US3781732A

US3781732A - Coil arrangement for adjusting the focus and/or correcting the aberration of streams of charged particles by electromagnetic deflection, particularly for sector field lenses in mass spectrometers

Info

Publication number: US3781732A
Application number: US00226479A
Authority: US
Inventors: H Wollnik
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-02-18
Filing date: 1972-02-15
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25
Also published as: DE2107770C3; DE2107770A1; DE2107770B2

Abstract

A coil arrangement for adjusting the focus and/or correcting the aberration of streams of charged particles by electromagnetic deflection, particularly for sector field lenses in mass spectrometers, comprises flat coils produced in accordance with conventional printed circuit technology. Part of the turns of each coil is shaped to produce a locally changing magnetic field normal to the stream of charged particles for differentially affecting the particle paths according to their position in this magnetic field.

Description

Un ted States Patent 1191 1111 3,781,732

Wollnik Dec. 25, 1973 COIL ARRANGEMENT FOR ADJUSTING [56] References Cited THE FOCUS AND/OR CORRECTING THE UNITED STATES PATENTS ABERRATION 0F STREAMS 0F CHARGED 3,118,092 l/l964 Marley 336/200 x PARTICLES BY ELECTROMAGNETIC 3,139,566 6/1964 Marley 335/210 DEFLECTION, IC L OR 2,830,212 4/1958 Hanlet 335/213 X Corpcw SPECTROMETERS Inventor: Hermann Wollnik, No. 129,

Eichendorffring, Giessen, Germany Filed: Feb. 15, 1972 Appl. No.: 226,479

Foreign Application Priority Data Feb 18, 1971 Germany P 21 07 770.3

US. Cl. 335/213, 250/495 D Int. Cl. I-I0lf 5/00 Field of Search 335/210, 213;

Primary ExaminerGeorge Harris Attorney-Markva & Smith [57] ABSTRACT A coil arrangement for adjusting the focus and/or correcting the aberration of streams of charged particles by electromagnetic deflection, particularly for sector field lenses in mass spectrometers, comprises fiat coils produced in accordance with conventional printed circuit technology. Part of the turns of each coil is shaped to produce a locally changing magnetic field normal to the stream of charged particles for differentially affecting the particle paths according to their position in this magnetic field.

17 Claims, 37 Drawing Figures PATENIEU M825 1975 SHEET 10? 6 PATENTED BECZS 1975 SHEET 3 OF 6 FIG. 7a

PAIENTEDUEC25 1915 3.781. 732

' SHEET u 0? 6 PATENTEUBECPS m5 sum 5 OF ill FIG/4a FIG/4b FIG /5a FIG/6 1 COIL ARRANGEMENT FOR ADJUSTING THE FOCUS AND/OR CORRECTING TI-IE ABERRATION OF STREAMS OF CHARGED PARTICLES BY ELECTROMAGNETIC DEFLECTION, PARTICULARLY FOR SECTOR FIELD LENSES IN MASS SPECTROMETERS BACKGROUND OF THE INVENTION This invention relates to an arrangement of coils for adjusting the focus and/or correcting the aberration of bundles of charged particles by electromagnetic deflection, particularly for sector field lenses in mass spectrometers.

Aberrations have already been the subject of correction in optical image reproduction for a long time, and this has led to the development of the complicated now generally familiar multi-lens systems, as used for instance in photographic cameras and optical microscopes. Although in the case of axially symmetrical electromagnetic lenses for focusing charged particles, for instance in mass spectrometers, cathode ray tubes and electron microscopes, the same problem in principle also arises, no similarly successful solutions are as yet known in the art. The difficulty is explained by the fact that whereas in optical systems lens surfaces can in practice have arbitrary configurations, no such discontinuous boundary surfaces exist in electron deflecting systems. I

For an understanding of the present invention reference is made to the drawings wherein:

FIGS. 1a, lband 1c. show diagrammatically focusing of particles with an ideal lens, with second order aberrations, and with third order aberrations respectively;

FIGS. 2a, 2b and 20 show poleshoe plates of various configurations;

FIGS. 3a, 3b and 3c show various forms of entry and exit boundaries of a second field which are passed by particle bundles;

FIG. 4 shows a hexapole lens;

FIG. 5 is a perspective view ofa coil assembly in accordance with the present invention;-

FIGS. 6a and 6b show respectively one coil ofthe coil assembly of FIG. 5 and the magnetic field produced by this coil;

FIGS. 7a and 7b show respectively a second coil of the coil assembly of FIG. 5 and the magnetic field produced by this coil;

FIG. 8a and 8b show respectively another embodiment of the coil shown in FIG. 7a and the magnetic field produced by this coil;

FIGS. 90 and 9b show respectively a further embodiment of the coil shown in FIG. 7a and the magnetic field produced by this coil;

FIGS. 10a and 10b show respectively still another embodiment of the coil shown in FIG. 7a andthe magnetic field produced by this coil;

FIGS. 11a and 11b show respectively still a further embodiment of the coil shown in FIG. 7a and the magnetic field produced by this coil;

FIGS. 12a and 12b show respectively a third coil of the coil assembly of FIG. 5 and the magnetic field produced by this coil;

FIGS. 13a and 13b show respectively another embodiment of the coil shown in FIG. 12a and the magnetic fieldproduced by this coil;

FIGS. 14a and 1412 show respectively a fourth coil of the coil assembly of FIG. 5 and the magnetic field produced by this coil;

FIGS. 15a and 15b show respectively another embodiment of the coil shown in FIG. 14a and the magnetic field produced by this coil;

FIG. 16 shows changes in the field distribution of the coils;

FIGS. 17, I8 and 19 show various forms ofa currentconducting lamina for use with the embodiment of FIG. 9a;

FIG. 20 shows a coil provided with circular gaps; and

FIG. 21 shows another embodiment of the invention.

Referring for a moment to FIG. la of the accompanying drawings an ideal lens 63 would focus all particles emanating from a point source at a point 20 on a symmetry axis 2 with an intensity distribution In However, the image formed by a real lens is always subject to aberrations, aberrations of different orders having to be distinguished. In second order aberrations a lens 64 (FIG. 1b) will focus particle rays that are remote from the axis at a point 21 offset from the symmetry axis and an asymmetrical image 21 having the intensity distribution Jb will be formed. This aberration can be compensated by additionally slightly deflecting the particles that pass through the lens at points more remote from the axis in the same direction for positive and negative values ofy (cf. the arrows 35 in FIG. lb). The deflecting force must increase with the square of the distance from the symmetry axis. Such measures, however, are complicated and difficult to perform.

Third order aberrations are illustrated in FIG. 10. In this instance, rays distant from the symmetry axis are indeed focused by a lens 65 at a point 22 situated on the symmetry axis, but this point does not coincide with the point of focus of rays that are nearer to the axis. Thus, an asymmetrically widened image 22 having the intensity distribution Jc is formed. This aberration can be reduced by additionally deflecting particles that pass through the lens at points that are distant from the symmetry axis in contrary directions for positive and negative values ofy (cf. the arrows 36 in FIG. 1c). The deflecting force must increase with the cube of the distance from the axis of symmetry, again a result that is very awkward and difficult to achieve.

Analogously, there are aberrations of the fourth, fifth and nth orders. Generally speaking the rule that applies is that all aberrations of even-numbered orders cause the image to be asymmetrically widened, whereas all aberrations of odd-numbered orders cause the image to widen symmetrically. Moreover, it has been established that aberrations of even-numbered orders vanish in non-deflecting systems, such as electron microscopes, but'that they'are of significant magnitude in all deflecting systems, such as in mass spectrometers or cathode ray tubes.

When speaking of aberrations of the second to the .nth order, the action of an ideal lens may be described as a first order effect which consists in that rays remote from the axis of symmetry are refracted differently to rays that are axially proximate, the deflecting force rising linearly with increasing distance from the axis of symmetry.

A pure deflection which is the same for all rays of a bundle of'rays may then be described as a zero order effect.

The correction of aberrations in particle optics is usually so performed that the distribution of themagnetic or of the electrostatic field in the lens is slightly modified. For instance, instead of constructing a magnetic sector field (FIG. 2a) from two

poleshoe plates

37 and 38 extending in parallel, it is formed from poleshoes which in cross section are coned' 39 and 40 (FIG. 2b) or toroidal 41 and42 (FIG. This preserves the zero order deflection properties, but the first order properties as well as the second, third and higher order aberrations are modified. Nearly the same effect can be achieved by deforming the entry and exit boundaries 44 of a sector field that are passed by a particle bundle 43 at right angles as shown in FIG. 30. If, as shown in FIG. 3b, a particle bundle 45 does not pass the field boundaries 46 perpendicularly but at an angle or if, as shown in FIG. 30, a particle bundle 47 passes field boundaries that are arched as at 48 in FIG. 30, then the zero order properties will have been preserved but the first order properties and the aberrations of second and higher orders are modified.

Another known method which is entirely different relies on the employmentof so-called multipole elements. A conventional deflecting field lens has two poles and its deflecting field may therefore be regarded as being a dipolefield. A tetrapole lens has four poles and hence a tetrapole field, a hexapole lens has six poles 49 to 54 having windings 66 to 71 and hence a hexapole field (FIG. 4), an octapole has eight poles and so forth. By suitably associating or serially connecting a large number of such elements a fully corrected image field can finally be achieved. One property of multipole elements is of special importance, viz:

A real dipole has zero and first order properties and produces image aberrations of second and higher orders,

A real tetrapole has properties of the first order and aberrations of the second and higher orders, but lacks zero order properties.

A real hexapole (FIG. 4) has aberrations of the second, third and higher orders but no properties of zero and first order. I

A real octapole has aberrations of the third and higher orders, but no zero and first order properties nor second and higher order aberrations.

Consequently, provided a system is skilfully designed, it is possible by adjusting the intensity of the dipole field to adjust the deflection to the desired value.

By adjusting the intensity of the tetrapole field the first order focus of the entire system can then be adjusted as desired without changing the deflection effect.

By adjusting the intensity of the hexapole field second order aberrations can be reduced without changing deflection or focus, i.e. without changing the zero and first order properties.

By adjusting the intensity of the octapole field the third order aberrations of the overall system can be reduced without changing either the zero and first order properties or the second order aberrations, i.e. whilst preserving the focus, and without in any way adversely affecting the previously corrected second order aberration.

M ultipole elements hitherto used in the form of separate auxiliary elements and located in the path of the rays of a particle spectrometer or of a deflecting system, are generally composed of separate tetrapole, hexapole, octapole etc. elements.

However, it has already been proposed to construct them in form of one element having many poles, the windings being so placed around the poles that each pole firstly has its own windings, that every two poles then have a common winding, that each three neighbouring poles again have a common winding and so forth. This arrangement provides an element, in which a group of windings affects the strength of the tetrapole, another group of windings the strength of the hexapole and yet another group of windings that of the octapole, and so forth.

All multipole elements hitherto used are nevertheless very complex and not easy to produce. In fact they call for very difficult mechanical forms of construction and make very high demands upon precision besides involving a complicated winding technique. Despite the efforts that have already been made it is a disadvantage of all known multipole systems that they still give rise to a certain degree of ripple in the field distribution.

SUMMARY OF THE INVENTION It is the object ofthe present invention to eliminate these drawbacks of known types of multipole systems.

To attain this object the present invention provides a coil arrangement which exhibits two features in combination, namely a. that the coils are flat coils produced according to conventional printed circuit technology;

b. that part of the turns of each coil is appropriately shaped to produce a locally changing magnetic field normal to the stream of charged particles (particle bundles) for differentially affecting the particle paths according to their position in this magnetic field.

These flat coils, produced for instance by etching according to printed circuit techniques, have the advantage of being producible with high precision at the expense of a reasonable amount of effort and cost. Moreover, owing to their flat shape they can be conveniently cooled and for the same reason they are very economical in their spatial requirements, so. that they can be built into existing instruments. Owing to the great precision with which the coils can be produced, ripple in the field distribution can be completely avoided provided the current-conducting surfaces (cf. FIGS. 7a to 15a) are sufficiently narrow and the creation of an ideal field distribution is not expected to be provided directly at the surface of the correcting coils. The coil arrangement as proposed by the invention is therefore very advantageously applicable to all such cases in which it is important by adjustment or correction to produce particular field intensity distributions for eliminating existing aberrations in systems, for instance in mass spectrometers, electron microscopes, cathode ray tubes, particle guidance systems in accelerator technology and so forth.

If the two above-mentioned features are both observed as proposed by the invention the detail design and configuration of the coils is, as such, entirely arbitrary. However, it is a major advantage so to design and shape the coils that their windings substantially conform with the contour lines of a rectangle. Moreover, it is also preferred that the windings of that part of each coil which generates a locally modified magnetic field normal to the particle bundle are widened to form current-conducting laminae in that region in which they extend substantially parallel to the axis of the particle bundle.

According to yet another feature of the invention the coils of desired different orders may be contained in a vertical or horizontal stack within the hollow interior of a yoke of ferromagnetic material, said yoke having preferably the form of for instance an oblong box or of a hollow cylinder. Accordingly, the coils may with advantage be fiat or arched to conform with the interior of the yoke.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Other features and advantages of the invention will emerge as the following description of embodiments of the invention proceeds with reference to the drawings.

FIG. 5 is a perspective view of the overall construction of a coil assembly inside the cavity 24 of an oblong box-shaped yoke 23 of ferromagnetic material. In this cavity 24, in this instance more particularly at the top and at the bottom of the hollow interior,

flat coils

1, 2, 3 and 4 are located, the paths of the particles passing through this hollow interior in the direction of the symmetry axis z; the reference characters x and y denote the relatively perpendicular coordinates of the system.

The structural configuration of the differently acting coils which are stacked in the yoke 23 so that the direction of the current in the current-conducting surfaces is parallel to the direction of the particle paths will be understood by reference to FIGS. 6a to 15a and 17 to 19 of the drawings.

Coil l which is shaped like a normal coil (FIG. 6a) produces a homogeneous field 6 (FIG. 6b) parallel to the y-axis in FIG. 5 and deflects all the particles by a like adjusted amount, analogous to the effect of a diole.

p Coil 2 may be shaped substantially as shown in FIG. 7a. In the y plane it generates a field 7 which linearly increases in strength, and which has first order properties analogous to a tetrapole lens. This is achieved because the windings of that part of the coil 2 which produce a locally modified magnetic field normal to the bundle of particles is widened to form current-conducting laminae 25 where the turns are substantially parallel to the axis of the particle bundle. The coil 2 comprises a plurality of coplanar interposed codirectional turns. Owing to the spaces between the several current-conducting ribbons the field distribution is not entirely uniform (cf. FIG. 7b). A different method of designing such a coil is that shown in FIG. 80. Although this coil which comprises only one turn and a single current-conducting lamina 34 has the advantage of producing an absolutely uniform field distribution (cf. FIG. 8b), it has the drawback that its internal resistance is too low. Another alternative form of construction is a coil shaped as illustrated in FIG. 9a. In order to achieve a uniform field distribution across the entire surface in this embodiment it is desirable to provide on two opposite sides of the current-conducting lamina, where the current connections are situated, either (cf. FIG. 18) openings 28 of appropriate shape and disposition near the

edges

26 and 27 of a currentconducting lamina 18 or (FIG. 19)

current conductors

29 and 30 which have a lower ohmic resistance than their current-conducting lamina 19 itself. Alternatively a current carrying lamina 17 may be provided with slits 33 parallel to the axis of the particle bundle, either in the middle of the current-conducting lamina (FIG. 17) or completely dividing the current-conducting lamina into separate ribbons 31 (FIG. 9a) to generate a field distribution as shown in FIG. 9b.

However, if it is desired to provide a linear field distribution according to FIGS. 10b or 11b, then it is advisable to use a coil consisting of two mirrorsymmetrical components as indicated in FIGS. 1011 or 11a connected in parallel in such a way that the current flows through them in opposite hands.

Coil 3 is constructed substantially as indicated in FIG. 120 or FIG. 13a, but in this case a linearly rising field distribution, as in the case of coil 2, is not the aim, but rather a field distribution 12 and 13 which in the centre plane, i.e. in the plane y 0, changes as the square ofx (cf. FIGS. 12b, 13b). Whereas in the case of coils according to FIG. 100 the current-conducting laminae and the intervening spaces are equal, the width of the laminae and the distances between them in coils accordingvto FIG. 12a, and hence the current density, decreases linearly. For this purpose it is preferred to make use of a coil comprising two mirror-symmetrically identical component coils as in FIG. 12a or 13a which are connected in series in such a way that the current circulates through both halves in the same direction. The field distribution curves of FIGS. 12b and 13b as well as those in the following FIGS. 14b and 15b which produce fields l4 and 15 are drawn in smooth curves, i.e. without ripples.

Coil 4 is one having a configuration as illustrated in FIGS. 14a or 15a. The current-conducting field is so designed that a field distribution varying as x is produced in the centre plane (y 0) (cf. FIGS. 14b and 15b). This is attained because the width of the currentconducting laminae and 56 respectively 57 and 58 and the spaces between them increase as x.

Hence coil 1 functions like a dipole,

coil 2 functions like a tetrapole,

coil 3 functions like a hexapole, and

coil 4 functions like an octapole.

An element constructed as indicated in FIG. 5 therefore quite obviously provides the above discussed possibilities of correcting aberrations.

' With the aid of the coils illustrated in FIGS. 7a to 15a the above-described multiple pole which had been assumed to be separate from the sector field magnet provided in at least many cases, can be further developed. If the current-conducting ribbons or laminae are suitably arched for the coils to produce zero field strength along a curve of particular radius instead of along a straight line then these coils can be accommodated within the poleshoes of the sector field. This radius should exactly correspond to the radius of curvature along which the particle bundle travels within the sector field. The strength of the dipole and hence the deflecting effect of the existing sector field can be slightly modified. Major changes are difficult to achieve because of the relatively low current-carrying capacity of printed circuits. Low currents will also not change the tetrapole strength very much. However, the hexapole and the octapole strengths can be considerably varied already with the aid of relatively low currents. In such a case each coil 59 should be provided with circular gaps 60, 6l'and 62 (cf. FIG. 20). Only with the aid of these gaps can the supplementary field strength be 7 made to rise in proportion to the 1st, 2nd, 3rd or a higher power of the radius.

Another embodiment of the invention which is particularly suited for cases in which the cross section of the particle bundles is substantially circular, as is the case for instance inan electron microscope, is illustratively shown in FIG. 21. In principle this arrangement corresponds to FIG. 5. However, the yoke is substantially circular in cross section and the several coils 32 that are provided likewise conform to this circular cross section. By connecting several such multiple elements in series, possibly relatively slightly twisted or by using helically etched coils this arrangement can be made to simulate the effect of Scherzer correcting elements.

The field distribution 7 to in FIGS. 7b to 15b is strictly correct only in proximity with the coils and it changes as indicated in FIG. 16 towards the middle. However, if necessary, these deviations can be easily eliminated by slightly changing the current density in the coil. The necessary current density distribution will then not be exactly proportional to x"". Minor terms containing x"', x"' etc. down to x will also appear. The value of these terms is obtained by prescribing the field strength in the middle of the correcting element or magnet, calculating the consequent field strength where the correcting coils are located and then producing this field strength by suitable current density distributions in the coils. In detail the current density'is calculated to be proportional to the field strength component parallel to the correcting coil.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments are therefore to be considered in all respects as illustrative and not restrictive.

.What is claimed is:

l. A coil arrangement for adjusting the focus and cor-- recting the aberration of streamsof charged particles by electromagnet deflection, particularly for sector field lenses in mass spectrometers, comprising the two following features in combination:

a. a plurality of flat coils produced by conventional printed circuit techniques, and

b. a part of the turns of each coil is appropriately shaped, with the turns of each coil being different from the turns of the other coils, to produce a locally modified magnetic field normal to the stream of charged particles for differentially affecting the paths of the particles according to their position in the magnetic field,

whereby the magnetic field may be varied both in strength and distribution by adjusting the currents in the different coils individually.

2. A coil arrangement as set forth in claim I, wherein the coils are so designed and shaped that their windings substantially conform with the contour lines of a rectangle.

3. A coil arrangement as set forth in claim 1, wherein the windings of that part of each coil which generates a locally modified magnetic field normal to the particle bundle are widened to form current-conducting laminae in that region in which they extend substantially parallel to the axis of the particle bundle.

4. A coil arrangement as setforth in claim 3, wherein each coil comprises a plurality of codirectionally wound interposed turns comprising current-conducting laminae generating a field of linearly with rising strength normal to the particle bundle and having the properties of a tetrapole lens.

5. A coil arrangement as set forth in claim 3, wherein each coil has only one turn comprising a currentconducting lamina generating a field of linearly with rising strength normal to the particle bundle and having the properties of a tetrapole lens.

6. A coil arrangement as set forth in claim 5, wherein the current-conducting lamina is a rectangle provided on opposite sides, where the current connections are situated, with openings near the edges so arranged and contrived that the current distribution in the lamina is approximately uniform over the entire lamina surface.

7. A coil arrangement as set forth in claim 5, wherein the current-conducting lamina is a rectangle provided on opposite sides, where the current connections are situated, with current conductors that have a lower ohmic resistance than the current-conducting lamina itself.

8. A coil arrangement as set forth in claim 5, wherein the current-conducting lamina is divided into ribbons by slits extending parallel to the axis of the particle bundle, the width of said ribbons being so chosen that the current distribution in the several ribbons is as uniform as possible across the entire conducting surface.

9. A coil arrangement as set forth in claim 8, wherein the slits between the current-conducting ribbons are provided only in the middle of the current-conducting lamina.

10. A coil arrangement as set forth in claim 3, wherein each coil comprises two like mirrorsymmetrical coils connected in parallel in such a way that the current flows through them in opposite hands and produces a field which in the centre plane varies in proportion to x to provide the properties of an octapole lens. 7

11. A coil arrangement as set forth in claim 3, wherein each coil comprises two like mirrorsymmetrical coils so connecting in series that the current flows through them in the same hand of rotation to generate a field which in the centre plane has a field distribution varying with x and thus providing the properties of a hexapole lens.

12. A coil arrangement as set forth in claim 3, wherein the coils having parts widened to form currentconducting laminae are associated with anotherflat coil produced according to the conventional techniques of printed circuit production, said additional coil being likewise shaped to conform with the contours of a rectangle but being evenly wound and lacking current-conducting laminae, as such to produce a homogeneous field parallel to the y-axis which, when affecting one particle bundle, causes the same deflection for all particle paths.

13.A coil arrangement as set forth in claim 1, wherein coils of different orders-are stacked in the hollow interior of a yoke made of a ferromagnetic material and so arranged that a desired field distribution for effecting a correction is achieved across the direction of the charged particles.

14. A coil arrangement as set forth in claim 13, wherein the yoke has the form of an oblong box.

15. A coil arrangement as set forth in claim 13, wherein the yoke has the form of a hollow cylinder.

16. A coil arrangement as set forth in claim 15, wherein the coils of the same order are so located inside the yoke that they face each other in pairs.

17. A coil arrangement as set forth in claim 16, wherein the surface of the coils matches the internal shape of the yoke.

Claims

1. A coil arrangement for adjusting the focus and correcting the aberration of streams of charged particles by electromagnet deflection, particularly for sector field lenses in mass spectrometers, comprising the two following features in combination: a. a plurality of flat coils produced by conventional printed circuit techniques, and b. a part of the turns of each coil is appropriately shaped, with the turns of each coil being different from the turns of the other coils, to produce a locally modified magnetic field normal to the stream of charged partiCles for differentially affecting the paths of the particles according to their position in the magnetic field, whereby the magnetic field may be varied both in strength and distribution by adjusting the currents in the different coils individually.

2. A coil arrangement as set forth in claim 1, wherein the coils are so designed and shaped that their windings substantially conform with the contour lines of a rectangle.

4. A coil arrangement as set forth in claim 3, wherein each coil comprises a plurality of codirectionally wound interposed turns comprising current-conducting laminae generating a field of linearly with rising strength normal to the particle bundle and having the properties of a tetrapole lens.

5. A coil arrangement as set forth in claim 3, wherein each coil has only one turn comprising a current-conducting lamina generating a field of linearly with rising strength normal to the particle bundle and having the properties of a tetrapole lens.

10. A coil arrangement as set forth in claim 3, wherein each coil comprises two like mirror-symmetrical coils connected in parallel in such a way that the current flows through them in opposite hands and produces a field which in the centre plane varies in proportion to x3 to provide the properties of an octapole lens.

11. A coil arrangement as set forth in claim 3, wherein each coil comprises two like mirror-symmetrical coils so connecting in series that the current flows through them in the same hand of rotation to generate a field which in the centre plane has a field distribution varying with x2 and thus providing the properties of a hexapole lens.

12. A coil arrangement as set forth in claim 3, wherein the coils having parts widened to form current-conducting laminae are associated with another flat coil produced according to the conventional techniques of printed circuit production, said additional coil being likewise shaped to conform with the contours of a rectangle but being evenly wound and lacking current-conducting laminae, as such to produce a homogeneous field parallel to the y-axis which, when affecting one particle bundle, causes the same deflection for all particle paths.

13. A coil arrangement as set forth in claim 1, wherein coils of different orders are stacked in the hollow interior of a yoke made of a ferromagnetic material and so arranged that a desired field distribution for effecting a correction is achieved across the direction of the charged particles.