce a magnetic field which may be of the order of 500 gauss. The tield extends through the entire length of the tube and is parallel to its longitudinal axis. The purpose of this magnetic eld is to keep the electron stream focused or constrained throughout the active length of the tube.

In its operation, an input microwave signal is applied through input waveguide 34, inducing a signal potential on lead 14. The matching element 35 is adjusted to produce maximum signal voltage at the input helix 30. As in conventional traveling wave tubes, the axial phase velocity of the traveling wave through the helix is a fraction of the velocity of light, the actual phase velocity being determined by the pitch and diameter of the helix. The velocity of the electron stream is usually adjusted so that it is slightly greater than the phase velocity of the wave passing through the helix. The interaction of the electron stream and the wave on the helix results in a density and velocity modulation of the electron stream. Resistive coating 61 of the impedance member 38 has a resistance such that an electric wave propagated through member 38 is very highly attenuated in the absence of the electron stream because of the relative dimensions of member 38 with respect to a wavelength at the frequency of the microwave signal. Hence, nearly all of the energy transmitted along the axis of member 38 is in the form of a space charge wave propagated by the electron stream, the signal energy existing in the form of electron bunching of the stream electrons. Currents induced by the hunched electron stream in the resistive coating 61 and in the slow-wave material 60 produce electric fields which act on the stream electrons so as to increase still further the electron bunching, an increase in the electron bunching being equivalent to an increase in the signal amplitude. Thus, the magnitude of the modulation of the electron stream representative of the signal amplitude continuously increases as the stream electrons move along contiguously to the resistive coating 61.

To explain more adequately the functioning of the present invention, an analogy may be made to a shorted transmission line. As is commonly known, the input impedance of the transmission line is a function of the electrical distance to the shorted termination and the losses of the line. In the case of a quarter wavelength shorted transmission line, the input impedance is essentially infinite, provided there are no losses. Similarly, in the present invention, a wave is propagated exterior to the resistive coating 61 to a reiiecting surface provided by the highly conductive coating 62, thus producing any desired reactive impedance in the plane of resistive coating 61 dependent upon the spacing and the intervening attenuation of the reilected wave. As in the case of transmission lines, the impedance in the plane of resistive coating 61 can be made inductive by decreasing the spacing between resistive coating 6l and conductive coating 62 below that corresponding to an electrical one-quarter wavelength, and made capacitive by increasing the spacing beyond an electrical one-quarter wavelength. Hence, in order to obtain an inductive impedance, it is only necessary to use an appropriate spacing which is just less than an odd multiple of an electrical one-quarter wavelength. The resistive coating 6l, of course, is electrically in parallel with this inductance because the coating is physically coincident with the surface at which this inductive impedance appears. These paralleled impedances determine the actual impedance which impedance member 38 presents to the electron stream.

The aforementioned increasing space charge wave is propagated axially along member 38 by the electron stream and emerges at the far end where it induces a voltage on helix 44, transforming a portion of the signal energy in the electron stream into the form of an electromagnetic growing wave on the output helix. The stream electrons, as they proceed toward ferrule 46, continue to impart energy to the growing wave on the helix. After transferring most of the signal energy from the stream to the helix, the electrons are collected by collector electrode 47. The electric field induced by the electromagnetic wave on lead 20 connected to the output helix 44 is parallel with the electric field of the fundamental mode desired to be excited in the waveguide, hence, amplified signal energy is transferred from the output helix 44 to the output waveguide 52.

An analysis of impedance wall amplifiers indicates that amplification of a wave in decibels per unit length is proportional to the imaginary part of the quantity where Y: Yw )'Bi is an admittance made up of the parallel connection of the wall admittance, Yw, and the capacitive susceptance, Bs, of the space occupied by the electron stream, In is the current in the electron stream, Vo is the potential through which the stream electrons are accelerated, e is the absolute dielectric constant, and w is the angular frequency of the amplified wave.

Reference is made, for example, to Waves in Electron Streams and Circuits, by J. R. Pierce, Bell System Technical Journal, July 1951. Examination of the foregoing expression representative of amplification shows that when Y is purely capacitive, there will be n0 amplification. On the other hand, for Y small and purely inductive, there will be a large gain, and for Y resistive-capacitive, resistive, or resistive-inductive, there will be gain, an increased amount of gain being associated with the latter admittance.

Inasrnuch as the capative susceptance, Bs, of the space occupied by the electron stream never vanishes, a small amount of inductive susceptance added by an inductive wall may still leave admittance Y capacitive. Also, a large inductive susceptance may cancel susceptance BS entirely, leaving admittance Y purely resistive provided that the Wall admittance, YW. has a resistive part. A still larger inductive susceptance would make admittance Y resistive-inductive.

In accordance with the present invention, the admittance, Y, obtained by combining an inductive reactance, in parallel with the resistive coating 61 having a very large resistance per unit area, results in what is essentially a large inductance. As previously mentioned, an inductive rcactance in the plane of resistive coating 61 may be produced by positioning the electromagnetic refiecting surface 62 just less than an odd number of electrical quarter wavelengths from resistive coating 61 with slow-wave material 60 disposed in the intervening space. The gain per unit length of a tube incorporating such an impedance member presenting a high inductive reactance to the electron stream may then be of the order of tens of decibels per centimeter for currents and voltages well Within the ranges encountered in practice.

To explain more adequately the function of the reflecting surface provided by coating 62 of impedance member 38, reference is made to Figs. 2 and 3 wherein equivalent circuit diagrams arel illustrated for the impedance presented to the electron stream by an impedance member with and without the reflecting surface, respectively.

Referring to Fig. 2, there is shown an equivalent circuit diagram of the impedance presented to the electron stream by resistive coating 61 acting alone, resistive elements 86, 87, 88 and 89 representing, in lumped parameter form, the series resistance of resistive coating 61, and tubular elements 80, 81, 82, 83 and 84 representing the electrical coupling of the electron stream to the walls. In addition to the foregoing, there is also a distributed series capacitance between adjacent tubular elements of resistive coating 61 which is represented in lumped parameter form by capacitors 91, 92, 93 and 94. It is to be noted that this capacitance would exist even in the absence of the resistive coating 61, the capacitance being that of the space along the electron stream.

In order to obtain maximum gain in an electron stream amplifier tube, it is necessary to maintain the impedance presented to the electron stream as high as possible consistent with the frequency bandwidth desired. It can readily be seen by inspection of Fig. 2 that the admittance presented to the electron stream consists of conductance plus capacitive susceptance. To increase the gain of the tube, it is necessary to decrease the capacitive susceptance in Jaralle! with the resistance. One means of accomplishing this, as explained previous-ly, is to position a metal Wall one electrical one-quarter wavelength or less behind resistive coating 61, in addition to interposing a slow-wave material between the resistive coating and the highly conductive metal wall which acts as a reflector. In reality, this wave reflection has the same function as a parallel resonant circuit in that it reduces the capacitance mentioned in one frequency range, cancels the capacitance completely at one predetermined frequency, and places an inductive impedance in parallel with the resistance of the coating in another frequency range. The structure, therefore, may be illustrated as a series of parallel elements as shown in Fig. 3, that is, inductive elements 96, 97, 98 and 99 in parallel, respectively, with capacitors 91, 92, 93 and 94, and resistive elements 86, 87, 83 and 89.

Since it has been previously pointed out in the Haeff application that it is preferable to present a resistiveinductive impedance to an electron stream rather than a resistive-capacitive impedance, an electron stream amplified tube embodying the disclosed invention could be designed to be resonant near the upper limit of the frequency `range to be amplified by the tube in that amplification is greater in the resistive-inductive regions. In this manner, a resistive-inductive impedance is presented to the electron stream throughout the rangs of frequencies required for a given signal, which results in a uniform amplification of all components of the signal being amplified.

What is claimed as new is:

l. In an electron stream tube for amplifying a space charge wave propagated by an electron stream, an element for presenting an impedance to the electron stream comprising a wall composed of a slow-wave material having a substantially uniform thickness of substantially an odd multiple of one-quarter of a wavelength in said material at the frequency of said wave, said wall having a first surface positioned contiguous to the electron stream and an opposite surface; a resistive coating deposited on said first surface whereby said space charge wave propagated by said electron stream induces currents in said resistive coating to generate concomitant electric fields which interact with said electron stream to amplify said space charge wave; and a conductive coating deposited on said opposite surface for presenting, in conjunction with said wall composed of a slow-wave material, a high irnpedance in parallel with said resistive coating to said electron stream.

2. The element for presenting an impedance to the election stream as defined in claim` l wherein the product of the relative dielectric and permeability constants of said slow-wave material with respect to free space is not less than 256 divided by the kilo electron volts through which the electrons comprising said electron stream are accelerated.

3. In an electron stream tube for amplifying a space charge wave propagated by an electron stream, an element for presenting an impedance to the electron stream comprising a wall composed of a slow-wave material having a predetermined dielectric and permeability constant and a substantially uniform predetermined thickness, said wall having a rst surface positioned contiguous to the electron stream and an opposite surface; a resistive coating deposited on said first surface whereby said space charge wave propagated by said electron stream induces currents in said resistive coating to generate concomitant electric elds which interact with said electron stream to amplify said space charge wave; and a conductive coating deposited on said opposite surface for presenting to said electron stream in conjunction with said slow-wave material, in parallel with said resistive coating an impedance determined by said predetermined thickness of said wall.

4. The element for presenting an impedance to the electron stream as defined in claim 3 wherein said predetermined thickness of said wall is greater than an even multiple of one-quarter of an electrical wavelength of said wave and less than an odd multiple of one-quarter electrical wavelengths.

5 An electron stream tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for directing said electron stream along a predetermined path; means for modulating said electron stream in response to microwave signals to produce a corresponding space charge wave propagated along said path by said electron stream; a member having for presenting an impedance to said electron stream, said member a resistive surface extending contiguous to said modulated electron stream for at least several wavelengths of said space charge wave whereby said space charge wave induces currents in said resistive surface to generate concomitant electric fields which interact with said electron stream to amplify said space charge wave, an element having a conducting surface positioned a predetermined distance from and parallel to said resistive surface, and a slow-wave material interposed between said resistive surface and said conducting surface, whereby said conducting surface in conjunction with said slowwave material presents a predetermined impedance dependent upon the electrical distance between said resistive surface and said conducting surface to said electron stream to increase the amplification of said space charge wave; and output means coupled to said electron stream for deriving a microwave output signal from said amplified space charge wave.

6. The electron steam tube as defined in claim 5 wherein said predetermined distance between said conducting surface and said resistive surface is greater than an even multiple of one-quarter electrical wavelengths and no more than one-quarter odd multiple of electrical wavelengths.

7. An electron stream tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for modulating said electron stream in response to microwave signals to produce a corresponding space charge wave propagated by said electron stream; a member for presenting an impedance to said electron stream having a resistive surface contiguous to said modulated electron stream for at least several wavelengths of said space charge wave, said space charge wave inducing currents in said resistive surface to generate concomitant electric fields whereby said electric fields interact with said electron stream to produce amplification of said space charge wave, a conducting surface disposed substantially an odd multiple of one-quarter electrical wavelengths at the frequency of said microwave signal from said resistive surface, and a slow-wave material interposed between said resistive surface and said conducting surface, whereby said conducting surface, in conjunction with said slow-wave material, presents a high impedance in parallel with said resistive surface to said electron stream; and output means coupled to said electron stream for deriving a microwave ouput signal from said amplified space charge wave.

8. In an electron stream tube for amplifying a spacecharge wave propagated by an electron stream, an element for presenting an impedance to said electron stream comprising a wall composed of a slow-wave material having a thickness greater than an even multiple of electrical one-quarter wavelengths in said material at the frequency of said wave and no more than an odd multiple of electrical one-quarter wavelengths, said wall having a first surface positioned contiguous to said electron stream and an opposite surface; a resistive coating deposited on said first surface for maintaining said surface at a uniform potential; and a highly conductive coating deposited on said opposite surface for presenting an inductive impedance to said electron stream.

9. In an electron stream tube for amplifying a space charge wave propagated by an electron stream, an element for presenting an impedance to said electron stream comprising a wall including a material of predetermined dielectric and magnetic characteristics and having a substantially uniform thickness of an odd multiple of onequarter wavelengths in said material at the frequency of said wave, said wall having a first surface positioned contiguous to the electron stream and an opposite surface; a resistive layer disposed on said first surface whereby said space charge wave propagated by said electron stream induces currents in said resistive layer to generate concomitant electric fields which interact with said electron stream to amplify said space charge wave; and a conductive layer disposed on said opposite surface for presenting a high impedance in parallel with said resistive layer to said electron stream.

l0. The electron stream tube for amplifying microwave signals as defined in claim 7 wherein said resistive surface of said member is disposed concentrically about said modulated electron stream.

ll. An electron stream amplifier tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for directing said electron stream along a predetermined path; means for modulating said electron stream in response to microwave input signals; an element for amplifying the modulations of said electron stream, said element including a hollow dielectric cylinder of uniform thickness disposed concentrically about and contiguous to said path, a resistive coating disposed on the inner surface of said cylinder, and a conductive coating disposed on the outer surface of said cylinder; and output means coupled to said electron stream for deriving a microwave output signal from the amplified modulations of said electron stream.

l2. An electron stream amplifier tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for directing said electron stream along a predetermined path; means for modulating said electron stream in response to microwave input signals; a member having a resistive layer disposed on `said member concentrically about and contiguous to said path, said modulations inducing currents in sai-d resistive layer which generate electric fields, said fields interacting with said electron stream to amplify said modulations, a dielectric wall of uniform thickness disposed exterior to and in contact with said resistive layer, and a conductive layer disposed on the outer surface of said dielectric wall, the portions of said electric fields, exterior to said resistive layer, propagating radially outwards to said conductive layer whence said fields are reflected back to said resistive layer to cancel the spatial capacitance exterior thereto thereby increasing the amplification of said modulations; and output means coupled to said electron stream for deriving a microwave output signal from the amplified modulations of said electron stream,

13. An electron stream amplifier tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for directing said electron stream along a predetermined path; means for modulating said electron stream in response to microwave input signals; a member for presenting a resistiveinductive impedance to said electron stream to amplify said modulations, said member having a resistive layer disposed on said member concentrically about and contiguous to said path, said modulations inducing currents in said resistive layer which generate electric fields, said fields interacting with said electron stream, a dielectric wall of uniform thickness disposed exterior to and in contact with said resistive layer, and a conductive layer disposed on the outer surface lof said dielectric wall, the portions of said electric fields, exterior to said resistive layer, propagating radially outwards to said conductive layer whence said fields are reected back to said resistive layer to produce an inductive impedance in parallel therewith thereby increasing the amplification of said modulations; and output means coupled to said electron stream for deriving a microwave `output Signal from the amplified modulations of said electron stream.

King Apr. 16, 1940 Cassen Dec. 8, 1942 12 2,367,295 Llewellyn Jan. 16, 1945 2,584,802 Hansell Feb. 5, 1952 2,602,148 Pierce July 1, 1952 2,611,101 Wallauschek Sept. 16, 1952 2,616,990 Knol et al. Nov. 4, 1952 2,630,547 Dodds Mar. 3, 1953 2,636,148 Gorham Apr. 2l, 1953 2,652,513 Hollenberg Sept. l5, 1953 2,661,441 Mueller Dec. l, 1953 FOREIGN PATENTS 969,267 France May 17, 1950 OTHER REFERENCES Article by Von Hippel et al., pp. 1097-1109, Industrial and Engineering Chemistry, for November 1946, v01. 38, No. 1l.

Article by Broekman, pp. 1077-1080, Electrical Engineering, for December 1949.

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