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Performance of a Twin Cold Finger Gifford-McMahon Cryocooler
PERFORMANCE OF A TWIN COLD FINGER GIFFORD-McMAHON CRYOCOOLER J.M. Pfotenhauer, O.D. Lokken, & P.E. Gifford* Applied Superconductivity Center, University of Wisconsin- Madison 1500 Engineering Dr., Madison, WI 53706 USA *Cryomech, Inc. 113 Falso Dr., Syracuse NY 13211 USA A single stage Gifford McMahon cryocooler with two identical cold fingers electrically isolated from each other has been fabricated to cool a pair of 1500 A, HTS current leads. 60 watts of cooling capacity are provided at 70 K on each of the cold fingers. In that the necessary voltage isolation is provided in a region where heat is transferred by the helium gas internal to the cryocooler, thermal inefficiencies associated with an electrical isolator in the cold bus conduction path are avoided. Initial performance results of the cryocooler are reported in terms of measured heat loads at various operating temperatures.
INTRODUCTION In the development of high temperature superconducting (HTS) current leads used in conjunction with low temperature superconducting magnets, it has been recognized [ 1,2] that a thermal intercept is required above the HTS section of the current lead to ensure a minimum total refrigeration load associated with the operation of the current lead. The thermal intercept is often accomplished through the use of a cryocooler thermally connected to the current lead. In order to avoid a short circuit of the current path at this intercept, it is common to include an electrical isolation piece between the single cold finger of the cryocooler and the thermal intercept point on each of the current leads (see figure 1a). Unfortunately, the electrical insulator is typically also a thermal insulator and significant temperature drops can be realized across this member during operation of the current lead. For example, the thermal load from the upper stage of a 1.5 kA conduction cooled current lead operating between room temperature and 70 K is expected to be ~ 60 watts on each polarity. The temperature drop across a G-10 insulator with nominal dimensions of 0.5 mm thickness and 8 x 10 -3 m 2 cross sectional area is in excess of 10 K even without consideration of contact resistance. A decreased cryocooler efficiency will be associated with this thermal resistance since the cryocooler must operate at a temperature 10 K colder to provide the 70 K intercept temperature at the current lead. The added thermal resistance represents a decrease in efficiency for a Carnot refrigeration cycle from 0.3 to 0.25, or a minimum increase of 22% in the equivalently required room temperature power. It is therefore of significant advantage to eliminate this thermal resistance. The present paper presents a means of providing the necessary electrical isolation between the current leads through the use of a twin-cold-finger Gifford McMahon cryocooler which has been fabricated and run through initial performance tests. In this approach, heat is carried from the cold finger to the warm end of the GM cooler via the internal helium gas, while the electrical isolation is built in to the structure of the cooler itself. A pure metal to metal contact is allowed between each cold finger and their respective current leads - thus eliminating the previous thermal resistance, and the electrical insulation within the body of the cryocooler has no influence on the means of transferring the heat away via the helium gas. The internal structure of the GM cooler is designed to avoid an electrical breakdown in the helium gas. 363
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HTS current lead
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Figure 1. a) conventional configuration for voltage isolation between current lead and cryocooler, b) incorporation of voltage isolation within cryocooler. INITIAL OPERATION OF THE TWIN FINGER GM COOLER A scaled representation of the twin cold finger GM cooler is shown in figure 2. This device, jointly developed by the UW Applied Superconductivity Center and Cryomech has been tested both in a standalone configuration at Cryomech and within a current lead test rig at the University of Wisconsin - Madison. In the current lead test rig configuration, the twin cold finger cooler (model PF01), and an additional standard 60 watt cooler (model AL60) are both powered by the 5 kW model CP640 compressor. Expansion chamber volumes have been designed for a nominal cooling power of 60 watts @ 70 K at each cold finger. The refrigeration capacity performance map for both the stand-alone test and the current lead installation test is shown in figure 3. Note that capacity per cold finger is shown here. Refrigeration capacity values in the stand alone test have been obtained by applying a measured heat load directly to each cold finger and monitoring the resulting temperature. The method for determining the refrigeration capacity in the current lead test rig was somewhat more involved and less direct, but the results are in good agreement with the stand alone values. Determination of cooling power from cooldown process The twin cold finger GM cooler has been installed into a mating twin-finger cold well on the UW-Madison current lead test rig to investigate the ability of changing the cryocooler without breaking the vacuum on the test dewar. During the initial operation of the cryocooler in which the copper bus alone was attached to the cryocooler (no current leads installed), it has been observed that a significant thermal resistance is associated with the specific design of the cold well - cryocooler contact. While the cold well will require re-design, the result of this configuration is that the temperatures measured on the copper cold bus between the cryocooler and the current lead are significantly higher than the cryocooler cold finger. Nevertheless, the rate of temperature drop observed during the cooldown process have been useful for determining the cryocooler power. Total cooldown time with the two copper buses attached (14.6 kg) is 2 hours. The thermal resistance of 0.8 K/W determined to exist between the cryocooler cold finger and the copper bus is significantly higher than the thermal resistance associated with the geometry of the copper bus Rt, bus = L / (k A)
(1)
ICEC16/ICMC Proceedings
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allowing a "lumped capacitance" treatment of the copper bus cooldown. Here L and A are respectively the length in the direction of, and cross sectional area perpendicular to, the heat flow, and k is the thermal conductivity of the copper. From the lumped capacitance model, the heat flow (q, in watts) is simply proportional to the cooldown rate, the mass (M) and specific heat (Cv) of the copper bus according to
dT q=MCv d--T"
(2)
The associated values of temperature for each value of q were obtained from the copper bus temperature data by correcting for the thermal resistance of the bus - cold finger contact. As can be seen in figure 3 the agreement is quite good with the values obtained in the stand-alone test. In addition, when combined with the performance of the AL60 driven simultaneously by the same compressor, the total refrigeration power provided at 70 K is 170 watts; a value which compares well with the 180 watt capacity at 70 K of the single AL200 cold head driven by the same compressor. Before installing the current leads into the test rig, the voltage isolation between the two cold fingers was measured using a standard HIPOT device. With all parts at ambient temperature a voltage isolation of 2.8 kV was measured.
366
ICEC16/ICMC Proceedings
INTEGRATION WITH UPPER STAGE OF CURRENT LEADS The twin cold finger cooler has been used to cool the upper (conventional) stage of the current leads before attaching the lower HTS stages. The goal of this measurement was to characterize the resistance of the leads in their cold operating state and to confirm that sufficient cooling power can be provided by the cryocooler for the upper stage heat load. Voltage taps have been installed which span the distance from the current lead flag to the bottom, or thermal intercept point. A 1000 amp supply was used on the first test of this setup and the resulting voltages across the (+) and (-) leads, as well as the connection bus between them, are shown in figure 4. The data display that the two current leads have significantly different resistances, and that there will be need to monitor the (-) lead closely during subsequent operation. At all values of current, the flag temperature was maintained below 25 ~ demonstrating that sufficient cooling is provided by the cryocooler. A
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Figure 4. Resistance measurements of current leads cooled by conduction to GM cryocooler alone. SUMMARY A single stage, twin cold finger GM cryocooler has been successfully developed for use in cooling the upper stage of HTS current leads. A nominal cooling power of 60 watts is provided at each cold finger at 70 K, and a room temperature voltage isolation of 2.8 kV is provided. The refrigeration performance of the twin cold finger cooler has been mapped both with a direct measurement, and by using the transient cooldown data. Both methods of quantifying the capacity agree with each other. The twin finger cryocooler has been demonstrated to provide sufficient cooling for operation of 1 kA current leads. REFERENCES 1. Wesche, R. and Fuchs, A.M., "Design of Superconducting Current Leads," Cryogenics, 33 (1993), 714-718. 2. Yang, S. and Pfotenhauer, J.M., "Optimization of the Intercept Temperature for High Temperature Superconducting Current Lead," presented at 1995 CEC, Columbus Ohio.
INTRODUCTION In the development of high temperature superconducting (HTS) current leads used in conjunction with low temperature superconducting magnets, it has been recognized [ 1,2] that a thermal intercept is required above the HTS section of the current lead to ensure a minimum total refrigeration load associated with the operation of the current lead. The thermal intercept is often accomplished through the use of a cryocooler thermally connected to the current lead. In order to avoid a short circuit of the current path at this intercept, it is common to include an electrical isolation piece between the single cold finger of the cryocooler and the thermal intercept point on each of the current leads (see figure 1a). Unfortunately, the electrical insulator is typically also a thermal insulator and significant temperature drops can be realized across this member during operation of the current lead. For example, the thermal load from the upper stage of a 1.5 kA conduction cooled current lead operating between room temperature and 70 K is expected to be ~ 60 watts on each polarity. The temperature drop across a G-10 insulator with nominal dimensions of 0.5 mm thickness and 8 x 10 -3 m 2 cross sectional area is in excess of 10 K even without consideration of contact resistance. A decreased cryocooler efficiency will be associated with this thermal resistance since the cryocooler must operate at a temperature 10 K colder to provide the 70 K intercept temperature at the current lead. The added thermal resistance represents a decrease in efficiency for a Carnot refrigeration cycle from 0.3 to 0.25, or a minimum increase of 22% in the equivalently required room temperature power. It is therefore of significant advantage to eliminate this thermal resistance. The present paper presents a means of providing the necessary electrical isolation between the current leads through the use of a twin-cold-finger Gifford McMahon cryocooler which has been fabricated and run through initial performance tests. In this approach, heat is carried from the cold finger to the warm end of the GM cooler via the internal helium gas, while the electrical isolation is built in to the structure of the cooler itself. A pure metal to metal contact is allowed between each cold finger and their respective current leads - thus eliminating the previous thermal resistance, and the electrical insulation within the body of the cryocooler has no influence on the means of transferring the heat away via the helium gas. The internal structure of the GM cooler is designed to avoid an electrical breakdown in the helium gas. 363
364
ICEC 16/ICMC Proceedings I
HTS current lead
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I
cryocooler I cold finger I
thermal, intercept
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Ill
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Figure 1. a) conventional configuration for voltage isolation between current lead and cryocooler, b) incorporation of voltage isolation within cryocooler. INITIAL OPERATION OF THE TWIN FINGER GM COOLER A scaled representation of the twin cold finger GM cooler is shown in figure 2. This device, jointly developed by the UW Applied Superconductivity Center and Cryomech has been tested both in a standalone configuration at Cryomech and within a current lead test rig at the University of Wisconsin - Madison. In the current lead test rig configuration, the twin cold finger cooler (model PF01), and an additional standard 60 watt cooler (model AL60) are both powered by the 5 kW model CP640 compressor. Expansion chamber volumes have been designed for a nominal cooling power of 60 watts @ 70 K at each cold finger. The refrigeration capacity performance map for both the stand-alone test and the current lead installation test is shown in figure 3. Note that capacity per cold finger is shown here. Refrigeration capacity values in the stand alone test have been obtained by applying a measured heat load directly to each cold finger and monitoring the resulting temperature. The method for determining the refrigeration capacity in the current lead test rig was somewhat more involved and less direct, but the results are in good agreement with the stand alone values. Determination of cooling power from cooldown process The twin cold finger GM cooler has been installed into a mating twin-finger cold well on the UW-Madison current lead test rig to investigate the ability of changing the cryocooler without breaking the vacuum on the test dewar. During the initial operation of the cryocooler in which the copper bus alone was attached to the cryocooler (no current leads installed), it has been observed that a significant thermal resistance is associated with the specific design of the cold well - cryocooler contact. While the cold well will require re-design, the result of this configuration is that the temperatures measured on the copper cold bus between the cryocooler and the current lead are significantly higher than the cryocooler cold finger. Nevertheless, the rate of temperature drop observed during the cooldown process have been useful for determining the cryocooler power. Total cooldown time with the two copper buses attached (14.6 kg) is 2 hours. The thermal resistance of 0.8 K/W determined to exist between the cryocooler cold finger and the copper bus is significantly higher than the thermal resistance associated with the geometry of the copper bus Rt, bus = L / (k A)
(1)
ICEC16/ICMC Proceedings
200
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Figure 2. Twin Cold Finger GM Cryocooler
Figure 3. Refrigeration performance per cold finger.
allowing a "lumped capacitance" treatment of the copper bus cooldown. Here L and A are respectively the length in the direction of, and cross sectional area perpendicular to, the heat flow, and k is the thermal conductivity of the copper. From the lumped capacitance model, the heat flow (q, in watts) is simply proportional to the cooldown rate, the mass (M) and specific heat (Cv) of the copper bus according to
dT q=MCv d--T"
(2)
The associated values of temperature for each value of q were obtained from the copper bus temperature data by correcting for the thermal resistance of the bus - cold finger contact. As can be seen in figure 3 the agreement is quite good with the values obtained in the stand-alone test. In addition, when combined with the performance of the AL60 driven simultaneously by the same compressor, the total refrigeration power provided at 70 K is 170 watts; a value which compares well with the 180 watt capacity at 70 K of the single AL200 cold head driven by the same compressor. Before installing the current leads into the test rig, the voltage isolation between the two cold fingers was measured using a standard HIPOT device. With all parts at ambient temperature a voltage isolation of 2.8 kV was measured.
366
ICEC16/ICMC Proceedings
INTEGRATION WITH UPPER STAGE OF CURRENT LEADS The twin cold finger cooler has been used to cool the upper (conventional) stage of the current leads before attaching the lower HTS stages. The goal of this measurement was to characterize the resistance of the leads in their cold operating state and to confirm that sufficient cooling power can be provided by the cryocooler for the upper stage heat load. Voltage taps have been installed which span the distance from the current lead flag to the bottom, or thermal intercept point. A 1000 amp supply was used on the first test of this setup and the resulting voltages across the (+) and (-) leads, as well as the connection bus between them, are shown in figure 4. The data display that the two current leads have significantly different resistances, and that there will be need to monitor the (-) lead closely during subsequent operation. At all values of current, the flag temperature was maintained below 25 ~ demonstrating that sufficient cooling is provided by the cryocooler. A
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Figure 4. Resistance measurements of current leads cooled by conduction to GM cryocooler alone. SUMMARY A single stage, twin cold finger GM cryocooler has been successfully developed for use in cooling the upper stage of HTS current leads. A nominal cooling power of 60 watts is provided at each cold finger at 70 K, and a room temperature voltage isolation of 2.8 kV is provided. The refrigeration performance of the twin cold finger cooler has been mapped both with a direct measurement, and by using the transient cooldown data. Both methods of quantifying the capacity agree with each other. The twin finger cryocooler has been demonstrated to provide sufficient cooling for operation of 1 kA current leads. REFERENCES 1. Wesche, R. and Fuchs, A.M., "Design of Superconducting Current Leads," Cryogenics, 33 (1993), 714-718. 2. Yang, S. and Pfotenhauer, J.M., "Optimization of the Intercept Temperature for High Temperature Superconducting Current Lead," presented at 1995 CEC, Columbus Ohio.