GB2152740A

GB2152740A - Microwave amplifiers and oscillators

Info

Publication number: GB2152740A
Application number: GB08013605A
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: FR2563389A1; GB2152740B

Abstract

An amplifier or oscillator has a circular waveguide (WG), a solenoid (46) for producing a magnetic field parallel to the axis of the guide, and an electron gun (40) for producing a hollow electron beam 45 which periodically varies in spacing from the axis. The guide (WG) is arranged to propagate an RF field in the TM02 mode. Rings (44) in the guide suppress other TM modes and slots (43) in the wall of the guide suppress the TE modes. <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 l.E.E.E. Transactions on Electron Devices Vol. ED-13 March 1 966 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 beam is caused to periodically vary in diameter and interact with the wave.

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, and means for causing the guide to propagate an electromagnetic wave in the TMo2 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 TMo1 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 TMo2 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, and Figure 6 is a schematic diagram explaining the operation of the electron gun.

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 TM01 circular waveguide mode see Figure 1 This mode pattern has a maximum Ez at the axis AX falling off to zero at the guide edge ED.

The electron beam ripples (Figure 2a) allow the regions of synchronism to occur when the beam is near the axis, i.e. high Ez, and the regions of non-synchronismm to occur when the beam is near the beam tunnel where Ez is small. The method of synchronism can be seen by considering Figure 2. Figure 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 E,. Mid-way between the points, the beam radius is a maximum and the electrons are in a region of low E2.

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.

Figure 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 2fwr + a (or more generally 2nso + 0, 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 0/2, and the phase of the beam signal is 0/2 + 7r. 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 TMo 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 Figure 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 Figure 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 desctructive 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 ccmpeting modes, which are: TE, TM TE21 To01, TM" TE3, TM2, TE4, TE,2 TMo2 All TE modes are easily suppressed by a longitudinal slot in the waveguide wall.

Figure 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 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 Figure 4 and in Figure 3, the rings are positioned so that the electron beam 45 ripples through them and so that their walls are arranged at the intermediate point 1 P 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.

The electron gun and input section 40 will now be described in detail with reference to Figure 5 which shows a similar, but different, construction to that of Figure 4.

In a previously proposed electron-gun for use in the known device operating in the TMo 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 Figure 5 (and of Figure 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 rippling.

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 Figures 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 annulus of porous tungsten having a triangular crosssection 52, the face of the annulus along a line N at an angle a to the 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 Figure 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 Figure 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 external connections (not shown).

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

The angle a 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 einsel lens may be used.

Although the inventive microwave device of Figure 4 has been described in relation to an amplifier, it may be applied to an oscillator.

The inventive electron gun may be used either with the inventive ripple beam device of Figure 4 employing the TMo2 mode or with the known device employing the TMo1 mode.

The solenoid 46 may be superconductive.

Claims

CLAIMS 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, and means for causing the guide to propagate an electromagnetic wave in the TMo2 mode, to interact with the beam. 2. A device according to Claim 1, wherein the causing means includes conductive rings arranged within the waveguide to suppress all TM modes other than the TMo2 mode. 3. A device according to Claim 1 or 2, wherein the causing means includes longitudinal slots in the wall of the waveguide to suppress all the TE modes. 4. A device according to Claim 3, wherein the slots communicate with an annular cavity, the wall of which have a coating for dissipating any modes which pass through the slots. 5. A device according to Claim 1, 2, 3 or 4, wherein the electron beam forming means comprises, means for producing a magnetic field parallel to the axis, and means for producing an electron beam which is directed along a line at an angle to the axis and intersects the field. 6. A device according to Claim 5, further comprising means for causing the field to be parallel to the said line in the region adjacent the line. 7. A device according to Claim 5 or 6, comprising means for causing the electron beam to be convergent along the said line. 8. A device according to Claim 7, wherein the causing means comprises an electrostatic lens whose potential is externally controllable. 9. A device according to Claim 5, 6, 7 or 8, wherein the 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. 1 0. A device according to Claim 9, wherein the said face of the annular cathode is planar perpendicular to the said line. 11. A device according to Claim 9 or 10, wherein the cathode is supported by an annular support which is coaxial with the said axis. 1 2. A device according to Claimn 9, 10 or 11, wherein an input for introducing an RF signal into the waveguide is coaxial with, and surrounded by the annular cathode. CLAIMS New claims filed on 21st October 1982 replacing original claims 1-12.

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 and means for causing the Have guide to propagate an electromagnetic wave in the TMo2 mode, to interact with the beam.

2. A device according to Claim 1, wherein the causing means includes conductive rings arranged within the waveguide to suppress all TM modes other than the TMo2 mode.

3. A device according to Claim 1 or 2, wherein the causing means includes longitudinal slots in the wall of the waveguide to suppress all the TE modes.

4. 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.

5. A device according to Claim 1, 2, 3 or 4, wherein the electron beam forming means comprises, means for producing a magnetic field having field lines which are parallel to the axis, and means for producing a linear electron beam in the form of a cone which is inclined at an angle to the axis and intersects the magnetic field lines.

6. A device according to Claim 5, further comprising means for causing the field lines to be parallel to the electron beam in the region of the cone.

7. A device according to Claim 5 or 6, comprising means for causing the electron beam to be convergent along said cone.

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

9. A device according to Claim 5, 6, 7 or 8, wherein the electron beam producing means comprises an annular cathode coaxial with the said axis and having an annular emissive face facing the axis along the cone.

10. A device according to Claim 9, wherein the said face of the annular cathode is planar perpendicular to said cone.

11. A device according to Claim 9 or 10, wherein the cathode is supported by an annular support which is coaxial with the said axis.

12. A device according to Claim 9, 10 or 11, wherein an input for introducing an RF signal into the waveguide is coaxial with, and surrounded by the annular cathode.

1 3. A device substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.