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Construction details of a liquid helium cryostat for a superconducting objective and cold stage
Ultramicroscopy 12 (1983) 79-82 North-Holland Publishing Company
79
LETTER TO T H E E D I T O R C O N S T R U C T I O N DETAILS OF A LIQUID H E L I U M CRYOSTAT FOR A SUPERCONDUCTING OBJECTIVE AND COLD STAGE M.K. LAMVIK, R.E. WORSHAM *, D.A. K O P F and J.D. ROBERTSON Department of Anatomy, Duke University Medical Center, Durham, North Carolina 27710, USA
Received 15 July 1983
An electron microscope with a superconducting objective lens and a field emission electron source was constructed at Oak Ridge National Laboratory. The microscope is currently located at Duke University, where it is being applied to a program of low temperature electron microscopy. The progress of its construction was described in the proceedings of the Electron Microscopy Society of America (e.g., refs. [1,2]), and the microscope was reviewed by Hawkes and Valdr6 [3]. This material was recently reviewed again by Lefranc et al. [4], but their report allows the impression that it describes the current status of the instrument. Because there has been much current discussion concerning low temperature electron microscopes, we consider it important to describe the current configuration of our cryostat and stage to correct any inaccurate impressions that might be drawn from the review of Lefranc et al. For example, they comment that all of the normal lenses in the microscope are taken from a Siemens Elmiskop I; this statement was true in 1971, when the first cryostat and the first objective lens were tested using the illumination system and projector from an Elmiskop I [5]. A normal intermediate lens was soon added. In 1973-74 [1], the thermionic gun and condensers were replaced with a field emission gun and a single normal condenser lens for operations up to 150 kV. Simultaneously, the cryostat [2] was modified and the
* Present address: Universityof British Columbia, Vancouver, BC, Canada V6T 2A3.
objective was replaced entirely to improve on the first design. Finally, the framework was built entirely anew with support above the center of gravity, and in 1975 pneumatic isolation was added. Although the other lenses were replaced, the old Siemens projector remains in use and is working very well. The objective lens and specimen stage are located within a liquid-helium-cooled cryostat. All of the components that require liquid helium temperature are in the liquid, if possible, or are bonded to the bottom plate of the cryostat, which is heavily gold-plated, 2.5 cm thick copper that is always necessarily in contact with the liquid helium. The copper coil form that contains the N b - T i lens coil is also necessarily filled with liquid helium to allow superconducting operation of the objective lens. A horizontal hole on the lens median plane passes through the coil form to allow insertion of the specimen carrier. The illumination alignment deflection coils and illumination stigmator are in direct contact with the liquid helium vessel, and all of their electrical leads pass through the liquid helium vessel. The liquid helium vessel and its attachments are surrounded by two concentric radiation shields. These shields are made of 3 mm copper and are heavily gold-plated, as is the helium vessel. Liquid nitrogen was not used to cool the shields, in an effort to eliminate the possibility of vibration and to better utilize the space in the column. Instead, the concentric shields are cooled by the helium vapor. T h i s cooling provides a low-temperature surface adjacent to the helium vessel that (1) re-
0304-3991/83/0000-0000/$03.00 © 1983 North-Holland
80
M.K. Lamvik et al. / Construction details o/liquid helium crvostat
duces radiation losses, (2) provides a long, lowtemperature tie point for the electrical leads, and (3) gives solid low-temperature anchors for the posts and drives. The temperature of the shields will vary during the cool-down/boil-off cycle. During design, the shields were given the nominal values of 20 and 90 K, respectively. Germanium resistors and thermocouples, as appropriate, are mounted on the shields to allow temperature measurements. With active helium flow, the inner shield has been measured to be 19 K; after stopping the flow, the inner shield was 24 K, and the other shield was 128 K; and several hours later, the inner shield has warmed to 30 K. The earlier version of the cryostat used epoxyglass hollow cylinders to clamp the cryostat parts, but this was not sufficiently stable. With the large-diameter, thin-wall posts of epoxy-glass and highly stressed stainless steel rods that are presently installed, no motion has been detected. Hence, there is no problem in using a superconducting objective and a normal intermediate. The compressive strength of epoxy-glass is much greater and the thermal conductivity is much less in the direction perpendicular to the planes of the glass fibers, and the construction here takes full advantage of these properties. This enhances the extreme ruggedness and the low heat losses of the cryostat. Space for ten standard electron microscope grids is provided in a specimen carrier that can be inserted into the microscope through an airlock. In its normal operating position, the specimen holder is surrounded by a cylindrical support that is clamped at each end to the stage ring that goes around the objective lens. The specimen carrier is positioned into a dovetail groove within the support and is held by spring pressure. Direct cooling contact is provided by copper foils that are clamped to the stage ring at one end and to the copper bottom of the liquid helium vessel at the other. A germanium resistance temperature sensor is also installed on the stage ring. It is held in a copper stud facing the inner shield. The stage ring rests on three sapphire ball bearings that are held on a supporting table that is attached to the lower part of the objective lens. This base, like the stage ring, is tied thermally to the copper bottom of the liquid
helium vessel with copper foils. In this way, the specimens are surrounded by surfaces at liquid helium temperature (the iron and the lens coils above and below) and are cooled by direct conduction through the stage ring and the copper foils to the liquid helium vessel. All parts of the stage and specimen holders are made of berylliumcopper. At one time in the past, after removal of all of the copper foils connected to the stage, a heater was installed on the stage ring. It was possible to operate with specimen temperatures as high as 30 K while maintaining superconducting operation of the objective lens [6]. The boil-off rate was greatly increased and stage motion prevented more than medium resolution in images. The diffraction mode was usable, allowing a transition in TaS 2 to be observed in the region of 20 K. The ability to operate over a range of temperatures is extremely helpful, should it be necessary to investigate transition temperatures in specimen radiation damage properties that may occur above 4.2 K. It is also possible to reduce temperature below 4.2 K by pumping on the helium [5]. The construction details show that the specimen in the Duke University cryomicroscope is surrounded by surfaces at liquid helium temperature, and the stage is directly connected with the liquid helium vessel through copper conductors. These are exactly the requirements expected of a "cryomicroscope". As Heide [7] has pointed out, however, many factors influence the cooling rate of the specimen in a cold stage, including the conductivities of all parts and junctions in the chain from the liquid helium to the specimen. We are not aware that the ultimate specimen temperature has been measured in any "cryomicroscope", and though we are beginning to make some preliminary measurements in our instrument, it is a difficult thing to do accurately. Without measurements, it is doubtful that strong claims can be made for the temperature of any particular cyromicroscope stage. Until such measuremens are complete, we propose that both the construction details reported here and our earlier measurements of radiation damage effects [8] suggest that there is no significant difference in specimen temperature between the cryomicroscopes discussed by Lefranc
M.K. Lamvik et al. / Construction details of liquid helium cryostat
et al. [4], i n c l u d i n g cryomicroscope.
the
Duke
University
T h i s w o r k is s u p p o r t e d b y c o n t r a c t N 0 1 - G M - 0 2116 f r o m N I H .
References [1] R.E. Worsham, W.W. Harris, J.E. Mann, E.G. Richardson and N.F. Ziegler, in: Proc. 32nd Annual EMSA Meeting, St. Louis, 1974 (Claitor's, Baton Rouge, LA, 1974) pp. 412-413.
81
[2] R.E. Worsham and J.E. Mann, in: Proc. 34th Annual EMSA Meeting, Miami Beach, 1976 (Claitor's, Baton Rouge, LA, 1976) pp. 532-533. [3] P.W. Hawkes and U. Valdr& J. Phys. El0 (1977) 309. [4] G. Lefranc, E. Knapek and I. Dietrich, Ultramicroscopy 10 (1982) 111. [5] R.E. Worsham, J.E. Mann and E.G. Richardson, in: Proc, 29th Annual EMSA Meeting, 1971 (Claitor's, Baton Rouge, LA, 1971)pp. 10-11. [6] J. Narayan, Appl. Phys. Letters 29 (1976) 223. [7] H.G. Heide, Ultramicroscopy 10 (1982) 125. [8] M.K. Lamvik, D.A. Kopf and J.D. Robertson, Nature 301 (1983) 332.
79
LETTER TO T H E E D I T O R C O N S T R U C T I O N DETAILS OF A LIQUID H E L I U M CRYOSTAT FOR A SUPERCONDUCTING OBJECTIVE AND COLD STAGE M.K. LAMVIK, R.E. WORSHAM *, D.A. K O P F and J.D. ROBERTSON Department of Anatomy, Duke University Medical Center, Durham, North Carolina 27710, USA
Received 15 July 1983
An electron microscope with a superconducting objective lens and a field emission electron source was constructed at Oak Ridge National Laboratory. The microscope is currently located at Duke University, where it is being applied to a program of low temperature electron microscopy. The progress of its construction was described in the proceedings of the Electron Microscopy Society of America (e.g., refs. [1,2]), and the microscope was reviewed by Hawkes and Valdr6 [3]. This material was recently reviewed again by Lefranc et al. [4], but their report allows the impression that it describes the current status of the instrument. Because there has been much current discussion concerning low temperature electron microscopes, we consider it important to describe the current configuration of our cryostat and stage to correct any inaccurate impressions that might be drawn from the review of Lefranc et al. For example, they comment that all of the normal lenses in the microscope are taken from a Siemens Elmiskop I; this statement was true in 1971, when the first cryostat and the first objective lens were tested using the illumination system and projector from an Elmiskop I [5]. A normal intermediate lens was soon added. In 1973-74 [1], the thermionic gun and condensers were replaced with a field emission gun and a single normal condenser lens for operations up to 150 kV. Simultaneously, the cryostat [2] was modified and the
* Present address: Universityof British Columbia, Vancouver, BC, Canada V6T 2A3.
objective was replaced entirely to improve on the first design. Finally, the framework was built entirely anew with support above the center of gravity, and in 1975 pneumatic isolation was added. Although the other lenses were replaced, the old Siemens projector remains in use and is working very well. The objective lens and specimen stage are located within a liquid-helium-cooled cryostat. All of the components that require liquid helium temperature are in the liquid, if possible, or are bonded to the bottom plate of the cryostat, which is heavily gold-plated, 2.5 cm thick copper that is always necessarily in contact with the liquid helium. The copper coil form that contains the N b - T i lens coil is also necessarily filled with liquid helium to allow superconducting operation of the objective lens. A horizontal hole on the lens median plane passes through the coil form to allow insertion of the specimen carrier. The illumination alignment deflection coils and illumination stigmator are in direct contact with the liquid helium vessel, and all of their electrical leads pass through the liquid helium vessel. The liquid helium vessel and its attachments are surrounded by two concentric radiation shields. These shields are made of 3 mm copper and are heavily gold-plated, as is the helium vessel. Liquid nitrogen was not used to cool the shields, in an effort to eliminate the possibility of vibration and to better utilize the space in the column. Instead, the concentric shields are cooled by the helium vapor. T h i s cooling provides a low-temperature surface adjacent to the helium vessel that (1) re-
0304-3991/83/0000-0000/$03.00 © 1983 North-Holland
80
M.K. Lamvik et al. / Construction details o/liquid helium crvostat
duces radiation losses, (2) provides a long, lowtemperature tie point for the electrical leads, and (3) gives solid low-temperature anchors for the posts and drives. The temperature of the shields will vary during the cool-down/boil-off cycle. During design, the shields were given the nominal values of 20 and 90 K, respectively. Germanium resistors and thermocouples, as appropriate, are mounted on the shields to allow temperature measurements. With active helium flow, the inner shield has been measured to be 19 K; after stopping the flow, the inner shield was 24 K, and the other shield was 128 K; and several hours later, the inner shield has warmed to 30 K. The earlier version of the cryostat used epoxyglass hollow cylinders to clamp the cryostat parts, but this was not sufficiently stable. With the large-diameter, thin-wall posts of epoxy-glass and highly stressed stainless steel rods that are presently installed, no motion has been detected. Hence, there is no problem in using a superconducting objective and a normal intermediate. The compressive strength of epoxy-glass is much greater and the thermal conductivity is much less in the direction perpendicular to the planes of the glass fibers, and the construction here takes full advantage of these properties. This enhances the extreme ruggedness and the low heat losses of the cryostat. Space for ten standard electron microscope grids is provided in a specimen carrier that can be inserted into the microscope through an airlock. In its normal operating position, the specimen holder is surrounded by a cylindrical support that is clamped at each end to the stage ring that goes around the objective lens. The specimen carrier is positioned into a dovetail groove within the support and is held by spring pressure. Direct cooling contact is provided by copper foils that are clamped to the stage ring at one end and to the copper bottom of the liquid helium vessel at the other. A germanium resistance temperature sensor is also installed on the stage ring. It is held in a copper stud facing the inner shield. The stage ring rests on three sapphire ball bearings that are held on a supporting table that is attached to the lower part of the objective lens. This base, like the stage ring, is tied thermally to the copper bottom of the liquid
helium vessel with copper foils. In this way, the specimens are surrounded by surfaces at liquid helium temperature (the iron and the lens coils above and below) and are cooled by direct conduction through the stage ring and the copper foils to the liquid helium vessel. All parts of the stage and specimen holders are made of berylliumcopper. At one time in the past, after removal of all of the copper foils connected to the stage, a heater was installed on the stage ring. It was possible to operate with specimen temperatures as high as 30 K while maintaining superconducting operation of the objective lens [6]. The boil-off rate was greatly increased and stage motion prevented more than medium resolution in images. The diffraction mode was usable, allowing a transition in TaS 2 to be observed in the region of 20 K. The ability to operate over a range of temperatures is extremely helpful, should it be necessary to investigate transition temperatures in specimen radiation damage properties that may occur above 4.2 K. It is also possible to reduce temperature below 4.2 K by pumping on the helium [5]. The construction details show that the specimen in the Duke University cryomicroscope is surrounded by surfaces at liquid helium temperature, and the stage is directly connected with the liquid helium vessel through copper conductors. These are exactly the requirements expected of a "cryomicroscope". As Heide [7] has pointed out, however, many factors influence the cooling rate of the specimen in a cold stage, including the conductivities of all parts and junctions in the chain from the liquid helium to the specimen. We are not aware that the ultimate specimen temperature has been measured in any "cryomicroscope", and though we are beginning to make some preliminary measurements in our instrument, it is a difficult thing to do accurately. Without measurements, it is doubtful that strong claims can be made for the temperature of any particular cyromicroscope stage. Until such measuremens are complete, we propose that both the construction details reported here and our earlier measurements of radiation damage effects [8] suggest that there is no significant difference in specimen temperature between the cryomicroscopes discussed by Lefranc
M.K. Lamvik et al. / Construction details of liquid helium cryostat
et al. [4], i n c l u d i n g cryomicroscope.
the
Duke
University
T h i s w o r k is s u p p o r t e d b y c o n t r a c t N 0 1 - G M - 0 2116 f r o m N I H .
References [1] R.E. Worsham, W.W. Harris, J.E. Mann, E.G. Richardson and N.F. Ziegler, in: Proc. 32nd Annual EMSA Meeting, St. Louis, 1974 (Claitor's, Baton Rouge, LA, 1974) pp. 412-413.
81
[2] R.E. Worsham and J.E. Mann, in: Proc. 34th Annual EMSA Meeting, Miami Beach, 1976 (Claitor's, Baton Rouge, LA, 1976) pp. 532-533. [3] P.W. Hawkes and U. Valdr& J. Phys. El0 (1977) 309. [4] G. Lefranc, E. Knapek and I. Dietrich, Ultramicroscopy 10 (1982) 111. [5] R.E. Worsham, J.E. Mann and E.G. Richardson, in: Proc, 29th Annual EMSA Meeting, 1971 (Claitor's, Baton Rouge, LA, 1971)pp. 10-11. [6] J. Narayan, Appl. Phys. Letters 29 (1976) 223. [7] H.G. Heide, Ultramicroscopy 10 (1982) 125. [8] M.K. Lamvik, D.A. Kopf and J.D. Robertson, Nature 301 (1983) 332.