A REFRIGERATED DEWAR FOR THE JOSEPHSON ARRAY VOLTAGE CALIBRATION SYSTEM

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• Publication Date: April 1, 1991
• Content Type: MAGAZINE

Approved For Release ~A~~'~I~NS ~~s~~>~~F;~A7ND MEASURENfE7VT, ~OL0~6of~~.~.
A Refrigerated Dewar for the Josephson Array
Voltage Calibration System
Abstract-A refrigerated dewar has been used successfully with the
Josephson array voltage calibration system. It has been used to re-
liquify helium with no degradation of the calibration system's perfor-
mance. The independence of the array voltage from temperature has
been confirmed to three parts in 10g per kelvin over the range 1.6 K
s T < 4.6 K. No significant change in the array operation on a quan-
tized state near 1.018 V was detected when its surrounding bath was
pumped superfluid, nor when it was cooled by helium vapor at 4.6 K.
INTRODUCTION
SERIES arrays of Josephson junctions have been developed
which are capable of producing quantized voltage levels over
a wide voltage range [1]. These arrays have been used in a volt-
age calibration system since February 10, 1987, to maintain the
U.S. Legal Volt [2] at the National Institute of Standards and
Technology (NIST, formerly NBS). A similar system within the
Primary Standards Laboratory (which is operated for the De-
partment of Energy, Albuquerque Operations Office by Sandia
National Laboratories (SNL) has been in operation since July
1989. Complete automation of these and other such systems
will require the development of an automated cryogenic sub-
system. Here we present details on the design and performance
of a new refrigerated dewar to maintain the Josephson junction
array near 4K with no liquid helium loss and no routine operator
intervention. No degradation in the stability of the
array-based voltage calibration system was noticed while it was
used with the refrigerated dewar, even when re-liquifaction of
helium was underway. Careful measurements of the quantized
array voltage near 1.018 V were made over a wide temperature
range to confirm the expected independence of the array voltage
from temperature, and in so doing to insure that the refrigerator
temperature would not systematically affect the array voltage
standard.
SYSTEM DESIGN
The refrigerated dewar design is displayed in Fig. 1. It was
manufactured for SNL by the Research and Manufacturing Co.,
Inc. (RMC) of Tucson, AZ. The dewar's helium space consists
of a 3.81-cm diameter neck, a 25.4-cm diameter belly section
with a 15-1 capacity, and a 4.45-cm diameter tail section, all
made of stainless steel. The liquid helium region is surrounded
by two coaxial copper heat shields which attach to the neck at
their appropriate positions. Each shield is wrapped with a few
Manuscript received June 11, 1990.
The author is with the Electrical Standards Division, Sandia National
Laboratories, Albuquerque, NM 87185.
IEEE Log Number 9042441.
Mu-Metal Shields
Ta I I
Vacuum
He Compressor
--3,300~torr Storage
Fig. 1. Refrigerated dewar design. The compressor is located 15 m from
the dewar during operation.
turns of thin metalized plastic. The outer heat shield is cooled
to about 60K by the first stage of aGifford-McMahon cycle
(GM) refrigerator. The second stage of the GM cools the inner
shield to about 17 K. AJoule-Thomson cycle (JT) loop then
provides in excess of 1.5 W of useable refrigeration power to
the liquid helium bath through an annular heat exchanger lo-
cated at the top of the belly region. The temperature of the liq-
uid helium in the belly can be varied from 3.8 to 5.2 K by
varying the regulated return pressure on the JT line. Thermom-
eters are located on the two heat shields, the JT expansion noz-
zle, and on the tail section of the dewar. Their temperatures are
read out by a controller which may be automated over an IEEE-
488 bus. The flow rate of helium gas in the JT loop is monitored
by a flow meter with a digitizable analog output. Finally, the
compressor's supply tank pressure may be monitored through
the analog output of a pressure transducer. Hence all parameters
critical to the performance of the refrigerated dewar may be
computer monitored. If necessary the JT return line pressure
regulator can be computer actuated, resulting in fully automated
temperature control of the dewar. The thermometer in the tail
section of the dewar is located 8.9 cm above the bottom of the
helium space, which places the thermometer at the same height
as the array during voltage calibrations. Consequently this ther-
mometer will accurately read the array temperature even if a
vertical temperature gradient exists in the column of liquid he-
lium.
A set of coaxial bellows separates the top flange from the
neck of the dewar. This permits the apparatus mounted on the
top flange to be mechanically isolated from the rest of the dewar
which is rigidly mounted to the refrigerator. Two coaxial
mu-metal shields were located in the dewar to protect the array
from stray magnetic fields. One was located next to the outer
0018-9456/91/0400-0326$01.00 ?1991 IEEE
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wall of the vacuum space while the other was located within the
helium space in the tail section of the dewar.
The array used in this work contains 2076 Josephson junc-
tions connected in series [1]. It was manufactured at NIST in
Boulder, CO, on February' 4, 1987 and was numbered 211-03.
The SNL array voltage calibration system is quite similar to the
NIST system [2]. Only the differences between these systems
are mentioned here. The SNL system utilizes a probe which
features cryogenic filtering, and a lower heat loss than conven-
tional probes which also use metal waveguides [3]. The SNL
system operates with the dewar pressure approximately 50 torr
above ambient, while the NIST system operates near ambient
pressure. Here the array temperature in the SNL system was
about 70 mK warmer than the NIST system when they were
both operated side-by-side in Gaithersburg, MD. The SNL sys-
tem uses an automated switch while the NIST system uses a
manual switch to inter-compare the array and unknown volt-
ages. The measurement procedures described in [2] are identi-
cal to those used in this paper, except that these data were taken
manually during development.
The entire SNL array system was transported between Al-
buquerque, NM, Gaithersburg, MD, and Tucson, AZ, during
1989. The SNL system was operated with the refrigerated de-
war only in Tucson. The results of using the SNL system to
calibrate a single DC voltage reference over the last year are
displayed in Fig. 2. Both the SNL system and the do reference
appear to transport nicely, as evidenced by the good repeat-
ability between the different locations. While in Gaithersburg,
the SNL system was compared with the NIST system. Both sys-
tems and the do voltage reference of Fig. 2 were located in the
NIST screen, room, The do reference voltage was measured
throughout a 100-h interval, with the two array systems Caking
turns making the measurements. Only one calibration system
was attached to the voltage reference at a time. Both systems
experienced the same 0.01-?V scatter (1 sigma) during 12 cal-
ibrations of this 1.018-V reference, as displayed in Fig. 3. Each
calibration followed the procedure described in [2]. and con-
sisted of 200 s of data averaging. The SNL system was found
to read 0.026 ?V below the NIST system, and hence a 0.026-
ppm difference has been taken into account for the SNL system
calibrations. The source of this difference has not been deter-
mined.
The refrigerated dewar was used with the SNL system in Tuc-
son to check for any dependence of the quantized array output
voltage on its temperature. The do reference of Fig. 2 was cal-
ibrated five times within a 13-h interval and with the array at
different temperatures on day 318. The do reference of Fig. 2
displayed a drift in its norminal 1.018 V output of -0.005 ?V
per day from day 318 to day 391, during which time it was used
with the refrigerated dewar. The array temperature was set by
varying the JT return pressure. The array was biased with
83 440 600 000 (5) Hz radiation, and the n = 5900 step of the
array was used to create a quantized array voltage of
1.018002194 V, using the 2e/h value of 0.483593420 GHz/?V
(which was the NIST accepted value prior to 1990 ). The results
of these five measurements are shown in Fig. 4. No systematic
variation of the array voltage with temperature was detected.
These measurements were taken as the array temperature was
.
?
I
jl
t
~
1
?
t
?
?
?
0 100 200 300
Days since January 30, 1989
Fig. 2. SNL array system calibrations of the voltage reference versus time.
The data were taken at NIST through day 32, at SNL through day 304, at
RMC through day 318, and at UNM through day 391.
Fig. 3. Comparison of the SNL system (solid dots) to the NIST system
(open dots) which is used to maintain the US legal volt. These data were
taken in collaboration with Richard Steiner of NIST.
(?V)
z.l~
96 120
Time (Hours)
4,2 4.4 4.6
Array Temperature (K)
Fig. 4. Temperature-independence of the array voltage. The calibration
system noise level is indicated.
raised, lowered, and raised again. This indicates that any tem-
perature coefficient of the array voltage must be less than 0.03
?V /K over the range 3.94 K G T < 4.59 K. The 4.35 (1) K
measurements in Fig. 4 were made at 1.0 bar and 1.7 bar, and
no systematic variation of the array voltage with pressure was
detected. Two similar measurements were made at the Univer-
sity of New Mexico (UNM) in Albuquerque two months later.
They were taken at 1.56 and 4.48 K and displayed no system-
atic temperature variation, consistent with the previous mea-
surements. Differences in the helium bath temperatures between
NIST and SNL array calibration sytems cannot account for the
0.026-ppm discrepancy displayed in Fig. 3.
The measurement at 1..56 K was taken with the surrounding
helium bath in is superfluid phase. This dramatic improvement
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in the bath's thermal conductivity did not effect the stability of
the array or the value of its quantized voltage outpuk. The bath
supertluid transition did, however, slighly reduce the noise level
in the array's do electrical characteristics [4].
At 4.6 K the array was operated with no liquid helium in the
dewar. The array was surrounded by helium vapor at 4.6 K
which was cooled by the 4.2 K heat exchanger at the top of the
belly region, This convecting helium vapor provided sufficient
coaling to keep the array stable at a quantized voltage output
near 1,018 V.
This refrigerated dewar is capable of producing over four
liters of liquid helium from room-temperature gas per day. Cal-
ibrations using the array voltage standard were not disrupted by
helium liquifaction. Hence, 4He gas may be recovered from
other cryogenic apparatus (i.e., a cryogenic current comparator
or a quantum Hall effect resistance standard) and re-liquified by
this refrigerated dewar. With all refrigeration to this dewar off,
the steady state liquid helium lass rate was less than 21 per day.
ACKNOWLEDGMENT
The assistances of Dr. Richard Steiner of NIST, of Dr. E. T.
Swartz of RMC Cryosystems, and of the Physics and Astron-
omy Department of UNM are gratefully acknowledged.
REFERENCES
[1] F. L. Lloyd et al., "A Josephson array voltage standard at 10 V,"
IEEE Electron Device Letters, vol. EDL-8, pp. 449-450, Oct.
1987. The design and fabrication of the 2076 junction array used
in this work was reported in R. L. Kautz, C. A. Hamilton, and F.
L. Lloyd, "Series-array Josephson voltage standards," IEEE
Trans. Magn. , vol. MAG-23, pp. 883-890, Mar. 1987.
[2] R. L. Steiner and B. F. Field, "Josephson array calibration sys-
tem: Operational use and verification," IEEE Trans. Instrum.
Meas. , vol. 38, pp. 296-301, Apr. 1989.
[3] R. Duncan, "A new cryoprobe for the Josephson junction array
voltage standard," Bull. Am. Phys. Soc., vol. 34, pp. 1535-1536,
June 1989.
[4] -, "Thermal effects on the Josephson series-array voltage stan-
dard," Physica B, vol. 165, pp. 101-102, Aug. 1990.
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