EP0438738B1 - Quasi optical component for microwave radiation - Google Patents

Description

Technical field

The invention relates to a quasi-optical component for microwave radiation with a quasi-optical element which emits incident microwave radiation along a main axis and which has a characteristic transverse dimension which is less than 50 times a wavelength.

State of the art

Very high powers (1-30 MW) are required in the range from approx. 50 GHz for the use of microwaves for heating fusion plasmas. As studies have shown, these services can best be mastered with so-called quasi-optical components. The term quasi-optical describes the principle that the microwaves are no longer guided through conductive walls, but rather spread out under open space conditions.

When heating plasmas with microwaves, such quasi-optical components can be used at different points, for example in the microwave source (quasi-optical or cylindrical gyrotron) or in the transmission path (cf. "Design of the CIT Gyrotron ECRH Transmission System") , JA Casey et al., 13th Int. Conf. On Infrared and Millimeter Waves, 5-9 Dec 1988, pp. 123-124). The so-called Vlasov converter is of particular importance in connection with the cylindrical gyrotron. Such a quasi-optical element is described, for example, in the publications "An X-Band Vlasov-Type Mode Convertor", BG Ruth et al., 13th Int. Conf. on Infrared and Millimeter Waves, 5-9 Dec 1988, pp. 119-120, and "A quasi-optical convertor for efficient conversion of whispering gallery modes into narrow beam waves", A. Möbius et al., 13th Int. Conf. on Infrared and Millimeter Waves, 5-9 Dec 1988, pp. 121-122. The fact that quasi-optical components also cause problems is explained using the example of the gyrotron.

In the gyrotron, an electron beam gun generates an electron beam that reaches a resonator via a drift path. There part of the kinetic energy of the electrons is converted into the desired microwave radiation.

The quality of the electron beam plays a central role in the optimal excitation of the microwaves. In order to impair the beam quality on the drift path as little as possible, it must be ensured that the electrons there always sense an electrical potential. In principle, this can be achieved by means of a cylindrical or possibly conical metal tube that is a few millimeters larger in diameter than the electron beam.

However, this tube can also resonate in addition to the correct resonator. This would result in a drastic deterioration in the beam quality. It must therefore be ensured with suitable means that no microwaves can be generated in this area. In addition, this area has the task of damping microwaves that run from the resonator to the cannon.

There are currently two solutions to this problem. One is based on the published patent application EP-0 301 929 A1 forth. In the case of a cylindrical gyrotron, a conical beam guide with a ribbed metallic inner surface is arranged in the drift path. Absorbent rings made of magnesium oxide are arranged between the inwardly projecting metal ribs.

This solution has the following principle of operation. The copper ring protruding somewhat inwards forms the electrical surface. The damping ring behind it does not affect the electron movement, but dampens the microwaves. The disadvantage of this mostly used solution is the high price of the damping ceramic and the poor thermal coupling of the ceramic to a heat sink. In addition, the inside of this beam conductor is difficult to pump.

The second solution is known from patent CH-664,044 A5. The electrically conductive surface of the beam guide is achieved here by a metal grid enclosing the electron beam. The structure receives the property of resonance damping through the openings in the grating. They are dimensioned so that they allow the microwaves to be attenuated. One problem with this solution is the undefined absorption of the microwaves.

Further problems arise in connection with microwaves, which feed back from the resonator into the electron beam space and can have a similarly disruptive effect.

Presentation of the invention

The object of the invention is to provide quasi-optical components of the type mentioned at the outset which avoid the problems existing in the prior art.

• a) eine Elektronenkanone zum Erzeugen eines Elektronenstrahls,
• b) eine Driftstrecke mit einer Strahlführung für den erzeugten Elektronenstrahl, welche eine den Elektronenstrahl umschliessende, elektrisch leitende Innenfläche mit Oeffnungen zum Dämpfen unerwünschter Mikrowellenstrahlung aufweist,
• c) und ein Resonator angeordnet sind, in welchem kinetische Energie des Elektronenstrahls in gewünschte Mikrowellenstrahlung umgewandelt wird,

in welchem Gyrotron der Elektronenstrahl auf der Driftstrecke ohne Beeinträchtigung der Qualität geführt wird.In particular, it is also an object of the invention to provide a gyrotron in which one behind the other in an evacuated vessel on an electron beam axis

• a) an electron gun for generating an electron beam,
• b) a drift path with a beam guide for the generated electron beam, which has an electrically conductive inner surface enclosing the electron beam with openings for damping undesired microwave radiation,
• c) and a resonator are arranged, in which kinetic energy of the electron beam is converted into the desired microwave radiation,

in which gyrotron the electron beam is guided on the drift path without impairing the quality.

According to the invention, the solution is that the component comprises a cooled absorption device which is arranged close to the quasi-optical element in such a way that at least one powerful secondary maximum of the diffraction caused by the characteristic transverse dimension is destroyed.

The essence of the invention is that the interfering microwaves are attenuated as close as possible to their point of origin (mirror, converter, etc.). be destroyed before they can act in an uncontrolled manner on the electron beam or on any sensitive components of the gyrotron. According to the invention, the damping body is preferably attached at the locations of the expected secondary maxima. It should be able to dissipate the high power (typically between 1% and 10% of the beam power). The damping body essentially consists of a dielectric vessel with relatively small losses for the microwaves (transparent) and a dielectric liquid (absorption). The absorption capacity of the liquid is on the one hand not too great, so that film boiling cannot occur, and on the other hand not too small, so that the secondary maxima can still be essentially destroyed. Such liquids are known, for example, from the technology of microwave calorimeters.

The absorption device comprises a vessel which is transparent to microwaves, in particular made of ceramic (e.g. aluminum oxide ceramic), which is filled with a cooling liquid, in particular water, which absorbs the microwaves. The quasi-optical element is preferably a focusing mirror or a Vlasov converter.

A gyrotron according to the invention is characterized in that a cooled absorption device enclosing the beam guide is provided for absorbing the microwave radiation emerging through the openings of the beam guide.

A major advantage of this embodiment is that the microwaves are first radially scattered away, which results in the attenuation of the microwave radiation in the interior, and are then absorbed by separate means. Because of their spatial separation from the actual beam guidance, the latter means can be designed in a simple manner for the required cooling capacity. The microwave energy is also destroyed in a well-defined room. Finally, the absorbent structure can be actively cooled in the invention.

According to an advantageous embodiment, the beam guide has a plurality of metal rings axially spaced apart on the axis mentioned. An advantage of this embodiment is that the interior of the beam guide can be pumped out well.

At low frequencies (less than about 70 GHz) it is particularly advantageous if the beam guidance has a section with metal rings and a section with a jacket around it said axis arranged metal rods. Then both TE and TM modes can be decoupled well.

It is particularly advantageous if the cooled absorption device is formed by a double-walled hollow cylinder, the inner and outer walls of which consist entirely of a material which is transparent to microwaves, preferably of an aluminum oxide ceramic, and which is flowed through by a cooling medium, preferably water, which absorbs the microwaves. The vessel is completely housed in the evacuated tube vessel. Such an absorption device can be integrated without problems in a gyrotron of a known type. The cost of this absorption device is much lower than that of a solution known from the prior art.

The metal rings are preferably copper rings which are kept at a distance by means of pins. The optimal axial distance between the metal rings and thus the space between two metal rings is at least half a wavelength of the microwave radiation to be attenuated. These measures ensure the desired good damping properties of the beam guidance. The gaps are largely free of obstructions.

At low frequencies (<70 GHz), i.e. for long wavelengths, the distance mentioned does not necessarily have to correspond to half a wavelength, but can also be smaller. In this case, however, make sure that the supporting metal pins are at least half a wavelength apart. The microwaves are then coupled out by gaps in the form of long (transverse to the axis), thin (longitudinal to the axis) slots.

Instead of metal rings, metal rods are also suitable, which likewise surround the electron beam in a jacket-like manner and are kept at a distance by suitable holding rings.

In addition to the beam guide made of metal rings according to the invention, the grid beam guide known per se from the patent specification CH-664,044 A5 is also suitable.

It is particularly advantageous if, in a gyrotron according to the invention, the inner wall of the cooled absorption device forms a section of the wall of the evacuated vessel and the outer wall (made of metal) of the hollow cylinder is placed on the outside of the said vessel. In such an embodiment, fewer sealing problems occur because no coolant supply lines have to be introduced into the evacuated vessel 12 and only two vacuum-tight connections (at both ends of the ceramic cylinder) occur.

Further advantageous embodiments result from the totality of the dependent patent claims.

Brief description of the drawings

The invention will be explained in more detail below on the basis of exemplary embodiments and in connection with the drawings.

Fig. 1

schematisch einen Axialschnitt durch ein erfindungsgemässes Gyrotron mit integrierter Absorptionsvorrichtung;

Fig. 2

eine Strahlführung für kleine Frequenzen;

Fig. 3

eine quasi-optische Komponente umfassend einen fokussierenden Spiegel;

Fig. 4

eine Transportleitung mit zwei quasi-optischen Komponenten;

Fig. 5

eine quasi-optische Komponente mit einem Vlasov-Konverter; und

Fig. 6

schematische einen Axialschnitt durch ein Gyrotron mit Absorptionsstrukturen im Resonator.

Show it:

Fig. 1

schematically an axial section through an inventive gyrotron with an integrated absorption device;

Fig. 2

a beam guide for low frequencies;

Fig. 3

a quasi-optical component comprising a focusing mirror;

Fig. 4

a transport line with two quasi-optical components;

Fig. 5

a quasi-optical component with a Vlasov converter; and

Fig. 6

schematic an axial section through a gyrotron with absorption structures in the resonator.

The reference symbols used in the drawing and their meaning are summarized in the list of designations. In principle, the same parts are provided with the same reference symbols in the figures.

Ways of Carrying Out the Invention

The principle of the invention can be explained most simply with the aid of FIG. 3. The quasi-optical component shown comprises a focusing mirror 16a as a quasi-optical element and a hollow cylindrical vessel 17 as an absorption device. The microwaves are incident along a predetermined direction of incidence 18. The mirror 16a essentially reflects the microwaves in the direction of a main axis 19. It has a diameter D (= transverse dimension) which is typically less than 50 times a wavelength. The wavelength in turn is in the millimeter or submillimeter range, i.e. approximately between 10 and 0.1 mm. The relatively small transverse dimension results in diffraction at the mirror as a whole. The corresponding secondary maxima, which contain between 1% and 10% of the total beam power (1-30 MW), are no longer negligible (e.g. at 1 MW without further ado 20 kW and more).

According to the invention, the absorbent vessel 17 is arranged as close as possible to the quasi-optical component, ie the mirror 16a, in such a way that the undesired secondary maxima are absorbed. The energy distribution in the microwave beam is indicated in the figure. The first, in this case strongest secondary maximum 20 is just damped. Other secondary maxima also disappear in vessel 17.

3 shows the general case where the direction of incidence and the direction of failure (main axis) do not coincide. This case occurs, for example, during the quasi-optical transmission of the microwave radiation from a source (gyrotron) to a consumer (fusion reactor). At certain intervals, focusing mirrors are set up that focus the diverging beam again. In this way it is e.g. possible to transport the microwaves over a longer distance (10⁴-10⁵ times the wavelength).

4 shows an embodiment suitable for the transmission of the microwaves. Two mirrors 16a and 16b are provided, which bring about the desired focusing of the radiation. They are e.g. housed in a transport line 22, which itself does not act as a waveguide (quasi-optical case), but only provides protection against accidental interruption of the beam path. According to the invention, near the mirrors 16a, 16b, the wall of the transport line 22 is shielded with absorption devices 21a, ..., 21d. Depending on the shape of the wall, it can be flat, disc-shaped vessels or curved (sectors of a double-walled hollow cylinder). They are preferably flushed with water as the cooling medium. The undesired secondary maxima are therefore eliminated immediately after they have arisen.

5 shows a further example of a quasi-optical component according to the invention. A Vlasov converter 23 emits the modes guided in the tube as a Gaussian wave in the direction of a main axis 19. A rotationally symmetrical absorption device 21e (for example, a water-filled vessel) that surrounds the main axis destroys the disturbing secondary maxima 20.

The invention is also applied with great advantage to a gyrotron. There are basically two different aspects. On the one hand, it is about protecting the electron beam against "vagabond" microwaves, and on the other hand, to suppress stray radiation in the resonator. The first is the problems related to the electron beam.

A gyrotron with a grating beam guidance is known from the already mentioned patent specification CH-664,044 A5. The invention now provides an improved possibility for beam guidance. In principle, however, the agents according to the invention are accommodated in the gyrotron at the same location as the beam guidance in the prior art. It is therefore sufficient if the known features of the gyrotron are only mentioned briefly.

In Fig. 1 an electron beam gun 1, e.g. a magnetron injection cannon (MIG for short) known as such. It creates e.g. annular electron beam 2 with a diameter of a few millimeters. This runs along an electron beam axis 3, passes through a resonator 4 and finally ends in a collector 13. A strong static magnetic field compresses the electron beam 3 and forces the electrons to gyrate.

In the resonator 4, the electrons running on spiral paths excite a desired alternating electromagnetic field. The microwave radiation thus obtained from the kinetic energy of the electrons is coupled out of the resonator 4 and fed to a consumer. In Fig. 1, the resonator 4 is designed in a quasi-optical manner, i.e. it essentially consists of two mirrors opposite one another on a resonator axis, the resonator axis being perpendicular to the electron beam axis 3.

It should be noted at this point that the invention is equally suitable for a cylindrical gyrotron. At As is known, the resonator in the form of a waveguide lies coaxially with the electron beam axis 3.

There is a drift path between the electron beam axis 3 and the resonator 4. The electron beam 2 must be guided on this without deteriorating its quality (in particular its energy sharpness). A beam guide 5 according to the invention as described below is used for this.

Several metal rings 6.1, 6.2, ..., 6.5 are arranged coaxially to the electron beam axis 3. With their inside they form the metallic inner surface necessary for guiding the electron beam. They have a given mutual distance d. The gaps created by this are empty. They represent the openings (diffraction gaps) in the inner surface of the beam guide, which ensure that the microwave radiation is coupled out, which has been undesirably excited in the area within the metal rings.

The metal rings 6.1, 6.2, ..., 6.5 are preferably made of copper. They should also be thin in the radial direction in order to facilitate the coupling out of the microwave radiation. The number of metal rings results from the required length of the beam guidance (e.g. approx. 300 mm for a quasi-optical gyrotron with an operating frequency of 100 GHz), the distance d and the width of the rings.

According to a preferred embodiment, the metal rings 6.1, 6.2, ..., 6.5 are kept at a distance by means of metal pins 7.1, 7.2. The thin metal pins 7.1, 7.2 have the advantage that the passage of the outcoupled microwave radiation is largely unimpeded.

The space between the metal rings must be dimensioned so that the unwanted microwave radiation can pass through well. This is the case when the openings in at least have a dimension of about half a wavelength or more in one direction. Mainly at small wavelengths, it is the distance d between the rings that is greater than half the wavelength of the microwaves generated in the gyrotron. If, on the other hand, the wavelength is relatively large (frequency less than 70 GHz), it is sufficient if the metal pins are at a distance of at least half a wavelength from one another. The axial distance between the rings may then be smaller.

So that both TE and TM modes can leave the beam guidance well, especially at low frequencies (<70 GHz), it is advisable to design the latter in the manner described below.

2 shows a beam guide for low frequencies. It has at least two sections, of which the first metal rings 6.1, 6.2, 6.3 of the type described and the second comprises a plurality of parallel metal bars 14.1, 14.2, ..., 14.5. The metal rods 14.1, 14.2, ..., 14.5 of the second section are fixed by suitable retaining rings 15.1, 15.2 and also surround the electron beam (electron beam axis 3) in a jacket-like manner (i.e. like the metal rings). The mutual distance between the metal rods 14.1, 14.2, ..., 14.5 may be less than half a wavelength. The retaining rings 15.1, 15.2, however, should not be less than this minimum distance.

In the case of the beam guidance described, the TE modes are coupled out particularly well in the first section and the TM modes in the second section. If necessary, several such sections can be alternately connected in series.

At high frequencies (> 70 GHz) there is no selective decoupling of certain modes. The beam guidance can then either consist only of rings or only rods.

There is a preferred upper limit for the distance d. It is determined by half the difference between the inner radius of the beam guidance, i.e. the relevant metal rings, and radius of the electron beam 2.

The inner radius of the beam guide is determined by the maximum possible drop in potential of the electron beam. Once the inner radius is fixed, the distance d between the metal rings in the frame shown can be selected.

According to the invention, the microwave radiation passing through the intermediate spaces is now destroyed by a cooled absorption device 8 which surrounds the beam guide. The absorption device 8 encloses the beam guide 5 in the form of a jacket. According to an advantageous embodiment, it is embodied by a double-walled hollow cylinder. The hollow cylinder has an inner wall 9 which consists of a ceramic which is transparent to microwaves. The outer wall 10 and the ceiling and floor of the hollow cylinder are made of metal. The hollow cylinder is flushed with a cooling medium 11 (e.g. water) which absorbs the microwaves.

The microwave radiation scattered radially from the beam guide 5 is absorbed by the cooling medium 11 in the hollow cylinder. The metallic outer wall ensures that the unwanted electromagnetic radiation cannot escape from the gyrotron. It should be noted that there is no risk of thermal overloading of the ceramic due to the flow cooling. It is therefore not critical if the ceramic is not optimally transparent to the microwaves and absorbs part of it. The commercially available and inexpensive aluminum oxide ceramics are therefore quite suitable for the present purposes.

As is known, the electron beam gun 1, beam guide 5 and resonator 4 must be accommodated in an evacuated vessel 12. This is, at least in the area of the drift section, mostly cylindrical or possibly conical. In general, the absorption device is accommodated in the vessel 12, which must be provided with suitable passages for the coolant supply and removal.

Fig. 6 shows a corresponding embodiment. The absorption device 8 is a completely ceramic (double-walled) hollow cylinder, which is accommodated in the space between the beam guide 5 and the metallic wall of the vessel 12. Depending on requirements, another such absorption device 8b can be located behind the resonator 4, i.e. be installed on the electron beam axis 3 between the resonator 4 and the collector 13. This space can also be "contaminated" by microwaves which have a disruptive effect on the electron beam 2.

• 1. aus dem Spiegelresonator verlorengegangene Mikrowellenleistung nicht die gekühlten Wände des Kryostaten für den supraleitenden Magneten unzulässig aufheizen, sondern gezielt in einem leistungsfähigen Absorber vernichtet werden (Mikrowellenverluste aus dem Resonator lassen sich nicht ganz vermeiden), und
• 2. die Herstellung von doppelwandigen Dämpfungskörpern aus Materialien gleicher thermischer Ausdehnung einfacher ist.

The advantages of an absorption structure made entirely of ceramic are that

• 1. microwave power lost from the mirror resonator does not heat up the cooled walls of the cryostat for the superconducting magnet inadmissibly, but is specifically destroyed in a powerful absorber (microwave losses from the resonator cannot be completely avoided), and
• 2. The manufacture of double-walled damping bodies from materials of the same thermal expansion is easier.

The absorption device 8a therefore has a double function: on the one hand, it attenuates the radiation coupled out from the beam guide 5 and, on the other hand, the radiation coming out of the resonator.

6 also shows the use of the quasi-optical component according to the invention in the resonator 4. It each comprises a mirror 16c, 16d (of the resonator) and a cylindrical, double-walled vessel 17c, 17d. These vessels 17c, 17d are in trained in the manner already described and absorb the powerful secondary maxima.

According to a preferred embodiment (FIG. 1), the inner wall of the hollow cylinder forms part of the wall of the evacuated vessel 12. The vessel 12 thus has a cylindrical ceramic insert in the area of the drift path. This means that the vessel 12 is transparent to microwaves in the area of the drift path. The outer wall of the hollow cylinder is then simply placed on the outside of the vessel 12. This embodiment is based on the experience that watertight connections are easier to implement than vacuum-tight connections. In the present case, only two vacuum-tight seams are necessary. Additional openings in the evacuated vessel 12 are completely eliminated.

Instead of the spaced metal rings, a lattice beam guide can also be used, as is known as such from the cited patent CH-664,044 A5.

The beam guidance is generally not limited to the section between the electron beam gun and the resonator. Rather, it can be continued after the resonator. Accordingly, an absorption device of the type described can also be located after the resonator, so that at least the microwave radiation is also absorbed in this area (cf. FIG. 6).

The beam guidance according to the invention considerably improves the pump path compared to the prior art. The gaps also allow radial pumping, which is not possible with pipes made of metal and ceramic rings.

Although in the best case an absorption device should be arranged at the point of impact of a secondary maximum, a highly conductive metal wall can also be provided as a reflector be. The microwave power is then led to the absorber via this metal wall (and possibly via further reflectors).

The invention creates the prerequisites which are necessary in order to be able to generate microwaves of high power and to transmit them safely. With a 1 MW quasi-optical gyrotron, for example, the diffraction losses are approximately 20 kW. This performance would hit the liquid nitrogen shield of the cryostat unhindered, which would have to carry this performance away. This would result in a disproportionately high consumption of liquid nitrogen. Furthermore, the microwave power wandering in an uncontrolled manner in the gyrotron could be absorbed or coupled in at further, undesirable points, such as e.g. with electron gun, electron beam, resonator, HF window, vacuum seals, cable connections, diagnostic systems (for temperature, level, etc.), high-voltage insulators, and there lead to malfunctions or damage. Ultimately, these microwaves could also emerge from the gyrotron at undesirable locations and thus endanger people and devices in the vicinity.

In summary, it can be stated that the invention has created the possibility of guiding a high-quality electron beam in a gyrotron.

LIST OF DESIGNATIONS

1 - electron beam gun; 2 - electron beam; 3 - electron beam axis; 4 - resonator; 5 - beam guidance; 6.1, ..., 6.5 - metal rings; 7.1, 7.2 - metal pins; 8 - absorption device; 9 - inner wall; 10 - outer wall; 11 - cooling medium; 12 - vessel; 13 - collector; 14.1, ..., 14.5 - metal rods; 15.1, 15.2 - retaining rings.

Claims (10)

1. Quasi-optical component for microwave radiation comprising a quasi-optical element which radiates incident microwave radiation along a major axis and which has a characteristic transverse dimension which is smaller than 50 times one wavelength, characterized in that the component comprises a cooled absorption device which is arranged closely in front of the quasi-optical element in such a manner that at least one high-power secondary peak of the diffraction due to the characteristic transverse dimension is destroyed.
2. Quasi-optical component according to Claim 1, characterized in that the absorption device is a vessel transparent to microwaves, particularly of ceramics, which is filled with a cooling liquid absorbing the microwaves, particularly water.
3. Quasi-optical component according to Claim 1, characterized in that the quasi-optical element is a focusing reflector or a Vlasov-type convertor.
4. Device for guiding an electron beam in a gyrotron along an axis from an electron gun to a collector, the device exhibiting as beam guide an electrically conductive inside surface enclosing the electron beam and having openings for damping unwanted microwave radiation, characterized in that a cooled absorption device enclosing the beam guide is provided for the absorption of the microwave radiation emerging through the openings in the beam guide.
5. Device according to Claim 4, characterized in that the beam guide exhibits several metal rings axially spaced apart with intermediate spaces along said axis, which are held at a distance with the aid of pins.
6. Device according to Claim 4, characterized in that the beam guide exhibits a section with metal rings and a section with metal rods arranged in the form of a jacket around said axis so that both TE and TM modes are easily coupled out even with lower frequencies of the microwave radiation.
7. Device according to Claim 4, characterized in that the cooled absorption device is formed by a double-walled hollow cylinder, the inside and outside wall of which consists of a material transparent to microwaves, particularly of an aluminum oxide ceramic, and through which a cooling medium absorbing the microwaves, particularly water, flows.
8. Device according to Claim 5, characterized in that the axial spacing of the metal rings and thus the intermediate space between two metal rings each is in each case at least one half wavelength of the microwave radiation to be damped.
9. Gyrotron in which in an evacuated vessel,

   a) an electron gun for generating an electron beam,

   b) a drift system with a beam guide for the electron beam generated, which exhibits an electrically conductive inside surface enclosing the electron beam and having openings for damping unwanted microwave radiation,

   c) and a resonator are arranged behind one another along an electron beam axis, in which resonator kinetic energy of the electron beam is converted into desired microwave radiation, characterized in that

   d) a cooled absorption device enclosing the beam guide is provided for the absorption of the microwave radiation emerging through the openings in the beam guide.
10. Gyrotron according to Claim 9, characterized in that the inside wall of the cooled absorption device forms a section of the wall of the evacuated vessel and the outside wall of the hollow cylinder consists of metal and is placed externally onto the vessel.

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