Couplers for extracting RF power from a gyrotron cavity directly into fundamental mode waveguide

Abstract: In a gyrotron cavity resonator, generated energy is extracted into a symmetric set of fundamental-mode waveguides (34) by ports (30, 32) disposed to couple energy in phase from the operating electromagnetic mode but in anti-phase with respect to an unwanted mode of lower cutoff frequency than the operating mode, thereby neutralizing coupling to the unwanted mode. A second set of interspersed ports may be disposed to load degenerate, orthogonal modes.

Description: The invention pertains to a gyrotron, using beam-interaction cavity circuits operating in higher order modes. The generated wave energy is separated from the beam into an output waveguide. In present cyclotron art, the cavity is excited in a circular-electric field mode, TE onm . The generated TE on waveguide wave is extracted by passing axially through the beam collector to an output window. The electron beam is spread out and collected on the wall of the waveguide, which is usually enlarged in this region to reduce the dissipated power density. The separation has posed many problems. Unseparated electrons go out the waveguide and bombard the dielectric vacuum window. Also, the TE on is not the fundamental waveguide wave, so directing and utilizing it entails problems of mode conversion and mode interference. US-A-4 200 820 describes a method of diverting the output waveguide from the electron-beam channel by a diagonal mirror with an aperture for beam passage. This brings some problems in that local, non-propagating fields generated at the aperture distort the wave "reflection" in the mirror generating competing lower-order mode in the interaction cavity. US-A- 4 460 846 describes a method of diverting the electrons into an enlarged collector while allowing the circular wave to pass through a waveguide, smaller than the collector, to the external load. For many uses, such as feeding antennae, it is still necessary to convert the power into a fundamental mode such as the TE₁₀ in rectangular waveguide. Many mode converters are known in the waveguide art, but they suffer from waveguide mismatches, narrow bandwidth or limited power-handling capacity. An object of the invention is to provide coupling from a higher-order mode in a cavity into fundamental modes in output waveguides. A further object is to provide output coupling which is inherently free from exciting lower-order modes in the cavity. A further object is to provide phase-locked coupling into a plurality of output waveguides. These objects are realized by a plurality of output ports disposed to neutralize coupling to cavity modes of cutoff frequency lower than the cavity operating frequency. This is done by locating each pair of similar ports symmetrically with respect to the fields of the cavity mode such that the couplings to lower-order modes excited by their coupling impedances are exactly out of phase. FIG. 1 is an axial section of a gyrotron embodying the invention. FIGS. 2, 3, 4 and 5 are sketches of the field lines of the pertinent modes in a cylindrical cavity. FIG. 6 is a graph of field intensity in a TE nm2 mode. FIG. 1 is an axial section of a gyrotron embodying the invention. A cathode structure 10 has a truncated electron-emissive surface 12 heated by an interior radiant heater (not shown) fed through an insulated lead-in 14. A hollow conical anode 16 supported by a hollow dielectric cylinder 18 from the metallic vacuum envelope 19 draws a hollow beam of electrons 20 from emitter 12. An axial magnetic field deflects beam 20 to produce an azimuthal motion component and limit its radial motion. Anode 16 may have a greater taper than emitter 12 to improve focusing of hollow beam 20 and give it an axial motion component. After leaving anode 16, beam 20 may be further accelerated by axial electric field to an apertured end-plate 21 of vacuum envelope 19. In this region, the axial magnetic field increases to reduce the beam diameter and increase the transverse velocity at the expense of axial velocity. Beam 20 passes through an input aperture 22, preferably of diameter to be cut off as a waveguide for the operating frequency. Beyond aperture 22, beam 20 passes through an interaction chamber 24 and leaves through an output aperture 26 into an enlarged beam collector 28. In collector 28, the axial magnetic field decreases rapidly so the beam expands under magnetic and space-charge forces before being dissipated on the walls of collector 28, which are in contact with a fluid coolant. Cavity 24 is resonant in a TE mode to interact with transverse components of electron motion. The generated electromagnetic wave energy is extracted through apertures 30, 32 leading via waveguides 34 and dielectric vacuum windows 36 to useful loads (not shown). The described above, prior-art gyrotrons usually operated in TE o cavity modes and the power was extracted through the cylindrical collector into a TE o mode in axial, circular waveguide to prevent mode conversion by any parts which are not circularly symmetric. To get the wave into a fundamental-mode waveguide where it could be handled by known methods requires elaborate mode convertors which are imperfect, narrow-band, power lossy and subject to power-limiting arcing. The present invention provides means for coupling directly into TE₁₀ waveguide, thereby eliminating mode converters and window failure by beam electrons leaking through the collector. The simplest of these means is illustrated in FIG. 2. The unperturbed field patterns in circular waveguide 40 are shown for the TE o1 . The other modes having lower cutoff frequencies are the TE₁₁ and TE₂₁ shown in FIGS. 3 and 4. The TE₁₁ and TE₂₁ have longer cut-off wavelengths than the TE o1 and can resonate in a waveguide designed for TE o1 and hence can be coupled to the TE o1 mode by any mechanical asymmetries. Higher order modes of cutoff frequencies higher than the TE o1 generally cannot resonate in the TE o1 resonator which is cut off for them. Electric field lines 42 in the plane of the paper are shown. Magnetic lines are not shown. FIG. 2 is the TE o1 mode used in many conventional gyrotrons, where electric field lines 42 are closed, coaxial circles. FIG. 3 is the lowest-order or "dominant" mode, the TE₁₁. It corresponds topographically to the TE₁₀ in rectangular waveguide. FIG. 4 is the TE₂₁ mode which may be used as the operating mode in gyrotrons embodying the invention. Consider a resonator operating in the TE o1 mode. If we place a coupling aperture 30 in waveguide 40, the surface current 46, which flows in a circular path in the wall, creates a localized electric field across aperture 30 which extends into circular guide 40, falling off with distance from aperture 30. This unsymmetric field couples to and excites the TE₁₁ mode of FIG. 3 which, having a lower cutoff frequency than the operating TE o1 mode, can build up and resonate in a variety of axial variations depending on resonator length and terminations. According to the invention, a second coupling aperture 32 is positioned 180 degrees in azimuth from first aperture 30 and at the same axial position. The wall current 46 is in the opposite direction from that at aperture 30, so the excitation of the lower-order mode TE₁₁ is exactly 180 degrees out of phase and the combination of the two apertures neutralizes the excitation of TE₁₁. In the cavity at its TE o1 cutoff frequency, i.e. in a resonant state, only two other modes are above their cutoffs, the TE₁₁ described above and the TE₂₁ of FIG. 4. For the TE₂₁, it is evident that at any two points in the wall 180 degrees apart the wall currents 46 are in the same direction, so the two coupling apertures 30, 32 of FIG. 2 would couple the TE₂₁ to the TE o1 . If, however, a second pair of opposed apertures 50, 52 are placed 90 degrees from the first set 30, 32, their wall currents 46 are in opposite rotational sense from those at apertures 30, 32 so that the coupling to the TE o1 mode is neutralized. The coupling to TE₁₁ is also neutralized because in the TE₁₁ opposite wall currents are always in opposite direction and in the TE₂₁ they are always in the same direction. The fact that mode decoupling is based on these fundamental symmetries shows that this neutralization is valid independently of the azimuthal rotation of the modes. The TE₁₁ has a two-fold degeneracy in that a 90 degree rotation produces an orthogonal mode uncoupled from the original. The TE₂₁ has a 4-fold degeneracy in that a 45 degree rotation produces an orthogonal mode. The mode polarization set up in a cylindrical resonator is generally determined by asymmetric excitation and loading conditions. In an oscillator, the mode with the lowest loading generally prevails. Of course, two degenerate modes can coexist. If their fields are 90 degrees out of phase, they form a circularly polarized wave. According to the invention, power is extracted from a TE o1 or TE₂₁ resonator without exciting any other modes which can resonate, four identical waveguides are coupled to the resonator through four identical coupling apertures 30, 32, 50, 52. To preserve the symmetry, waveguides 36 lead to identical loads. If desirable, the power in the guides can be combined into a single guide by symmetric combining circuits well known in the art. To eliminate effects of imperfect matches in the combiners, the guides are preferably of the same electrical length. To combine in the same polarization may require phase or polarization inverters. Gyrotron operation does not require any particular mode pattern in the resonator because the cyclotron orbits of the electrons are generally small compared to the field pattern. TE on modes have prevailed in the prior art because the cavity losses are relatively small, the symmetry allows convenient damping of spurious non-circular modes, the electric field maxima are removed from the wall so the convenient, hollow electron beam can be at field maxima without undue interception on the wall, and all parts of the beam can interact with the same electric field. However, in pursuit of higher power at higher frequency, higher order modes allow larger structures and larger beams. For example, the TE₂₁ becomes feasible with the balanced couplings of the present invention. The TE₁₁ can resonate in the TE₂₁ resonator, but coupling is neutralized. The TE₂₁ resonator may be made larger, allowing the TE o1 to be above cutoff, but coupling to it also is neutralized. The invention provides decoupling for still higher order TE nm modes. For one of these, n pairs of output apertures are needed, evenly spaced in azimuth. There is a second mode problem in most overmoded resonators. This is degenerate modes. For each TE nm mode, for example, there is an identical degenerate mode whose field pattern is rotated by 90/n degrees. FIG. 5 shows the TE₂₁ mode degenerate to the one shown in FIG. 4. By the symmetry of the field patterns, these two degenerate modes are uncoupled from each other. If the resonator is to operate in a first mode as in FIG. 4, it will be loaded to extract energy from this first mode, but then the loading apertures will be at points of zero wall currents for the second degenerate mode of FIG. 5. No energy will be coupled out from this second mode, so the resonant impedance will be very high and the oscillation will build up in the unloaded degenerate mode rather than the desired operating mode. The invention comprises means for loading the unwanted degenerate modes more heavily than the desired operating modes, a process called "mode suppression". Additional loading apertures 54,56, 58 and 60 are provided, azimuthally midway between the output ports described above. These ports are heavily coupled to dissipative loads, such as well-known waveguide waterloads or dry lossy material such as plastic or ceramic containing carbon or metallic carbides. By following the same symmetry pattern as that of the useful mode, these mode suppressors do not disturb the fields of the desired modes by mode interference. A somewhat different embodiment is to have the loading impedance at the secondary ports 54, 56, 58 and 60 exactly equal to that at primary apertures 30, 32, 50 and 52 and coupling the secondary apertures to useful loads. Then both degenerate modes are used so their relative strengths are immaterial. The secondary outputs will be 90° out of phase with the primary ones, so combining the two sets requires 90° phase shifters in the waveguide. It may become physically impractical to place so many waveguides around the resonator at the same axial position. In that case, the set of mode suppression load apertures 54, 56, 58 and 60 is displaced from the set of power-output apertures 30, 32, 50, 52. The oscillating mode is then preferably a TE mn2 . FIG. 6 is a graph of the axial variation of electric field strength (squared) 62 inside the cavity 24′. For simplicity, only one load aperture 30′ and one mode-suppression aperture 54′ are indicated. In this 2-dimensional graph, they are shown in the same axial plane. In 3 dimensions, the two are displaced by 45 degrees as in FIG. 5. Each set of apertures 30′ et al and 54′ et al is placed at an axial maximum of electric field and hence of wall current. The two maxima 66, 68 may be somewhat different in amplitude due to axial growth of the wave, but as long as each set has the required azimuthal symmetry the operation is not impaired. In fact, as described above, it is preferable to have the mode-suppression loading at apertures 54, 56, 58 and 60 heavier than the useful output loading at apertures 30, 32, 50 and 52, so the mode-suppression apertures 54, 56, 58 and 60 are downstream from the load apertures. The fact that the fields are out-of-phase at the mode suppression apertures is immaterial because in proper operation there is no excitation of the unwanted degenerate mode.

Claims: A gyrotron comprising an interaction cavity (24) for supporting a transverse electric field wave in a higher-order mode in a cavity resonator in energy-exchanging relation with an electron beam, means for extracting electromagnetic energy from said cavity into the fundamental modes of a plurality of waveguides, said means comprising at least a pair of coupling apertures (30, 32, 50, 52, 54, 56, 58 60) in the cavity wall located at positions where the wall currents of said higher-order mode are equal in amplitude and phase, and the wall currents of an unwanted lower-order mode are equal in amplitude and phase, but reversed in direction between said apertures with respect to said wall currents of said higher-order mode. The gyrotron of claim 1 wherein said cavity (24) is an axial cylinder and said pair of coupling apertures (30, 32) are at the same axial position and differ in azimuth by 180 degrees. The gyrotron of claim 2 wherein said higher-order mode is a TE nm mode, and comprising n pairs of said apertures (30, 32, 50, 52) at the same axial position and equally spaced azimuthally where n is the azimuthal mode number of the desired mode. The gyrotron of claim 2 wherein said higher-order mode is a TE on mode and said lower-order mode is a TE nm mode and comprising n pairs of said apertures (30, 32, 50, 52) at the same axial position and equally spaced azimuthally where n is the azimuthal mode number of the desired mode. The gyrotron of claim 3 further comprising a second set of n pairs of said coupling apertures (54, 56, 58, 60) at the same axial position and azimuthally spaced equally from said apertures of said first set. The gyrotron of claim 5 further comprising waveguide means for conducting energy from said second set into waveguides with identical load impedance. The gyrotron of claim 6 wherein said load impedance is such as to provide heavier loading at said second set of apertures than at said first set. The gyrotron of claim 6 wherein said load impedance of said second set is equal to said load impedance of said first set and both sets are connected to useful loads. The gyrotron of claim 8 further including means for combining wave energy from said two sets, said combining means comprising differential phase shifter means for combining said wave energy in phase. The gyrotron of claim 3 or claim 4 further comprising a second set of n pairs of aperture (54, 56, 58, 60) equally spaced azimuthally from said apertures of the first set and at an axial position removed from said axial position of said first set. A gyrotron as claimed in any one of claims 1 to 10 wherein said coupling apertures couple wave energy into fundamental-mode waveguides with equal load impedance. The gyrotron of claim 11 further including means for combining the output of at least one pair of said waveguides into one fundamental-mode waveguide.

Inventors: IVES ROBERT LAWRENCE, JORY HOWARD ROBERTS, LARUE ALBERT D, IVES, ROBERT LAWRENCE, JORY, HOWARD ROBERTS, LARUE, ALBERT D
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1989
Main classification: H01J23
IPC codes: H01J23, H01P1, H01J25