US2758243A

US2758243A - Electron beam tubes

Info

Publication number: US2758243A
Application number: US291225A
Authority: US
Inventors: Rolf W Peter
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1952-06-02
Filing date: 1952-06-02
Publication date: 1956-08-07
Anticipated expiration: 1973-08-07
Also published as: NL87433C; DE954276C; NL178756B; GB726267A; FR1076777A; BE520379A

Description

Aug. 7, 1956 R. w. PETER ELECTRON BEAM TUBES Filed June 2, 1952 [NI 'E \"I'OR. RUL W. PETER Uaited States Patent 2,758,243 Patented Aug. 7, 1956 Fice ELECTRON BEAM TUBES Rolf W. Peter, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 2, 1952, Serial No. 291,225

Claims. (Cl. 315--3.5)

This invention relates to electron beam tubes, and particularly, to helix type traveling wave amplifier tubes.

The object of the invention is to provide improved means for coupling the circuit helix of a traveling wave tube to the input and output transmission lines.

In the drawing:

Fig. l is a schematic view of a conventional helix type traveling wave tube; and

Fig. 2 is an axial sectional view of a helix type traveling wave tube embodying the invention.

In a conventional traveling wave amplifier tube of the helix type, an electron beam is projected along an elongated helix at a velocity approximately equal to the axial phase velocity of waves traveling along the helix. By successive processes of electron velocity modulation and inductive output, the beam is modulated by the electric fields set up along the helix by the signal wave, and the modulated beam gives up energy to the wave to increase its amplitude.

Suitable means are provided at the input and output ends of the helix for coupling the helix to input and output transmission lines. These transmission lines may be waveguides or coaxial lines. Fig. 1 shows a conventional tube in which the input end of the helix 1 is coupled through the dielectric envelope 4 to an external input waveguide 3 extending transversely of the tube axis by means of an axially-extended variable-pitch transition portion 1a of the helix extending parallel to the electric field component E1 in the waveguide 3. A conventional electron gun G, including a cathode 5, focusing electrode 7 and accelerating electrode 9, is mounted in an enlarged end portion 4a of the dielectric envelope 4, in axial alignment with the helix 1. The distance from the accelerating electrode 9 to the end of the variable-pitch transition portion 1a, where the active portion of the helix begins, is indicated by the letter L1. The opposite end of the helix 1 is coupled through the envelope 4 to an external output waveguide 11 by means of an axiallyextended variable-pitch transition portion 1b of the helix, similar to transition portion 1a, extending parallel to the electric field component E2 in the waveguide 11. A collector electrode 13 is mounted adjacent to the end of the transition portion 16. The distance from the end of the variable-pitch portion 111 to the collector 13 is indicated by L2. The active length of the helix is indicated by La.

It is of greatest importance to make a traveling wave tube as short as possible for a given gain. No axial space should be used by anything that has no importance for beam forming and beam-field interaction. In conventional designs considerable axial space is used up to provide a broad-band match between the feeding lines and the helix, as illustrated in Fig. 1.

The conventional coupling arrangement shown in Fig. 1 has several disadvantages. First, the parts of the electron beam that extend along the two transition portions 1a and 1b are not electrically shielded from the surrounding dielectric envelope wall, and hence, variable potentials are produced along the inner wall of these parts of the envelope during operation. These variable potentials cause undesirable variations in beam velocity along the tube axis in these regions. A second disadvantage of the arrangement of Fig. 1 lies in the increased cost of providing a suitable axial focusing magnetic field for preventing spreading of the beam and consequent collection of electrons by the helix. The gain of a traveling wave amplifier tube is proportional to the active length of the helix. However, the focusing magnetic field must be provided over substantially the entire beam path, as indicated by the arrows H in Fig. 1. Hence, any reduction in the overall length of the beam path, such as by reducing the distances L1 and L2, results in lower cost for a tube of given active length, or gain. Third, the relatively large distance L1 may impose a serious limitation on the signal-to-noise ratio of the tube. In order to obtain lowest noise figure, it may be necessary that the distance L1 be as small as possible. The shorter the active helix, the larger the relative length of the coupling antenna and the more important to eliminate the latter.

In accordance with the present invention, the input end of the active helix is positioned close to the accelerating electrode of the electron gun and the helix is inductively coupled to the external input transmission line by means of a conductor located entirely within the tube envelope and extending laterally from the end of the helix and back along the helix away from the electron gun. In the case of waveguide input, the helix extends at least partly through the input waveguide to a point close to the electron gun and the coupling conductor extends back across the waveguide substantially parallel with the high frequency electric field therein. Furthermore, the output end of the active helix is positioned close to the collector electrode and the helix is inductively coupled to the output transmission line by means of a similar conductor located entirely within the envelope and extending laterally from the end of the helix and back along the helix away from the collector electrode and toward the input coupling conductor. In waveguide output, the helix extends at least partly through the output waveguide and the coupling conductor extends back across the waveguide substantially parallel with the high frequency electric field therein. The helix itself does not couple to the electric fields Within the waveguides. Therefore, these fields are not disturbed by the helix running across them.

As shown in Fig. 2, the active helix 1 extends across the input and output waveguides 3 and 11 to points close to the accelerating electrode 9 and collector electrode 13, respectively. The ends of the helix 1 are inductively coupled to the fields within the input and output waveguides by conductors 15 and 17 which extend laterally from the ends of the helix and then back along the helix toward each other in directions substantially parallel with the electric fields E1 and E2, respectively, within the two waveguides. In the arrangement shown in Fig. 2, the electric fields E1 and E2 and the coupling conductors 15 and 17 are parallel with the axis of the helix. The coupling conductors 15 and 17 may be either integral extensions of the helix wire, as shown in Fig. 2, or separate conductors connected to the helix ends. The conductors 15 and 17 are located entirely within the envelope of the tube 4.

In Fig. 2 is shown a magnet structure suitable for use with waveguide type input and output lines for focusing the electron beam. This structure comprises two

pole pieces

19 and 21 connected by one or more permanent magnet rods or bars 23. The various electrodes of the tube are provided with suitable supporting means and external leads for applying suitable direct-current potentials. These have not been shown in the'd'rawing since they are not part of the present invention.

The present invention provides a traveling wave tube in which each of the distances L1 and L2 is reduced to a small fraction of the corresponding distance in the conventional tube of Fig. 1, so that the overall beam path is not much longer than the active helix length L3.

The arrangement shown does not involve any mechanical connection between the traveling Wave tube and the surrounding waveguides or magnet structure. Therefore, the tube is readily insertable and removable from various waveguide and magnet structures.

Instead of projecting the electron beam through the interior of the helix of the tube, as shown, an annular electron gun may be provided to project a hollow beam along the outside of the helix, in which case the coupling conductor or antenna would be folded back inside the helix instead of outside the latter.

What is claimed is:

l. A traveling wave electron tube comprising an elongated dielectric envelope containing an elongated conductive helix, an electron gun close to one end of said helix for projecting a stream of electrons along said helix for interaction with waves traveling thereon, and means for inductively coupling said end of said helix through said envelope to an external input transmission line, said means including a coupling conductor located entirely within said envelope and extending laterally from said end and back over said helix away from said electron gun.

2. A traveling wave electron tube comprising an elongated dielectric envelope containing an elongated conductive helix, means for projecting a stream of electrons along said helix for interaction with waves traveling thereon, means for coupling an. external input transmission line to the end of said helix adjacent to said projecting means, a collector electrode in the path of said stream close to the opposite end of said helix, and means for inductively coupling said opposite end through said envelope to an external output transmission line, said lastnamed means including a coupling conductor located entirely within said envelope and extending laterally from said opposite end and back over said helix away from said collector electrode.

3. A traveling wave electron tube comprising an elongated dielectric envelope containing an elongated conductive helix, an electron gun close to one end of said helix for projecting a stream of electrons along said helix for interaction with waves traveling thereon, a collector electrode in the path 'of said stream close to the opposite end of said helix, and means for inductively coupling the two ends of said helix through said envelope to external input and output transmission lines, said means including a coupling conductor located entirely within said envelope and extending laterally from each of said ends and back over said helix toward each other.

4. A traveling wave electron tube comprising an elongated dielectric envelope containing an elongated conductive helix and an electron gun close to one end of said helix for projecting a stream of electrons along said helix for interaction with waves traveling thereon, and means for coupling an external high frequency signal transmission line to said end of said helix, said means including a pair of conductors external to said envelope and axially spaced along said helix adjacent said end thereof to form an axial gap and adapted to be coupled to said line to establish a high frequency electric field across said gap and parallel to the axis of said helix, and a coupling conductor located entirely within said envelope and extending laterally from said end and back along said helix parallel to said axis to couple inductively through said envelope with said field.

5. A traveling wave tube comprising an elongated dielectric envelope containing an elongated conductive helix, a section of waveguide of rectangular cross section external to and extending transversely of said envelope and having a pair of apertures in opposite Walls thereof through which said envelope extends, said helix extending through at least one of said apertures and having one end positioned close to and in alignment with the other aperture, a coupling conductor located entirely within said envelope and extending laterally from said end and back over said helix away from said other aperture for inductively coupling through said envelope with the transverse high frequency electric field within said waveguide, and an electron gun in said envelope close to said helix for projecting a stream of electrons along said helix for interaction with waves traveling thereon.

References Cited in the file of this patent UNITED STATES PATENTS Re. 21,739 Llewellyn Mar. 4, 1941 2,238,770 Blumlein Apr. 15, 1941 2,575,383 Field Nov. 20, 1951 2,584,802 Hansell Feb. 5, 1952 2,623,129 Lerbs Dec. 23, 1952