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Suppression of magnetic field fluctuations in cryogenic experiments
RESEARCH AND TECHNICAL NOTES THE PREVENTION OF SONIC VIBRATIONS IN HELIUM SIPHONS A. I. S H A L ' N I K O V Institute for Physical Problems, Academy of Seiences, Moscow, U.S.S.R. Received 16 May 1966t
E v ER VO N E who employs low temperatures must encounter sonic vibrations in vessels containing liquid helium, which leads to its violent evaporation. This unpleasant phenomenon arises when tubes (for example, filling siphons) are introduced into the liquid helium and are closed at one end. In studying the sonic vibrations in siphons (with the help of a microphone) we found a simple means of preventing it-introducing into the lower end of the siphon a small t R e c e i v e d b y PTI~ E d i t o r , 11 J u n e 1964: l~ksperimenta N o . 4, p. 251 (1965).
Pribory i Tekhnika
(3-5 cm) length of woollen thread. This simple arrangement makes it even possible to leave the siphon permanently in the container without introducing a noticeable increase in evaporation. Apparently the thread acts as a damper, preventing self-oscillation of the column of liquid inside the tube. This note has been especially translated for CRYO(3ENICSand is included by permission of the Editors of Pribory i Tekhnika Eksperimenta.We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover-to-cover translation of PTI~ by arrangement with the Russian publisher.
SUPPRESSION OF MAGNETIC FIELD FLUCTUATIONS IN CRYOGENIC EXPERIMENTS J. VOTRUBA~- and M. SOTT Nuclear Research Institute, Academy of Sciences, [¢e~, Czechoslovakia Received 1 July 1966
i N some kinds of low temperature experiments a very high stability is required of an applied magnetic field (especially in magnetic cooling and nuclear orientation). However, it is extremely difficult to stabilize the field of large solenoids (copper coils with very low inductance and impedance) fed by motor-generators or rectifiers. 1 One method for ripple-suppression is based on the application of large condenser batteries and chokes, but this rather bulky and expensive equipment has poor efficiency at low ripple frequencies. Another possibility is to use tubes or ringlets made of a highly conducting material; these compensate the field fluctuations inside by induced eddy currents. A high purity copper ,shield of 2 mm thickness reduces a ripple field of 100 c/s by a factor of tAt present: Physical Prague, Czechoslovakia. CRYOGENICS
Institute,
• OCTOBER
Academy
of
about 20 at liquid helium temperature. 2 Further improvements on this result would require considerable increase of the wall thickness, 3 especially for lower frequencies. This reduces the experimental space available for the sample inside the cryostat. In this note a new modification of such a shield is described. It has a much higher efficiency at low frequencies than does a high purity copper shield of the same dimensions.
Principle of operation E d d y currents induced in a metallic tube~ tend to cancel the changes of magnetic flux inside--their time dependence is given by the known exponential relation It = I o e x p [ - ( R / L ) t ] . . . (1)
Sciences,
+ We assume a cylindrical tube with the axis parallel to H; 1966
299
If effective shielding is required at the ripple frequency f, then the characteristic time of the shield
300
. . . (2)
= L/R
should not be shorter than T = f-1. The inductance L of the tube is mainly determined by the geometry and is roughly independent of the material used. Thus to fulfil the condition
L/R > f-1
...
200
100
(3)
at low frequencies, the resistance R must be decreased. In our case this is achieved by means of a tube made
t
t
/; / /
100
50
se
/,.2" K
3I
0
o
,
16o f
Figure
1.
2. Pb-Bi tube 4. Cu strip
Experimental arrangement diagram, Dewar omitted)
(schematic
from superconducting material with a narrow copper strip sandwiched in it (Figure 1). When this tube is immersed in liquid helium, the resistance R for eddy currents (induced in a plane perpendicular to the axis) is given entirely by the width of the copper strip. Obviously, the applicability of this type of shield is limited by the critical field of the superconductor used.
Experimental arrangement and results We have investigated the shielding properties of lead-bismuth (60--40) tube with a copper strip 1 mm wide. The whole tube (diameter 33 ram, wall 1.5 ram, 220 mm long) consisted of five pieces (each about 45 mm long) prepared by casting the lead-bismuth alloy in a cylindrical mould, into which the tinned copper strip was pushed beforehand. The shielding tube was mounted inside the helium Dewar and placed into the hole of the water-cooled solenoid (0.2 ~), which gives 60 kG at 1 MW (2 000 A, 500 V). In the ripple produced by the motor-generator 300
0
260
'=
(c/s)
Figure 2. Ripple penetration at 77° K and 4.2° K and shielding effect K at different frequencies: U, detector coil signal; K = U(77)/U(4.2); f, frequency of the RC generator. (The d.c. field is zero in this case, amplitude of a.c. constant at all frequencies)
!]2
1. Solenoid 3. Checking coil
'
different frequencies occur with an amplitude of the order of 0.1 per cent of the d.c. voltage--the lowest frequency observed was 16 c/s. A small coil was placed inside the tube (in the middle of solenoid, Figure 1) to measure the penetration of the ripple. Figure 2 shows the frequency dependence of the voltage induced in this coil when the solenoid was fed by an auxiliary R C generator. Data were taken at 77 ° K where the shield is almost ineffective (as seen by a rapid increase of the induced voltage with frequency) and at 4.2 ° K. The ratio K = U(77)/U(4) thus characterizes the shielding efficiency at a given frequency f.
50
/
/
25
/ --'~J
10
t
2=0 H~
~
3'0
'
40 (kO)
Figure 3. Penetration of the motor-generator ripple as a function of the d.c. magnetic field H CRYOGENICS
• OCTOBER
1966
Finally, the shielding test was made under real conditions (with the 1 MW motor-generator). Figure 3 shows that the shielding (integrated over all frequencies present) is quite effective up to 12 kG, where the S-N transition occurs; this is in reasonable agreement with the properties of a lead-bismuth alloy reported elsewhere. 4 This result makes it possible to use such a shield for magnetic cooling, for example, and for other kinds of cryogenic experiments, in which H < 12 k G is applied. However, it should be possible to use the same method with other superconducting alloys like niobium-tin
and niobium-zirconium, thus extending the shield's working range to much higher fields. The authors are indebted to Dr. M. Kol~i6 and Mr. F. Soukup for discussion and assistance in the experiments. REFERENCES 1. High Magnetic Fields; Proc. Int. Conf. on High Magnetic Fields, M.LT., 1961 (Wiley/M.LT. Press, 1962) 2. KURTI,N. Private communication 3. Sourva, F. To be published 4. LITOMiSK~',M., and SmOVATKA,J. To be published
LETTERS TO THE EDITORS Low Temperature Thermoelectric Power of Gold-Iron versus Silver Normal Thermocouples I N an earlier letter 1 we have shown that thermocouple calibrations which use certain commercially calibrated germanium resistors as temperature standards can yield fictitious anomalies. In particular, we felt that anomalies in the thermoelectric power of gold-0.02 per cent (at.) iron between 5 and 13° K, which were found not only by Rosenbaum, Oder, and Goldner (ROG) 2 but also in our work, were probably fictitious. This conclusion was not, however, supported by -161
f -12
.--8..L. m _ ~
•
/ /
-10
1-8 -6 -4 -2 0
__
0
I
J
[
1
I--
r
I
I
:
I
2
/~
6
8
10
12
1/,
16
18
20
T~ (°K ) Figure 1. Thermoelectric power of gold-O.O2 per cent (at.) iron alloy. The solid line represents early work in this laboratory, and has been arbitrarily smoothed between 5 and 13° K. The dashed line is taken from the work of MacDonald, Pearson, and Templeton. 4 The solid circles represent the present measurements, where the temperature standard is a germanium resistor calibrated at N.B.S. according to the 1965 Provisional Scale CRYOGENICS"
OCTOBER
1966
direct experimental evidence, for at that time a reliable temperature reference for temperatures between 5 and 13° K was not available in our laboratory. The facts that both ROG and ourselves used gold-0.02 per cent (at.) iron wire drawn from the same bar (Sigmund Cohn Corporation, Mount Vernon, N.Y., Bar No. 1) and that, to the best of our knowledge, no other thermoelectric power measurements have been reported for this bar made it desirable to determine the thermoelectric power Of this material with the use of a reliable temperature standard. We have recently obtained a germanium resistor (manufactured by CryoCal Inc., Riviera Beach, Fla.) which has been calibrated at the National Bureau of Standards according to the 1965 Provisional Scale3 from 2-3 to 20.0 ° K. Using this resistor as a temperature standard, we have recalibrated a gold-0.02 per cent (at.) iron versus silver normal (silver-0.37 per cent (at.) gold) thermocouple from about 3 to 20 ° K. The experimental details are essentially the same as those described earlier; one thermocouple junction was kept at a known temperature of about 4.2 ° K while the temperature of the other junction was varied through the region of interest. The resulting voltage-temperature data were fitted by a least-squares method to a polynomial of the form, V = ~ AnT ~ ~
0
and the computed values of An were used to calculate dV/dT. The thermoelectric power S was then obtained from the relationship S = SAg
dV dT 301
E v ER VO N E who employs low temperatures must encounter sonic vibrations in vessels containing liquid helium, which leads to its violent evaporation. This unpleasant phenomenon arises when tubes (for example, filling siphons) are introduced into the liquid helium and are closed at one end. In studying the sonic vibrations in siphons (with the help of a microphone) we found a simple means of preventing it-introducing into the lower end of the siphon a small t R e c e i v e d b y PTI~ E d i t o r , 11 J u n e 1964: l~ksperimenta N o . 4, p. 251 (1965).
Pribory i Tekhnika
(3-5 cm) length of woollen thread. This simple arrangement makes it even possible to leave the siphon permanently in the container without introducing a noticeable increase in evaporation. Apparently the thread acts as a damper, preventing self-oscillation of the column of liquid inside the tube. This note has been especially translated for CRYO(3ENICSand is included by permission of the Editors of Pribory i Tekhnika Eksperimenta.We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover-to-cover translation of PTI~ by arrangement with the Russian publisher.
SUPPRESSION OF MAGNETIC FIELD FLUCTUATIONS IN CRYOGENIC EXPERIMENTS J. VOTRUBA~- and M. SOTT Nuclear Research Institute, Academy of Sciences, [¢e~, Czechoslovakia Received 1 July 1966
i N some kinds of low temperature experiments a very high stability is required of an applied magnetic field (especially in magnetic cooling and nuclear orientation). However, it is extremely difficult to stabilize the field of large solenoids (copper coils with very low inductance and impedance) fed by motor-generators or rectifiers. 1 One method for ripple-suppression is based on the application of large condenser batteries and chokes, but this rather bulky and expensive equipment has poor efficiency at low ripple frequencies. Another possibility is to use tubes or ringlets made of a highly conducting material; these compensate the field fluctuations inside by induced eddy currents. A high purity copper ,shield of 2 mm thickness reduces a ripple field of 100 c/s by a factor of tAt present: Physical Prague, Czechoslovakia. CRYOGENICS
Institute,
• OCTOBER
Academy
of
about 20 at liquid helium temperature. 2 Further improvements on this result would require considerable increase of the wall thickness, 3 especially for lower frequencies. This reduces the experimental space available for the sample inside the cryostat. In this note a new modification of such a shield is described. It has a much higher efficiency at low frequencies than does a high purity copper shield of the same dimensions.
Principle of operation E d d y currents induced in a metallic tube~ tend to cancel the changes of magnetic flux inside--their time dependence is given by the known exponential relation It = I o e x p [ - ( R / L ) t ] . . . (1)
Sciences,
+ We assume a cylindrical tube with the axis parallel to H; 1966
299
If effective shielding is required at the ripple frequency f, then the characteristic time of the shield
300
. . . (2)
= L/R
should not be shorter than T = f-1. The inductance L of the tube is mainly determined by the geometry and is roughly independent of the material used. Thus to fulfil the condition
L/R > f-1
...
200
100
(3)
at low frequencies, the resistance R must be decreased. In our case this is achieved by means of a tube made
t
t
/; / /
100
50
se
/,.2" K
3I
0
o
,
16o f
Figure
1.
2. Pb-Bi tube 4. Cu strip
Experimental arrangement diagram, Dewar omitted)
(schematic
from superconducting material with a narrow copper strip sandwiched in it (Figure 1). When this tube is immersed in liquid helium, the resistance R for eddy currents (induced in a plane perpendicular to the axis) is given entirely by the width of the copper strip. Obviously, the applicability of this type of shield is limited by the critical field of the superconductor used.
Experimental arrangement and results We have investigated the shielding properties of lead-bismuth (60--40) tube with a copper strip 1 mm wide. The whole tube (diameter 33 ram, wall 1.5 ram, 220 mm long) consisted of five pieces (each about 45 mm long) prepared by casting the lead-bismuth alloy in a cylindrical mould, into which the tinned copper strip was pushed beforehand. The shielding tube was mounted inside the helium Dewar and placed into the hole of the water-cooled solenoid (0.2 ~), which gives 60 kG at 1 MW (2 000 A, 500 V). In the ripple produced by the motor-generator 300
0
260
'=
(c/s)
Figure 2. Ripple penetration at 77° K and 4.2° K and shielding effect K at different frequencies: U, detector coil signal; K = U(77)/U(4.2); f, frequency of the RC generator. (The d.c. field is zero in this case, amplitude of a.c. constant at all frequencies)
!]2
1. Solenoid 3. Checking coil
'
different frequencies occur with an amplitude of the order of 0.1 per cent of the d.c. voltage--the lowest frequency observed was 16 c/s. A small coil was placed inside the tube (in the middle of solenoid, Figure 1) to measure the penetration of the ripple. Figure 2 shows the frequency dependence of the voltage induced in this coil when the solenoid was fed by an auxiliary R C generator. Data were taken at 77 ° K where the shield is almost ineffective (as seen by a rapid increase of the induced voltage with frequency) and at 4.2 ° K. The ratio K = U(77)/U(4) thus characterizes the shielding efficiency at a given frequency f.
50
/
/
25
/ --'~J
10
t
2=0 H~
~
3'0
'
40 (kO)
Figure 3. Penetration of the motor-generator ripple as a function of the d.c. magnetic field H CRYOGENICS
• OCTOBER
1966
Finally, the shielding test was made under real conditions (with the 1 MW motor-generator). Figure 3 shows that the shielding (integrated over all frequencies present) is quite effective up to 12 kG, where the S-N transition occurs; this is in reasonable agreement with the properties of a lead-bismuth alloy reported elsewhere. 4 This result makes it possible to use such a shield for magnetic cooling, for example, and for other kinds of cryogenic experiments, in which H < 12 k G is applied. However, it should be possible to use the same method with other superconducting alloys like niobium-tin
and niobium-zirconium, thus extending the shield's working range to much higher fields. The authors are indebted to Dr. M. Kol~i6 and Mr. F. Soukup for discussion and assistance in the experiments. REFERENCES 1. High Magnetic Fields; Proc. Int. Conf. on High Magnetic Fields, M.LT., 1961 (Wiley/M.LT. Press, 1962) 2. KURTI,N. Private communication 3. Sourva, F. To be published 4. LITOMiSK~',M., and SmOVATKA,J. To be published
LETTERS TO THE EDITORS Low Temperature Thermoelectric Power of Gold-Iron versus Silver Normal Thermocouples I N an earlier letter 1 we have shown that thermocouple calibrations which use certain commercially calibrated germanium resistors as temperature standards can yield fictitious anomalies. In particular, we felt that anomalies in the thermoelectric power of gold-0.02 per cent (at.) iron between 5 and 13° K, which were found not only by Rosenbaum, Oder, and Goldner (ROG) 2 but also in our work, were probably fictitious. This conclusion was not, however, supported by -161
f -12
.--8..L. m _ ~
•
/ /
-10
1-8 -6 -4 -2 0
__
0
I
J
[
1
I--
r
I
I
:
I
2
/~
6
8
10
12
1/,
16
18
20
T~ (°K ) Figure 1. Thermoelectric power of gold-O.O2 per cent (at.) iron alloy. The solid line represents early work in this laboratory, and has been arbitrarily smoothed between 5 and 13° K. The dashed line is taken from the work of MacDonald, Pearson, and Templeton. 4 The solid circles represent the present measurements, where the temperature standard is a germanium resistor calibrated at N.B.S. according to the 1965 Provisional Scale CRYOGENICS"
OCTOBER
1966
direct experimental evidence, for at that time a reliable temperature reference for temperatures between 5 and 13° K was not available in our laboratory. The facts that both ROG and ourselves used gold-0.02 per cent (at.) iron wire drawn from the same bar (Sigmund Cohn Corporation, Mount Vernon, N.Y., Bar No. 1) and that, to the best of our knowledge, no other thermoelectric power measurements have been reported for this bar made it desirable to determine the thermoelectric power Of this material with the use of a reliable temperature standard. We have recently obtained a germanium resistor (manufactured by CryoCal Inc., Riviera Beach, Fla.) which has been calibrated at the National Bureau of Standards according to the 1965 Provisional Scale3 from 2-3 to 20.0 ° K. Using this resistor as a temperature standard, we have recalibrated a gold-0.02 per cent (at.) iron versus silver normal (silver-0.37 per cent (at.) gold) thermocouple from about 3 to 20 ° K. The experimental details are essentially the same as those described earlier; one thermocouple junction was kept at a known temperature of about 4.2 ° K while the temperature of the other junction was varied through the region of interest. The resulting voltage-temperature data were fitted by a least-squares method to a polynomial of the form, V = ~ AnT ~ ~
0
and the computed values of An were used to calculate dV/dT. The thermoelectric power S was then obtained from the relationship S = SAg
dV dT 301