US2758241A - Travelling wave tube - Google Patents

Description

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7, W56 N. W. ROBINSON TRAVELLING WAVE TUBE Filed Aug. 28, 1950 INVENTOR.

NORMAN, WEIGHT ROBINSON AGENT TRAVELLING WAVE TUBE Norman Wright Robinson, 'Salfords, near Redhill, England, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application August 28, 1950, Serial No. 181,754

Claims priority, application Great Britain September 1, 1949 6 Claims. (Cl. SIS-3.5)

This invention relates to travelling wave tubes.

Known travelling wave tubes comprise a long, usually tightly wound wire helix and an electron gun adapted to direct a beam of electrons in a direction parallel to the axis of and within the helix. An electro-magnetic wave may be impressed on the helix at the end adjacent the cathode and caused to pass along the helix travelling in the same sense as the electron beam. The operation of the travelling wave tube depends on the fact that the electro-magnetic wave and the electrons travel along the travelling wave tube with about the same velocity. The electrons, entering the helix with a velocity slightly greater than the axial velocity of the electro-magnetic wave, tend to become bunched as they travel through the helix. A power gain for propagation in the direction of electron motion may thus be obtained, energy being abstracted from the tube at about the end of the helix remote from the electron gun. The final anode of the electron gun usually consists of a copper-nickel tube of the same internal and external diameter as the helix, and electrical contact between the metal tube and the helix is made by welding. A similar conection to a second metal tube is required to be made at the output end of the helix.

The helix may be supported along the whole of its length, together with the final anode and the second metal tube at the output end of the helix, by quartz or glass tubing of circular internal cross section. In this case, the quartz or glass tubing has to be slightly larger in internal diameter than the outer diameter of the helix in order to accommodate the extra thickness at the welds. The axis of the helix, therefore, tends to depart from the linear. A further disadvantage of the use of quartz or glass tubing of circular cross section is that there is a region of high dielectric constant in close contact with the helix, which gives rise to distortion of the longitudinal field in the centre of the helix. irregularities in the thickness of the quartz or glass also tend to cause distortion.

Alternatively, the helix may be spaced from the glass or quartz tubing and supported at intervals by glass or quartz dimples provided internally of the quartz or glass tubing. The main disadavntage of this construction of travelling wave tube is that, since the mechanical strength of the helix is small, the helix tends to sag between adjacent points of support so that the axis of the helix is not linear.

The invention provides a travelling wave tube in which the axis of the helix is substantially linear, the helix is in contact with a material of high dielectric constant at only a few points in each turn and the effects of irregularities in the material of high dielectric constant are minimised.

According to the invention, a travelling wave tube comprises a long wire helix, means to generate an electron beam and to direct said beam in a direction parallel to the axis of and within the helix and a quartz, glass or ceramic tube supporting said helix, the internal cross section of the quartz, glass or ceramic tube being a regu- States Patent 2,758,241 Patented Aug. 7, 1956 lar figure having three or more sides, the outer circumference of the cross section of the helix forming the inscribed circle in the regular figure. The sides of the regular figure may be linear or may be concave or convex.

Preferably the cross section of the quartz, glass or ceramic tube is a three sided regular figure. Above a limit of ten sides little advantage is gained since the cross section is then approximately a circle.

In a travelling wave tube according to the invention, the welded joints between the helix and the two tubes, which may be copper nickel tubes, one at each end of the helix, may be accommodated in the corners of the glass tubing externally of the circle inscribed in the regular figure.

Two examples will now be given of the manner in which suitable quartz, glass or ceramic tube may be manufactured.

Starting with quartz or glass tubing of substantially circular internal cross section having at any point along its length an internal circumference not greater than the internal perimeter required in the finished tubing, a suit ably shaped mandrel is caused to travel through the tubing under the action of a substantially constant driving force, the tubing being heated at or near the zone of engagement with the mandrel so that the heated zone travels along the tubing as the mandrel progresses. The mandrel tapers towards its leading end to a size smaller than the original bore of the quartz or glass tubing. The internal cross section of the finished tubing may, of course, be obtained by way of a number of intermediate stages. This manner of manufacture is suited to the production of quartz or glass tubes the cross section of which is a regular figure having four or more sides. With a three sided figure wear of the mandrel is more pronounced. In any case the bore of the tube should be such that the helix is regular to about /2%. Wear of the mandrel at the corners is not objectionable; in this case a regular figure having curved instead of linear sides is produced. A typical helix for use in a travelling wave amplifying tube comprising such a quartz or glass tube and operating in the 7,000-10,000 mc./s. region is approximately 10" long and 2.5 mm. outside diameter, the helix being wound with 48 turns per inch of wire 0.25 mm. in diameter.

Starting with a solid strip, being a right cylinder of rectangular cross section, of quartz or radio frequency ceramic, such for example as that available commercially under the name Frequentite, a V is machined at about the centre of one longitudinal surface of the strip. A second strip of quartz or radio frequency ceramic is secured to the first. The walls of the tube so provided may be thick compared with the dimensions of the cross section of the bore. The very regular bore of triangular cross section so produced is such that a helix approximately 10 long and 1 mm. outside diameter may be used, the helix being wound with turns per inch of wire 0.1 mm. in diameter.

In order that the nature of the invention may be more readily understood reference will now be had to the accompanying diagrammatic drawing, in which:

Fig. 1 is a longitudinal section of a first travelling wave tube,

Fig. 2 is a section taken on the line Il'II of Fig. 1.

Fig. 3 is a detail view showing the connection between one end of the helix and the final anode of the electron gun, and

Fig. 4 is a cross section of a second travelling wave tube.

Referring now to Fig. 1, the travelling wave tube com- 0 prises an electron gun 1, including a cathode 21, grid 22,

accelerating member 23 and final anode 2, and a Wire helix 3 connected at one end to the final anode 2 and at the other end to a copper-nickel tube 4 at the output end. The whole is enclosed in an outer tube 5, for example of glass, having a collector electrode 6 at the end of the tube remote from the electron gun 1, the helix being supported in position by a glass or quartz tube 7 of square inner and outer cross section. The helix is connected electrically by a weld 8 to the final anode 2 and by a weld 9 to the metal tube 4 at the output end. An attenuator 10 consisting of a colloidal graphite deposit on the inner surface of the glass or quartz tube, is provided in known manner to prevent self oscillation when the travelling wave tube is in use. A choke 11 is provided at each end of the tube.

Fig. 2 shows the outer tube 5, the quartz or glass tube of square cross section 7 and the helix 3. It will be seen that the outer circumference of the cross section of the helix 3 forms the inscribed circle in the inner square and makes contact with the tube 7 at four points only per turn.

Fig. 3 shows the final anode 2 held in position within the tube 7 by a shim 12, the helix 3 and the joint 8. The joint 8 at any point along its length is accommodated in one angle of the inner square externally of its inscribed circle.

Fig. 4 shows the outer tube 5 and the helix 3 of a second travelling wave tube. The helix is held in position between two strips 13 and 14 of quartz or Frequentite. In the strip 14 a V shaped slot has been machined such that in section the slot 15 and the adjacent longitudinal surface of the strip 13 form an equilateral triangle, the outer circumference of the cross section of the helix 3 forming the inscribed circle in the triangle.

What I claim is:

1. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron-beam and connected at one end to the anode, a conducting tube surrounding the axis of the electron beam connected to the other end of the helix, a collector electrode adjacent the end of the tube remote from the helix, and a tube of electrically insulating material having a length at least as great as said given length enclosing and supporting the entire helix, said tube having a regular polygon internal cross-section, whereby each turn of the helix is only engaged at a few spaced points.

2. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron beam and connected at one end to the anode, a conducting tube surrounding the axis of the electron beam connected to the other end of the helix, a collector electrode adjacent the end of the tube remote from the helix, and a tube of vitreous material having a length at least as great as said given length enclosing and supporting the entire helix, said tube having an internal cross-section of a regular polygon of at least three and at most ten sides, whereby each turn of the helix is only engaged at a few spaced points.

3. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron-beam and connected at one end to the anode, a conducting tube surrounding the axis of the electron beam connected to the other end of the helix, a collector electrode adjacent the end of the tube remote from the helix, and a tube of vitreous material having a length at least as great as said given length enclosing and supporting the helix, said tube having a triangular internal cross-section, whereby each turn of the helix is only engaged at a few spaced points.

4. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron beam and connected at one end to the anode, a conducting tube surrounding the axis of the electron beam connected to the other end of the helix, a collector electrode adjacent the end of the tube remote from the helix, and a tube of vitreous material having a length at least as great as said given length enclosing and supporting the helix, said tube comprising a rod having a V-shaped recess and a flat plate covering the recess thereby forming a channel having a crosssection in the form of an equilateral triangle for accommodating and supporting the conductive helix, whereby each turn of the helix is only engaged at a few spaced points.

5. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron-bearn and connected at one end to the anode by a welded joint, a conducting tube surrounding the axis of the electron beam connected to the other end of the helix by a welded joint, a collector electrode adjacent the end of the tube remote from the helix, and a tube of vitreous material having a length at least as great as said given length enclosing and supporting the helix, said tube having a regular polygon internal crosssection, whereby each turn of the helix is only engaged at a few spaced points, the welded joints being accommodated within the angles formed by the sides of the polygon.

6. An electric discharge tube of the travelling wave type, comprising an electron gun including an anode structure for directing a beam of electrons along a given axis, a hollow conductive helix having a given length and having a given axis parallel to the axis of and surrounding the electron beam and connected at one end to the anode, a conducting tube surrounding the axis of the electronbeam connected to the other end of the helix, a collector electrode adjacent the end of the tube remote from the helix, and a tube of vitreous material having a length at least as great as said given length enclosing and supporting the helix, said tube having an internal rectangular cross-section, whereby each turn of the helix is only engaged at a few spaced points.

References Cited in the file of this patent UNITED STATES PATENTS 1,901,703 Crowley Mar. 14, 1933 2,064,469 Haeif Dec. 15, 1936 2,300,052 Lindenblad Oct. 27, 1942 2,575,383 Field Nov. 20, 1951 2,611,101 Wallauschek Sept. 16, 1952

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