US3305752A - Fast wave crossed field travelingwave tube - Google Patents

Description

Feb. 21, 1967 K W. FRlz 3,305,752

FAST WAVE cRossED FIELD TRAVELING-WAVE TUBE Filed Dec. 6, 1963 INVENTOR. Fifi wm rn? me z 4 IL w BY 4.4L.

24@ zs/Wv- E "L S-4 fam@ United States Patent() 3,305,752 FAST WAVE CROSSED FIELD TRAVELING- j WAVE TUBE Walter Friz, Greene County, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Filed Dec. 6, 1963, Ser. No. 328,763 1 Claim. (Cl. 315-39) structure is a rectangular waveguide Wound in a linear v spiral and having a slit along its plane of symmetry for the passageof a disc-shaped electron beam for interacting with the wave propagated in the guide. The purpose of this invention is t provide a traveling-wave tube of the above type S0 constructed that the phase relation between the propagated wave and the electron beam modulation along any radius of the beam is such that a transfer of energy occurs from the beam to the wave. In accordance with the invention, this is accomplished by decreasing the larger dimension of the waveguide as the radius of the spiral increases in order to increase the phase velocity of the propagated wave as required to maintain the proper phase relation between the wave and the beam modulation.

The invention will be described in more detail with reference to the specific embodiment thereof shown in the accompanying drawings in which FIGS. 1 and 2 are cross sectional views of a travelingwave tube incorporating the invention,

FIG. 3 shows the electrical connections to the electrodes of the tube in FIGS. 1 and 2, and

FIG. 4 illustrates a method of coupling to the spiral waveguide.

Referring to FIGS. 1 and 2, the traveling wave tube is contained in an annular housing of 4magnetic material having an upper half and a lower half 11. Centrally located in these halves are magnetic pole pieces 12 and 13 which extend toward each other, leaving space between their ared en ds for an annular concave cathode 14. Series connected magnet coils 15 and 16 surround the pole pieces 12 and are supplied with direct current from a suitable source (not shown). The coil winding directions are such that the ploe pieces from similar magnetic poles, north poles for instance, so that they magnetic flux passes radially outward from the cathode of the tube parallel to the plane of symmetry 17 of the tube to the outer rims of the upper and lower halves 10 and 11 of the housing, and thence equally through-the upper and lower halves back to the pole pieces. The concave cathode 14 together with the accelerating electrodes 18 and 19 act as an annular electron gun causing the electrons emitted by the cathode to converge into a thin discshaped beam 20 centered on the plane of symmetry 17 and extending to collector electrode 21, the electrons being constrained to follow paths generally radial and parallel to the plane of symmetry by the above described magnetic field. The electron beam system power supply circuit is shown schematically in FIG. 3. The cathode may be insulated and supported between the pole pieces 12 and 13 in any suitable manner.

The traveling-wave structure of the tube consists of a rectangular waveguide, having a greater dimension a and a lesser dimension b, wound in a linear spiral defined by the equation 3,305,752 Y Patented Feb. 21, 1967 where R0 is the starting radius, 6 is measured in radians and b-j-s as shown in FIG. 2.

This structure may be formed in two halves 22 and 23 joined at plane 17 to form a structure symmetrical with respect to this plane. The waveguide walls in each half of the structure are made of such length as, when the structure is assembled, to leave a slit at the plane lof symmetry 17 for passage of electron beam 20. The two halves 22 and 23 are joined to each other and to collector electrode 21 at their circumferences and t0 pole pieces 12 and 13 at their centers by soldering, brazing, welding or other suitable method providing gas-tight joints so that the traveling wave structure may be evacuated.

The spiral waveguide may be coupled to external circuits by waveguides 24 and 25 which may be joined to the spiral waveguide in the manner illustrated in FIG. 4. In each case, a gas-tight window 26 is provided to seal the interior of the traveling-wave structure.

The rectangular waveguide is operated in its dominant mode, the TELO mode. In this mode the electric field is transverse to the direction of propagation and its lines are normal to the larger dimension lof the waveguide with maximum intensity at the center. The electric eld lines are therefore parallel to the electron beam and densest on either side of the beam. The lines of the magnetic ield form closed loops parallel to the wider surface of the waveguide.

In order for the electron beam and the propagated wave in a traveling-wave tube to interact in such a way that the wave energy is increased, it is necessary that the phase velocity of the electromagnetic wave be approximately equal to the average velocity of the electrons. Applying this requirement to a tube of the type shown in FIG. 2, in order for this condition to obtain along any radial filament of the beam, such as the filament along line 27 in FIG. 2, it is necessary that the beam see substantially the same phase of the electromagnetic wave in each successive transit of the waveguide along line 27. With a constant phase velocity in the guide this would be impossible except along certain radial filaments of the beam because of the increasing lengths of the spiral turns. In accordance with the invention, the' phase velocity is increased by a continuous reduction of the dimension a of the guide as required t-o provide the proper phase relation 4along all radii of the disc beam. The required value of a at any point in the spiral may be determined as follows:

vIn order that the phase relation between the electromagnetic -wave and the beam density modulation wave, which results from the velocity modulation of the beam electrons by the electric field in the waveguide, shall remain substantially the same, the phase shift encountered by the wave in one spiral turn should differ from the phase shift encountered by the beam modulation wave in traversing the radial distance between turns only by the angle mr where n is an even integer of either sign. This is expressed by the equation where r is the average radius of a spiral turn, g is the phase constant of the waveguide, ,6e is the phase constant of the beam modulation wave and h is the increment in the spiral radius in one turn, these terms being further defined as follows:

RWE-

Substituting the above value of r in (2) and rearranging The phase constant of a rectangular waveguide in terms of its greater dimension a is where is the free space wavelength of the electromagnetic wave. Using the relationships 21r and c 21rc )WFT when is the free space phase constant of the wave and c is the velocity of light, Equation 4 becomes 5 fylrgy Equating (3) and (S) 1rC 2 (Lfmjtl'ba) Solving (5) for a gives h Heem Since, as indicated above, the value `of r in Equation 2 is the average radius of the spiral turn ending at the spiral angle 0, the value of a given by Equation 7 is that value of a required to give the proper phase shift in one complete turn of a rectangular waveguide of constant cross section formed in a circle of radius r. However, in order to obtain the proper phase relations along all radii of the disc beam, the cross section of the spiral waveguide in FIGS. 1-2 cannot remain constant in any given turn of the spiral but must continuously decrease throughout the spiral as 0 increases. Therefore, the value of a computed from Equation 7 is an effective value of a for the spiral turn ending at 0, with the actual value of a being greater than the effective value in the earlier part of the turn and less in the latter part. Assuming the actual and effective values to become substantially equal midway of the turn, the actual value of a at any point 0 in the spiral, measured from line 27 in FIG. 2, may be determined by substituting 0=0+vr in Equation 7. Thus, to determine the values of a at the points where line 27 intersects the spiral waveguide (0=0, 2r, 41r, 61r, etc.) the values of 6 to be substituted in equation 7 are 1r, 3f, 51r, 71r etc. By drawing a smooth curve through these values of a plotted against 0', the value of a at any point 0' in the spiral may be determined. From this information a spiral rectangular waveguide having the proper taper of its longer dimension may be constructed.

It will be obvious to those skilled in the art that the positions of cathode 14 and collector 21 may be interchanged. This would not require any change in the construction of the waveguide.

I claim:

A traveling-wave tube comprising: means including an annular cathode and an annular collector electrode for producing a thin disc-shaped beam of electrons between said cathode and collector; a rectangular waveguide wound in a linear spiral and situated between said cathode and said collector, said spiral waveguide being concentric with said disc-shaped beam and symmetrically positioned with respect to the central plane of said beam, the larger dimension of said waveguide being normal to said central i plane and said waveguide being slit at the center of its larger dimension to permit passage of said beam; means for introducing an electromagnetic wave at one end of said spiral waveguide and'for removing said electromagnetic wave from the other end of said spiral waveguide after propagation therethrough, said propagated wave interacting with said beam to produce a velocity modulation of its electrons and a resulting density modulation of the beam; and said spiral waveguide further having its larger dimension vary continuously as an inverse function of the spiral angle such that the difference between the phase shift incurred by said electromagnetic wave in traveling around any complete turn of said spiral waveguide and the phase shift incurred by the density modulation of said beam in traveling the radial distance between turns of said spiral waveguide is substantially equal to mr radians where n is an even integer.

References Cited by the Examiner UNITED STATES PATENTS 2,654,004 9/1953 Bailey 315--4 X HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, J R. Assistant Examiner,

Images