US3916201A

US3916201A - Electron microscope having a plurality of coaxial cryogenically cooled lenses

Info

Publication number: US3916201A
Application number: US440332A
Authority: US
Inventors: Karl-Heinz Herrmann; Reinhard Weyl; Helmut Zerbst; Isolde Dietrich
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1973-02-16
Filing date: 1974-02-07
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: FR2218651A1; NL179247C; DE2307822C3; DE2307822A1; FR2218651B1; GB1459281A; NL7401645A; JPS49115264A; NL179247B; DE2307822B2

Abstract

The present invention relates to a lens system for particle radiation devices, particularly electron microscopes, which includes means for adjusting the particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in which the lenses are received and a central supporting tube in the cryostat on which the lenses are mounted. An electron microscope produced according to the present invention is substantially free from thermal drift phenomena, is substantially insensitive to mechanical vibrations, can be readily protected against the influence of external magnetic fields, and can be assembled and disassembled conveniently for cleaning or exchanging parts.

Description

' United States Patent Herrmann et al.

ELECTRON MICROSCOPE HAVING A PLURALITY OF COAXIAL CRYOGENICALLY COOLED LENSES Inventors: Karl-Heinz Herrmann, Berlin;

Reinhard Weyl, Assling; Helmut Zerbst; Isolde Dietrich, both of Munich, all of Germany Siemens Aktiengesellschaft, Berlin & Munich, Germany Filed: Feb. 7, 1974 Appl. No.: 440,332

Assignee:

Foreign Application Priority Data Feb. 16, 1973 Germany 2307822 US. Cl. 250/396; 250/311 IntJCl. G21K 1/08 Field of Search 250/310, 311, 396, 397, 250/398, 399

References Cited UNITED STATES PATENTS ll/196l Buchhold 250/396 1 Oct. 28, 1975 Dietrich et a1 250/396 Dietrich et a] 250/396 Primary Examiner-Davis L. Willis Attorney, Agent, or Firm-Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT The present invention relates to a lens system for particle radiation devices, particularly electron micro? scopes, which includes means for adjusting the particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in whichthe lenses are received and a central supporting tube inthe cryostat on which the lenses are mounted. An electron microscope produced according to the present invention is substantially free from thermal drift phenomena, is substantially insensitive to mechanical vibrations, can be readily protected against the influence of external magnetic fields, and can be assembled'and disassembled conveniently for cleaning or exchanging parts.

24 Claims, 6 Drawing Figures 1 Sheet 1 of 4 3,916,201 I US. Patent Oct. 28, 1975 US. Patent 0ct.28,1975 sheetzom 3,916,201

Fig.2

US. Patent Oct. 28, 1975 Fig.3

Sheet 3 of 4 U.S. Patfi'lt Oct. 28, 1975 Sheet4 o f4 3,916,201

Fig.6 2009i! ill ZUUL ELECTRON MICROSCOPE HAVING, A PLURALITY OF COAXIAL CRYOGENICALLY COOLEI) LENSES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of lens systems for particle radiation applications, particularly electron microscopes involving the use of cryogenically cooled magnetic lenses which are received in a cryostat and are supported on a central support tube located in the cryostat.

2. Description of the Prior Art The prior art has described columns for an electron microscope with superconducting lenses, including a plurality of condenser lenses, intermediate lenses, an objective lens and a projective lens. Each individual superconducting lens is arranged in a helium cryostat which can be displaced horizontally on a platform in order to enable alignment of individual lenses in relation to the column axis and the objective lens. This prior art arrangement is not only relatively elaborate but is also sensitive to thermal drift phenomena. It is also difficult to dismantle. Dismantling of this type of column is necessary if the internal components such as the pole pieces and the diaphragms are to be cleaned or replaced.

SUMMARY OF THE INVENTION The present invention provides a column for an electron microscope or the like using superconducting lenses, the structure being such that it is substantially free of thermal drift phenomena, is largely insensitive to mechanical vibrations, can readily be protected against the influence of externalmagnetic fields such as alternating current fields, and whose individual components which are located in proximity to the electron beam can be readily cleaned and/or exchanged in relatively simple fashion. The present invention also provides a structure which is substantially more compact than prior art structures.

The lens system of the present invention utilizes a plurality of superconducting magnetic lenses with means for magnetic adjustment of the particle beam. The lenses of the new systemare arranged in a cryostat and are assembled on a central supporting tube located in the cryostat. In the simplest device, the system consists of the objective' lens and an intermediate lens which is used for secondary magnification. In the preferred form of the invention, two or more lens systems designed in accordance with the present invention are provided which form component parts of an overalllens system, namely, the overall electron microscope. '55

Inside the support tube, there are diaphragms and/or pairs of pole pieces and/or adjusting systems for adjusting the axis of the particle beam.

It is a relatively simple matter to clean a lens system in accordance with the present invention or to exchange parts inside the system, particularly in that form of the invention in which there is a sleeve of electrically conductive material provided inside the'support tube, the sleeve being designed to be removable from the support tube but fitting relatively closely inside the q tube. Components which are arranged inside the support tube are located inside the sleeve for ready removability.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the inven tion will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although'variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

FIG. 1 is a view partly in elevation and partly in crosssection of an electron microscope assembly according present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I v FIG. 1 illustrates a column 1 of an electron microscope employing superconducting lenses. A cryostat housing is provided in which the lens system components, to be described subsequently, are located and into which liquid helium is introduced in order to cool the components.

Housing sections

2, 4 and 6 aresuperimposed together to form a closed cryostat housing with an opening 8 for the introducion of'liquid helium and with an opening 9 for gaseous helium and for the passage of electrical leads into the interior of the cryostat housing.

Reference numerals

3 and 5 have beenapplied to flanges through which the

sections

2, 4 and 6 for the passage of liquid helium through the complete cryostat housing.

Reference numeral 104 has been applied to a plate which contains a bore sealed in vacuum'tight relationship for the introduction 'of the specimen. The bore 105, as seen in FIG. I, is horizontally disposed. The

meral 106, servingfor the passage of the helium through the interior of the cryostat housing.

Condenser lenses have been identified at

numerals

11, 12 and 13, and the intermediate lenses at numerals l5, l6 and 17. The objective lens because of the presf ence of the plate 104 is split into two

parts

14 and 114 located respectively above and below the plate 104 and together forming a lens. In the embodiment of FIG. 1,

the housing, there is a support tube 24 to which the

lens portions

14 and 1 l4 and the parts of the first intermediate lens 15 are attached. Inside section 6 of the housing I there is a support tube 26 upon which the

lenses

16, 17

and 18 are arranged, lens 18 being a projective lens.

In accordance with the preferred embodiment of the present invention, the

support tubes

22, 24 and 26 have

individual sleeves

23, 25 and 27 fitted into them. These sleeves are provided without any substantial clearance between the outer diameter of the sleeve and the inner diameter of the tube, but the sleeves are nevertheless conveniently removable from the support tubes.

Inside the sleeve 23, for each of the

lenses

11, 12 and 13, a pair of

pole pieces

31, 32, and 33, respectively, are provided, the pole pieces being made of ferromagnetic material. Preferably, these pairs of pole pieces are arranged to be removable from the interior of the sleeves so that they can be individually cleaned. In addition to the pairs of pole pieces, there are a number of

diaphragms

41, 141, 42 and 43 inside the sleeve 23 which are used to stop out the electron beam. These diaphragms, in addition, are preferably made to be removable. The attachment and centering of the diaphragms and pole pieces can be effected by means of adjuster mechanisms which are operated following removal. Pole pieces can also be arranged inside the tube, in the manner shown with respect to the lens 15.

Inside the sleeve 27 in the support tube 26, there are pairs of

pole pieces

36 and 37 as well as

diaphragms

46 and 47. Inside the sleeve 25 which is inside the support tube 24 of the housing, there is a further diaphragm 45 and

screening elements

46 and 47 composed of superconductive material. These screening elements are used to shape the magnetic focusing field of the screening lens operating as the objective lens.

Because the

sleeves

23 and 27 can be quite readily removed, the sleeve 25 which is located inside the support tube 24 of the section 4 can also be readily extracted from the overall lens system. In the focusing zone of the

objective lens

14, 114, that is, in the neighborhood of the

screening elements

46 and 47, it is important to insure that the walls are free of any contaminating foreign bodies which could develop electrical charges and thus give rise to an unwanted superimposition of an electric field.

The structure of the lens systems using support tubes in accordance with the present invention, and particularly in the matter of removable sleeves, also facilitates the exchange of individual parts which for electrooptical reasons are subjected to modification or wear, for example, the consequence of electron impact.

Non-mechanical adjusting systems can be provided in the column to correct errors which may arise. In FIG. 1,

reference numerals

51, 52, 53, 54, 55, 56 and 57 are applied to adjusting systems of this kind arranged between the individual lenses and constituting deflection systems by means of which the electron beam can be deflected laterally in any direction, and/or stigmators by means of which deviations on the part of the lens fields from rotational symmetry can be corrected.

The adjusting

systems

51, 52, 53 and 54 are located inside the support tube 22 and inside the slide-in sleeve 23. Each individual adjusting system can consist, for example, of four individual coils arranged in one plane at right angles. Using the four coils, three of which are shown in the figure, a magnetic deflecting field can be generated by means of which an electron beam which is deviating from the axis can be deflected back to the system axis.

The deflection systems referred to previously can, as mentioned, be arranged inside the support tube or the sleeve or outside the support tube. The latter arrangement has been illustrated with respect to the adjusting system 58 in the direct vicinity of the objective lens. Adjusting systems can readily be positioned outside the support tube if the tube has a relatively small diameter. A small internal diameter is convenient particularly for the support tube 24 of the lens system, consisting of the

objective lens

14, 114, and the first secondary magnification lens 15, because the geometric dimensions of the objective lens in the neighborhood of the specimen are involved in the imaging properties. Because of the small internal diameter of the tube 24, the pairs of pole pieces of the intermediate lens 15 are also outside the tube.

Using the structure of the present invention, a column with a large number of condenser lenses and a large number of additional secondary magnification lenses can be produced. Because of the low installed height and the large refractive power of superconducting lenses, the installed length of the column in accordance with the invention can be kept very short, that is, a very precise, stable and vibration resistant structure is provided.

Using the principals of the present invention, it is possible to build a column for a beam voltage of3 megavolts, which has at least two condenser lenses and three intermediate lenses, giving a magnification of about 1,000,000 times, the length of the column being less than 1 meter. Comparable known systems with non-superconducting lenses, on the other hand, have lengths up to 5 meters.

To shield the individual lens systems and therefore the overall lens system against external magnetic fields, in particular, alternating current fields, it is advisable to provide superconducting shielding cylinders inside the cryostat housing. A shielding cylinder 202 of this kind has been shown in FIG. 1 located in housing section 2. Corresponding shielding cylinders can also be arranged in the other housing sections. They are located in the liquid helium and are therefore superconducting.

In connection with

lenses

11 and 12, there has been shown an arrangement for introducing the electric current flowing through the magnetic winding of the lens. The other lenses can be electrically connected in a similar fashion. A single current supply unit can be employed to energize different lenses or correcting systems, successively.

Leads

212 and 218 can be taken out of the interior of the cryostat through an opening such as the opening 9. Also inside the cryostat is a superconducting short-

circuiting line

214 or 219 which produces short circuits, respectively, between the

lines

212 and 213 or 213 and 218, respectively. However, this short circuit is absent as long as the flow of current through the winding 215 surrounding the line 214 or the winding 220 surrounding the line 219 produces sufficient heat for the

line

214 or 219 to retain a normally conducting condition. A controllable generator 216 is provided for generating current and by operation of

switches

221 and 222, the current flow is either through the winding 215 or through the winding 220. While the short circuit 214 is inoperative as a consequence of the heat developed in the winding 215 with normal conduction occurring, the generator 217 feeds current into the winding of the lens 11 independently of the load. The coil of lens 12 is not energized because the current flow is through the short circuit line 219. As soon as the current in the lens 11 has built up, the current in the winding 215 is interrupted and the short circuit 214 becomes operative so that a permanent superconduction current flows in the winding of:the lens 11. Subsequently, in the same fashion, the current can be introduced into thecoil 12; I r

A support tube used in accordance with the present invention can be made up in sections of different material. The material of the support tube may be superconducting, ferromagnetic or neither superconducting nor ferromagnetic in a given section. If a sleeveis provided inside a support tube, the same applies to the sleeve. Which material is used at which point in the support tube and in the sleeve depends upon whether, at the particular location, ferromagnetic directing of magnetic lines of force or screening of a magnetic field using the phenomenon of superconduction is required, is acceptable or has to be avoided. For example, in the case of pole pieces which are arranged inside the support tube or the sleeve, it is necessary that between the pole pieces and the magnetic surround of the lens, for example, between 31 and 211, the magnetic lines of force should close. Consequently, the portions of the support tube 22 and the sleeve 23 in proximity to the ferromagnetic surround 211 on the lens 11 are ferromagnetic in nature so that the lines of force passing through 211 can cross to the pole piece 31 without substantial difficulty. On the other hand, the support tube and sleeve at the location of the gap between the pole pieces of the pair 31 should be non-ferromagnetic because otherwise there would be more or less a short circuit in respect to the particular magnetic field designed to develop between the pole pieces of the pair 31 in the neighborhood of the axis of the lens system. In the neighborhood of the

objective lens

14, 114, the support tube 24 and the sleeve 25 must be superconducting in the region of the

screening elements

46, 47. This superconducting condition is necessary so that no gap is created of the kind through which the lines of force could penetrate between the superconducting screen 2140 of the lens 14 and the screening element 46. On the other hand, in the lens gap between the section 14 and the section 1 14, the magnetic field must enter the center of the lens in order to exert a focusing action there. The support tube and sleeve cannot therefore be superconducting or ferromagnetic in this zone. In the

superconducting shielding lens

14, 114, the shielding

elements

46 and 47 act as far as focusing is concerned in the manner comparable with the way the pairs of pole pieces act. In the neighborhood of the pole pieces of the lens 15, the support tube and sleeve must be ferromagnetic. However, between the pole pieces of the lens 15, the support tube and sleeve must be free of ferromagnetic or superconducting properties. The support tube and sleeve can, however, be superconducting in some cases in the neighborhood of the pole pieces of the lens 15.

In the embodiment shown in FIG. 1, as mentioned, adjusting systems are provided inside and outside the sleeve and support tube. Insofar as these adjusting systerns, which consist of field coils, are located internally, the sleeves and/or support tubes should be ferromagnetic in order to produce an external annular pattern of closing of the lines of force for the individual coils, for example, the adjusting system 51. Whenthe adjustof the axis of the column. It is convenient in the case of external adjusting systems such as system 58 to provide for magnetic return externally around the adjusting system, although this has not been shown in FIG. 1. If required, it is also possible to use a ferromagnetic casing 2ll.of the lens as a return means.

The support tube and the sleeve are provided with bores as necessary or expedient. For example, in the support tube 24 and the sleeve 25 there are bores arranged in continuation of the bore 105 in the plate 104. Through the bore 105 and the associated bores in the support tube and sleeve, the specimen is introduced into the object plane of the

objective lens

14, 114.

FIG. 2 illustrates an embodiment of the invention in which a support tube 522 passes centrally through the complete column. Inside the support tube 522 there is a through sleeve 523. Inside the sleeve 523, the lens structures of the type shown in FIG. 1 have been employed, and correspondingly numbered. In the same way as plate 104, a plate 1 104 is provided in which

horizontal bores

1105 and 1106 are formed. Through these bores slides 1110 and 1111 can be passed. These slides can be used as the object mounts and/or as diaphragm mounts. The objective lens consists of a lens split into two

parts

504 and 514 and is provided with a superconducting winding. The two lens sections are surrounded by a housing 516 of ferromagnetic material. Pole piece 534 is provided for the objective lens.

The first intermediate lens 515 produces secondary I magnification and has pole pieces 535 which are located inside the support tube 'and inside the sleeve 23. Support tube 522 and sleeve 523 at the locations where housing 516 and

pole pieces

534 and 535 touch each other or approach each other are made of ferromagnetic material. In the neighborhood of the gap between the individual pole pieces of the

pairs

534 and 535, the materials of the support tube and sleeve are not ferromagnetic so that no magnetic short circuit can occur at this location.

FIG. 3 illustrates a further modified form of the invention including a housing section 1304 which corresponds essentially to section 4 of the embodiment shown in FIG. 1, with a support tube 1224 with a stepped down construction directly below the objective lens. The objective lens consists of the

sections

604 and 614 and is of the type previously described in connection with FIGS. 1 and 2. In the embodiment of FIG. 3, a superconducting shielding lens is-provided with a superconducting shielding housing 1314 and with superconducting shielding elements1246 and 1247. These cylindrical shielding elements surround the inner portion of the support tube 1224. The support tube 1224 has a diameter as small as possible so that the

shielding elements

1246 and 1247 are in as close proximity as possible to the specimen or object, with their mutually opposite edges forming the magnetic field, the specimen being located at' the tip of the slide 1210 which is introduced through a bore 1206 in the plate 1204. An opening 1206 is provided for the introduction of liquid helium.

Inside that portion of the support tube 1224 which has the larger cross-section, there is a sleeve 1225 of ing system is arranged outside the support tube as, for

the type previously described in connection with the other embodiments. Inside the sleeve 1225 there is a diaphragm 1245. In the construction shown in FIG. 3, the sleeve 1225 can only be removed from the support tube 1224 from below, but this does not constitute any appreciable restriction. The other details of the embodiment shown in FIG. 3 correspond with the structure shown in FIG. 1, and the same reference numerals have been applied.

FIG. 4 is a fragmentary view ofa further embodiment of the invention in which the shielding lens acts as an objective lens. In the embodiment of FIG. 4, the shielding lens is split into two

parts

704 and 714 between which a plate 1704 is located, the latter corresponding essentially to the

plates

1104 and 1204.

Superconducting shielding elements

716 and 717 are also provided, and the coils have been identified at

reference numerals

718 and 719.

A support tube 724 is provided about which the

casings

716 and 717 are arranged. Those parts of the support tube 724 indicated by

reference numerals

726 and 728 are made of superconducting material. In the

areas

704 and 714, the parts 730 and 731 which are arranged inside the support tube 724 act as shielding elements as explained in connection with the embodiment shown in FIGS. 1 and 3. The sections 727 and 729 of the support tube 724 consist of a material which is neither superconducting nor ferromagnetic but does have good thermal conductivity. The material of the sections 727 and 729 therefore has no influence upon the shaping of the magnetic field in that zone in which the

shielding elements

726 and 728 are located opposite each other. The sections 727 and 729 can, for example, be an integral part of the support tube 724 and the plate 1704. The parts of the support tube 724 can preferably be integrally attached by welding. A pair of

rings

732 and 733 serve to cool those sides of the shielding elements which are disposed toward the gap. They should be made of material having good thermal conductivity which is neither ferromagnetic nor superconducting. In operation, the interior of the support tube 724 is evacuated for the passage of the electron beam and consequently the welds provided in this area should be made vacuum tight.

The embodiment of FIG. 5 illustrates a structure with pole pieces both inside a support tube 822 and inside a sleeve 823. A housing 816 surrounds a lens winding 818. The heavily

cross-hatched sections

841 and 842 of the support tube 822 and a sleeve 823 consist of ferromagnetic material so that closure of the magnetic field between the

pole pieces

834 and 835 and the housing 816 is possible. The section 843 of the support tube and the corresponding section of the sleeve is neither ferromagnetic nor superconducting so that the magnetic field which is to be developed between the tips of the

pole pieces

834 and 835 is not short circuited.

FIG. 6 is a schematic illustration of an electron microscope with a lens system in accordance with the present invention. Only the cryostat housing 2000 of the lens system has been shown, this housing corresponding to the

sections

2, 4 and 6 of FIG. 1. A vacuum tight chamber 2002 contains the parts of the support tube or several individual support tubes, plus any extensions thereof in the upward or downward directions. In the top end of the vacuum chamber 2002, there is an electron source 2003. This, for example, may be an incandescent filament. A viewing chamber 2004 is provided at the bottom end of the vacuum tight chamber 2002. Inside the viewing chamber there is a viewing screen for inspecting the enlarged electron microscope image. The interior of the vacuum tight chamber 2002 is connected to a pipe 2006 to a vacuum pump 2007.

The annular anode for the assembly is identified at reference numeral 2008. A voltage source 2009 is used to maintain a voltage between the electron source 2003 and the anode 2008. The broken line 2009 indicates the path of the electron beam inside the vacuum tight chamber through the lens system in accordance with the present invention.

It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

We claim as our invention:

1. A lens system for a beam of particle radiation comprising means for adjusting the particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in which said lenses are received, and a central support tube in said cryostat on which said lenses are mounted.

2. The system of claim 1 in which said magnetic lenses include an objective lens and an intermediate lens having coinciding optical axes.

3. The lens system of claim 1 in which there is at least one diaphragm within said support tube.

4. The lens system according to claim 1, wherein said lens system includes at least one magnetic lens with ferromagnetic pole pieces arranged inside said support tube, said support tube in proximity to said pole pieces being composed at least partially of ferromagnetic material.

S. A lens system according to claim 1 in which shielding elements of superconducting materials associated with at least one magnetic lens are located inside said support tube, and the support tube in the vincinity of said shielding elements is composed at least partially of superconducting material.

6. A lens system according to claim 1 which includes at least one adjusting system within said support tube.

7. A lens system according to claim 1 including at least one magnetic lens with pole pieces located outside said support tube.

8. A lens system according to claim 1 including at least one magnetic lens with shielding elements located outside said support tube.

9. A lens system according to claim 1 including at least one adjusting system located outside said support tube, said support tube being neither ferromagnetic nor superconducting in proximity to said adjusting system.

10. A lens system according to claim 1 which includes a sleeve of electrically conductive material in said support tube, said sleeve being closely fit into said support tube but removable therefrom.

11. A lens system according to claim 10 in which a diaphragm is positioned within said sleeve.

12. A lens system according to claim 10 in which a pair of pole pieces is positioned within said sleeve.

13. A lens system according to claim 10 in which an adjusting system is positioned within said sleeve.

14. A lens system according to claim 1 including an objective lens, a plate in proximity to said objective lens, said plate containing at least one radial bore for the introduction of a specimen, and containing at least one axially extending bore for the circulation of cool-- ant in said cryostat.

15. A lens system according to claim 1 including several magnetic lenses connected in series with respect to their energizing means.

16. A lens arrangement which includes a plurality of lenssystems in superimposed relation, each of said lens systems comprising means for adjusting a particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in which said lenses are received, and a central support tube in said cryostat on which said lenses are mounted.

17. A lens arrangement according to claim 16 in which said lens arrangement includes a condenser lens system, a lens system consisting of an objective lens and a first intermediate lens, and a magnification lens system including at least one intermediate lens and a projective lens.

18. A lens arrangement according to claim 16 in which the individual lens systems are assembled so as to provide a single internal space for coolant.

19. A lens arrangement according to claim 16 in which all the lens systems are disposed in a single cryostat.

20. A lens arrangement according to claim 16 in which all the lens systems are arranged on a single support tube.

21. The lens arrangement according to claim 16 which includes ferromagnetic polepieces of at least one magnetic lens arranged inside of the support tube, the support tube in proximity to said pole pieces being composed at least partially of ferromagnetic material.

22. A lens arrangement according to claim 16 in which shielding elements of superconducting materials associated with at least one magnetic lens are located inside said support tube, and the support tube in the vicinity of said shielding elements is composed at least partially of superconducting material.

23. A lens arrangement according to claim 16 which includes at least one adjusting system within said support tube.

-24. A lens arrangement according to claim 16 which includes a sleeve of electrically conductive material in said support tube, said sleeve being closelyfit into said

Claims

5. A lens system according to claim 1 in which shielding elements of superconducting materials associated with at least one magnetic lens are located inside said support tube, and the support tube in the vincinity of said shielding elements is composed at least partially of superconducting material.

14. A lens system according to claim 1 including an objective lens, a plate in proximity to said objective lens, said plate containing at least one radial bore for the introduction of a specimen, and containing at least one axially extending bore for the circulation of coolant in said cryostat.

16. A lens arrangement which includes a plurality of lens systems in superimposed relation, each of said lens systems comprising means for adjusting a particle beam, a plurality of superconducting cryogenically cooled magnetic lenses, a cryostat in which said lenses are received, and a central support tube in said cryostat on which said lenses are mounted.

24. A lens arrangement according to claim 16 which includes a sleeve of electrically conductive material in said support tube, said sleeve being closely fit into said support tube but removable therefrom.