US3535584A - Micro-wave crossfield electron tube device - Google Patents

Description

@ct. @wrmm 3,535,584

MICRO-WAVE CROSSFIELD ELECTRON TUBE DEVICE Filed April 5 was 2 Sheets-Sheet 1 ,(r o) T INVENTOR ALAN B. Cpwvms BY ATTORNEYS 0a. 20, 1970 A, B, CUTTING Y 3,535,584

MICRO-WAVE CROSSFIELD ELECTRON TUBE DEVICE Filed April 3 1968 2 Sheets -Sheet 2 F/a4. F/G5 INVENTOR Aym B. Cay-mus T J llTTORNEY United States Patent Office 3,535,584 Patented Oct. 20, 1970 3,535,584 MICRO-WAVE CROSSFIELD ELECTRON TUBE DEVICE Alan 13. Cutting, Lewisham, England, assignor to English Electric Valve Company Limited, London, England, a

British company Filed Apr. 3, 1968, Ser. No. 718,519

Claims priority, application Great Britain, Apr. 4, 1967,

15,264/ 67 Int. Cl. H01j 25/34, 25/50 US. Cl. 31539.3 23 Claims ABSTRACT OF THE DISCLOSURE A crossfield device such as a magnetron or carcinotron in which the interaction space formed between the cathode and the anode is conical in shape or is symmetrical aoout an axis and is so shaped that the electric field direction in the interaction space is inclined at an angle greater than degrees and less than 90 degrees to the direction of the axis. In one embodiment the anode is constituted by the digital elements of a digital line slow-wave structure coupled to an output waveguide loop.

This invention relates to crossfield devices particularly magnetrons, carcinotrons or microwave amplifiers.

It has been proposed to construct a magnetron having a central cathode, an annular anode including a number of resonant cavities, and an interaction space between the cathode and the anode, and in such a construction to have an axial magnetic field and a radial electric field. This type of magnetron has the disadvantages of a limited cathode area, a limited maximum number of resonant cavities, restriction of choice of outputs, a high level of stored energy, a configuration leading to complications of access, output coupling, tuning and cooling, and high manufacturing costs.

It is an aim of the present invention to eliminate or at least reduce some or all of the above-mentioned disadvantages.

According to one feature of the invention a cross field device comprising a cathode and an anode with an interaction space between said cathode and said anode said interaction space being symmetrical about an axis and the direction of the electric field between the cathode and the anode in said interaction space being at an angle other than 0 degree or 90 degrees to said axis.

According to a further feature a device comprising an anode, a cathode, an interaction space being formed between said anode and said cathode so as to be symmetrical about an axis, means for connecting a source of potential to said anode and to said cathode to produce an electric field in said interaction space and means for producing a magnetic field substantially at right angles to said electric field in said interaction space, the arrangement being such that said electric field is inclined to said axis at an angle different from 0 degrees or 90 degrees while said device is operating.

Preferably the cathode, anode and interaction space are all annular and are all symmetrically arranged about the said axis. The annular interaction space may be a parallelogram in cross-section. Also preferably a magnet, which may for example be a permanent magnet, is arranged to provide a magnetic field in the interaction space having a direction which is in a radial plane about the said axis, and at right angles to the direction of the electric field. The permanent magnet may conveniently have coaxial poles.

According to a further feature of the invention there is provided a crossfield device comprising a cathode and Gil an anode formed by an annular slow wave structure, an interaction space being formed between said anode and said cathode so that said space is symmetrical about an axis the direction of the electric field in said interaction space being inclined at an angle other than 0 degrees and degrees to the said axis when said device is operating.

The slow-wave structure may for example be a digital line comprising a number of fingers having the end surfaces adjacent to the interaction space normal to the direction of the electric field. The fingers may conveniently be a quarter wavelength long in a direction parallel to the said axis and the mean radius of the fingers about the said axis may be equal to the mean radius of the cathode, a surface of which is preferably parallel to the said ends. Means may be provided to select a mode of propagation.

According to a further feature of the invention there is provided a crossfield device including an annular slow wave structure forming an anode, a cathode such that an interaction space symmetrical about an axis is formed between said anode and said cathode, means for selecting a predetermined mode of propagation in said slow-wave structure comprising a closed waveguide loop having a resonance at the frequency of said predetermined mode and means for coupling said waveguide loop to said slowwave structure the arrangement being such that the direction of the electric field in said interaction space is inclined at an angle other than 0 degrees or 90 degrees to said axis during the operation of said device.

The closed rectangular cross-section waveguide loop is preferably arranged with its broad walls in a plane normal to the said axis, to have a resonance at the frequency of the selected mode and to be coupled to the slow-wave structure by a number of slots, in a said broad wall. The number of slots is preferably a factor of the number of fingers.

The slots are preferably all arranged so that the energy therein is in phase. The selected mode may be the 1r-mode and the number of slots may be a factor of half the number of fingers.

The amount of energy stored in the waveguide loop may be reduced by providing asymmetry of the coupling by the slots. This may be achieved by asymmetry of the slots and/ or of their position in relation to the waveguide loop and/ or slow-wave structure.

The device may be tuned by varying the phase velocity in the waveguide loop and this may be achieved by the introduction of a dielectric cylinder into the loop or by constructing the waveguide loop from ridged waveguide with the ridge on the unslotted broad side of the waveguide. In the latter case the ridged wall is deformed to change the gap between the ridge and the slotted wall to vary the tuning.

The output of the device may conveniently be by way of a rectangular waveguide directionally coupled to the waveguide loop.

According to a further feature of the invention there is provided a crossfield device including a cathode an anode and an interaction space between said cathode and said anode, said interaction space being of conical form.

Preferably the device according to this feature is symmetrically arranged about an axis and, when in operation, the electric field in the interaction space is at an angle, excluding 0 and 90, to the said axis.

A crossfield device according to the invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a part sectioned diagrammatic perspective view of a magnetron.

FIG. 2 is a fragmentary sectional elevation along section line II-Il of FIG. 1;

FIG. 3 is a diagram indicating the axes of reference for mathematical analysis.

FIG. 4 is a sectional detail illustrating one method of tuning the wave guide, and

FIG. 5 is a sectional detail illustrating an alternative tuning method.

FIGS. 1 and 2 a slowawave structure in the form of a digital line 1 of annular form has a number of fingers 2, arranged in a circle, each approximately a quarter wavelength along in a direction parallel to the axis 3 of the digital line 1. The fingers 2 are rectangular in cross-section with the major dimension of the rectangle coincident with a radial line from the axis 3.

The fingers 2 are constructed of an electrically conducting nonmagnetic material and are rigidly attached at one end to the outside of a wall 4 of a rectangular waveguide loop 5. The wall 4 is parallel with the major dimension of the waveguide cross-section and is in a plane normal to the axis 3. The other ends of the fingers 2 are chamfered to form end faces each having an angle, relative to a line normal to the axis 3, of a.

The rectangular waveguide loop 5 is coupled to the digital line 1 at a number of points by slots 6 each orientated to have its major dimension coincident with a radial line from the axis 3.

A cathode 7, in the form of a ring centered on the axis 3, is positioned with an electron emitting surface 8 in spaced parallel relationship to the chamfered ends of the fingers 2 and having a mean radius equal to the mean distance of the fingers from the axis 3. A cathode heater 9 is of annular form and is located within the cathode 7. The annular space between the cathode and the chamfered end faces of the fingers 2 forms an interaction space of conical form. The annular interaction space has a cross-section in the form of a parallelogram.

A permanent magnet 10 has an annular cavity 11 in [Which is housed the cathode 7 and the chamfered end portions of the fingers 2. The magnet 9 is arranged to provide a magnetic field which everywhere in the interaction space between the cathode 7 and chamfered ends of the fingers 2 is substantially parallel to the surface 8 of the cathode 7.

A rectangular output waveguide 12 is coupled to the waveguide loop 5 by a directional coupler in the form of slots 13.

In operation the heater 9 is supplied with energy and heats the cathode 7 and an electric field, having a direction at right angles to the said magnetic field, is produced between the cathode 7 and fingers 2 by a high voltage D.C. supply 20.

The slow-wave structure is a closed resonator and therefore has an integral number (N) of periodicities. The resonances of this structure will occur when the phase change around the structure is 21m where n, the number of wavelengths round the structure, is integral and the phase change/ period of the line is then 271'11/ N. The resonant frequencies can readily be determined for all appropriate values of n. In general there will be a number of modes which can propagate and it will be necessary to select a single mode, for example the 1r-1'I1Ode for which a N/ 2. This mode selection is achieved by the coupling of the structure at a number of points by slots 6 to the waveguide loop 5'. The waveguide loop 5 is a travelling wave cavity and is designed to have a resonance at the frequency of the selected mode. The Waveguide loop 5 is a fast-wave structure and for resonance must have a phase delay around the structure of 2m where m is an integral number. m is much smaller than N and can conveniently be a factor of N. In this case m slots 6, equally spaced, will not affect the symmetry of the device and if m is also a factor of n the slots 6 will all be in phase.

The slow-wave structure and the waveguide loop 5 have circular symmetry so that waves in clockwise or counterclockwise directions will have the same phase velocities and hence the same resonant frequencies. The magnetic field produces a definite direction of motion for the electrons and depending on the coupling of the electron beam to the forward or backward wave components of the slow-wave structure, one of these components will predominate; the symmetry of the coupling to the waveguide loop 5 will excite both components in the Waveguide loop 5 i.e. a standing Wave will be set up. The waveguide loop 5 however cannot be considered as a normal resonant cavity since if there are no reflections the positions of maxima and minima of the E field (electric field) are arbitrary and the waveguide loop 5 has resonances when the length is a number of guide wavelengths not a number of half wavelengths. By using a directional coupler to couple power out of the waveguide loop 5, the relative amplitude and phase of the two travelling wave components can be determined. Depending on the results of this, the output could be designed to couple by slots 13 to one or both components of the Wave. By introducing some asymmetry into the coupling, for example, by adjusting size, shape, orientation or position of the slots 6, between slow-wave structure and waveguide loop 5, only one component of the fast-wave would be excited, thus halving the stored energy in this loop and decreasing the RF. (radio frequency) pulse rise time.

With tight coupling between the slow-wave structure ad the waveguide loop 5, the device may be tuned by varying the phase velocity in this loop.

One method of tuning is carried out as shown in FIG. 4 by introducing a dielectric ring 21 into the waveguide loop 5. Thus a device having the same dimensions for the waveguide loop 5 may be employed at any one of a plurality of spot frequencies by introducing a respective dielectric ring 21 of the appropriate dimensions into the waveguide 5. Another method of tuning is illustrated in FIG. 5 wherein the waveguide 5 is provided with a ridge 22 on the unslotted broad side of the Waveguide loop 5. Deforming this wall will change the gap between the ridge and the opposite wall and hence alter the phase velocity of the waveguide loop 5.

1 A simplified mathematical analysis of the device folows:

The potentiial distribution without space change Consider right-handed cylindrical co-ordinates (r, 0, z, shown in FIG. 3 in which the axes are chosen so that the 0 direction is directed into the paper). In these c0- ordinates Laplases equation is 0% in 1% n 'r D1 2 a2 a2 Since (p has symmetry about the z axis ago 5 m+6 and Equation 1 reduces 5 1 ago 5 0. 7 ar+r O with the assumption that (r) +g0 (z) where (p (r) is independent of z and p (z) is independent of r A solution of Laplaces equation is:

(P(l',Z) +A log r+B +C where A, B and C are constants. For the boundary conditions that +0 at z-l-O,

r=r and that the field at r is in a direction making an angle at with the z axis.

(r, z) +E(z cos DC+TO sin a log r/r (3) Putting r +rr and neglecting terms of order in the expansion of log z)=E(z cos [-r sin or) (4) It should be noted that the assumption of independent components of to does not lead to equipotentials being generators of coaxial cones but that the approximation neglecting terms of order Since the E and B fields are substantially constant in the interaction region the linear magnetrom theory may be applied to the device except that an equilibrium condition for constant velocity at a constant radius must be applied.

The force on an electron in the given field is F =e (E-i-uxB) where a has components (r, r0, z) and B has components (B cos, 0, -B sin) and radial acceleration is then 2-2 1' r (E s1n+r0B sm) Now the only forces acting on an electron are perpendicular to the cathode surface so that if it starts from a point (r,,, O, 0) it always has co-ordinates (r d-a sin, 0, r -l-a cos) and for some value of a, the electron velocity has no 1'- or components.

At this point where V is the anode voltage and d is the anode-cathode clearance. Equation 5 then becomes Now is this is to be an equilibrium orbit i'=i"=0 and Equation 6 becomes 0 V u R (ti Sm (r -Fa sin a) Since u is a function of V, Equation 7 gives the relationship between B and V for the equilibrium orbit to be a circle about the axis r=r,,.

It should be noted that for 5::0, sin =0 and the equilibrium condition is the trivial solution u=O'. For a 0, there is always a value of B which can be found to give a solution to Equation 7 for any value of V or M. It should be noted however that if the angle or is made too small so that it approaches zero, it may be difficult or even impossible to construct a practical form of the device.

Also for r slightly greater than the equilibrium value r and for a constant u, the final term in Equation 6 will be reduced and the bracketed terms unaltered. Hence for r, r r 0.

Similarly for r r '1 O i.e. the equilibrium radius r is a stable orbit.

I claim:

1. A crossfield device, comprising an annular cathode, an annular anode arranged about the same axis as the cathode and spaced axially from said cathode to provide an interaction space symmetrical about the said axis and having an axial extension from said cathode to said anode, means operative to produce an electric field between said cathode and said anode, and means producing a magnetic field within said interaction space substantially at right angles to said electric field, said cathode and said anode being so formed that the direction of the electric filed between said cathode and said anode in said interaction space is inclined at an angle greater than 0 degrees and less than degrees to said axis.

2. A device according to claim 1 wherein the radial cross-section of said interaction space is in the form of a parallelogram.

3. A device according to claim 1 wherein a magnet is arranged to provide a magnetic field in said interaction space having a direction which is in a radial plane about the said axis and at right angles to the direction of the electric field.

4. A device according to claim 3 wherein said magnet is a permanent magnet.

5. A device according to claim 3 wherein said magnet is provided with coaxial pole pieces.

6. A crossfield device comprising an annular cathode, an annular slow wave structure constituting an anode and arranged about the same axis as the cathode and spaced axially from said cathode to provide an interaction space symmetrical about the said axis and having an axial extension from said anode to said cathode, said anode and said cathode producing an electric field in said interaction space, and means producing a magnetic field within said interaction space substantially at right angles to said electric field, the direction of said electric field in said interaction space being inclined at an angle greater than 0 degrees and less than 90 degrees to said axis.

7. A device according to claim 6 wherein said sloW- 'wave structure is a digital line comprising a plurality of fingers of which the end surfaces adjacent said interaction space are perpendicular to the direction of the electric field.

8. A device according to claim 7 wherein said fingers are a quarter wavelength long in a direction parallel to the said axis.

9. A device according to claim 7 wherein the mean radius of the fingers about said axis is equal to the mean radius of said cathode.

10. A device according to claim 7 wherein the surface of said cathode is parallel to the end surfaces of said fingers.

11. A device according to claim 6 wherein means are provided to select a pre-determined mode of propagation in said slow-wave structure.

12. A crossfield device comprising an annular slow wave structure constituting an annular anode, an annular cathode arranged about the same axis as the anode and spaced axially from said anode to provide an interaction space which is symmetrical about the said axis and has an axial extension from said anode to said cathode, a closed waveguide loop having a resonance at a frequency corresponding to a predetermined mode of propagation to select said predetermined mode of propagation in said slow-wave structure, means coupling said waveguide loop to said slow-wave structure, means operative to produce an electric field within said interaction space, and means producing a magnetic field substantially at right angles to said electric field, the direction of said electric field in said interaction space being inclined at an angle greater than 0 degrees and less than 90 degrees to said axis during the operation of said device.

13. A device according to claim 12 wherein said waveguide is of rectangular cross-section and is arranged with the broad walls thereof situated in a plane perpendicular to the said axis, said coupling means taking the form of a plurality of slots in said broad walls.

14. A device according to claim 13 wherein said slowwave structure is a digital line comprising a plurality of fingers of which the end surfaces adjacent to said interaction space are perpendicular to the direction of the electric field and the number of said slots is an integral sub-multiple of the number of said fingers.

15. A device according to claim 13 wherein said slots are all arranged so that the energy therein is in-phase.

16. A device according to claim 14 in which said selected mode is the ar-mode and the number of slots is half the number of fingers.

17. A device according to claim 13 wherein the coupling provided by said slots is made asymmetrical whereby the amount of energy stored in the waveguide loop is reduced.

18. A device according to claim 17 wherein said slots are formed asymmetrically.

19. A device according to claim 17 wherein said slots are of symmetrical form, an asymmetry being introduced into the position of said slots in relation to said waveguide loop and said slow-wave structure.

20. A device according to claim 12 wherein tuning is effected by means for varying the phase velocity in said waveguide loop.

21. A device according to claim 20 wherein said tuning means comprises a dielectric cylinder which can be selectively introduced into said waveguide loop.

22. The device according to claim 20 wherein said waveguide loop is constructed from ridged waveguide having References Cited UNITED STATES PATENTS 2,600,509 6/1952 Lerbs 31539.3 X 2,889,488 6/1959 Reverdin 315-393 2,890,372 6/1959 Dench 31539.3 X 3,223,882 12/1965 Thal 3153.5 X

ELI LIEBERMAN, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R.

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