US3008044A

US3008044A - Application of superconductivity in guiding charged particles

Info

Publication number: US3008044A
Application number: US11032A
Authority: US
Inventors: Buchhold Theodor Adam
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1960-02-25
Filing date: 1960-02-25
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07

Description

Nov. 7,1961 T. A. BUCHHOLD 3,008,044

APPLICATION OF SUPERCONDUCTIVITY IN GUIDING CHARGED PARTICLES Filed Feb. 25. 1960 2 Sheets-Sheet 1 Fig. 6. In venzior' 7Zeo doP r9. flue/150%? by M 0 NOV. 7, 1961 uc o 3,008,044

APPLICATION OF SUPERCONDUCTIVITY IN GUIDING CHARGED PARTICLES Filed Feb. 25, 1960 2 Sheets-Sheet 2 Fig. 2

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Umted Smtes Patent Gfiice 3,008,044 Patented Nov. 7, 1961 3,008,044 APPLICATION OF SUPERCONDUCTIVITY IN GUIDING CHARGED PARTICLES Theodor Adam Buchhold, Schnectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Feb. 25, 1960, Ser. No. 11,032 11 Claims. (Cl. 250-495) The present invention relates to a magnetic lens for use in focusing electron beams and other charged particles.

More particularly, the invention relates to a new and improved magnetic lens employing superconducting material for focusing charged particles such as electron beams, and which is suitable for use in electro-optical instruments such as the electron-microscope, electrondiifraction instruments, and the like.

In magnetic electro-optical instruments such as those identified above a number of magnetic lens structures are arranged along an electron-beam path to focus the electron beam and to direct it against a specimen examined. Thereafter an objective lens structure selects electrons scattered or transmitted through the specimen, and directs them against a viewing device such as a fluorescent plate. To accomplish the above, it is necessary for the magnetic lens structures to be able to produce rather finely controlled magnetic fields for guiding the electron beam along desired paths. Because the guiding magnetic fields thus formed, determine to a great extent the ultimate resolution obtainable with instruments of this type, it is the purpose of the present invention to provide a new and improved magnetic lens structure wherein the magnetic field configuration produced by the lens can be controlled to a much greater extent than heretofore pos sible with existing lens structures.

It is therefore the primary object of the present invention to provide a new and improved magnetic lens which employs superconducting guides that can be shaped to provide any desired magnetic field configuration with relative ease, and which assures that the magnetic field configuration cannot be readily modified or altered in an undesired manner from extraneous eifects.

In practicing the invention, a magnetic lens is provided which comprises a magnetizable core structure having a central opening to accommodate an electron beam and superconducting shields for flux shaping to reduce lens errors. If desired the lens can be built to include superconducting guide members which are shaped to provide a desired specially shaped magnetic field configuration.

Other objects, features and many of the attendant advantages of the invention will be appreciated more readily as the same becomes better understood by reference to the following detailed description when considered connection with the accompanying drawings, wherein fike parts in each of the several figures are identified by the same reference character, and wherein:

FIGURE 1 is a sectional view of a magnetic electron microscope employing a novel superconducting magnetic lens structure as the objective lens assembly thereof;

FIGURE 2 is a perspective partially-broken away view of the new and improved superconducting magnetic objective lens structure employed in the electron microscope shown in FIGURE 1, and comprising a part of the present invention;

FIGURE 3 is a fragmentary view of the end of the pole pieces of a conventional magnetic objective lens assembly for an electron microscope;

FIGURE 4 is a fragmentary side view of the end of the pole pieces of the superconductive magnetic objective lens assembly illustrated in FIGURE 2 and used in the electron microscope of FIGURE 1;

FIGURE 5 is a crosssectional view of an alternative magnetic lens structure, employing specially shaped superconductive guides;

FIGURE 6 is a cross-sectional view of still another form of superconductive magnetic field forming lens assembly;

FIGURE 7 is a cross-sectional view of still a fourth form of magnetic lens structure employing superconductive field guiding members;

FIGURE 8 is a plan view of a superconductive disk comprising a part of the lens assembly shown in FIG- URE 7;

FIGURE 9 is a plan view of a second form of superconductive disk comprising a part of the magnetic lens assembly shown in FIGURE 7; and

FIGURE 10 is a schematic diagram of a suitable energization circuit for use with the new and improved mag netic lens structure.

The electron microscope shown in FIGURE 1 includes an outer housing 11 having an enlarged mid-portion 12, the purpose of which will be appreciated more fully hereinafter. The housing 11 is adapted to be evacuated through an opening 13, and supports an electron beam source 14 at the upper end thereof. The electron beam source 14 may be of any standard construction for use in electron optical instruments such as the source described on page 177 of the text book entitled Electron Optics and the Electron Microscope by Zworykin, Morton, Ramberg, Hillier, and Vance, published by John Wiley and Sons. The electron beam produced by the electron source 14 is directed down through a condenser lens assembly shown at 15 which is of conventional construction. It is the primary function of the condenser lens assembly 15 to regulate the intensity and convergence of the electron beam produced by the source 14, and for directing the beam upon the target specimen shown at 16 supported immediately above the objective lens assembly 17, to be described more fully hereinafter. The objective lens assembly is supported within the enlarged midportions of the housing 11 and serves to collect the electron images of the target specimen 16 transmitted through the target specimen 16 and to direct the image upon an intermediate image lens assembly 18. Because the ulti mate resolution of the electron microscope is mainly dependent upon the construction of the objective lens assembly, the present invention has been devised to greatly improve resolution of instruments of this type. The resolution of conventional objective lenses is limited by lens errors which cannot be reduced sufliciently by existing lens construction. Through the use of superconductive shields in the lens structure in the manner to be described hereinafter, the magnetic field produced by the lens can be properly shaped to greatly reduced the lens error and thereby improve the resolution of the lens structure. Additionally, if the magnetic field producing coil comprising a part of the lens is fabricated from superconductive wire, a particular value magnetizing current can be frozen into the coil in a manner to be explained hereinafter, and thereby produce a very stable magnetic field that can be maintained indefinitely. Since such a coil is superconducting and at the low temperatures involved thermal movements are reduced to practically zero, additional stability in the lens structure is obtained. Because of these accumulated advantages, it is deemed possible that electron microscopes can be built employing lens structures made possible by the invention which might be capable of viewing atoms.

The electron image produced by the objective lens assembly and directed to the intermediate image lens assembly 18 is thereafter imaged on the projector lens assembly 19. The projector lens assembly serves to focus the resultant electron image on a fluorescent plate 21 or other viewing device which can be viewed through a window 22. The construction and operation of the intermediate image assembly 18, the projector lens assembly 19 and fluorescent

viewing screen arrangement

21 and 22, are all conventional, and a further description of these elements can be obtained from the above identified Zworykin et al., text book.

The details of construction of the magnetic objective lens assembly 17 are shown in FIGURE 2 of the drawings. The magnetic lens assembly is formed by a generally tire-shaped toroidal core member of magnetizable material such as iron 23 which has a pair of spaced apart

leg members

24 and 25. The

legs

24 and 25 terminate in a pair of spaced apart pole-pieces which define an aperture for receiving or passing the electron beam. Supported within the toroidal core member 23 is an electric coil 26 which is preferably comprised of a large number of turns of superconducting wire such as niobium or other superconducting material of high critical magnetic field strength. The exact number of turns of superconducting wire used to form coil 26 depends upon the turns ratio desired to produce the particular magnetic field required by the instrument. The turns of the coil 26 are preferably insulated from each other as well as being insulated from the iron core member 23. If desired, the coil 26 may be formed from a number of turns of insulated copper wire or other wire which does not possess superconducting characteristics. However, at the temperature region in which the lens is designed to operate, the conductivity of the copper is greatly increased, and hence thermal losses are proportionally lessened. This factor alone provides a considerable advantage in that even the slightest variations in size or. spacing of the pole pieces due to thermal expansion can adversely affect the magnetic field configuration of the lens, and hence its performance. It is preferred that the coil 26 be fabricated from superconducting wire, in which event it is possible to freeze in a magnetizing current into the coil with an excitation circuit such as that shown at FIGURE 10. The excitation circuit of FIGURE 10 comprises the superconducting coil 26 which is excited from a battery 2 connected across the coil 26 through a knife switch 3 and variable resistor 4 for controlling the value of the magnetizing current supplied through coil 26. Also connected across the coil 26 is a gate element 5 of superconducting material having a low critical magnetic field strength surrounded by a field coil 6. The field coil 6 is connected to a battery 7 through a switch 3 and variable resistor 9. The gate 5 exhibits the normal superconductive tendency to lose its superconductive characteristics while disposed in a magnetic field so that while the field coil 6 is energized, the gate element 5 loses its superconductivity and will appear as an open circuit to current supplied to coil 26 from battery 2 upon switches 3 and 8 being closed. Thereafter if the switches are opened, gate 5 will appear as a superconductive short circuit across coil 26 to trap current flowing in the coil. The current thus trapped will produce a very stable magnetic field which can be maintained indefinitely because of the lossless character of the closed circuit which includes the coil 26.

Supported over the upper leg 25 of core member 23 is a superconductive disk 27 which is made from superconducting material of very high critical field strength such as niobium-tin, and which has a centrally formed aperture therein which is aligned with the opening defined by the spaced apart pole pieces of the core member 23. The specimen 16 to be examined is seated over the opening in the superconductive disk 27 and is held in position there by a suitable specimen holder formed by a pair of

concentric rings

28 and 29, which intermesh and retain the specimen 16 in place over the opening in the magnetic lens structure. The

specimen holder

28 and 29 is adapted to be slidably moved over the surface of the superconductive disk 27 by a manipulator (not shown in FIG- URE l) to position the specimen 16 at an optimum location, relative to the electron beam with which it is being examined.

The magnetic lens structure 17 shown in FIGURE 2 is completed by a set of

superconductive shields

31, 32 and 33 which are placed on opposite sides of the

leg members

24 and 25 of the core 23, particularly along the surfaces thereof which define the spaced apart pole pieces as best shown in FIGURE 4 of the drawings. The function of the

superconductive plates

31, 32 and 33, in conjunction with the superconductive disk 27 can best be appreciated from an examination of FIGURE 3 of the drawings. Referring to FIGURE 3, a pair of spaced apart

pole pieces

34 and 35 are shown which define an opening such as would be required in a magnetic lens structure similar to that disclosed in FIGURE 2. It is assumed that in fabricating the pole pieces shown in FIGURE 3, which are constructed of iron or other suitable magnetic core material, no superconductive plates have been used. As a consequence, it can be seen from an examination of the lines of flux indicated at 36 that such lines will emanate from the spaced apart pole pieces at points along the surface other than the very tips of the pole pieces. As a consequence of this phenomenon a rather erratic and poorly defined magnetic field of flux lines is produced. Because the ultimate resolution of a magnetic electron microscope or similar electron optic instrument employing magnetic lens structures is dependent upon the magnetic field configuration obtainable with spaced apart pole pieces such as 34 and 35, such field configuration as is exhibited in FIGURE 3 does not provide a lens construction suitable for optimum resolution. For this reason, the magnetic field confining

superconductive shields

27, 31, 32, and 33 have been added to the spaced apart

legs

24 and 25 of core member 23 in the new and improved magnetic lens provided by the invention. These superconductive plates due to their well-known characteristic of not permitting magnetic lens of flux to penetrate them when in a superconductive condition serve to confine the lines of magnetic flux to the open end of the spaced apart pole pieces formed by the

legs

24 and 25. Because of this confinement, a much more regular and better defined flux pattern can be developed by the spaced apart pole pieces. As a con sequence, it is believed that by proper use and shaping of superconductive shields such as those shown at 27, 31, 32 and 33, greatly improved resolution can be obtained with a magnetic lens structure employing such shields.

The manner in which the new and improved superconductive magnetic lens structure 1'7 shown in FIG URE 2 is supported within the electron microscope of FIGURE 1, and is maintained in a superconductive condition can best be understood by reference to FIGURE 1. The lens structure is supported within a toroidally shaped tank 41, which may be fabricated from a material such as copper, has both a bottom 42 and a top 43, through which a central opening is formed to allow for passage of the electron beam through these members. It is to be noted that while the tank 41 is toroidal in shape, the

bottom 42 and top 43 extend over most of the opening enclosed by the tank 41 so as to define a cold temperature space. In order to reduce the temperature of this space, the toroidal tank 41 is filled with a suitable liquid refrigerant such as liqm'd nitrogen through an insulated filling tube 40 that passes through the housing 11. The toroidal tank 41 is supported within the enlarged midportion 12 of the housing 11 by means of a plurality of tensioning stay wires 44 which are secured to the toroidal tank 41 at different points around its periphery and extend to tensioning posts 45 secured around the outer periphery of the enlarged mid-portion 12 of the housing 11. It is anticipated that a similar series of tensioning stay wires 44 would have to be arranged around the top portion of the toroidal tank 41 in order to rigidly secure it in position within the enlarged mid-portion 12 of the housing or container 11; however, these top tensioning stay wires 44 have not been illustrated because it was believed that inclusion of such wires would unduly complicate the drawings. The stay wires 44, while they do provide a rigid mechanical support for the toroidal tank 41, at the same time constitute a very high resistance to thermal leakage so as not to waste the liquid nitrogen or other liquid refrigerant supplied to the toroidal tank 41. It is of course possible to provide other means for supporting tank 41 within the housing 11; provided however, that such means must be a very poor conductor of heat.

Supported Within the outer toroidal tank 41 is a second toroidal shaped tank 46 which may be fabricated from copper and which contains a lower temperature liquid refrigerant such as liquid helium which is introduced into tank 46 through an insulated inlet tube 47 that passes through the outer housing wall 11. The inner toroidal tank 46 may be supported on the floor or bottom of the outer tank 41 by means of a plurality of insulating blocks shown at 48, or if desired, may be secured in place by suitable stay wires similar to the stay wires 44. The magnetic lens structure 17 is then disposed within the inner toroidal tank 46 in a manner such that its central opening is aligned with the electron beam path. By this arrangement, the magnetic lens structure 17 is maintained at a temperature in the neighborhood of 4.20 Kelvin (the temperature of liquid helium at atmospheric pressure) so that the superconducting shields 27, 31, 32 and 33, and coil 26 are maintained in a superconducting condition. Energization of the magnetic lens structure is achieved by suitable lead-in conductors not shown in FIGURE 1 connected to the coil 26 from an energization circuit such as shown at FIGURE 10. After the superconductive magnetic field producing coil 26 and the

superconductive shields

27, 31, 32 and 33 have attained superconductive temperature, a particular value magnetization current can be frozen into coil 26 in the manner described with relation to FIGURE 10, and the magnetic lens structure 17 will function in the manner illustrated in FIGURE 4 of the drawings to develop a very well-defined and properly shaped magnetic field flux for guiding the electron beam passing therethrough. Accordingly, it can be appreciated that the invention makes available a new and improved magnetic lens construction for developing closely controlled and well-defined magnetic field configurations for improving the resolution attainable with magnetic lens structures. Further, because of the extremely low temperatures at which the lens structure is designed to operate, thermal losses are reduced substantially to zero so that the guiding magnetic field produced by the lens can be maintained in an extremely stable manner over indefinite periods.

A second form of magnetic lens constructed in accordance with the invention is shown in FIGURE 5 of the drawings. The lens of FIGURE 5 includes a generally tire-shaped toroidal core 51 of magnetizable material such as iron having a pair of spaced apart leg portions 52 and 53 the ends of which form pole pieces defining a central opening to accommodate an electron beam or other charged particle path. An electromagnetic field producing coil 54 is supported within the toroidal core member 51, and preferably is fabricated from a number of turns of superconductive wire having the turns insulated from each other as well as being insulated from the core 51. It is of course possible to fabricate the coil 54 from copper or other non-superconducting material; however, in such event it would not be possible to freeze in the magnetizing current in the manner described with relation to FIGURE 10. The upper and lower faces of the legs 52 and 53 of the core member 51 have

concentric rings

55 and 56 respectively of superconducting material surrounding the opening defined by the ends of the leg portions 52 and 53. In addition a superconducting guide plate 57 is provided which extends between the inner ends of the leg portions 52 and 53, and has a cylinder-like concave configuration which closely matches that of a desired magnetic field configuration. As a consequence of this arrangement, magnetic lines of flux indicated at 58, which are produced when the superconducting coil 54 is excited, follow the surface configuration of the guide 57. Since the magnetic lines of flux cannot penetrate the superconductive guide 57 or the superconductive

concentric rings

55 and 56, the magnetic flux lines will be closely confined to the desired configuration built into the shape of the guide 57.

A similar magnetic lens construction is illustrated in FIGURE 6 of the drawings, wherein it can be seen that a superconductive guide member 59 that is positioned between the ends of the leg portions 52, 53 of the core 51 to provide a concave inner surface that the magnetic lines of flux indicated at 60 tend to follow to thereby produce outward divergence in the center magnetic field path which closely approximates or follows the surface configuration of the guide 59. In both of the species of magnetic lens structures shown in FIGURE 5 and FIGURE 6, it is necessary that a slit be formed which extends across the entire cross section of the guide 57 or the guide 59 to prevent the development of circulating currents in the superconducting guide that would otherwise prevent the passage of lines of magnetic flux through the opening defined by the superconductive guide. The guides 57 and 59 cause the magnetic lines of flux to follow their surface configuration because of its natural characteristic which prevents penetration of lines of flux through a superconducting surface. The lines of magnetic flux produced at the end of the legs 52 and 53 of the core member will try to cut through the superconducting members 57 or 59; however, these members because of their superconducting nature will develop counter currents which will prevent the lines of flux from entering and passing through the superconductor. As a consequence, the lines of flux will follow along the surface configuration of the guides 57 or 59, until it reaches a point, namely the opposite pole plate, where it can form a closed path. Accordingly, it can be appreciated from an examination of FIGURES 2, 5, and 6 that the invention makes available a greatly improved magnetic lens construction which is entirely flexible and can be readily formed to provide any desired configuration of the magnetic lines of flux produced by the lens structure.

Still another form of a superconductive magnetic lens structure constructed in accordance with the invention is shown in FIGURE 7, and includes a housing 61. The housing 61 comprises an iron core 62 for supporting a set of four superconducting magnetic field producing coils, and has a pair of spaced apart

leg portions

63 and 64 which form pole pieces that in turn define a central opening to accommodate an electron beam to be focussed. The four superconducting magnetic field producing coils disposed in the core 62 comprise a first coil 65 which may be formed from two separate coils each having 525 turns of .01" diameter niobium wire wound so as to surround the

indented leg portions

63 and 64 of the core member. A second field producing coil 66 7 likewise encircles the indented leg portions of the core members 63 and 64', and is formed from two separate coils each having 900 turns of .01" diameter niobium wire. Supported within the space between the

leg portions

63 and 64 of the core are a pair of superconductive disks 69 and 71 respectively, which have a central opening approximately 6 millimeters in diameter which matches the opening defined by the leg portions 63 and 64- to accommodate the electron beam. A plan view of one of the disks 63 is shown in FIGURE 8 of the drawings wherein it can be seen that a slit '72 is formed across its entire cross section to prevent the development of circulating electric currents. If desired, a similar slit could be made along a line diametrically opposite the slit '72 so that the disk 69 would be formed from two halves. A second set of

superconductive disks

73 and 74 are seated over the two disks 69 and 71. A plan view of one of the second sets of superconductive disks 73 is shown in FIGURE 9 of the drawings wherein it can be seen that a slit '75 is cut across its entire cross section to prevent the development of undesired circulating currents. The four

disks

69, 71, 73 and 74 are stacked adjacent each other surrounding the opening defined for the electron beam path between the

leg portions

63 and 64 of the core member 62. As examples of the size of the disks which are fabricated from .08" thick niobium sheet mem bers, the disks 69 and 71 have an inside diameter of .24 and an outside diameter of 3.15". In place of a single slit '72, it is of course possible that the disk 69 be slit across its entire cross section so as to form two semicircular members. The disks 73 and '74 may have an inside diameter of .240" and an outside diameter of 1.355, and also are made of .08" thick niobium sheet members. Disposed over the upper surfaces of the

leg members

63 and 64 of core 62 are a second set of

superconductive disks

76 and 77 having a central opening of about .240" in diameter, and having an outside diameter of 1.355". The structure thus comprised may be held together by means of a set of

flange members

78 and 79 that are press-fit over the inside corners of the core member 62 to retain the entire structure in assembled relationship with the core member 62 being secured by welding or otherwise to the housing member 61. In operation, the coils 65 through 68 are connected in series circuit relationship, and upon excitation produce a magnetic field flux between the ends of the

leg members

63 and 64 with substantially no leakage occurring outside the gap designed to accommodate the electron beam. It is of course to be understood that the magnetic lens structure of FIGURE 7 and similarly the lens structures of FIGURES and 6 are to be disposed within a cooling arrangement such as is shown in FIGURE 1 of the drawings for retaining the temperature of the superconductive disk thereof at superconducting temperature. In this fashion, the superconducting disks will serve as shields to prevent penetration of magnetic lines of flux, and thereby confine the lines of flux to predetermined paths.

From the foregoing description it can be appreciated that the invention provides a new and improved magnetic lens structure wherein the magnetic field produced by the lens can be precisely shaped to a desired configuration, and can be confined to desired lines of travel. Should it be desired, the lens structure can include an open-circuited shield or guide of superconducting material that has its surface formed in the shape of the desired magnetic field configuration thereby assuring precise conformation of the produced magnetic field to its predesigned form.

Having described several embodiments of a new and improved magnetic lens structure constructed in accordance with the invention, it is believed obvious that other modifications and variations of the present invention are possible in the light of the above teaching. It is therefore to be understood that changes may be made in the particular embodiments of the invention described which are within the full intended scope of the invention as defined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States:

1. The combination comprising an electron beam source, an electron beam magnetic lens comprising a guide of superconducting material positioned from said electron beam source and adapted to have the electron beam directed therethrough, and means for maintaining the guide of superconducting material in superconducting condition.

2. The combination set forth in claim 1 wherein the guide is efiectivcly open-circuited across the entire crosssection to prevent the development of undesired circulating currents.

3. A charged particle focusing device comprising a guide of superconducting material formed in the shape of a desired magnetic field configuration, means for maintaining said material in a superconducting condition, and magnetic field producing means surrounding said guide of superconducting material.

4. The combination set forth in claim 3 wherein the guide of superconducting material is effectively open-circuited across its entire cross-section to prevent the development of undesired circulating currents.

5. The combination including an electron beam source, a magnetic lens comprising an open-circuited guide of superconducting material formed in the shape of a desired magnetic field configuration and having a cooling jacket containing a cooling fluid surrounding the loop for maintaining it in a superconducting condition, and a magnetic field producing coil surrounding the guide of superconducting material.

6. The combination comprising an evacuated container, an electron beam source disposed in said container, at least one magnetic lens positioned in said container adjacent the electron beam source comprising a body of superconductive material which is generally cylindrical and formed in the shape of a desired magnetic field configuration and which has a longitudinally extending opening whereby the body forms an open-circuited guide of superconductive material, a cooling jacket surrounding the guide of superconductive material for maintaining the same in a superconducting condition, and a magnetic field producing coil surrounding the guide of superconducting material.

7. The combination comprising a magnetic lens structure including a core of magnetizable material defining an opening for a charged particle, and at least one member of superconducting material secured to said core about said opening for shaping and defining the magnetic field produced in the opening by said core.

8. The combination set forth in claim 7 further characterized by magnetic field producing means positioned adjacent the core of magnetizable material, and cooling means surrounding the core for maintaining the superconducting member in superconducting condition.

9. The combination comprising a magnetic lens structure including in combination a core member of magnetizable material having at least a pair of spaced apart pole pieces defining an opening for guiding charged particles, and a set of superconducting members secured to opposite sides of each of said spaced apart pole pieces for shaping and defining the magnetic field produced in the opening by said pole pieces.

10. The combination set forth in claim 9 further characterized by magnetic field producing means positioned adjacent the core of magnetizable material, and cooling means surrounding the core for maintaining the superconducting members in superconducting condition.

11. The combination comprising an evacuated container, a electron beam source disposed in said container, at least one magnetic lens positioned in said container adjacent the electron beam source and adapted to have the electron beam directed therethrough, said magnetic lens comprising a core member of magnetizable material having at least a pair of spaced apart pole pieces defining an opening for guiding charged particles, and a set of superconducting members secured to opposite sides of each of said spaced apart pole pieces for shaping and defining the magnetic field produced in the opening by said pole pieces, a cooling jacket surrounding the magnetic lens structure, a magnetic field producing coil surrounding the loop of superconducting material, an electron beam target disposed in the container intermediate the electron beam source and the magnetic lens structure, and a fluorescent screen positioned in the container 10 on the side of the magnetic lens opposed from the target whereby an electron image of the target specimen passes through the magnetic lens and is focussed upon the fluorescent viewing screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,247,524 Schuchmann et al. July 1, 1941 10 2,362,515 Weigend Nov. 14, 1944 2,858,444 Leisegang Oct. 28, 1958