GB2109986A - Gyro amplifier - Google Patents

Abstract

A gyro amplifier includes a waveguide (1), circular in cross-section, a solenoid (7) for generating an axial in magnetic field within the waveguide and an injector (8) for directing a hollow beam (4) of electrons, in the form of a cone, into the waveguide. The electron beam interacts in the waveguide with radiation having a circularly polarised electric vector. Such radiation is introduced into the waveguide by a coaxial transformation means (30) which converts incident RF radiation, having a linearly polarised electric vector, into radiation having a circular polarised electric vector. A relatively wide bandwidth can be achieved with this arrangement as compared with that achieved using the hitherto known technique involving combining two degenerate linear polarised modes. The transformation means may comprise the series arrangement of a rectangular-to-circular waveguide transformer (31) a Faraday rotator (32) and a circular polariser (33) (Fig. 4, not shown). <IMAGE>

Description

SPECIFICATION Gyro amplifier The present invention relates to a gyro amplifier e.g. a gyro-TUlT or a gyro klystron.

For a better understanding of the background to the invention, the operation of a gyrotron device will be described initially by reference to Figures 1 and 2 I ofthe accompanying drawings, in which: Figure lisa section through a waveguide of a gyrotron device, and Figure 2 illustrates an electron gyrating in an elec tric field and in a magnetic field.

The manner of operation of a conventional gyrot ron device is, in outline, as follows: Referring to Figure la a conventional gyrotron device comprises a circular waveguide 1 dimen sioned to be an interaction region and to operate in the TEo1 mode at a chosen frequency in the RF range.

The TE01 mode electric field is shown by dashed lines 2 in Figure 1. An axial magnetic field 3 of strength B is applied to the waveguide and a hollow electron beam, the inner and outer bounds of which are indi cated by thick lines 4, is passed along the waveguide.

As shown at 5 in Figures 1 and 2, an individual electron 6 is caused to gyrate under the influence of the magnetic field.

The electron gyrates at the so-called cyclotron fre quency.

o, =e B/m (i) Wheree is the electronic charge, B is the magnetic field strength, and m the relativistic mass of the elec tron.

The radius of the orbit is given by r= mv (ii) eB where v is the tangential velocity of the electron.

The magnitude of the electric field is given by E = E0 cos xot (iii) where zO is the angular frequency associated with the applied RF field.

At time t = 0, the electric field is at a maximum givenbyE=E0.

An electron at position A will experience a max imum retarding field, whereas an electron at posi tion B will experience a maximum accelerating field.

Half a cycle later, at time = wd the electric field will once again be at a maximum, but in the opposite direction, i.e. E = -E,.

tf the angular frequency of the electron, Wc, is equal to the angle to the angular frequency o, of the applied R.F. field then the electron that started at A will now be at B, and once again experiencing a retarding field, whereas the electron that started at B will now be a A and once again experiencing and accelerating field.

In the conventional gyrotron device, electrons in the beam have, at least when they are initially in the waveguide, many different phases relative to the RF field.

It can be seen that all electrons starting at time t = O over the sector CAD, will experience a net decelerating field over a cycle. Therefore their velocity will decrease, as will their mass and hence, from equation (i) their frequency of rotation, zc, will increase, so that they will advance in phase with respect to the applied r.f. electric field.

Electrons in this sector will therefore advance in phase, moving cycle by cycle, towards point C. Also from equation (ii), as the electron's mass and velocity decreases, so its radius of gyration will decrease.

Conversely, all electrons starting at time t = 0 over the sector C B D will experience a net accelerating field. Their mass will increase and hence their frequency of rotation, xc, will decrease, causing them to retard in phase with respect to the applied r.f. electric field. So electrons in this sector also will, cycle, by cycle, tend to move towards point C, with an ever increasing radius.

Hence there is cycle, by cycle, a bunching of all the electrons towards point C.

In a conventional gyrotron device, the cyclotron frequency c is slightly less than the angular frequency o,, e.g. xO = 1.029 and the phase of the bunched electrons relative to the field is adjusted so that the electrons give up nett energy to the RF field in excess of cavity losses so an oscillation results and output power is available. The output power is dependent on the numbers of electrons bunched in the appropriate phase to give up energy to the RF field.

An application of a gyrotron device of the abovedescribed kind is the gyro amplifier, e.g. the gyro Travelling Wave Tube (TWT) or the gyro klystron in which a propagating waveguide is used to enable continuous interaction of the electron beam with an RF field. Gyro amplifiers are known in which the electron beam interacts with a TEo1 waveguide mode; however interaction with the circularly polarised TEt, dominant waveguide mode is preferred since this is the lowest frequency mode and is not, therefore, affected by interference from other modes. Hitherto, it has been possible to propagate a circularly polarised TE11 mode by combining two degenerate linearly polarised TE't modes excited, in space and time, in phase quadrature.Maintenance of an accurate phase relationship between the modes proves difficult, however, and a relatively narrow band width results.

It is an object of the present invention to provide a gyro amplifier in which the above-identified difficulties are substantially alleviated.

According to the present invention there is pro vided a gyro amplifier comprising: a waveguide which is circular in cross-section and is dimensioned to accommodate radiation conform ing to a circularly polarised electric wave mode and to act as an interaction region at a predetermined RF frequency, means for receiving RF radiation, intended for amplification, having a transverse elec tric mode and for transforming said radiation into radiation having a circularly polarised electric mode, the waveguide being aligned coaxially with the transformation means to receive circularly polarised radiation emanating therefrom, means for generating an axial magnetic field, in the interaction region of strength to cause electrons to gyrate at a predetermined cyclotron frequency and means for injecting into the waveguide an electron beam having such a preset component of velocity perpendicular to the axis of the waveguide as to cause the electrons in the beam to gyrate in the magnetic field of the said strengh at the cyclotron frequency and such a component of velocity parallel to the axis as to produce a plurality of cycles of the beam in the cavity.

In an embodiment, the injection means comprises an annular electron gun coaxial with the axis of the waveguide and arranged to direct a hollow linear electron beam, having a preset beam velocity, towards the said axis at an angle of incidence thereto defining the said velocity components and means for causing the said magnetic field to be parallel to the beam where the beam is linear.

In order that the invention may be more fully understood and carried into effect a specific embodiment thereof is now described by way of example only by reference to the Figures of the accompanying drawings in which, Figure 3 shows cross-sectional view through a gyro amplifier, Figure 4 shows an exploded view of polariser arrangement, and, Figure 5 is a vector diagram useful in understanding operation of the polariser arrangement.

Referring to Figure 3 a waveguide 1 of circular cross-section is dimensioned as an interaction region to operate in the circularly polarised TE1' mode at a frequency zO of RF radiation propagating therein.

As described in articles by Chu, Barnett and Granatstein (IEEE Trans on Electron Devices Vol. Ed 28 p866 875) the waveguide may be tapered. Electrons are injected into the waveguide by an injector, shown generally at 8, and are caused to gyrate at the cyclotron frequency xc in response to a magnetic field of strength B passing substantially axially along the waveguide. The magnetic field is generated by a solenoid 7 surrounding the waveguide.

The injector 8 generates a hollow beam 4 of electrons in the form of a cone which intersects the axis of waveguide 1 at an angle a. This configuration is especially suitable for use in the gyro amplifier operating in the TE'1 mode since circularly polarised radiation can be fed into the device, for amplification, along the axis of the waveguide, the elements forming the injector being located around the axis.

In this example, RF radiation is introduced into waveguide 1 using a three component polariser arrangement shown generally at30 in Figures 3 and 4 and illustrated on an enlarged scale in Figure 5.

Linearly polarised radiation, intended for amplification in the gyro amplifier, is incident at rectangular to - circular waveguide transformer 31 and passes to a Faraday rotator 32. The rotator includes a ferrite rod 34 positioned atthe centre of a length of circular waveguide 35. A coil of wire 36 surrounds the waveguide so that an axial magnetic field can be applied to the rod by means of a drive current The purpose of the rotator is to rotate the incident field through an angle of 45 , either in the clockwise sense or the anticlockwise sense according to the direction of the applied drive current. The radiation is then incident on a circular polariser 33.The illustrated polariser 33 is constructed from a circular waveguide with two plates 37, 38 inserted, as illustrated, so that the guide has a width different in the horizontal and vertical principal planes. The incident electric vector E at 45" to the broad face of the plate can be resolved into two equal componets having E vectors at right angles to and parallel to that face.

The polariser operates to introduce a phase delay of 90 to one component so that the emergent components are in time and spece quadrature i.e. the wave is circularly polarised. The hand of polarisation can be reversed by using the Faradey rotator to rotate the E vector, incident on the polariser, through 90".

If desired, the Faraday rotator 32 may be omitted from the polariser arrangement, radiation being passed directly from the waveguide transformer 31 to the polariser33.

Figure 5 is a vector diagram illustrative of the arrangement of Figure 4 and shows the orientation of the electric vector Eat the different positions in the arrangement beneath which the respective vectors are aligned.

Referring again to Figure 3 the injector 8 comprises an annular thermionic cathode 9, of triangular cross-section, coaxial with the axis 10 of the waveguide 1, the cathode having a flat annular emissive surface 11 facing the axis 10, the normal 12 to the surface 11 having an angle of incidence a to the axis. An annular heater 13 is provided for the cathode 9.

A control grid 14 is annular and spaced from, and parallel to, the emissive surface 11 of the cathode, being in the form of a truncated hollow cone having many apertures 15 in itforthe passage of electrons therethrough. An annular anode 16 having apertures 17 in it for the electrons is also provided.

The electrons in the beam are consl:-pired to followthe normal 12 by producing a magnetic feld directed parallel to the normal 12. This field is produced by modifying the lines of force of the magnetic field of the solenoid using some form of magnetic field modifier. In the example, an annular magnetic coil 18 on that side of the cathode 9 remote from the solenoid is used. The modification produces a magnetic field which is as nearly parallel to the normal 12 as possible with an abrupt transition parallel to the axis 10. In order to ensure the coherence of the beam, if required, an additional annular elecrode is provided on the grid 14. This additional electrode may take the form Of two annular wires 19 positioned at the respective sides of the grid 14. Each wire may be replaced by an annular eiectrode having a humped cross-section as shown at 20. The poten tials applied to the cathode 9, the control grid 14, the additional electrode 19 or 20 and the anode 16 are chosen to produce an electron beam having a desired beam current and a desired beam velocity.

It will be appreciated from Figure 3 that the elements which constitute the injector 8 surround, and are spaced from the axis 10, and as described hereinbefore this permits the use of the polariser arrangement, described in relation to Figures 4 and 5, which is mounted coaxially with the waveguide.

The beam velocity and angle a of incidence to the axis 10 is chosen so that: the component of velocity normal to the axis produces gyration of the electrons in the beam at the cyclotron frequency Zc= eB m required for interaction with the RF field of frequency sO; and the component of velocity parallel to the axis is such that a plurality of cycles of the gyrating beam exist in the resonant cavity. In this example, the ratio of normal to parallel components of velocity can be respectively 1:5. As the electron beam 4 passes along the waveguide 1 it progressively gives up energy to the RF field propogating in the circularly polarised TE" mode in the waveguide, which is thereby amplified. Magnetic coils 21 are provided to cause the beam to diverge once it has emerged from the waveguide 1 to impinge on the collector region 22 of an output waveguide 23 of the amplifier which is sealed by a window 24.

Circularly polarised radiation emergent from the waveguide 1 may, if required, be converted back to the linearly polarised TEo1 mode by using the polariser arrangement of Figure 4 in reverse.

A gyro amplifier constructed in accordance with the present invention provides a convenient arrangement for achieving interaction of a circularly polarised RF field with a gyrating electron beam.

Claims (8)

1. A gyro amplifier comprising: a waveguide which is circular in cross-section and is dimensioned to accommodate radiation conforming to a circularly polarised electric wave mode and to act as an interaction region at a predetermined radio frequency (RF), means for receiving RF radiation, intended for amplification, having a transverse electric mode and for transforming said radiation into radiation having a circularly potarised electric mode, the waveguide being aligned coaxially with the transformation means to receive circularly polarised radiation emanating therefrom, means for generating an axial magnetic field, in the interaction region of strength to cause electrons to gyrate at a predetermined cyclotron frequency and means for injecting into the waveguide an electron beam having such a preset component of velocity perpendicular to the axis of the waveguide as to cause the electrons in the beam to gyrate in the magnetic field of the said strength at the cyclotron frequency and such a component of velocity parallel to the axis as to produce a plurality of cycles of the beam in the cavity.

2. A gyro amplifier according to Claim 1 wherein the injection means comprises an annular electron gun coaxial with the axis of the waveguide and arranged to direct a hollow linear electron beam, having a preset velocity, towards said axis at an angle of incidence thereto defining said velocity components and means for causing said magnetic field to be parallel to the beam where the beam is linear.

3. A gyro amplifier according to Claim 1 or Claim 2 wherein the transformation means comprises the series arrangement of a rectangular - to - circular waveguide transformer for receiving RF radiation having a transverse electric mode and a circular polariserfor producing therefrom radiation having a circularly polarised electric mode.

4. A gyro amplifier according to Claim 3 including a Faraday rotator between the rectangular - to circular waveguide transformer and the circular polariser.

5. A gyro amplifier according to any one of Claims 2 to 4 wherein the means for causing the magnetic field to be parallel to the beam where the beam is linear comprises an annular magnetic coil arranged coaxially with the waveguide on the side of the injection means remote from the means for generating said axial magnetic field.

6. A gyro amplifier according to any one of Claims 1 to 5 wherein the waveguide tapers in the direction approaching the output thereof.

7. A gyro amplifier according to any one of Claims 1 to 6 including means for converting circularly polarised radiation generated at the output of the waveguide to radiation having a transverse electric mode.

8. A gyro amplifier substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.