Magnet arrangement for influencing a beam of charged particles

Abstract

The invention relates to a magnet arrangement for influencing a beam of charged particles, which comprises at least 2 n alternating magnetic poles, with n ≥ 2. Each magnetic pole is formed from at least one permanent magnet, and a yoke with a coil is arranged for each pole formed from at least one permanent magnet. The invention is characterized in that at least one gap is formed per pole, delimited by two permanent magnet surfaces, and wherein at least one plate made of a soft magnetic material and a means for motorized translation of the plate are arranged for each pole, such that the at least one plate can be moved in and out of the at least one gap using the translation means. The fields provided by the magnet arrangement according to the invention can be varied by up to ± 20%, based on the operating point.

Description

[0001] The invention relates to a magnet arrangement for influencing a beam of charged particles, such as is used in particle accelerators such as synchrotron radiation sources and such as are known from the prior art.

[0002] Magnet arrangements of the type used for the invention relate to those with 2 n, n ≥ 2, alternating poles arranged symmetrically in a plane, e.g. quadrupoles and sextupoles, as presented and described in the review article by K. Wille (Synchrotron radiation sources, Reports on Progress in Physics Vol. 54, 1991, pp. 1005-1068).

[0003] Depending on the design, devices for providing a magnetic field to focus a beam of charged particles comprise, in particular, permanent magnets and / or electromagnets. The number of magnetic poles in the devices corresponds exclusively to an even number. A distinction must be made between devices that have only two magnetic poles, dipoles, which are used for beam deflection, and so-called transverse optics (influence fields perpendicular to the direction of propagation of the beams), particularly quadrupoles and sextupoles, which are used for beam focusing.Devices of the type described in the invention comprise a symmetrical arrangement of an even number of permanent magnets and are designed as transverse optics for focusing beams of charged particles, with a number of alternating poles z ≥ 4, given by the number of alternating poles z = 2 n, with n ≥ 2, in particular quadrupoles and sextupoles with z = 4 or z = 6. To the applicant's knowledge, the upper limit for n achieved to date is n = 14. Devices of the type described in the invention also comprise devices in which the magnetic field is largely determined by permanent magnets in the device and in which the permanent magnets are optionally connected to yokes - permanent hybrid magnets - in which a magnetic field is co-determined by coils and the magnetic field can be changed by changing the coil current, which contributes to fine-tuning the magnetic field in the device.Such a device, which represents the closest state of the art, is disclosed in Paper 1 by J. Völker et al. (Variable permanent hybrid magnets for the Bessy III storage ring. In: Frank Zimmermann [Ed.]: IPAC2022: Proceedings of the 13th International Particle Accelerator Conference, in Bangkok, Thailand, 12-17 June 2022, Geneva: JACoW, pp. 2763-2766.).

[0004] In Paper 2 by PP Sanchez et al. (Concepts of tunable magnets using permanent magnetic material for synchrotron radiation sources, Nuclear Instruments and Methods in Physics Research A, Vol. 778, 2015, pp. 67-76), some solutions for fine-tuning magnetic fields in devices that include permanent magnets are presented, as is Paper 3 by BJA ​​Shepherd (Permanent magnets for accelerators. In: Proceedings / 11 thInternational Particle Accelerator Conference, 10-15 May 2020, Caen, France, 2021, Geneva: JACoW Publ., pp. 1-5). The fine-tuning of the magnets is made possible in the examples, for example, by a combination of permanent magnets and coils, movably arranged permanent magnets, and so-called magnetic shunt plates (or shimming plates) made of magnetically permeable material. The magnetic short-circuit plates made of magnetically permeable material serve here as a passageway for the magnetic fields. Due to the passageway for the fields, neighboring permanent magnets can be decoupled and the magnetic field strength of directly or initially adjacent permanent magnets is weakened. The application of such magnetic short circuits is described, for example, in the state of the art for dipoles to compensate for the effects of temperature fluctuations on the magnetic fields provided by the permanent magnets, such asin Paper 4 by T. Taniuchi et al. (Permanent Dipole Magnet R&D for SPring-8-II, 6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA, pp. 2883 - 2885) and also in Paper 1.

[0005] The following description explains the interpretation of the terms used in this application regarding materials for the magnet arrangement. All parts in the magnet arrangement that form the magnetic field (yoke, pole shoes, pole fingers, etc.) are made of ferromagnetic materials such as iron, cobalt, or nickel, which can conduct the generated magnetic fluxes well. This property is represented by a material-specific hysteresis curve of the magnetic flux density as a function of the external magnetic field. The local increase in this curve is also described as relative permeability. Ferromagnetic materials have a relative permeability of > 1 (unlike air or vacuum, which has a value of 1). Permeability describes the quality of a magnetic conductor in terms of the extent to which an external magnetic flux can be conducted through the material.the degree to which the magnetic flux density is amplified by an external magnetic field compared to a vacuum. The exact value depends heavily on the selected alloy of the ferromagnetic material, the production method, and the technical and thermal pretreatment, and can realistically assume values ​​of approximately 100,000. A characteristic quantity of the hysteresis curve is the saturation flux density, above which the relative permeability decreases sharply, and in extreme cases, drops to a value of 1. Naturally, this leads to permanent magnetization of the material. This refers to the generation of its own magnetic flux without an external magnetic field. This flux density is referred to as remanent magnetic flux density.The strength of this magnetization, generated by the remanent magnetic flux density, is called the coercive force and describes the external magnetic field strength required to reset (zero) the remanent magnetic flux. Alloys with coercive field strengths < 1000 A / m are referred to as magnetically soft (soft magnetic). Such soft magnetic materials are often used in magnets as yoke or pole piece material to generate a reproducible magnetic field without major losses in the field region. In contrast, magnetically hard alloys with very high coercive field strengths of up to a few MA / m are very well suited to producing permanently magnetized materials, which can then be referred to as permanent magnets. For the development of a magnet arrangement for influencing a beam of charged particles, the selection of these materials with the corresponding characteristics (rel.Permeability, remanent flux density and coercive field strength) are also crucial.

[0006] So-called permanent hybrid magnets are a combination of permanent magnets and a magnetically conductive iron yoke. They exhibit excellent long-term stability of the magnetic fields at constant temperature without the use of external energy sources, such as electric coils (electromagnets).

[0007] Permanent magnets are made of ferromagnetic materials, with additional additives, usually rare earth elements such as samarium, neodymium, or gadolinium. The resulting alloys (e.g., neodymium-iron-borium or cobalt-samarium) are considered magnetically hard and exhibit very high coercive field strengths, so that remagnetization is only possible with considerable energy expenditure. Magnetization of these permanent magnet alloys leads to high remanent flux densities of up to 1.5 T, which allows very high magnetic flux densities to be achieved in the permanent magnet and its surroundings, even without an external magnetic field. The various alloys and manufacturing processes determine the two characteristic quantities of a permanent magnet: the coercive field strength H c and the remanent flux density B R . These two quantities are temperature dependent. Above a critical temperature T cIt is possible to partially or completely demagnetize the permanent magnet. At room temperature, this relative temperature dependence of the characteristic values ​​of the permanent magnet is in the range of -10 -4 to -10 -3per °K. This temperature effect is compensated by so-called thermal shims (compensating pieces, usually made of NiFe), which have a small saturation flux density with a similar temperature dependence to the parameters of the permanent magnets. The arrangement of shims is complex and expensive, increases the total volume of the corrected magnet, and reduces the variability of the magnet's operating parameters. The use of shims for fine-tuning magnetic structures is also described in article 5 by J. Chavanne et al. (Recent developments in insertion devices at the ESFR: working toward diffraction-limited storage rings. In: Synchrotron radiation news, Vol. 28, No. 3, 2015, pp. 15-18).

[0008] A key characteristic of permanent magnets is their excellent long-term stability in temperature-stable and low-radiation environments, allowing permanent magnet-based magnet arrays to generate magnetic fields consistently for many years. However, there are cases where it is necessary to vary the field strength of the magnets during use (e.g., machine tests, subsequent corrections, and modifications to the standard optics of particle accelerators). In the case of electromagnets, this can be achieved by changing the coil current. Depending on the magnet and the application, variations of 5% to 20% of the field strength are required, which is very difficult to achieve with permanent magnets using correction coils alone.

[0009] To compensate for temperature effects, the state of the art describes the use of NiFe sheets, which are installed parallel to the permanent magnets in hybrid magnet arrangements, as described, for example, in Article 6 by K. Bertsche and J.-F. Ostiguy (Temperature considerations in the design of a permanent magnet storage ring, Proceedings of the 1995 Particle Accelerator Conference, Dallas, TX, USA, 1995, pp. 1381-1383). Depending on the material of the permanent magnet, between 8% and 18% of the total permanent magnet volume is required. The sheets act as a magnetic short circuit, whose magnetic saturation value depends strongly on temperature. By carefully positioning the NiFe sheets at the appropriate locations on the permanent magnets, thermal effects can be almost completely compensated.

[0010] For the fine tuning of complete magnet arrangements, so-called trimming sheets are added to the arrangements or additional gaps are inserted in iron yokes of permanent hybrid magnets in order to correct the small production-related variations in the magnetic parameters of the permanent magnets, which affect the entire hybrid magnets and also the entire arrangement, as described, for example, in article 7 by P. N'gotta et al. (Hybrid high gradient permanent magnet quadrupole, Physical Review Accelerators and Beams, Vol. 19, 2016, 122401 - 1-9) and in article 8 by J. Völker et al. (Development of the first permanent bending magnet at Bessy II, In Ralph Assmann [Ed.]: Conference Proceedings / 14th international particle accelerator conference, 7-12 May 2023, Venice, Italy, Geneva: JACoW, pp. 3878-3881). Since the fine-tuning of permanent hybrid magnets is very complex, it is rarely used.Mostly, trimming is done using electrical coils (on the iron yokes), as in a prototype at the Brazilian Synchrotron Light Source (Sirius), as described in paper 9 by G. Tosin et al. (Super hybrid quadrupoles, Nuclear Instruments and Methods in Physics Research A, Vol. 674, 2012, pp. 67-73). However, this requires relatively large coils, as the efficiency of the coils is considerably reduced by the use of permanent magnets in the arrangements. Another possibility is the complete displacement of the permanent magnets within the arrangement, as described, for example, in paper 10 by B.J.A. Shepherd et al. (Design and Measurement of a Low-Energy Tunable Permanent Magnet Quadrupole Prototype, Proceedings of IPAC2014, Dresden, Germany, 2014, pp. 1316-1318) and paper 11, also by B.J.A. Shepherd et al. (Novel adjustable permanent magnet quadrupoles for the CLIC drive beam decelerator.In: IEEE transactions on applied superconductivity, Vol. 22, No. 3, 2012, Art. No. 4004204, 4 pp.), the latter for a quadrupole with a high gradient. However, the required travel distances in the magnet arrangement and the finite precision of the iron yoke (with regard to the variation of the enormous magnetic forces) lead to very limited field strength ranges and significant field errors (additional multipole fields that can degrade the field quality). An overview and comparison of various implementations for fine-tuning quadrupoles and the underlying magnetic errors is also discussed in Paper 12 by G. Le Bec (High Gradient PM Technology for Ultra-High Brightness Rings, Proceedings of NAPAC2016, Chicago, IL, USA, 2016, pp. 1077-1082).

[0011] A special type of quadrupole tuning is described in Paper 13 by A. Ghaith et al. (Tunable high gradient quadrupoles for a laser plasma acceleration based FEL. In: Nuclear instruments and methods in physics research, A, Vol. 909, 2018, pp. 290-293). Here, two quadrupoles are arranged relative to each other, the first consisting of permanent magnets in a Halbach configuration in the form of a ring, and the second consisting of four permanent magnet cylinders that surround the ring and can rotate about their axis to achieve a gradient tuning of 50 T / m. A quadrupole arrangement structurally identical to Paper 10 is described in Paper 14 by F. Marteau et al. (Variable high gradient permanent magnet quadrupole (QUAPEVA), Applied physics letters, Vol. 111, 2017, Art.-No. 253503, pp. 1-5.) presented. Task

[0012] The object of the invention is to provide a magnet arrangement for influencing a beam of charged particles, with which, compared to the prior art, an adaptation of the magnetic field achieved by the magnet arrangement with reduced energy consumption compared to the prior art and, at the same time, with an increased adaptation range for a magnetic field compared to the prior art with regard to the achievable magnetic field.

[0013] The problem is solved by the subject matter of claim 1. Advantageous embodiments are the subject matter of the dependent claims.

[0014] The invention relates to a magnet arrangement for influencing a beam of charged particles comprising at least 2·n alternately arranged magnetic poles, with n ≥ 2, and wherein each magnetic pole is formed from at least one permanent magnet.

[0015] The invention is based on the introduction of plates made of soft magnetic material which are motorised and can be moved in and out of a gap defined or formed by at least two permanent magnet surfaces, e.g. formed from at least two permanent magnets, by means of a translation mechanism per pole. In a further embodiment, additional plates (i.e. a total of at least two plates) made of soft magnetic material are arranged per pole in the arrangement parallel to a respective gap and can be moved by the means for motorised translation. The plates can also be referred to as short-circuit plates. The provision of a one-piece permanent magnet with a gap solves this problem. However, the production of very strong and precise permanent magnets is very complex, which is why the shapes of these magnets are kept as simple as possible (cubes, bars, spheres, wedges, etc.).Therefore, the gap required here is preferably formed from at least two spaced-apart permanent magnets or two or more blocks, each formed from several permanent magnets.

[0016] The penetration depth of the plates made of soft magnetic material (shorting plates) defines the strength of a magnetic short circuit caused by the insertion of the plates into a permanent magnet (into the gap in the permanent magnet) or between the permanent magnets, which in turn influences the integrated strength of the magnetic sources, i.e. the permanent magnets or ensembles of permanent magnets. Calculations of the achievable variations show that small displacements of the plates relative to the permanent magnets, i.e. only a partial displacement in or out of each gap, are sufficient to compensate for temperature effects of the permanent magnets. Displacements on the order of the permanent magnets themselves, i.e. a complete displacement of the plates in or out of a respective gap, can change the field strengths by up to ± 20% relative to the operating point, or approximately 35% - 40% of the maximum.The plates are therefore advantageously dimensioned so that they can be fully inserted into a gap. The permanent magnets and all yoke components remain stable in one place, and no mechanical deformation of the entire magnet / magnet assembly occurs (minimal field errors). Individual fine-tuning can also be achieved by presetting the short-circuit plates or by using the individual process. This advantageously replaces three isolated and sometimes complex techniques for correcting the hybrid magnet prior to operation with a system that can be adjusted even during operation.

[0017] The device according to the invention for providing a magnetic field for focusing a beam of charged particles comprises at least an even number of permanent magnets which are sufficient to achieve a number of alternating poles in the device z = 2 n , with 2 ≦ n ≦ 20, i.e. at least 4 permanent magnets must be provided. The permanent magnets are arranged in such a way that they form a magnetic field transverse to the particle beam, i.e. the permanent magnets are arranged concentrically, with either 1. an orientation of one of their two poles (north and south pole) - polarity - in the direction of the center defined by the concentric arrangement, wherein the orientation of the poles alternates and all poles of all permanent magnets lie in one plane or 2.if at least, as is preferably the case, two permanent magnets together form one of the alternating poles, wherein the permanent magnets together cause an alignment of one pole (north or south) towards the center and the poles thus formed also alternate (north and south poles alternately aligned towards the center of the arrangement). The interacting permanent magnets are spaced apart from one another, and in particular a gap is formed between the permanent magnets of the same orientation. Consequently, more permanent magnets can be arranged in the device than the number of alternating poles. The arrangement of several permanent magnets per formed pole in the device can refer to an arrangement of several permanent magnets along the arrangement surrounding the center as well as perpendicular to the plane of the magnetic field formed in the device.Both possibilities of multiple arrangements of permanent magnets can also be combined. The numerical designation of the devices, such as "Quadru-", "Sextu-", etc., indicates the number of pole alternations (4, 6, etc.). The permanent magnets or extensions, e.g., yokes with or without pole tips, do not touch each other in the center. In the center of a device for focusing a particle beam, an open space is formed through which a beam of charged particles to be focused can pass. Connected permanent magnets in direct contact with one another and with the same polarity alignment are considered, for the purposes of the invention, to be considered a single permanent magnet - formed from several parts - due to their function within the device.

[0018] The plate thickness of the short-circuit plates and the soft magnetic material from which they are formed are selected as required. The plate thickness should be > 2 mm for stability reasons. Due to the overall size of the magnet arrangements, the plate thickness will not exceed 20 mm. In relation to the size of the permanent blocks, this corresponds to approximately 50% of the length of the blocks. The permanent magnets to be used for the inventive device have magnetic field strengths in the range of 0.5 T to approximately 2.0 T (field strengths of approximately 1.5 T have been realized so far). The permanent magnet blocks, which can be assembled from individual permanent magnets, have dimensions in the range of 1 mm. 3 up to 500 mm 3 , especially 10 mm 3 up to 100 mm 3 on.

[0019] The positioning systems provided according to the invention are motorized translation devices, e.g., based on synchronous motors. The positioning systems according to the invention have the advantage that they allow the short-circuit plates to be moved during operation of the magnet arrangement, and this with relatively low energy consumption.

[0020] The permanent magnets are each connected to a yoke for each alternating polarity, the magnetic flux of which can be additionally varied by a coil. This means that two or more permanent magnets, which form a pole in the device, can be connected to a yoke. The yokes are each equipped with coils that apply a magnetic field to the yoke. This serves to further fine-tune the magnetic field in the device. In particular, the yokes are provided with so-called pole tips at the end pointing towards the center. These serve to define the magnetic field more sharply and are made of soft magnetic materials such as CoFe, SiFe or others. The shape of the pole tips and their chamfer are optimized to suppress multipoles such as B12 and B20 and to increase B4 (quadrupole), for example.

[0021] The device according to the invention has the advantage that the provided magnetic field can be adjusted during operation of the device, up to an amount of ± 20% around the desired operating point. This adjustment can be carried out in an energy-saving manner using the provided positioning systems. The provision of a gap between permanent magnets or blocks of permanent magnets has a positive effect on the dimensioning of the magnet arrangement. The permanent magnets and the entire magnet arrangement can be made shorter, and the process-related complexity of the arrangement itself is reduced. Example

[0022] The invention is described in more detail using an embodiment and two figures.

[0023] The figures show: Fig. 1: Schematic representation of a section of an embodiment of a magnet arrangement according to the invention for influencing a beam of charged particles in an oblique top view in a 1st position of the short-circuit plates in which they remain ineffective Fig. 2: Schematic representation of a section of an embodiment of a magnet arrangement according to the invention for influencing a beam of charged particles in an oblique top view in a 2nd position of the short-circuit plates, in which they achieve the entire possible attenuation

[0024] In the Fig. Figure 1 schematically shows an embodiment of a magnet arrangement according to the invention for influencing a beam of charged particles 1 in an oblique top view in a first position of the short-circuit plates K1, K2, K3, K1', K2', K3', in which they remain ineffective. The short-circuit plates K1, K2, K3, K1', K2', K3' remain ineffective when they are completely moved out of the arrangements of permanent magnets PM, PM' and do not overlap with them. The section in the Fig. 1 and the Fig. Figure 2 shows approximately 1 / 4 of the entire magnet arrangement 1, which in the exemplary embodiment is a quadrupole. One of the four poles of the quadrupole is shown representatively. The remaining three poles are constructed identically to those shown. In the exemplary embodiment, two arrangements of permanent magnets PM, PM' are arranged per pole, each forming a gap (not visible in the figure). The middle of the three short-circuit plates K2, K2' per arrangement of permanent magnets PM, PM' is moved in and out of the gap formed by the permanent magnets PM, PM'. The two remaining short-circuit plates K1, K3, K1', K3' are moved externally, parallel to the arrangements of permanent magnets PM, PM', by the translation means T, T' (in Fig. 1 only T' is shown, for T see Fig. 2) with the arrangements of permanent magnets PM, PM' to varying degrees of overlap. These outer short-circuit plates K1, K3, K1', K3' reinforce the effect of the middle short-circuit plates K2, K2', i.e., when overlapping, the weakening of the magnetic field is adjusted according to the degree of overlap or the degree of the position of the inward or outward movement. Fig. 1, the short-circuit plates K2, K2' are fully extended from the gaps of the permanent magnet arrangements PM, PM' and the remaining short-circuit plates K1, K3, K1', K3' do not overlap with the permanent magnet arrangements PM, PM' (1st position as an extreme), which corresponds to the state of ineffectiveness of the short-circuit plates K1, K2, K3, K1', K2', K3' and thus to the strongest magnetic field in magnet arrangement 1. The positions in the poles of the quadrupole (not shown) of the exemplary embodiment are analogous. The magnet arrangement 1 also comprises a frame R. For each pole, a yoke J is arranged with a pole tip PT. In the exemplary embodiment, coils S are also arranged on the yoke J, with which fine tuning is possible. The coils are in the Fig. 1 and Fig. 2 is shown only as a cross-section for reasons of clarity. Fig. 1 and Fig. 2, a spacer made of aluminum - as a paramagnetic material without influence on the magnetic field - A is arranged between the arrangements of permanent magnets.

[0025] In the Fig. 2 are opposite the Fig. 1 the short-circuit plates K1, K2, K3, K1', K2', K3' are fully retracted into the gap or brought to complete overlap, which corresponds to a 2nd position as the second extremum of the greatest possible effect of the short-circuit plates K1, K2, K3, K1', K2', K3', the strongest attenuation of the magnetic field of the arrangements of permanent magnets PM, PM' (now covered by the short-circuit plates K1, K1') and thus the weakest magnetic field that can be adjusted with the magnet arrangement 1.

[0026] Between the Fig. 1 and Fig. 2 In addition to the extremes in the positioning of the short-circuit plates K1, K2, K3, K1', K2', K3' shown as an example to demonstrate how the arrangement works, all positions in between can be realized. The analogous positioning of the short-circuit plates K1, K2, K3, K1', K2', K3' given for the exemplary embodiment is also an example. By positioning the short-circuit plates K1, K2, K3, K1', K2', K3', differences per pole of the magnetic fields formed by the arrangements of permanent magnets PM, PM' can also be compensated for, so that the positioning of the short-circuit plates K1, K2, K3, K1', K2', K3' per pole and also per pole per arrangement of permanent magnets PM, PM' do not have to match. In the example, the short-circuit plates have a size of 100 mm x 120 mm x 10 mm.

[0027] The polarization of the arrangement of the permanent magnets PM, PM' in the section of the magnet arrangement 1 of the Fig. 1 and Fig. 2 points parallel to the edges of the permanent magnet arrangements PM, PM' inwards towards the center of the magnet arrangement 1, ie towards the pole tip PT, as illustrated by the arrows. Accordingly, the poles alternate in turn in the Fig. 1 and Fig. 2 not shown poles of the quadrupole.

[0028] In the exemplary embodiment, the arrangements of permanent magnets are made up of four cuboid permanent magnets made of NdFeB with the quality N42 (remanence B r : 12900-13200 Gauss (G), 1.29-1.32 Tesla (T); coercive field strength bH c : 10.8-12.0 kOe, 860-955 kA / m, iH c : ≥ 12 kOe, ≥ 955 kA / m; Energy product: 40-42 MGOe, 318-334 kJ / m 3; maximum operating temperature: ≤ 80 °C), with a gap between each two of the cuboids. The permanent magnets in the arrangement in the example have a size of 50 mm x 100 mm x 75 mm. They are preferably made from a single individual block, but production in which the blocks are joined from 2, 4 or 8 individual blocks is also possible. The pole tips are made of the so-called "Permendur", an alloy of equal parts cobalt and iron and 2% vanadium, with a preferred orientation of the crystallites. The pole tip is wedge-shaped with maximum dimensions (HBT) 55 mm x 74 mm x 160 mm. The aperture of the magnet arrangement is 10 mm with a length of 160 mm and the external dimensions of the frame R are 570 mm x 570 mm (without the means for translation). The pole tip gap is 8 mm and the pole tip shape is optimized to suppress multipoles such as B12 and B20 and to strengthen B4 (quadrupoles).Such a design is familiar to those skilled in the art from the prior art. The short-circuit plates K1, K2, K3, K1', K2', K3' are also made of Permendur, and the coils S are made of copper. The frame and yokes are made of 1010 steel, as is the translation means. The motorization (not shown in the figures) is provided by a synchronous motor, for example.

[0029] The magnetic fields that can be adjusted with the magnet arrangement of the exemplary embodiment lie in a range between 75 T and 125 T. The specific ranges of the magnetic fields vary in the fluctuation ranges of the permanent magnets used.

Claims

[1] Magnet arrangement for influencing a beam of charged particles comprising at least an even number of permanent magnets which are sufficient to fulfil a number z = 2 n of alternating poles in the device with 2 ≤ n ≤ 20 and wherein the permanent magnets are arranged such that all poles of all permanent magnets lie in one plane and are arranged concentrically and wherein a yoke with a coil is arranged for each pole formed from at least one permanent magnet and wherein at least one plate made of a soft magnetic material is arranged for each pole, characterized by that at least one gap, delimited by two permanent magnet surfaces, is formed per pole and a means for motorized translation of the at least one plate made of soft magnetic material is arranged, so that the at least one plate can be moved in and out of the at least one gap with the means for translation. [2] Magnet arrangement according to claim 1, characterized by that at least two plates made of soft magnetic material are arranged per pole.

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