Gyrotron having a mode converter

Abstract: A gyrotron having a mode converter located on the way of a wave propagating passage, said mode converter comprising a means (55) for mode-converting electromagnetic wave into radiation electromagnetic wave having an annular-shaped power distribution in a plane perpendicular to the direction in which the electromagnetic wave propagates, a ring-shaped mirror (annular mirror) (57) for reflecting the radiation electromagnetic wave which has been converted by the mode converting means, and a waveguide tube (50) having a kerf opposed to the annular mirror to receive the electromagnetic wave reflected by the annular mirror.

Description: The present invention relates to a gyrotron having a mode converter in the waveguide. There are various means for heating plasma in the fusion reactor. One of them is electron-cyclotron resonance heating by super high-power milimeter waves. This plasma heating calls for high power oscillators with a frequency at the band of millimeter waves. The gyrotron is deemed promising as this oscillator. In a case where output wave of the gyrotron is practically used to heat core plasma in the fusion reactor, the gyrotron is often separated from the core plasma by considerable distance. It is thus asked that the output wave mode of the gyrotron is converted into TE 01 mode which is smaller in transmission loss and that the wave thus converted in TE 01 mode is transmitted to the core plasma through a circular waveguide. This is the reason why attention has been paid to a system disclosed in a below-cited reference (1) and including a mode converter which is located on the way of waveguide passage in a circular waveguide and which is formed by a circular waveguide provided with periodic perturbations to convert the output wave oscillated under TE mn mode into that of the TE 01 mode. Reference (1): M. Thumm, et al. "In-Waveguide TE 01 -To-Whispering Gallery Mode Conversion Using Periodic Wall Perturbations" Recently, however, millimeter waves higher in frequency and larger in power are needed. The gyrotron of such large power type that can meet this need oscillates millimeter waves under that mode which is m >> 1, n ~ 1 under the TE mn mode and which is called the whispering gallery mode. It is difficult in this case to convert the output wave of this mode directly into that of the TE 01 mode by using the mode converter disclosed in the reference (1). In the case of the output wave of this whispering gallery mode, the output wave propagated through the circular waveguide tube is radiated like a beam into free space by the Vlasov launcher and this wave thus radiated is transmitted while being successively reflected and focused by plural curved mirrors, as disclosed in a below-cited reference (2). Or a system in which focused electromagnetic wave is entered into and transmitted in a waveguide provided with rows of grooves on the inner face in the circumferential direction thereof and called the corrugated waveguide tube has been studied. Reference (2): S.N. Vlasov, et al. "Transformation Of A Whispering Gallery Mode, Propagating In A Circular Waveguide, Into A Beam Of Waves". In short, the mode converter is formed by the Vlasov launcher and the curved mirrors. In the case of the waveguide passage formed as described above, however, high processing accuracy is needed in making the curved mirrors used to transmit the electromagnetic wave, the drive mechanism for adjusting optical axes, the corrugated waveguide tube and the like. The waveguide passage thus formed is therefore higher in cost as compared with the one formed by the circular waveguide tube. In a case where an electron beam collector for collecting electron beam is used together with the output waveguide tube in the gyrotron of the type which oscillates the output wave under the whispering gallery mode, the electron beam collector cannot resist against thermal load when the gyrotron is made to have a larger output. It has been therefore considered that the mode converter disclosed in the reference (2) is housed in this gyrotron, as shown in Fig. 7, to separate the electron beam collector from the output waveguide so as to make it possible to use a larger-sized electron beam collector. According to this gyrotron, gyrating electron beam shot from an electron gun is entered into and oscillated in a cavity resonator. Electromagnetic wave thus generated in the resonator is transmitted into a mode converter, which comprises the Vlasov radiator and the curved mirror, through the circular waveguide tube connected to the resonator. This electromagnetic wave is reflected by a reflecting mirror in a direction right-angled relative to the center axis of the cavity resonator and then sent as output electromagnetic wave through an output window. Reference numeral 18 in Fig. 7 denotes electromagnets for adding magnetic field needed to generate the gyrating electron beam, 19 electromagnets for adding magnetic field needed for oscillation, and 20 a collector for collecting electron beam. In the case of the gyrotron having the above-described arrangement, however, the mode converter 6 comprising the Vlasov converter 4 and the flat or curved mirror 5 is housed in the gyrotron. This makes the gyrotron complicated in structure and damages the axisymmetry of the gyrotron structure. In addition, reliability is reduced relative to the output wave transmitting axis in the gyrotron. The electromagnetic wave of the whispering gallery mode is hard to be transmitted with low loss to an intended position through the conventional waveguide passage. Further, when the electromagnetic wave of the whispering gallery mode is to be converted into that of the TE 01 mode in the conventional gyrotron and to be outputted through the gyrotron, the whole of the gyrotron also becomes complicated. Reference (3): EP-A-O 122 834 discloses a mode converter wherein a waveguide has a tapered form in its longitudinal direction and an elliptic cross-section. This elliptic cross-section is achieved by cutting out specific parts of a circular waveguide. It is an object of the present invention to provide a gyrotron having a mode converter on the waveguide passage to eliminate the above-mentioned drawbacks and, more particularly, a gyrotron capable of realizing a higher output and a higher efficiency without making the gyrotron complicated in structure. To solve this object the present invention provides a gyrotron as specified in claim 1. According to the gyrotron having the above-described arrangement, the shape of the reflecting surface of the annular mirror and the position and shape of the waveguide whose kerf is opposed to the annular mirror may only be selected to make it possible to convert the electromagnetic wave of the whispering gallery mode (TE mn , m >> 1, n ~ 1), for example, into that of other waveguide modes such as the TE 01 mode and to transmit it through the gyrotron. It is supposed that the electromagnetic wave radiated from the straight cut of the circular waveguide is a superposition of plane waves. Therefore, the wave vector (k) of this plane wave relative to the TE mn mode can be substantially obtained in the cylindrical coordinate system from the following equation. k = (k r , k  , k z ) wherein k r = [(x mn /a) 2 - (m/a) 2 ] 1/2 , k  = m/a, k z = [k2 - (x mn /a) 2 ] 1/2 and k = 2π/λ and where λ: the wavelength in free space, π: pi, Xmn: the n-th root of derivative of m-th order Bessel function of the first kind, m: the azimuthal mode number of wave in the waveguide, a: the waveguide radius. Particularly when the electromagnetic wave of the whispering gallery mode (m >> 1, n ~ 1) is radiated from the circular waveguide cut, it becomes radiation electromagnetic wave having an annular-shaped power distribution in a sectional plane perpendicular to the tube axis. Providing that the mode of wave wanted to obtain after the conversion is TE mn , the wave vector (k') of the plane waves superposed can be expressed as follows. k' = (k r ', k  ', k z ') wherein k r ' = [(x m ' n '/a) 2 - (m'/a') 2 ] 1/2 , k  ' = m'/a', and k z ' = [(k 2 - (x m ' n '/a) 2 ] 1/2 . The electromagnetic waves radiated the circular waveguide cut can be transmitted by reflecting with an appropriate annular mirror. Further, when the wave vector is changed from (k) obtained by the equation (1) to (k') obtained by the equation (2) on reflecting the electromagnetic wave by the annular mirror, most of the power of the TE mn mode can be converted into that of TE m'n' mode. The present invention is based on the above-described fundamental theory. When the mode converter having the above-described arrangement is located on the waveguide passage, therefore, the electromagnetic wave of the whispering gallery mode can be converted directly into that of the TE 01 mode. As the result, the waveguide passage thus formed can be smaller in transmission loss and simpler in structure. Further, the gyrotron in which the mode converter having the above-described arrangement is housed allows the electron beam collector to be separated from the output wave transmitting passage in the gyrotron without making the gyrotron complicated in structure and damaging the axisymmetry of the gyrotron structure. The electron beam collector can be thus made larger in size. This enables the gyrotron to have a larger output. Still further, an electrode for converting the kinetic energy of the electron beam to electric energy can be used to thereby increase the oscillation efficiency of the gyrotron to a greater extent. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view showing an arrangement of the waveguide passage formed according to an embodiment of the present invention; Fig. 2 is a view showing main components partly sectioned by which the waveguide passage is formed; Fig. 3A is a sectional view taken along a line X - X in Fig. 3B; Fig. 3B is a view showing an annular mirror located on the waveguide passage; Fig. 4 is a sectional view showing an arrangement of the waveguide passage formed according to another embodiment of the present invention; Fig. 5 is a view showing an arrangement of the waveguide passage formed according a further embodiment of the present invention; Fig. 6 is a view showing rows of grooves formed on the inner face of the annular mirror; and Fig. 7 is a sectional view showing an arrangement of the conventional waveguide. Fig. 1 shows the gyrotron provided with a mode converter 55 which will be described later according to an embodiment of the present invention. This gyrotron is of such type that oscillates under whispering gallery mode. More specifically, gyrating electron beam 52 produced by an electron gun 51 is injected into a cavity resonator 53 to oscillated electromagnetic waves in it. Electromagnetic wave of the whispering gallery mode created by the resonator 53 is transmitted into a mode converter 55 through a circular waveguide 54 which is connected to the resonator 53. The mode converter 55 includes a section which is shown in detail in Fig. 2. Namely, radiation wave radiated from a straight cut 56 of the circular waveguide 54 and having an annular-shaped power distribution in a plane perpendicular to the direction in which the radiation wave propagates is made incident on a non-axisymmetric annular mirror 57, which contributes to mode conversion, and its reflected waves 58 are introduced into a cut 60 of a tapered circular waveguide 59. The tapered circular waveguide 59 is smoothly connected to a linear circular waveguide 62 to which an output window 61 is attached. An electron beam collector 64 which serves to collect spent electron beam is arranged between and around the annular mirror 57 and the tapered circular waveguide 59. This electron beam collector 64 is cooled by a cooling system (not shown). The electron beam is introduced to the electron beam collector 64 by magnetic flux produced by superconducting magnets 65. The shape of the magnetic flux may be adjusted by additional super- or normal-conducting magnets located adjacent to the electron beam collector 64. Instead of the electron beam collector 64 located between the annular mirror 57 and the tapered circular waveguide 59, at least one annular electrode 66 may be used. By adding appropriate potential to the electrode 66, the spent electron beam can be collected with directly recovering its kinetic energy. An electromagnetic wave absorbing layer mode of silicon carbide material or formed by the chemical vapor deposition film of silicon carbide may be formed on a part or all of the inner surface of the structure which supports a circular waveguide tube 50, the annular mirror 57 and the tapered circular waveguide 59. Reference numeral 67 in Fig. 1 denotes electro-magnets for adding magnetic field to produce the gyrating electron beam. In the case of the gyrotron having the above-described arrangement, output wave of the gyrotron which oscillates electron beam under the whispering gallery mode is converted into that of TE 01 mode, which can be easily transmitted, by the mode converter 55 in the gyrotron and then outputted. Since the above-described mode converter 55 is incorporated into the gyrotron in this case, the gyrotron cannot be made complicated in structure. Further, the electron beam collector 64 can be separated from the output wave transmitting path in the gyrotron without damaging the axisymmetry of the gyrotron structure. Therefore, the electron beam collector 64 can be enlarged, thereby enabling the output of the gyrotron to be made higher. Still further, the electrode 66 which serves as a potential depressed collector to convert the kinetic energy of the electron beam 52 to electrical energy can be used. This enables the oscillation efficiency of the gyrotron to be increased to a greater extent. The output window 61 may be located between the annular mirror 57 and the tapered circular wave guide 59 or at an optional position in the tapered circular waveguide 59. Or it may be located adjacent to the kerf 60 of the tapered circular waveguide 59, which is large in sectional area, in order to make thermal load small. A tapered circular coaxial waveguide tube 42 shown in Fig. 4 may be used instead of the tapered circular waveguide tube 59. Fig. 2 partly shows the mode converter 55 according to am embodiment of the present invention in which the waveguide 50 is included. Figs. 3A and 3B are sectional and front views showing in an enlarged scale the annular mirror 57 which can be a characteristic of the present invention. The characteristic shape of this mirror 57 is apparent from Figs. 3A and 3B. This waveguide 50 has the mode converter 55 on its way and it is arranged to convert electromagnetic wave of TE 12 , 2 mode which is one of the whispering gallery mode into that of TE 01 mode by means of the mode converter 55 and then transmit the electromagnetic wave of TE 01 mode thus converted. The mode converter 55 is arranged in such a way that the circular waveguide 54 which guides the electromagnetic wave of TE 12 , 2 mode is provided with the kerf 56, that electromagnetic wave radiated from the kerf 56 is reflected by the annular mirror 57 located coaxial to the waveguide 54, and that the electromagnetic wave thus reflected is entered into the kerf 60 of the tapered circular waveguide 59. As shown in Figs. 3A and 3B, the annular mirror 57 has a non-axisymmetrical concave mirror 38 on the inner surface thereof. This concave mirror 38 is divided into 12 parts 39, same as the azimuthal mode number of input electromagnetic wave, so as to periodically change in the azimuthal direction of the mirror and a step 40 is formed at the border of each of the divided reflecting parts 39 of the mirror 38 with its adjacent one. Namely, the number of the periodic changes in the azimuthal direction is set same as the number (m) of the azimuthal direction modes which is defined at the time when the electromagnetic field distribution of the input electromagnetic wave has a factor of exp (± -1 m) in the cylindrical coordinate system (r, , z). Each of the divided reflecting parts 39 is formed to have such a curved surface that smoothly changes in the axial direction as well as in the azimuthal direction. The concave mirror 38 is formed in such a way that the unit normal vector erected from the divided reflecting part 39 can meet the following requisite. The unit wave vector (k) of the electromagnetic wave radiated from the kerf 56 of the circular waveguide 54 is calculated on the annular mirror 57 at first. The unit wave vector (k') of wave reflected at each of points on the annular mirror 57 is defined in such a way that the electromagnetic wave reflected by the annular mirror 57 is focused on a point on an optical axis 41 entering into the tapered circular waveguide 59 previously set. In order to convert the mode of the reflected wave into the TE 01 mode, it is needed that the optical axis 41 is in a (r, z) plane. The unit normal vector can be obtained from the wave vectors k and k' as follows. n = (k' - k) / |k' - k| The particularly shaped concave mirror 38 is formed on the inner surface of the annular mirror 57 on the basis of the unit normal vector thus obtained. The position, diameter and tapered angle of the kerf 60 of the tapered circular waveguide 59 are set in such a way that the electromagnetic field distribution of the electromagnetic wave reflected by the annular mirror 57 can become closely akin to that of the electromagnetic wave of the TE 01 mode at the kerf 60. When the waveguide 50 has the above-described arrangement, the electromagnetic wave of the whispering gallery mode can be converted on the basis of the above-mentioned reasons directly into that of the TE 01 mode by the mode converter 55. Therefore, a waveguide, simpler in construction, lower in cost and smaller in lost, can be formed. Although the electromagnetic wave reflected by the non-axisymmetrical annular mirror 57 which contributes to the mode conversion has entered into the tapered circular waveguide 59 in the case of the gyrotron shown in Fig. 1, it may be arranged that the electromagnetic wave reflected by the non-axisymmetrical annular mirror 57 is reflected by one or plural coaxial axisymmetrical annular mirror(s) and then entered into the tapered circular waveguide 59. Or it may be arranged that the electromagnetic wave radiated from the kerf 56 of the circular waveguide 54 is reflected by one or plural coaxial axisymmetrical annular mirror(s) and then entered into the non-axisymmetrical annular mirror 57 which contributes to the mode conversion, and that its reflected wave is entered into the tapered circular waveguide 59. Although the mode converter 55 has been interposed between the non-axisymmetrical annular mirror 57 and the kerf 60 of the tapered circular waveguide 59 to allow the electromagnetic wave reflected by the annular mirror 57 to be entered into the kerf 60 of the waveguide 59 in the case of the above-described embodiment of the present invention, a mode converter 32a may be interposed between the annular mirror 57 and a kerf 43 of the tapered coaxial circular waveguide 42 to allow the electromagnetic wave reflected by the annular mirror 57 to be entered into the kerf 43 of the waveguide 42, as shown in Fig. 4. Reference numeral 44 in Fig. 4 represents a support member made of ceramics or the like. An annular mirror 35a on the inner face of which rows of grooves are formed, as shown in Fig. 6, having a depth of about a quarter wavelength, a pitch smaller than a half wavelength and a width of about a half pitch is used as shown in Fig. 5. When the gyrotron has this annular mirror 35a as shown in Fig. 5, reflected wave can be linearly polarized relative to appropriate input radiation electro-magnetic wave, that is, radiation electromagnetic wave obtained when the electromagnetic wave of the TE 01 mode is radiated from the kerf of the circular waveguide, or radiation electromagnetic wave obtained when the electromagnetic wave of the TE 01 mode is introduced into a tapered coaxial waveguide 46 and then radiated from a kerf 47 of the waveguide 46, as shown in Fig. 5, or radiation electromagnetic wave obtained when the electromagnetic wave of the TE 01 mode is introduced into the mode converter 55 shown in Fig. 2 to produce mixed waves of the TE 01 and TE 02 modes and these mixed waves are radiated from the kerf of the circular waveguide connected to the mode converter 55. When the reflected wave is entered into a tapered corrugated waveguide 48 on the inner face of which rows of grooves are formed in the azimuthal direction thereof, or into a kerf 49 of the coaxial waveguide, therefore, its mode can be converted into HE 11 mode. According to the gyrotron of the present invention as described above, the electromagnetic wave of the whispering gallery mode can be converted directly into that of the TE 01 mode which is small in transmission loss. Therefore, the waveguide can be made simpler in construction and lower in cost. In addition, the electromagnetic wave of the TE 01 mode can be converted into that of other waveguide modes. Further, when one of the above-described mode converters is located on the wave guiding passage in the gyrotron, the gyrotron cannot become complicated in construction. In addition, the electron beam collector section can be separated from the output wave transmitting passage section in the gyrotron, if necessary, without damaging the axisymmetry of the gyrotron structure. The electron beam collector can be larger-sized, thereby enabling the gyrotron itself to have a larger output. Still further, the electrode which serves to collect a part of the energy of spent electron beam can be arranged in the gyrotron. This enables the gyrotron to have a still larger output and higher efficiency.

Claims: A gyrotron comprising along a tube axis thereof an electron gun (51) for producing an electron beam (52) which is injected into a resonator (53), said electron beam (52) exciting said resonator (53) to produce an electromagnetic wave, a waveguide (54) coupled to said resonator (53) and through which the electromagnetic wave produced by said resonator (53) propagates and, a mode converter (55) arranged in said waveguide (54), for converting the electromagnetic wave produced in the resonator into a radiated mode electromagnetic wave, characterized in that said mode converter (55) comprises: converting means (56) for converting the electromagnetic wave into the radiated mode electromagnetic wave which has an annular-shaped power distribution in a sectional plane perpendicular to an axial direction in which the electromagnetic wave advances; annular mirror means (57) having a circumferential inner surface for reflecting the radiated mode electromagentic wave which has been converted by said converting means (56); and a waveguide tube (59) located at an end portion of said waveguide (54) and having at an entrance of the waveguide tube a kerf (60) for receiving the radiated mode electromagnetic wave reflected by said annular mirror means (57), said kerf (60) being located at a portion spaced apart from said annular mirror means (57). A gyrotron according to claim 1, characterized in that a circumferential inner surface of said annular mirror means (57) comprises a plurality of reflecting surfaces (39) which are arranged in a circumferential direction to periodically change in the circumferential direction. A gyrotron according to claim 1, characterized in that a number of said reflecting surfaces which periodically change in the circumferential direction is equal to a circumferential mode index (m), defined when an electromagnetic distribution of an input electromagnetic wave being produced and converted has a factor of exp (± -1 m ) in a cylindrical coordinate system (r: radius, : angle, and z: major axial direction). A gyrotron according to claim 1, characterized in that said annular mirror means (57) is arranged in such a way that a differential coefficient in an axial direction of said reflecting surfaces is other than zero. A gyrotron according to claim 1, characterized in that rows of grooves for reflecting the electromagnetic wave in anisotropic manner are disposed circumferential inner surface of said annular mirror means (57). A gyrotron according to claim 1, characterized in that said means (55) for converting the electromagnetic wave into the radiated mode electromagnetic wave which has an annular power distribution is a waveguide tube section (59) having one of a circular and coaxial form and said kerf (60) is located facing an end portion of said annular mirror means (57). A gyrotron according to claim 1, characterized in that said waveguide tube (59, 42) is of a tapered circular or tapered circular coaxial type. A gyrotron according to claim 7, characterized in that said waveguide tube is a corrugated waveguide (48) having on an inner face thereof rows of grooves for anisotropically reflecting the electromagnetic wave.

Inventors: ITO YASUYUKI, ITO, YASUYUKI, ITO, YASUYUKI, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01J25
IPC codes: H01P1, H01J23, H01J25