Magnet assembly for influencing a charged particle beam

Abstract

The invention relates to a magnet assembly for influencing a charged particle beam, the magnet assembly comprising at least 2·n alternately arranged magnetic poles, where n ≥ 2. Here, each magnetic pole is formed at least from one permanent magnet and a yoke with a coil is provided for each pole formed from at least one permanent magnet. The invention is distinguished in that at least one gap, delimited by two permanent magnet surfaces, is formed for each pole and wherein at least one plate composed of a soft-magnetic material and a means for motorized translation of the plate are provided for each pole, so that the at least one plate can be moved into and out of the at least gap one by way of the means for translation. The fields provided by the magnet assembly according to the invention can be varied by up to ± 20 %, based on the operating point.

Description

[0001] title

[0002] Magnet arrangement for influencing a beam of charged particles

[0003] Description

[0004] 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.

[0005] 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).

[0006] 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 which are generically applicable to 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. As far as the applicant is aware, the upper limit for n achieved to date is n = 14. Devices which are generically applicable to the inventive device also include 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. In article 1 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) presents some solutions for fine-tuning magnetic fields in devices that contain permanent magnets. 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. The passageway for the fields allows neighboring permanent magnets to be decoupled and the magnetic field strength of directly or initially adjacent permanent magnets to be 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, as described, for example, in Paper 2 by T. Taniuchi et al. (Permanent Dipole Magnet R&D for SPring-8-l I, 6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA, pp. 2883 - 2885).

[0007] 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, in extreme cases dropping 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 called remanent magnetic flux density.The strength of this magnetization, created 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 appropriate parameters (rel.Permeability, remanent flux density and coercive field strength) are also crucial.

[0008] 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 temperatures without the use of external energy sources, such as electric coils (electromagnets).

[0009] 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 cand the remanent flux density ÖR. These two quantities are temperature-dependent. Above a critical temperature Tc, it is possible to partially or completely demagnetize the permanent magnet. At room temperature, this relative temperature dependence of the characteristic quantities of the permanent magnet is in the range of -10', depending on the alloy and procedure. 4 to -10' 3 per °K. This temperature effect is compensated by so-called thermal shims (compensating pieces, usually made of NiFe), which have a low saturation flux density and a temperature dependence similar 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.

[0010] 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, 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.

[0011] To compensate for temperature effects, the use of NiFe sheets is described in the state of the art, which are installed parallel to the permanent magnets in hybrid magnet arrangements, as described, for example, in Article 3 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, 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, the magnetic saturation value of which depends strongly on the temperature. By carefully adjusting the use of NiFe sheets at the appropriate locations on the permanent magnets, thermal effects can be almost completely compensated. For fine-tuning of complete magnet arrangements, the following methods are used: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 presented, for example, in Article 4 by P. N'gotta et al. (Hybrid high gradient permanent magnet quadrupole, Physical Review Accelerators and Beams, Vol. 19, 2016, 122401 - 1-9). Since the fine-tuning of the permanent hybrid magnets is very complex, it is only used very rarely. Trimming is usually done using electrical coils (on the iron yokes), for example in a prototype at the Brazilian Synchrotron Light Source (Sirius), as described in Article 5 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 significantly reduced by the use of permanent magnets in the array. Another possibility is the complete displacement of the permanent magnets within the array, as described, for example, in Paper 6 by BJA ​​Shepherd et al. (Design and Measurement of a Low-Energy Tunable Permanent Magnet Quadrupole Prototype, Proceedings of IPAC2014, Dresden, Germany, 2014, pp. 1316-1318). However, the required travel distances in the magnet array 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 considerable 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 can be found in Paper 7 by G.Le Bec (High Gradient PM Technology for Ultra-High Brightness Rings, Proceedings of NAPAC2016, Chicago, IL, USA, 2016, pp. 1077-1082).

[0012] Task

[0013] 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.

[0014] The problem is solved by the subject matter of claim 1. Advantageous embodiments are the subject matter of the dependent claims.

[0015] 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.

[0016] The invention is based on the introduction of plates made of soft magnetic material which are motorised by means of a translation means per pole and can be moved in and out into a gap which is delimited or formed by at least two permanent magnet surfaces, e.g. formed from at least two permanent magnets. 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 which is provided 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 permanent magnets spaced apart from one another or two or more blocks, each formed from several permanent magnets.

[0017] 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. However, 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% of 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 there is no mechanical deformation of the entire magnet / magnet assembly (minimal field errors). Individual fine-tuning can also be achieved by presetting the short-circuit plates or by using a customized 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.

[0018] 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 (alternating north and south poles 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 poles alternating. 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, such as yokes with or without pole tips, do not touch each other in the center. An open space is formed in the center of a device for focusing a particle beam, through which a beam of charged particles to be focused can pass. Connected permanent magnets in direct contact with each other and with the same polarity orientation are considered, for the purposes of the invention, to be considered a single permanent magnet—formed of multiple parts—due to their function within the device.

[0019] 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.

[0020] 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.

[0021] 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, forming 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 facing the center. These serve to more sharply define the magnetic field 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 enhance, for example, B4 (quadrupole).

[0022] 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.

[0023] Example

[0024] The invention is described in more detail using an embodiment and two figures.

[0025] The figures show:

[0026] 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

[0027] Fig. 2: Schematic representation of a section of an exemplary embodiment of a magnet arrangement according to the invention for influencing a beam of charged particles in an oblique plan view in a 2nd position of the short-circuit plates, in which they achieve the entire possible attenuation. Fig. 1 shows a schematic representation of an exemplary embodiment of a magnet arrangement according to the invention for influencing a beam of charged particles 1 in an oblique plan view in a 1st position of the short-circuit plates Kl, K2, K3, Kl', K2', K3', in which they remain ineffective. The short-circuit plates Kl, K2, K3, Kl', 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 Fig. 1 and Fig. 2 shows approximately 1 / 4 of the entire magnet arrangement 1, which is a quadrupole in the exemplary embodiment. One pole of the four poles of the quadrupole is shown representatively.The remaining three poles are identical in construction 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 Kl, K3, Kl', K3' are brought into different degrees of overlap with the arrangements of permanent magnets PM, PM' on the outside, parallel to the arrangements of permanent magnets PM, PM' by the translation means T, T' (only T' is shown in Fig. 1, for T see Fig. 2). These outer short-circuit plates Kl, K3, Kl', K3' reinforce the effect of the middle short-circuit plates K2, K2', i.e. when they overlap, the weakening of the magnetic field corresponds to the degree of overlap.the degree of the position of the inward or outward movement. In Fig. 1, the short-circuit plates K2, K2' are fully extended from the gaps of the arrangements of the permanent magnets PM, PM', and the remaining short-circuit plates Kl, K3, Kl', K3' do not overlap with the arrangements of the permanent magnets PM, PM' (1st position as an extreme), which corresponds to the state in which the short-circuit plates Kl, K2, K3, Kl', K2', K3' are ineffective 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. A yoke J with a pole tip PT is arranged for each pole. In the exemplary embodiment, coils S are also arranged on the yoke J, with which fine tuning is possible. For reasons of clarity, the coils are only shown as cross-sections in Figures 1 and 2.In Figures 1 and 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.

[0028] In Fig. 2, compared to Fig. 1, the short-circuit plates Kl, K2, K3, Kl', K2', K3' are fully retracted into the gap or completely overlapped, which corresponds to a 2nd position as the second extreme of the greatest possible effect of the short-circuit plates Kl, K2, K3, Kl', K2', K3', the strongest attenuation of the magnetic field of the arrangements of permanent magnets PM, PM' (now covered by the short-circuit plates Kl, Kl') and thus the weakest magnetic field that can be adjusted with the magnet arrangement 1.

[0029] Between the extremes in the positioning of the short-circuit plates Kl, K2, K3, Kl', K2', K3' shown in Figures 1 and 2 as examples to demonstrate how the arrangement works, all positions in between can be realized. The analogous positioning of the short-circuit plates Kl, K2, K3, Kl', K2', K3' given for the exemplary embodiment is also an example. By positioning the short-circuit plates Kl, K2, K3, Kl', K2', K3', differences per pole of the magnetic fields formed by the arrangements of permanent magnets PM, PM' can be compensated for, so that the positioning of the short-circuit plates Kl, K2, K3, Kl', 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.

[0030] The polarization of the arrangement of permanent magnets PM, PM' in the section of the magnet arrangement 1 in Figures 1 and 2 points inward, parallel to the edges of the permanent magnet arrangements PM, PM', toward the center of the magnet arrangement 1, i.e., toward the pole tip PT, as illustrated by the arrows. Accordingly, the poles alternate in turn in the poles of the quadrupole not shown in Figures 1 and 2.

[0031] 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 8 r : 12900-13200 Gauss (G), 1.29-1.32 Tesla (T); coercive field strength ÖH C : 10.8-12.0 kOe, 860-955 kA / m, / H 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 this example have a size of 50 mm x 100 mm x 75 mm. They are preferably made from a single block; 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, as well as 2% vanadium, with a preferred orientation of the crystallites. The pole tip is wedge-shaped with the maximum dimensions (HBT)

[0032] 55 mm x 74 mm x 160 mm. The aperture of the magnet assembly is 10 mm with a length of 160 mm and the outer dimensions of the frame R are

[0033] 570 mm x 570 mm (without the translation means). 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 shape is familiar to those skilled in the 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.

[0034] 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

Patent claims 1. Magnet arrangement for influencing a beam of charged particles comprising at least 2-n alternating magnetic poles, with n > 2 and n < 20, and wherein 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, 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 per pole, so that the at least one plate can be moved in and out of the at least one gap by means of the translation means.

2. Magnet arrangement according to claim 1, characterized in that at least two plates of soft magnetic material are arranged per pole.

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