EP0770264A1

EP0770264A1 - Magnetic system for gyrotrons

Info

Publication number: EP0770264A1
Application number: EP95924292A
Authority: EP
Inventors: Arnold MÖBIUS; Julius Pretterebner
Original assignee: Forschungszentrum Karlsruhe GmbH
Current assignee: Forschungszentrum Karlsruhe GmbH
Priority date: 1994-07-09
Filing date: 1995-06-20
Publication date: 1997-05-02
Anticipated expiration: 2015-06-20
Also published as: DE59502526D1; DE4424230C2; EP0770264B1; WO1996002064A1; DE4424230A1; US5828173A

Abstract

The invention relates to a device for generating the magnetic guide field in gyrotrons based on permanent magnets. The advantages with the layout of the magnets is that gyrotrons using hot magnets can be converted to permanent magnet operation without any substantial structural alterations to the gyrotron tube.

Description

Magnet system for gyrotrons

The invention relates to a magnet system for generating the axial magnetic constant field for gyrotrons between the emitter and collector area.

The purpose of the invention is to replace the current-operated current-operated gyrotron magnet system, be it conventional or superconducting electromagnets, with a maintenance-free permanent magnet system, without constructive modification measures on the gyrotron tube itself being necessary.

Gyrotrons are sources for the generation of high microwave powers at high frequencies, as are required for heating fusion plasmas. Typical orders of magnitude are 1 MW output power and frequencies in the range of 100 GHz.

Meinke-Gundlach shows the basic structure and description of a gyro tron in "Taschenbuch der Hochfrequenz¬ technik ¹¹ (Springer Verlag Berlin Heidelberg New York Tokyo 1986) on pages M82 - M85. Gyrotron oscillators can be inserted into the vacuum tube and disassemble the magnetic system generating the guide field, in particular high-performance gyrotrons are provided as additional heating for fusion plasmas (see pages S17 and S18).

So far there have only been theoretical works on the control of electron beam instabilities in gyrotrons. In Int. Journ. of Infrared and Millimeter Waves, Vol. 14, No. 4, 1993 is an article by AN Kuftin et al. under the title: "Theory of Helical Electron Beams in Gyrotrons". Permanent magnet systems for guiding helical electron beams are considered. The permanent magnet system consists of a central, axially polarized permanent magnet and permanent magnets which are oppositely axially polarized and which attach to its two end faces. This system still has one in the electron beam area of the gyrotron strong magnetic field reversal of the axial field and a strong increase in the field at the edges instead and a strong increase in the field at the surf.

If all electrons are to have the same starting conditions, the strong increase in the field in the area of the emitter only allows the use of effectively narrow emitter rings. The emitter and magnetic field must be adjusted exactly.

The field reversal at the collector results in restrictions in the design of the collector, in particular in the case of pretensioned collectors.

From a technical point of view, gyrotrons have currently reached an electrical efficiency of 50% (operation in the first harmonic of the cyclotron frequency). A further increase in it is at least not so urgent at the moment. However, gyrotrons for industrial use, such as. B. surface coatings and ceramic sintering interesting, so that the question of higher efficiency and associated with it the question of lower cooling capacity required and ge¬ lower cost of materials is of economic importance.

Parameters of such gyrotrons are relatively lower frequencies (e.g. 30 GHz) at low powers (e.g. 10 kW). Great efficiency losses occur in the gyrotron resonator, which acts as an interaction space, the largest cooling effort occurs at the collector, the second largest cooling effort occurs with gyrotrons in the magnet working with normally conducting magnets. The losses in the magnet can be reduced through the use. of permanent magnets drastically reduce.

The invention is based on the object of replacing the superconducting or normally conducting magnets used hitherto with permanent magnet arrangements which, unlike in the case of permanent magnet arrangements hitherto designed, do not require any additional scientific or constructive work in the gyrotron tube. which also restrict or make impossible the use of constructive developments of previously available gyrotrons (such as, for example, equipment with pretensioned collectors). In addition, electron beam reflections and electron beam instabilities in the gyrotron should be avoided.

The object is achieved by the characterizing features of claim 1. A central, radially polarized, an axially polarized ring magnet brought towards the collector area and a ring magnet arrangement which attaches to the opposite end face of the central ring magnet and blocks the breakout of the field fundamentally produce the magnetic field profile desired in the electron beam area. Due to the required field course, the geometry of the permanent magnets is determined with the help of a computer. A strong, but only meaningless field reversal now only takes place outside the electron beam area in the extension of the emitter. A second reversal of the magnetic field in the prior art of magnetic systems is avoided or its amplitude is meaningless. The mechanical bracing of the magnetic system is a technically known solution.

Subclaim 2 characterizes the simplest, namely symmetrical, but also a more material-intensive structure of the permanent system.

Claim 3 characterizes a material-saving, asymmetrical structure of the permanent system with which a strong magnetic field reversal is generated outside the electron beam range, but which has no influence.

With a simple, axially polarized permanent magnet in the emitter region, the axial emitter DC field, which is considerably weaker than the axial resonator DC field, can be set (claim 5). Current-operated solenoids and soft iron assemblies are still used for field correction and flux concentration (claims 6 and 7). Further known correction options on the axial DC magnetic field can be achieved with displaceable solenoids.

The device is to be explained and explained in more detail below. For this purpose, six figures are included in the drawing.

Show it:

1 shows the basic structure of a gyrotron with the inventive device for generating the static magnetic field.

Figure 2 shows the basic desired dependence of the magnetic guide field along the gyrotron axis.

3a and 3b and 4a and 4b the basic structure of the device according to the invention for generating the static magnetic field and the field along the axis.

In the gyrotron, FIG. 1, the electrons propagate as a hollow beam on helical paths - guided by a static magnetic field - from the cannon to the resonator and leave it as a "spent" beam to the collector, where the heat generated has to be dissipated.

The radius of the hollow electron beam R is determined by the magnetic guide field B through the relationship

BR ² = const

(see here and in the following Gyrotron Oscillators, Their Principles and Practice, edited by CJ Edgcombe, Taylor & Francis, 1993; in particular Chapter 5 of B. Pioscyk). With a given speed ratio α and magnetic field in the resonator and a selectable (triode) or fixed (diode) compression ratio (ratio of the hollow beam radii), the axial magnetic field is also fixed at the emitter. In the case of electrons propagating on helical paths, the ratio of the transverse to the axial speed component becomes

α =

through the equation

2

V_L

= const

B

fixed. If the transverse speed reaches the total speed, the electron beam is reflected (magnetic mirror).

The static magnetic field not only serves to guide the electron beam, but also sets according to the equation

eB (omega!) w _c = m

the cyclotron frequency of the electrons in the resonator is fixed, m is the relativistic mass of the electrons with the elementary charge e. B is the magnetic flux density. The frequency generated by the gyro tron is included

(omega!) w = nw _c .

n is an integer and is called the order of the cyclotron harmonics. For gyrotrons that generate microwave power at 30 GHz, the required magnetic field in the first harmonic is approximately 1.1 T in the second harmonic about 0.55 T. The field sought along the gyrotron axis can be seen in FIG. 2.

Generating the magnetic field with superconductors requires a lot of equipment and a constant need for helium or nitrogen during operation. Generating the magnetic field with normally conducting magnets requires high connection and cooling capacities. The energy consumption of the magnets should not be neglected compared to the power generated.

Generating the magnetic field with permanent magnets has brought about fundamental problems with the arrangements examined so far. In principle, these arrangements consist of a central, axially polarized and two radially polarized magnets (see FIG. 3 from Kuftin, Int. J. of Infrared and Millimeter Waves). The disadvantages of such systems are zero crossings on the axis and unused fields (poor efficiency), as well as steep drops at the edges. The steep drops at the edges have the disadvantage for the emitter side that the adjustment of the gyrotron magnet is critical and the effective emitter width is restricted. The result of the zero crossing is that, with an increasingly negative magnetic field, the electron beam can be reflected along the axis (magnetic mirror). This makes the design of the collector more difficult, the use of prestressed collectors for energy recovery is practically impossible.

Prestressed collectors are necessary to increase the efficiency. The ratio of the energy taken from the electrons to the original energy is the electrical efficiency n _e i. The overall efficiency can now be increased by letting the beam strike a prestressed collector, whereby part of the energy of the used beam is recovered with the efficiency n _c . The overall efficiency of a gyrotron with a biased collector is: _ ⁿ el n -

1 ~ n _c (1 ~ n _e _)

The use of pretensioned collectors is practically impossible or drastically complicated by a reversal of the sign of the axial magnetic guide field along the electron beam path.

On the cannon side - in order to also be able to use laminar cathodes and to keep adjustment problems to a minimum - the axial magnetic field should be locally constant in the area of the cathode, see FIG. 2.

FIG. 1 shows the basic schematic structure of a gyrotron. The essentials about the gyrotron can be read shortly in Meinke Gundlach, "Taschenbuch der HF-Technik, M82 ff.

Figure 2 shows, as already mentioned, the course of the desired axial magnetic constant field in the gyrotron areas: emitter, compression, resonator, decompression and collector. Depending on the design, the undulating course of the magnetic flux density is more or less provoked (see patent application Möbius / Dumbrajs), specifically because of the structure of the inner surface of the permanent magnet.

The field strength in the emitter area is about 5 - 25% of the axial constant field in the resonator area.

FIG. 3a shows a symmetrical arrangement of the permanent magnet system. Therefore, only the right half is shown as a computer printout, since it shows the magnetic field line course relevant for the gyrotron. The radially polarized central magnet, better the drawn axial half, is in contact via holders, which are not shown, with the right, axially polarized magnet, via the common conical surfaces. In the gyrotron area there is no or only a slightly compensable zero crossing of the field lines. The entire river is rotationally symmetrical to the z-axis.

3b shows the course of the constant field as a function of the z-axis, that is to say partly the gyrotron axis. This flux density curve is point-symmetrical to the axis origin and also has only a zero crossing (stagnation point) there, ie field reversal. Thus, the radially polarized permanent magnet half shown in FIG. 3a and the axially polarized permanent magnet adjoining it to the right are basically suitable for generating a constant magnetic field without zero crossing in the gyrotron range. All that is missing is the weaker constant field for the emitter area. However, half of the permanent magnet material is not used, just as the local magnetic field profile is symmetrical to the zero crossing.

The magnet system in FIG. 4a comes closer to the requirements of the constant field profile in the gyrotron, and in particular saves magnetic material. It consists of the central, radially polarized, ring-shaped permanent magnet. To the right (collector side) in the figure is the axially polarized, ring-shaped permanent magnet. On the left is the magnet arrangement blocking the outbreak of the field. This geometric shape enables the required field structure in the gyrotron area. The low constant field in the emitter zone is completely achieved by superimposing the small ring-shaped, axially polarized permanent magnet with a rectangular longitudinal section. Figure 4b shows the course according to strength and sign, depending on the location. Far behind the emitter, i.e. on the left, there is the strong, concentrated, inevitable field reversal. The gyrotron areas emitter, compression, resonator, decompression and collector are indicated. Now there is the DC magnetic field in the emitter area, the high DC field in the resonator and the DC field in the collector area, which is almost zero.

The field is thus more completely forced through the borehole of the magnet system. A reversal of the sign of the axial magnetic field does not take place in the gyrotron area or only to an insignificant degree and, moreover, only once. The electron beam can thus be guided in a stable manner from the emitter to the collector.

By fine structuring on the inner surface, a constant or a predetermined wavy magnetic field is generated in the center of the resonator. Spatially (locally) constant fields can be achieved in the area of the emitter (see FIG. 4a) and in the area of the collector by additionally axially polarized magnets. Zero crossings of weak fields can be suppressed in this way.

By combination with weak electromagnets, the magnetic field can be readjusted or further material savings can be achieved. Another possibility for tuning is radially and / or axially displaceable magnets.

For the list of characters

1 cathode

2 emitter ring

3 acceleration anode

4 interaction space, resonator

5 decoupling line

6 collector

7 device, solenoid, permanent magnet

8 gyrotron axis

9 emitter region

10 compression range

11 resonance range

12 decompression area

13 collector area

14 radial polarization

15 axial polarization

16 field lines

17 vacuum vessel

Claims

Claims:

1. Magnet system for generating the axial magnetic constant field for gyrotrons for guiding the electrons emerging from the emitter, characterized in that the magnetic system for generating the axial constant field in the resonator area is a permanent magnet system, the permanent magnet system consists of a central, radially polarized ring magnet , an axially polarized ring magnet attached to the end face of the collector and an annular magnet arrangement attached to the other end face, the latter being a subsystem blocking the outbreak of the magnetic field, which directly touch the partial magnets of the permanent system on their opposing surfaces , a constant or predefinable wavy magnetic field is generated by the geometry of the ring magnets and their mutual mechanical bracing in the region of the resonator, and none or at most a slight compensation up to the collector and emitter region axial magnetic field reversal occurs, the field reversal of the axial magnetic constant field takes place in the extension of the emitter region outside the region of the electron trajectories.

2. Magnet system for generating the axial magnetic field for gyrotrons according to claim 1, characterized in that

Permanent magnet system is symmetrical to an axis perpendicular to the system axis.

3. Magnet system for generating the axial magnetic field for gyrotrons according to claim 1, characterized in that the permanent magnet system is constructed asymmetrically and generates a strong magnetic field reversal in the extended emitter region, outside the electron beam generation.

4. Magnet system for generating the axial magnetic Gleich¬ field for gyrotrons according to claim 2 and 3, characterized in that the lying towards the collector, axially polarized ring magnet has a structured inner surface, whereby the magnetic DC field in the resonator area has a predetermined fine structure receives.

5. Magnet system for generating the axial magnetic field for gyrotrons according to claim 4, characterized in that there is an axially polarized, ring-shaped permanent magnet in the area of the emitter, with which the weaker in the emitter area, but locally con ¬ constant DC magnetic field is achieved.

6. Magnet system for generating the axial magnetic constant field for gyrotrons according to claim 5, characterized in that for correcting the axial magnetic field strength at least one current-operated solenoid is combined with the permanent magnet system.

7. Magnet system for generating the axial magnetic field for gyrotron according to claim 5, characterized in that soft iron configurations are braced with the permanent magnet system for correcting the axial magnetic field and for guiding the flux in the gyrotron region.