GB2152742A

GB2152742A - Microwave amplifiers and oscillators

Info

Publication number: GB2152742A
Application number: GB08013607A
Authority: GB
Inventors: Dudley Perring
Original assignee: EMI Varian Ltd
Current assignee: EMI Varian Ltd
Priority date: 1980-04-28
Filing date: 1980-04-28
Publication date: 1985-08-07
Also published as: FR2563390A1; GB2152742B

Abstract

A microwave device has a circular waveguide (WG) propagating an electromagnetic wave in the TM01 or TM02 mode, a solenoid (46) for producing an axial magnetic field in the guide (WG) and an electron gun (40), which directs electrons along a line inclined to the axial field to produce a beam 45 of periodically varying spacing from the axis. To reduce the intensity of the field required, a periodic magnetic structure is provided. In Fig. 4 it comprises alternate rings of magnetic and non magnetic material 70, 71 forming the waveguide (WG). Alternatives include superconductive rings and a periodic permanent magnet structure. <IMAGE>

Description

SPECIFICATION Microwave amplifiers and oscillators This present invention relates to microwave amplifiers and oscillators.

There is described in an article on pages 374 to 376 of I.E.E.E. Transactions on Electron Devices Vol. ED-13 March 1966 the ''Interaction between an Electron Beam of Periodically Varying Diameter and EM Waves in a Cylindrical Guide". The article describes a device comprising a waveguide of circular cross-section along which an electron beam is passed together with an electromagnetic wave the waveguide operating in a transverse magnetic (TM mode). The beam is caused to periodically vary in diameter and interact with the wave. The periodic variation in diameter is produced by interaction of the electron beam with an axial field produced by a solenoid.

According to the present invention, there is provided a microwave device comprising a waveguide, means for forming an electron beam which extends in the direction of the axis of the waveguide and which has a periodically varying spacing from the axis the beam forming means including a periodic magnetic structure for at least partially causing the periodic spacing of the beam from the axis, and means for causing the guide to propagate an electromagnetic wave in the TMol or To,,, mode, to interact with the beam.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, in which Figure 1 is a schematic illustration of the TM,,I mode of a circular waveguide; Figures 2, a, to C are a schematic illustration of the operation of a microwave device of the type with which the invention is concerned; Figure 3 is a schematic illustration of the Two., mode of a circular waveguide: Figure 4 is a diagram of an amplifier in accordance with the invention; Figure 5 is a diagram of the electron gun of the amplifier.

Figure 6 is a schematic diagram explaining the operation of the electron gun, and Figures 7a to e show, in schematic form, modifications of the amplifier of Fig. 4.

There will now be described the hitherto known operation of a microwave amplifier, hereinafter referred to as a "ripple beam" amplifier. In this known manner of operation, the amplification of the ripple beam amplifier is based on the electrons interacting with the z-component of the electric field of a TM1)1 circular waveguide mode, see Fig. 1. This mode pattern has a maximum E, at the axis AX falling off to zero at the guide edge ED.

The electron beam ripples (Fig. 2a) allow the regions of synchronism to occur when the beam is near the axis, i.e. high E,, and the regions of non-synchronism to occur when the beam is near the beam tunnel where E, is small. The method of synchronism can be seen by considering Fig. 2. Fig. 2a shows the hollow rippled electron beam. At points A,B,C,D, etc. the beam radius is a minimum and hence the electrons are in a high value of Ez. Mid-way between the points, the beam radius is a maximum and the electrons are in a region of low Ez.

The points A,B,C, etc. define the period of the rippled beam, and because the ripples are fixed in space interaction is considered over this period.

Fig. 2c shows the instantaneous phase of the r.f. signal on the electron beam, i.e. the fundamental component of the electron bunching. Because the signal travels slower in the beam than on the structure, the beam signal experiences a greater phase change per period. However, for a synchronous condition, the beam voltage, and hence the electron velocity, is so chosen that the phase change on the beam per period is 27r + 8 (or more generally 2n7r + 8, where n = 1.) This means that if the signal on the structure and beam are in phase at A, then they are also in phase at B.

Mid-way between A and B, the phase of the structure signal is 8/2. and the phase of the beam signal is 8/2 +"'. Hence the two signals are now in antiphase and a destructive interaction takes place. This destructive interaction occurs when the electrons have rippled to a position of low field. However, because the constructive interaction takes place where the electrons have rippled into a region of high field, then over the period there is a net constructive interaction. The device described operates in the TMo1 circular waveguide mode. The interaction mechanism suffers from two disadvantages both of which would be overcome by operating in the TMo2 waveguide mode.

As shown in Fig. 3 in the TMo2 circular waveguide mode the Ez component of the electric field has a high value on the axis AX falling to zero away from the axis at some intermediate point 1 P. The Ez component then reverses between the point 1 P and the edge ED or wall of the waveguide.

Firstly, as has been shown above, one half cycle of the interaction is destructive. It is only the fact that this takes place in a low electric field that causes the net interaction over the cycle to be constructive. However, by operating in the TMo2 mode, see Fig. 3 the beam ripples can be arranged so that the constructive half cycle can interact with the electric field near the axis as before, but what was the destructive half cycle now interacts with the outer field pattern, which is in antiphase to the axial field and hence has a further constructive interaction.

Therefore, if the beam ripple is chosen to match the TMo2 mode pattern there is no destructive interaction.

Secondly, in the TMo, mode of operation, the electrons in the constructive half cycle will be retarded whilst those in the destructive half cycle will be accelerated resulting in a net break up of the ripple structure. Whereas in the TMo2 mode of operation both half cycles are retarded resulting in an overall slowing of the electrons without break-up of the ripple structure. The change in interaction due to the overall slowing of the electrons can be compensated for by a velocity taper which would take the form of a controlled flaring of the waveguide towards the output end.

In order to operate in the TMo2 mode some means are provided to suppress all the intermediate competing modes, which are: TE11 TMoz TE2 To,,. TM TE3 TM TE4, TE,2 TMo2 All TE modes are easily suppressed by a longitudinal slot in the waveguide wall.

Fig. 4 shows an exemplary ripple beam amplifier in accordance with the invention.

The amplifier comprises an electron gun and r.f. input section 40, an interaction section 41, and an output and beam collector section 42.

The amplifier uses a hollow cylindrical electron beam 45 which has a periodically varying diameter as shown.

The interaction section 41 comprises a circular waveguide WG arranged to operate in the TMo2 mode. The guide has an odd number of longitudinal slots (only one 43 shown) in its wall to suppress all the TE modes. The slots 43 communicate with an annular cavity C the walls of which are coated with Kanthal (RTM) K to dissipate any modes which pass through the slots. Within the guide there are supported, on thin insulating supports, conductive rings 44 to suppress all TM modes other than the TMo2 mode. As shown in Fig. 4 and in Fig. 3, the rigs are positioned so that the electron beam 45 ripples through them and so that their walls are arranged at the intermediate point 1P of zero field in the TMo2 mode.

A solenoid 46 having pole pieces 46' and 46" surrounds the waveguide to produce a uniform axially directed magnetic field within the waveguide.

The electron beam collector in section 42 comprises an anode 47 which is surrounded by a water jacket 48 for cooling. A coupling 49 couples the amplified r.f. signal to an output waveguide 50.

An exemplary manner in which the 'rippled beam is produced, in accordance with the present invention, will now be described.

Firstly, the electron gun and input section 40 will now be described in detail with reference to Fig. 5 which shows a similar, but different.

construction to that of Fig. 4.

In a previously proposed electron-gun for use in the known device operating In the TMt, mode longitudinal energy in a cylindrical electron beam was converted into transverse energy by a conversion step using a magnetic field to cause the beam to "ripple" i.e. periodically vary its diameter.

However in the electron gun of Fig. 5 (and of Fig. 4), instead of converting longitudinal energy into transverse energy by means of a magnetic field step, a linear beam. which preferably converges. having approximately the correct proportion of longitudinal and transverse energy is launched along a similarly converging magnetic field. The proportions of longitudinal and transverse energy are defined by the angle the linear beam makes with the axis of the waveguide. The beam then enters the region of uniform magnetic field, produced by the solenoid and thus produces spiral electron orbits, with an unchanged longitudinal energy. Hence if the inner and outer trajectories start with the same longitudinal speed. a phase coherent ripple beam is produced.An approximation to the required field is to be found at the entrance to the polepiece 46' of the solenoid 46 with return path and polepieces. It will be appreciated that the above implies that all trajectories are launched at the same angle to the axis. and that the fundamental difference between this approach and previous proposals is that energy transfer is to be prevented rather than relied upon to cause the ripping.

Also many practical problems are eliminated by this approach-a larger gun. external to the solenoid, may be used and an input r.f.

coupler introduced via the centre of the gun.

This design lends itself more than any other to reduction of size for higher frequency of operation. Provided the magnetic field can be suitably shaped it is not necessary to reduce the size of the gun by the same factor as the rest of the tube, as was required in previous proposals.

Referring to Figs,. 5 and 6, the electron gun comprises an annular dispenser cathode 1 50 with a 'Pierce type' focussing electrode P. The cathode is coaxial with the axis AX of the waveguide WG and comprises an annuius of porous tungsten having a triangular crosssection 52, the face of the annulus facing along a line N at an angle a to the axis AX. A ring 53 with a groove accommodating a heater is placed in contact with the back of the annulus of tungsten. The cathode is supported on an inner cylinder of molybdenum 54 via a heat shield 57.

The grid assembly 55 comprises a tubular member a part 55' of which is flared outwardly parallel to the face of the tungsten annulus 52. In the part 55' is defined an annular grid of fine radial webs. The grid controls the cathode current to obtain a low perveance without making the accelerating field region too large. The grid in operation has a positive voltage (e.g. + 600V) relative to the cathode.

An anode 62, which is maintained at for example OV relative to earth, has an annular slot 63 in it aligned with the beam path from the cathode.

Adjacent the anode are the pole piece 46' of the solenoid and a magnetic field modifier 64. The modifier 64 comprises a cylindrical member of magnetic material arranged on the axis of the waveguide. The function of the modifier will be described hereinafter with reference to Fig. 6.

The waveguide mode used for the ripple beam interaction favours an axial input (or output) coupler. Advantage is therefore taken of the annular cathode and hollow beam by introducing the input coupler 60 via the centre of the electron gun. The one drawback of this is that the gun now has very high electric fields both outside and inside. making the shielding of sharp corners, e.g. heater connections, spot welds and brazes etc., much more of a problem than usual. As may be seen from Fig. 5. two ceramic insulators 58, 59 are required to complete the vacuum envelopes of the tube, and a special U-shaped flange 61 is required to allow for differential thermal expansion of the ceramics and the input coupler 60.

The cathode package 1 50 and grid assembly 55 are supported on respective separate coaxial cylinders 56 and 54, one (54) of molybdenum and the other (56) of stainless steel to give a degree of temperature compensation. All heater and grid connections are made in the interspace between these. The connections (not shown) extend between the cylinders and through a ceramic ring 61 insulating the cylinder from one another, and through a stainless steel ring 62 to extern, connections (not shown).

Referring to Fig. 6 the electron gun operates to produce a hollow beam which has a periodically varying diameter as follows.

The angle C made by the normal N to the face of the dispenser cathode 52 relative to the axis AX defines the proportions of transverse energy (i.e. perpendicular to AX) and axial energy in the electron beam. The beam thus forms a hollow cone as indicated by lines N.

The solenoid produces an axial field F in the waveguide WG parallel to the axis AX and the interaction of the beam with the field F causes the electrons in the beam to follow spiral paths along the field, thus producing the periodically varying diameter.

Outside the waveguide, i.e. adjacent the anode 63 and the cathode 52, the field is not parallel to the axis but would curve round, if the modifier 64 were not provided. The purpose of the modifier is to cause the field to be parallel to the lines N in the region between the anode 63 and the cathode 52 with as sharp a transition as possible between the field lines being parallel to the axis AX and parallel to the lines N.

It has been found that it is desirable to make the electron beam convergent along the lines N. For this purpose electrostatic focussing means may be provided on the grid 55.

As shown at 66 these means may be humps on the grid, or alternatively as shown at 67 an electrostatic lens whose potential is externally controllable may be used.

The electron gun as so far described produces, in conjunction with the solenoid 46, a spiralling electron beam. However, the magnetic field required is intense. Thus, in accordance with the present invention, a periodic magnetic structure is provided to reduce the intensity of the field.

In the example of Fig. 4, the wall of the waveguide WG is formed of alternate rings of soft magnetic material e.g. iron 70 and rings of non-magnetic material e.g. copper 71. The rings of iron locally reduce the intensity of the axial magnetic field in the waveguide due to the solenoid. This aids the production of a 'rippling' beam. In the shown example, the widths of the copper and iron rings are equal and equal half the period of the ripple on the electron beam. However, the widths may be unequal. Furthermore the rings are shown as having a rectangular cross-section, although as shown in Fig. 7a, the iron rings may taper radially inwardly, or be of any other cross section.

The iron rings may, as shown in Fig. 7d be replaced by superconducting rings 72, which again produce a periodic disturbance in the axial field (indicated by Ba in Figs. 7a and 7d) produced by the solenoid.

Figs. 7b and c show periodic magnetic structures which do not require the use of a solenoid and the production of an axial field.

Thus the solenoid 46 may be removed. However, means must then be provided to produce the magnetic field parallel to the lines N of Figs. 5 and 6.

In Fig. 7b a periodic arrangement of permanent magnets PM is shown. The magnets are held together with like poles adjacent, being separated by pole pieces 73. The use of just the magnets PM and pole pieces 73 would produce a roughly sinusoidally varying magnetic field along the axis. The field can be made 'squarer' by putting further pole pieces 74 in the gaps between the pole pieces 73 to introduce a third-harmonic of the basic sinusoid. The magnets are of rare-earth cobalt magnetic material.

In Fig. 7c the periodic magnetic structure comprises periodic superconducting current loops. A ring-bar structure or a bifilar helix structure is used to produce current loops which alternate in the direction of flow of current, thus produced the required periodic magnetic field.

It will be appreciated that the solenoid 46 of Fig. 4 may be superconductive.

It will also be appreciated that the periodic magnetic structure of Figs. 4 and 7 may be applied to the device using the TMo1 mode as well as to the device using the TMo2 mode.

Furthermore, although the invention has been described in relation to an amplifier, clearly it may be appiied to an oscillator.

CLAIMS (Filed on 15 April 1981) 1. A microwave device comprising a waveguide, means for forming an electron beam which extends in the direction of the axis of the waveguide and which has a periodically varying spacing from the axis the beam forming means including a periodic magnetic structure for at least partially causing the periodic spacing of the beam from the axis, and means for causing the guide to propagate an electromagnetic wave in the TMo1 or TMo2 mode, to interact with the beam.

2. A device according to Claim 1, wherein the beam forming means includes means for forming in the waveguide a magnetic field parallel to the axis, and means for producing a periodic disturbance in the said field to at least partially cause the periodically varying spacing of the beam from the axis.

3. A device according to Claim 2, wherein the means for producing a periodic disturbance comprises alternate rings of magnetic material and non magnetic material.

4. A device according to claim 3, wherein the wall of the waveguide is formed of the said rings.

5. A device according to Claim 3, wherein rings of magnetic material are superconductive.

6. A device according to Claim 1, wherein the beam forming means includes a periodic arrangement of permanent magnets.

7. A device according to Claim 6, wherein the beam forming means includes permanent magnets held together with like poles adjacent but separated by pole pieces.

8. A device according to Claim 7, comprising further pole pieces between the said pole pieces separating the magnets.

9. A device according to Claim 1, wherein the beam forming means includes means for producing periodic superconducting current loops.

1 0. A device according to Claim 9, wherein the beam forming means comprises a superconductive ring-bar structure or a bifilar helix structure.

11. A device according to any preceding claim, wherein the causing means causes the guide to support the TMo2 mode and suppress other modes.

1 2. A device according to Claim 11, wherein the causing means includes conductive rings arranged within the waveguide to suppress all TM modes other than the TM,,, mode.

13. A device according to Claim 11 or 12, wherein the causing means includes longitudinal slots in the wall of the waveguide to suppress all the TE modes.

14. A device according to Claim 3.

wherein the slots communicate with an annular cavity, the walls of which have a coating for dissipating any modes which pass through the slots.

15. A device according to any preceding claim, wherein the beam forming means includes means for directing a beam of electrons along a line inclined to the said axis and intersecting the magnetic field formed in the waveguide by the periodic magnetic structure to produce the said period spacing.

1 6. A device according to Claim 1 5, further comprising means for causing the field to be parallel to the said line in the region adjacent the line.

1 7. A device according to Claim 1 5 or 16, comprising means for causing the electron beam to be convergent along the said line.

1 8. A device according to Claim 1 7, wherein the causing means comprises an electrostatic lens whose potential is externally controllable.

19. A device according to Claim 1 5, 16, 17 or 18, whereinthe electron beam producing means comprises an annular cathode coaxial with the said axis and having an annular emissive face facing the axis along the said line.

20. A device according to Claim 19, wherein the said face of the annular cathode is planar perpendicular to the said line.

21. A device according to Claim 19 or 20, wherein the cathode is supported by an annular support which is coaxial with said axis.

22. A device according to Claim 1 9, 20 or 21, wherein an input for introducing an RF signal into the waveguide is coaxial with, and surrounded by, the annular cathode.

Claims

New claim filed on 25th April 1 983 Superseded claim 1.

1. A microwave device comprising a waveguide, means for forming an electron beam which extends in the direction of the axis of the waveguide and is spaced from said axis by a distance varying periodically along said axis, the beam forming means including a magnetic structure producing a periodic variation of magnetic field strength along said axis, for at least partially causing the periodic spacing of the beam from the axis, and means for causing the guide to propagate an electromagnetic wave in the TMo1 or TMO2 mode, to interact with the beam.