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Rubber band suspension for a nuclear demagnetization cryostat
Research and technical notes Table 1
Specifications for the charcoal pump and 3He pot
extra freedom in designing a cryostat for specific experimental requirements.
Charcoal pump
Mass of charcoal
40 g Packing factor 20% Volume 88 cm 3
Conclusions
3He pot Volume Surface
8 cm 3 of 140 pressed thin Cu discs
area
0.1 m2
Load Cu block, stainless steel calorimeter, total volume
500 cm 3
A demountable charcoal pump design has been described. Cooling of the pump relied on an adjustable pressed contact at low temperatures. Testing demonstrated the feasibility of obtaining low temperatures (0.6 K) over a short period of time with such a simple pump design. Even better results are expected with some improvements over the actual arrangements.
Acknowledgements of heat that must be extracted from the load to cool it from 1 to 0.3 K is about 30 times the amount needed to cool the 3He content in the pot at the beginning of the pumping for that same temperature interval. It is then not surprising that the 3He pot with a finite cooling power, can not handle the cooling of the large total thermal mass. Given this result, a much better performance can be expected with regards to run time if the load heat capacity is reduced. A reduction of the residual heat leak would lead to a lower minimum temperature. However, the demountable feature of the pump allows
This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and its programme 'Attach6 de Recherche'.
Rubber band suspension for a nuclear demagnetization cryostat
and the cryostat, and to increase the cryostat mass as much as reasonably possible 1. The resonance frequency of the cryostat system can thus be reduced to about 1 Hz, whereas vibration frequencies in walls and ceilings of buildings are typically at least an order of magnitude higher. So the combination of air springs and big cryostat mass is well suited to attenuate vibrations of the surroundings. A different approach to the problem is described in the following. As shown in Figure 1, in our apparatus the cryostat insert and the dewar hang from an aluminium base plate (A) which is resting in a support frame at three points (B), the frame being bolted to the ceiling of the laboratory (C). From each corner of the base plate a set of six rubber bands (E) and a pulley block (F) lead to the upper corners of the support frame. The rubber bands, which are normally intended for gymnastic exercises, are 3 cm wide and 0.4 cm thick, and have a typical length of 40 cm. They can be purchased in any major sport shop*. The four pulleys make it quite easy to raise the base plate with the cryostat by about 2 cm so that it is freely floating. The total load to the rubber bands is about 200 kg. The diffusion pump (NRC-VHS4) of the dilution refrigerator (D) is also mounted to this plate, as is another small diffusion pump which is needed to evacuate the cryostat before an experiment and to pump the exchange gas. This diffusion pump and most of the necessary plumbing are omitted from Figure 1 for clarity. The dilution refrigerator has been described elsewhere 2. The diffusion pumps are connected to forepumps which are situated in the basement of the laboratory by U-shaped bellows (G), one end
K. Uhlig and W. Hehn Walther Meissner Institut for Tieftemperaturforschung der Bayerischen Akademie der Wissenschaften, D8046 Garching, FRG Received 19 November 1987
In the course of the construction of a nuclear demagnetization refrigerator a cryostat suspension made from rubber bands was built to keep the mechanical vibrations of the surroundings from penetrating into the apparatus. Measurements of the attenuation behaviour of this rubber band suspension are reported, which demonstrate that this simple set-up works very effi-
ciently. Keywords: cryostats; heat leaks; suspension
Mechanical vibrations, among other things, can be the source of troublesome heat leaks into the nuclear stage of submillikelvin refrigerators. The common strategy to avoid vibrational heat leaks is to use big masses of concrete for the cryostat support, air springs between the support 0011-2275/88/090612-03 $03.00 © 1988 Butterworth & Co (Publishers) Ltd
612
Cryogenics 1988 Vol 28 September
References 1 Mate, C.F., Harris-Lowe, R., Davis, W.L. and Daunt, J.G. Rev Sci lnstrum (1965) 36 369 2 Esel'son, B.N., Lazarev, B.G. and Shevts, A.D. Cryogenics (1962) 2 279 3 Crisp, R.S. and Turner, P, Cryogenics (1985) 25 45
*The rubber bands described are manufactured by J.G. Schmidt Co., 565 Solingen, FRG
Research and technical notes
but the signal amplitude is reduced by about a factor of 20 for the above-mentioned frequencies in comparison to the 'down' position. To transfer helium into the dewar it has to be clamped to the frame, otherwise the motion of the dewar would be too large when the transfer line is inserted, and the cryostat would warm up. Fastening can be done very easily by just inserting spacers between the base plate and the support frame; it is not necessary to release the pulleys for a He transfer. It should be mentioned that the rubber bands become slightly longer with time when stretched. Typically in the first week of operation the cryostat sank by about 1 mm per day, and in the second week by about 0.5 mm on average. It is advisable to keep the rubber bands stretched permanently, that way the effect described above can be minimized. Finally, we checked if the running forepumps influenced the geophone spectrum. We found that even in the cryostat 'down' position a contribution from the pumps was not detectable. This is because the pumps rest on rubber 20 cm
20.
a
Figure I Schematic diagram of the cryostat suspension. A, Aluminium base plate; B, resting points; C, points where frame is bolted to the ceiling; D, diffusion pump; E, rubber band; F, pulley block; G, Ushaped bellows
of the bellows being bolted to the walls of the laboratory. This arrangement has the advantage that long, thick, vibration compensated lines between the refrigerator and the diffusion pumps can be omitted. However, before the demagnetization of the nuclear stage starts, the diffusion pumps have to be turned off as they generate considerable vibration of the system, and the dilution refrigerator is operated just with the forepump alone. Only a little cooling power is needed at this stage of the experiment to keep the radiation shields around the nuclear stage cold. This can easily be provided by the dilution refrigerator even if it is run only by the mechanical pump. Seismometric measurements of the system are depicted in Figure 2*. The seismometer was placed on the base plate, the plate resting in the support frame and the pulleys being released. Figure 2a shows a Fourier transform of the output of the 'geophone', the original signal is shown in the insert. The seismometer signals were recorded and analysed in a digital storage scope (Le Croy model 9400). The resonance of 16.8 Hz of the spectrum is caused by the compressor of the He liquefier which is located in a separate building about 50 m away from the laboratory. In Figure 2b the experimental conditions were the same as in Figure 2a, except that the cryostat was suspended from the rubber bands; clearly the compressor resonance had disappeared. In another experiment a small electric motor was clamped to the support frame and run at rotation speeds between 20 and 100 Hz, causing strong vibrations of the frame and the cryostat. In the cryostat 'up' position the motor resonance can still be detected with the seismometer * The seismometer is a "Moving-coil electromagnetic geophone' from Mark Products US, Inc., Area 713/498-0600, 10507 Kingherst Drive, Houston, TX 77099, USA
40
~'0
E m
li
- - ~
.2
"0
1 t .sec
I .6
--
6O
I
20
40
I
l
I
I
I
I
I
b
~'0 .2
~:
t,sec
.6
m
O
40
80
120
~;,Hz Figure 2 Fourier transforms of seismometer signals for two different experimental conditions; the original signals are depicted in the inserts. (a) Typical seismometer signal with the cryostat resting in the su pport frame; (b) corresponding signal with the cryostat suspended from the rubber bands
Cryogenics 1988 Vol 28 September
613
Research and technical notes
stoppers and the connections between the pumping lines and the pumps are made from soft rubber tubing.
Conclusions We find that a rubber band suspension is very efficient, inexpensive, and easy to build. It is especially recommended for consideration in smaller cryostats, where low vibrational heat leaks are required. However, this system could certainly be used in bigger applications, too, if dimensioned appropriately.
New possibility of magnetic ripple shielding for specific heat measurements in hybrid magnets Z. Tarnawski*, H.P. van der Meulen, J.J.M. Franse, K. Kadowaki, P.A. Veenhuizen and J.C.P. Klaasse Natuurkundig Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, 1018-XE Amsterdam, The Netherlands Received 10 November 1987
A test of the new high Tc superconducting materials for magnetic ripple shielding has been carried out. It was found that magnetic ripples of 0.0009 T (peakto-peak) in the frequency range below 20 kHz can be completely shielded in high static fields by a 2 mm thick Y-Ba-Cu-O screen.
Acknowledgement We would like to thank Dr Gebrande from the Institut fiir Allgemeine und Angewandte Geophysik of the LMU, Munich, for making available the geophone.
References 1 Mueller, R.M., Buchal, C., Folle, H.R., Kubota, M. and Pobell, F. 2
Cryogenics (1980) 20 395 Uhlig, K. Cryogenics (1987) 27 454
voltage a.c. line and amount to 2 x 10 - 4 RMS or by the power supply system in which case they amount to half of this value. In addition, field ripples of 600 Hz have been mentioned for the Nijmegen Laboratory for High Magnetic Fieldsa. Superconducting coils of Nb3Sn-Ti have been used as a passive filter to perform resistivity measurements up to 21 T (see Reference 1). However, for specific heat measurements, especially in magnetic fields above 20 T where critical currents in the superconducting coil become very small (even in the advanced Ti-alloyed Nb3Sn superconductor), an active type of ripple shielding is needed 3. This type of active coil with negative feedback was used for specific heat measurements at FBHF for fields up to 20 T. The noise at 6 and 84 Hz is reduced by two orders of magnitude in this way 2. The discovery of the new class of high T~ superconductors 5'6 with very high upper critical field ( 8 0 - 1 8 0 .9 T) 7 - lO has provided a new possibility for solving the ripple problem over a wide frequency range. In this note we report the shielding properties of the YBa2Cu30 7 superconductor.
Experiments Keywords: high Tc superconductivity; magnetic fields; shielding High precision transport as well as specific heat measurements at high magnetic field in a hybrid or Bitter-type magnet are very difficult to perform 1.2 or even unsuccessful up until now 3. These difficulties originate from temperature fluctuations induced by magnetic field ripples generated by the power supply system2,4 or by the noise included in the high voltage a.c. line ~. Experimental conditions concerning magnetic field ripples are quite different for the different high field laboratories. For example, field ripples with frequencies of 6 and 84 Hz occur at the Francis Bitter National Laboratory (FBHF) using the 36kA power supply system2. At the High Field Laboratory For Superconducting Materials of Tohoku University ~, field ripples between 1 and 10 Hz are generated either by the noise in the high *On leave from Solid State Physics Department, Academy of Mining and Metallurgy, al Mickiewicza 30, 30-059 Krakow, Poland 0011-2275/88/090614-03 $03.00 © 1988 Butterworth & Co (Publishers) Ltd
614
Cryogenics 1988 Vol 28 September
The material chosen for this investigation is the well characterized high T~superconductor YBa2Cu30 7. It was prepared by a standard ceramics technique, as previously described 11. The final product was a cylinder of 9 mm diameter with a density of 5.07 g c m - 3 compared to the theoretical density of 6.74 g cm- 3. Electrical, magnetic, transport, thermodynamic and structural properties have been described elsewhere x2,~. A hole (5 mm diameter) was drilled in the 10 mm high and 9 mm wide cylinder of YBa2Cu30 7 yielding a superconducting screen with a thickness, d, of 2 mm. A pick-up coil of 1I00 turns, n~, inside the screen and outer coil of 1400 turns, n2, were mounted in a sample holder provided with a 1900fl manganine heater and a platinum thermometer (Pt 1000). Measurements were performed in fields from 0 to 5 T in the temperature range between 4.2 and 100 K. Input (outer coil) current, J = Jo sin cot,and output (pick-up coil) signal were measured by a digital Keithley multimeter as well as observed by an oscilloscope. For temperatures less than 87 K, well below the superconducting transition temperature, the maximum values of Jo(T) for which the
Specifications for the charcoal pump and 3He pot
extra freedom in designing a cryostat for specific experimental requirements.
Charcoal pump
Mass of charcoal
40 g Packing factor 20% Volume 88 cm 3
Conclusions
3He pot Volume Surface
8 cm 3 of 140 pressed thin Cu discs
area
0.1 m2
Load Cu block, stainless steel calorimeter, total volume
500 cm 3
A demountable charcoal pump design has been described. Cooling of the pump relied on an adjustable pressed contact at low temperatures. Testing demonstrated the feasibility of obtaining low temperatures (0.6 K) over a short period of time with such a simple pump design. Even better results are expected with some improvements over the actual arrangements.
Acknowledgements of heat that must be extracted from the load to cool it from 1 to 0.3 K is about 30 times the amount needed to cool the 3He content in the pot at the beginning of the pumping for that same temperature interval. It is then not surprising that the 3He pot with a finite cooling power, can not handle the cooling of the large total thermal mass. Given this result, a much better performance can be expected with regards to run time if the load heat capacity is reduced. A reduction of the residual heat leak would lead to a lower minimum temperature. However, the demountable feature of the pump allows
This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and its programme 'Attach6 de Recherche'.
Rubber band suspension for a nuclear demagnetization cryostat
and the cryostat, and to increase the cryostat mass as much as reasonably possible 1. The resonance frequency of the cryostat system can thus be reduced to about 1 Hz, whereas vibration frequencies in walls and ceilings of buildings are typically at least an order of magnitude higher. So the combination of air springs and big cryostat mass is well suited to attenuate vibrations of the surroundings. A different approach to the problem is described in the following. As shown in Figure 1, in our apparatus the cryostat insert and the dewar hang from an aluminium base plate (A) which is resting in a support frame at three points (B), the frame being bolted to the ceiling of the laboratory (C). From each corner of the base plate a set of six rubber bands (E) and a pulley block (F) lead to the upper corners of the support frame. The rubber bands, which are normally intended for gymnastic exercises, are 3 cm wide and 0.4 cm thick, and have a typical length of 40 cm. They can be purchased in any major sport shop*. The four pulleys make it quite easy to raise the base plate with the cryostat by about 2 cm so that it is freely floating. The total load to the rubber bands is about 200 kg. The diffusion pump (NRC-VHS4) of the dilution refrigerator (D) is also mounted to this plate, as is another small diffusion pump which is needed to evacuate the cryostat before an experiment and to pump the exchange gas. This diffusion pump and most of the necessary plumbing are omitted from Figure 1 for clarity. The dilution refrigerator has been described elsewhere 2. The diffusion pumps are connected to forepumps which are situated in the basement of the laboratory by U-shaped bellows (G), one end
K. Uhlig and W. Hehn Walther Meissner Institut for Tieftemperaturforschung der Bayerischen Akademie der Wissenschaften, D8046 Garching, FRG Received 19 November 1987
In the course of the construction of a nuclear demagnetization refrigerator a cryostat suspension made from rubber bands was built to keep the mechanical vibrations of the surroundings from penetrating into the apparatus. Measurements of the attenuation behaviour of this rubber band suspension are reported, which demonstrate that this simple set-up works very effi-
ciently. Keywords: cryostats; heat leaks; suspension
Mechanical vibrations, among other things, can be the source of troublesome heat leaks into the nuclear stage of submillikelvin refrigerators. The common strategy to avoid vibrational heat leaks is to use big masses of concrete for the cryostat support, air springs between the support 0011-2275/88/090612-03 $03.00 © 1988 Butterworth & Co (Publishers) Ltd
612
Cryogenics 1988 Vol 28 September
References 1 Mate, C.F., Harris-Lowe, R., Davis, W.L. and Daunt, J.G. Rev Sci lnstrum (1965) 36 369 2 Esel'son, B.N., Lazarev, B.G. and Shevts, A.D. Cryogenics (1962) 2 279 3 Crisp, R.S. and Turner, P, Cryogenics (1985) 25 45
*The rubber bands described are manufactured by J.G. Schmidt Co., 565 Solingen, FRG
Research and technical notes
but the signal amplitude is reduced by about a factor of 20 for the above-mentioned frequencies in comparison to the 'down' position. To transfer helium into the dewar it has to be clamped to the frame, otherwise the motion of the dewar would be too large when the transfer line is inserted, and the cryostat would warm up. Fastening can be done very easily by just inserting spacers between the base plate and the support frame; it is not necessary to release the pulleys for a He transfer. It should be mentioned that the rubber bands become slightly longer with time when stretched. Typically in the first week of operation the cryostat sank by about 1 mm per day, and in the second week by about 0.5 mm on average. It is advisable to keep the rubber bands stretched permanently, that way the effect described above can be minimized. Finally, we checked if the running forepumps influenced the geophone spectrum. We found that even in the cryostat 'down' position a contribution from the pumps was not detectable. This is because the pumps rest on rubber 20 cm
20.
a
Figure I Schematic diagram of the cryostat suspension. A, Aluminium base plate; B, resting points; C, points where frame is bolted to the ceiling; D, diffusion pump; E, rubber band; F, pulley block; G, Ushaped bellows
of the bellows being bolted to the walls of the laboratory. This arrangement has the advantage that long, thick, vibration compensated lines between the refrigerator and the diffusion pumps can be omitted. However, before the demagnetization of the nuclear stage starts, the diffusion pumps have to be turned off as they generate considerable vibration of the system, and the dilution refrigerator is operated just with the forepump alone. Only a little cooling power is needed at this stage of the experiment to keep the radiation shields around the nuclear stage cold. This can easily be provided by the dilution refrigerator even if it is run only by the mechanical pump. Seismometric measurements of the system are depicted in Figure 2*. The seismometer was placed on the base plate, the plate resting in the support frame and the pulleys being released. Figure 2a shows a Fourier transform of the output of the 'geophone', the original signal is shown in the insert. The seismometer signals were recorded and analysed in a digital storage scope (Le Croy model 9400). The resonance of 16.8 Hz of the spectrum is caused by the compressor of the He liquefier which is located in a separate building about 50 m away from the laboratory. In Figure 2b the experimental conditions were the same as in Figure 2a, except that the cryostat was suspended from the rubber bands; clearly the compressor resonance had disappeared. In another experiment a small electric motor was clamped to the support frame and run at rotation speeds between 20 and 100 Hz, causing strong vibrations of the frame and the cryostat. In the cryostat 'up' position the motor resonance can still be detected with the seismometer * The seismometer is a "Moving-coil electromagnetic geophone' from Mark Products US, Inc., Area 713/498-0600, 10507 Kingherst Drive, Houston, TX 77099, USA
40
~'0
E m
li
- - ~
.2
"0
1 t .sec
I .6
--
6O
I
20
40
I
l
I
I
I
I
I
b
~'0 .2
~:
t,sec
.6
m
O
40
80
120
~;,Hz Figure 2 Fourier transforms of seismometer signals for two different experimental conditions; the original signals are depicted in the inserts. (a) Typical seismometer signal with the cryostat resting in the su pport frame; (b) corresponding signal with the cryostat suspended from the rubber bands
Cryogenics 1988 Vol 28 September
613
Research and technical notes
stoppers and the connections between the pumping lines and the pumps are made from soft rubber tubing.
Conclusions We find that a rubber band suspension is very efficient, inexpensive, and easy to build. It is especially recommended for consideration in smaller cryostats, where low vibrational heat leaks are required. However, this system could certainly be used in bigger applications, too, if dimensioned appropriately.
New possibility of magnetic ripple shielding for specific heat measurements in hybrid magnets Z. Tarnawski*, H.P. van der Meulen, J.J.M. Franse, K. Kadowaki, P.A. Veenhuizen and J.C.P. Klaasse Natuurkundig Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, 1018-XE Amsterdam, The Netherlands Received 10 November 1987
A test of the new high Tc superconducting materials for magnetic ripple shielding has been carried out. It was found that magnetic ripples of 0.0009 T (peakto-peak) in the frequency range below 20 kHz can be completely shielded in high static fields by a 2 mm thick Y-Ba-Cu-O screen.
Acknowledgement We would like to thank Dr Gebrande from the Institut fiir Allgemeine und Angewandte Geophysik of the LMU, Munich, for making available the geophone.
References 1 Mueller, R.M., Buchal, C., Folle, H.R., Kubota, M. and Pobell, F. 2
Cryogenics (1980) 20 395 Uhlig, K. Cryogenics (1987) 27 454
voltage a.c. line and amount to 2 x 10 - 4 RMS or by the power supply system in which case they amount to half of this value. In addition, field ripples of 600 Hz have been mentioned for the Nijmegen Laboratory for High Magnetic Fieldsa. Superconducting coils of Nb3Sn-Ti have been used as a passive filter to perform resistivity measurements up to 21 T (see Reference 1). However, for specific heat measurements, especially in magnetic fields above 20 T where critical currents in the superconducting coil become very small (even in the advanced Ti-alloyed Nb3Sn superconductor), an active type of ripple shielding is needed 3. This type of active coil with negative feedback was used for specific heat measurements at FBHF for fields up to 20 T. The noise at 6 and 84 Hz is reduced by two orders of magnitude in this way 2. The discovery of the new class of high T~ superconductors 5'6 with very high upper critical field ( 8 0 - 1 8 0 .9 T) 7 - lO has provided a new possibility for solving the ripple problem over a wide frequency range. In this note we report the shielding properties of the YBa2Cu30 7 superconductor.
Experiments Keywords: high Tc superconductivity; magnetic fields; shielding High precision transport as well as specific heat measurements at high magnetic field in a hybrid or Bitter-type magnet are very difficult to perform 1.2 or even unsuccessful up until now 3. These difficulties originate from temperature fluctuations induced by magnetic field ripples generated by the power supply system2,4 or by the noise included in the high voltage a.c. line ~. Experimental conditions concerning magnetic field ripples are quite different for the different high field laboratories. For example, field ripples with frequencies of 6 and 84 Hz occur at the Francis Bitter National Laboratory (FBHF) using the 36kA power supply system2. At the High Field Laboratory For Superconducting Materials of Tohoku University ~, field ripples between 1 and 10 Hz are generated either by the noise in the high *On leave from Solid State Physics Department, Academy of Mining and Metallurgy, al Mickiewicza 30, 30-059 Krakow, Poland 0011-2275/88/090614-03 $03.00 © 1988 Butterworth & Co (Publishers) Ltd
614
Cryogenics 1988 Vol 28 September
The material chosen for this investigation is the well characterized high T~superconductor YBa2Cu30 7. It was prepared by a standard ceramics technique, as previously described 11. The final product was a cylinder of 9 mm diameter with a density of 5.07 g c m - 3 compared to the theoretical density of 6.74 g cm- 3. Electrical, magnetic, transport, thermodynamic and structural properties have been described elsewhere x2,~. A hole (5 mm diameter) was drilled in the 10 mm high and 9 mm wide cylinder of YBa2Cu30 7 yielding a superconducting screen with a thickness, d, of 2 mm. A pick-up coil of 1I00 turns, n~, inside the screen and outer coil of 1400 turns, n2, were mounted in a sample holder provided with a 1900fl manganine heater and a platinum thermometer (Pt 1000). Measurements were performed in fields from 0 to 5 T in the temperature range between 4.2 and 100 K. Input (outer coil) current, J = Jo sin cot,and output (pick-up coil) signal were measured by a digital Keithley multimeter as well as observed by an oscilloscope. For temperatures less than 87 K, well below the superconducting transition temperature, the maximum values of Jo(T) for which the