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Current leads for HERA
Nuclear Instruments and Methods in Physics Research A276 (1989) 53-57 North-Holland, Amsterdam
53
CURRENT LEADS FOR HERA I. BEN-ZVI, B.V. ELKONIN and J .S . SOKOLOWSKI Department of Nuclear Physics, The Weizmann Institute of Science, Rehooot 76100,
Israel
D. SELLMANN Deutsches Elektronen Synchrotron, DES Y, Notkestrasse 85, 2000 Hamburg 52, FRG
Received 5 October 1988 Three prototypes of current leads (6500 A, 10 X 100 A and 4 X 100 A) for the HERA accelerator were designed, built and tested . The measured total cooling load of these current leads in the refrigerator mode of operation was 3 .2, 3 .9 and 2.4 W/kA, respectively . Subsequently, these current leads were produced in quantity by a commercial outfit and they are now in the process of assembly within the HERA cryogenic systems in DESY . The essential technical details and experimental results are described. 1. Introduction We describe here three types of current leads developed for the HERA (hadron electron ring accelerator) superconducting magnets presently under assembly in Hamburg. The main objective in the design of current leads is to minimize the heat input to the refrigeration system . The total refrigeration cooling load [1] is defined by : QI
- Qc +m hef+
where Q, is the total cooling load of the current lead, Q, is the heat flux from the cold end of the current lead into the coolant, th,,, is the mass flow of the cooling gas up the current lead (he = heat exchanger) and f is a factor characterizing the refrigerator in a refrigerator/ liquifier mixed duty operation. Another design objective is the stability of the current lead against thermal runaways . This is important in particular when interruptions in the cooling gas flow occur. The stability in our case is achieved either by using a copper conductor with the lowest thermal conductivity compatible with the design (e.g. phosphorus deoxidized copper in the case of the 10 x 100 A and the 4 x 100 A current leads), or by increasing the current lead's thermal mass (the 6500 A current lead). The current leads would be located in the accelerator beam tunnel and they had to be fabricated of high-radiation resistant materials. The prototypes of the three current leads were constructed at the Weizmann Institute of Science workshops and extensively tested . The series production (total of 72 units) by a commercial manufacturer (Ricor Ltd., Israel) was under the supervision of the authors. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
A number of 6500 A and 4 x 100 A current leads have been in operation for extended periods of time in the magnet test facility of HERA and their performance has been flawless . 2. Design and construction 2.1 . The 6500 A current lead
The 6500 A current lead belongs to the class of current leads in which the cooling fins serve for heat exchange and as thermal mass . This current lead consists of an electrical conductor and a stainless steel tube enclosing it . The electrical conductor is a rod made of electrolytic tough pitch copper with a measured RRR of 77 . A spiral-shaped groove is cut along the conductor (see fig. 1) . This groove provides the heat exchange surface between the copper and the cooling gas flowing through. This cooling channel is 2 mm wide and 10 .5 mm deep with a pitch of 5 mm. The bulk cross section of the spiral rod is 170 mmz. We are using two Kaptoncoated copper wires for measuring the voltage drop across the lead. The wire reaching the bottom part of the current lead is wound along the inner surface of the spiral conductor and is protected by a thin (0 .2 mm) copper ribbon from possible damage by the high velocity (up to 100 m/s) flow of the helium gas. The cooling gas leaves the current lead through a ceramic high voltage insulator. The high voltage capability is necessary for handling magnet quenches . The first pair of the current lead prototypes with shorter conductor length (420 mm) had its spiral turned
1. Ben-Zvi et al. /Current leadsfor HERA
54
current lead through a standard NW16 flange. The outer diameter of the heat exchanger is 67 mm and its length is 630 mm .
2.3. The 4 x 100 A current lead The 4 X 100 A current lead consists of a stainless steel tube enclosure around four electrical conductors made of the same type of copper tube as the one used for the 10 x 100 A current leads. The conductors are supported by insulating G10 spacers . Segmented openings in the spacers are staggered by 180 ° in order to alternate the flow direction of the helium gas . The G10 spacers for both, the 10 x 100 A and the 4 x 100 A current leads, were cut to shape with laser beam . A commercially available 4 X 150 A ceramicinsulated feedthrough is used for the electrical current . The outer diameter of the heat exchanger is 29 mm and its length is 760 mm . A photograph of the 4 x 100 A current lead is shown in fig . 3 . All the welds and the demountable joints (indium seal or conflat flange connections) are vacuum tight (the leak rate is less than 10 -9 std . cm3 helium per second) and they have been pressure tested up to 26 bar. The
Fig. 1 . The 6500 A current lead spiral conductor.
on a lathe . The present current leads with their conductor length of 760 mm were manufactured on a milling machine using a rotating metal slitting saw .
2.2 . The 10 x 100 A current lead The 10 x 100 A current lead consists of two coaxial stainless steel tubes enclosing 10 conductors made of phosphorus deoxidized copper tubing . We have used a 1/4 in . diameter commercial plumbing tubing with a measured RRR of 7 .7. We prefer the use of tubes because they have more heat exchange surface area for a given conductor cross section than an equivalent rod . Fig . 2 shows a photograph of the 10 x 100 A current lead before final assembly . The conductors are being held together and properly spaced by flat, ring-shaped, insulating G10 spacers with a rectangular gas flow slot in each spacer . The slot orientation alternates by 180 ° in order to change the direction of flow of the coolant (helium gas) . Commercial 150 A ceramic-insulated copper feedthroughs are used as electric current inlets . Ten pairs of thin copper wires with Kapton insulation are provided for the measurement of the voltage drop across each current lead . The helium gas exits from the
Fig. 2 . The 10 X 100 A current lead with its enclosure removed .
I. Ben-Zvi et al. / Current leads for HERA
55
tion and enclosed in a liquid nitrogen shield . The cold helium, supplied by the refrigerator as a mixture of liquid and gas, is separated by means of a phase separator . Liquid helium from the separator is fed into the measuring vessel, where a superconducting helium level detector determines the level of the liquid helium, and stabilizes it by means of a control valve in the bypass line . The pressure in the measuring vessel is controlled by the refrigerator . The cold end heat load of the current leads is determined by the total gas mass flow necessary to keep the liquid helium level constant. The tests of the current leads were performed at several currents around the design current of 6500 A . The state of equilibrium was defined by a constant voltage drop (within 1% change in 30 min) across the current leads . The measured total refrigeration cooling load per current was Q,/I = (3 .2 t 0 .3) mW/A at 6500 A and the cooling gas mass flow rh he = 340 mg/s (for the HERA refrigerator factor f = 34 J/g). The voltage drop was 29 mV .
3.2. The 10x 100 A current lead The tests of the 10 x 100 A current leads were conducted at the Weizmann Institute . An IBM PC-AT computer was programmed to maintain certain variables at specified levels, to change parameters, to display and to store data . Fig . 5 shows the test apparatus . The vertical cryostat consists of an outer vacuum vessel, nitrogen bath with a
Fig. 3 . The 4 x 100 A current lead with its heat exchanger exposed . insulation of each conductor to ground (for the 10 x 100 A and for the 4 x 100 A current leads) was tested at 600 V in helium atmosphere and no spark or leakage current was noted . This test voltage is by a factor of three higher than the peak voltage to be expected during a quench.
cold He from refrigerator cold He to refrigerator
3. Test procedure and experimental results
3.1 . The 6500 A current lead The test of the 6500 A current leads has been executed at DESY . The test apparatus, shown in fig. 4, was connected to a 900 W helium refrigerator. All components of the cryostat containing liquid or cold helium gas are covered with a multilayer superinsula-
-
liquid Helium
Fig . 4 . The test apparatus for 6500 A current leads .
56
I. Ben-Zvi et al. / Current leadsfor HERA
duces the heat flow into the current lead and protects the current lead from a variable thermal load due to the changing liquid helium level in the cryostat . The two flows are regulated by Matheson mass flow controllers . The value of the flow regulation point may be set by the computer . The main function of the bypass flow is to change the rate at which liquid helium enters the brass cup, enabling the level control . The flow controller set point of the bypass flow is adjusted by the computer according to the error signal from the liquid helium detector in the cup . When the current lead is at equilibrium, the bypass flow becomes constant . Then we have the relation Q~=h ( mhe+lnbp)1
Fig . 5 . The test apparatus for 10 X 100 A and 4 X 100 A current leads.
radiation shield and a helium tank . The radiation shield is made of aluminum, the other parts of the cryostat are fabricated of stainless steel. The flange of the current lead is connected to the top plate of the cryostat . The bottom part of the current lead is enclosed in a brass cup with a long 1/4 in . diameter copper tube extending to the bottom of the helium tank . Pressurizing of the tank causes the liquid to reach the cup through this tube . The pressure in the cryostat is controlled by two valves ; a pressure building valve and a pressure relief valve, which in turn are controlled by the computer . The pressure of 2 .5 psi was stable within ± 0 .02 psi . The stainless steel enclosure of the current lead heat exchanger is covered with a thermal insulation . The level of the liquid in the helium tank is measured using a set of carbon composition resistors . The level of the liquid nitrogen in the liquid nitrogen bath is maintained by a liquid nitrogen controller . A platinum resistance thermometer is located on the outer surface of the current lead heat exchanger, 170 mm below the current lead flange . The brass cup contains a helium liquid detector (an Allen Bradley carbon composition resistor) which is placed 20 mm below the entrance to the heat exchanger of the current lead . The liquid is maintained at the correct level by computer control . 10 mm below the entrance to the heat exchanger the helium vapour reaching the top part of the brass cup is divided into two separate flow paths : the ?h,, (he = heat exchanger) flows through the heat exchanger, the other one, rin bp (bp = bypass), through a copper tube soldered to the thermal radiation shield and exits at the top plate of the cryostat . The helium-cooled radiation shield re-
where h is the latent heat of the liquid helium at the operating pressure. The mass flows are measured and they provide the heat leak at the cold end of the current lead . An equivalent method of measurement is to keep rit bp constant and place mhe under computer control . At equilibrium the above equation still holds . The tests of the current leads were performed at a few current levels below, at and above the designed value . The switching from one current value to another was done by computer control after the system has reached equilibrium . The state of equilibrium was defined by the change rate of the m he (for rim by constant) through the current lead, of the voltage drop across the current lead and of the temperature measured by the platinum resistance thermometer . Each of these variables was allowed a maximum change of 2% during 45 min over two successive periods of 45 min each. The results of the 10 X 100 A current lead tests are shown in fig . 6 . The optimum current is 100 A, the optimum rim he is 66 .1 mg/s, and Q,/I = (3 .9 ± 0 .3) mW/A (for the HERA refrigerator factor f = 34 J/g) . This value of Q,/I is approximately (within 5%) equal to the mean measured value for the 12 X 100 A current leads of the Isabelle project [2] . The voltage drop at the optimum current is 24 .7 mV . 3.3. The 4X 100 A current lead
Tests of the 4 X 100 A current lead were performed using the same measuring system as for the 10 x 100 A current lead . The optimum rh he was 9 mg/s and Qt/I = (2 .4 ± 0 .5) mW/A. The voltage drop at the optimum current was 37 .1 mV . The measurement of the 4 X 100 A current leads was more difficult than the 10 X 100 A leads because the thermal load which is being measured is smaller . Since it was difficult to measure the optimum current we have assigned a nominal value of 4 X 100 A based on our experience that this lead was very stable and has been
I Ben-Zoi et al. / Current leads for HERA
57
4. Conclusions
01 68
70
72
74
, 76 .
mhe
01 72
74
76
78
, 80 . m he [m 9
mg
L secI
wi A5
(F)
4 82
84
86
88
90 "'he 80 [~l
100
120
Three types of current leads were developed for the HERA accelerator . A narrow and deep continuous spiral groove was cut along a relatively long 6500 A current lead's conductor fabricated from such ductile material as electrolytic tough pitch copper. The conductor was manufactured on a milling machine using a rotating metal slitting saw . A 1/4 in. diameter commercial plumbing phosphorus deoxidized copper tubing was used for electrodes of the 10 X 100 A and 4 X 100 A current leads . We prefer the use of tubes because they have more heat exchange surface for a given conductor cross section than an equivalent rod . The stability and a good figure of merit Q,/I of these current leads prove the advantage of phosphorus deoxidized copper as a conductor material for small current leads . The tests and next operation for extended period of time showed good heat performances and high stability of the current leads .
I40
Fig. 6 . The results of the 10X100 A current lead test : total cooling power Q, vs mass flow rate m he for five values of the current (curves A-E) and Q, /I vs current I at the optimal value of rim he (curve F). operated for periods of a few hours at up to 4 X 250 A without showing any instability . The longer conductor length of this lead as compared with the 10 X 100 A current lead (for the same type of copper tube for the individual conductors) as well as the better value of Qt/I at 100 A suggest that its "optimal current" should be somewhat smaller than 4 X 100 A, but in view of the results an optimum cannot be defined. The stability and good figure of merit Q,/I of the current lead show the advantage of phosphorus deoxidized copper as the conductor material for small current leads . We feel that current leads constructed from this type of material may be operated at higher currents than the theoretical optimum. In our case we have used this possibility to make a longer conductor at the given conductor cross-section and current, resulting in an improved heat exchanger.
Acknowledgements We are grateful for the support and encouragement of Prof. Y . Eisenberg who suggested the project . The mechanical workshops at the Weizmann Institute were extremely helpful in machining the prototypes. We thank N . Stem for his computer program which helped to calculate the 6500 A current lead, J . Mersel for writing and implementing the program for current lead tests and M . Sidi for the design and construction of the specialized electronic equipment for these tests. This work was supported in part by a grant from the Israel Ministry of Science and Development.
References [1] D. Guesewell and E .U. Haebel, Proc . 3rd Int. Cryogenic Conf . (1970) p. 187. [2] W.J . Schneider and A.P. Werner, Technical Note no. 272, Brookhaven National Laboratory (1981) .
53
CURRENT LEADS FOR HERA I. BEN-ZVI, B.V. ELKONIN and J .S . SOKOLOWSKI Department of Nuclear Physics, The Weizmann Institute of Science, Rehooot 76100,
Israel
D. SELLMANN Deutsches Elektronen Synchrotron, DES Y, Notkestrasse 85, 2000 Hamburg 52, FRG
Received 5 October 1988 Three prototypes of current leads (6500 A, 10 X 100 A and 4 X 100 A) for the HERA accelerator were designed, built and tested . The measured total cooling load of these current leads in the refrigerator mode of operation was 3 .2, 3 .9 and 2.4 W/kA, respectively . Subsequently, these current leads were produced in quantity by a commercial outfit and they are now in the process of assembly within the HERA cryogenic systems in DESY . The essential technical details and experimental results are described. 1. Introduction We describe here three types of current leads developed for the HERA (hadron electron ring accelerator) superconducting magnets presently under assembly in Hamburg. The main objective in the design of current leads is to minimize the heat input to the refrigeration system . The total refrigeration cooling load [1] is defined by : QI
- Qc +m hef+
where Q, is the total cooling load of the current lead, Q, is the heat flux from the cold end of the current lead into the coolant, th,,, is the mass flow of the cooling gas up the current lead (he = heat exchanger) and f is a factor characterizing the refrigerator in a refrigerator/ liquifier mixed duty operation. Another design objective is the stability of the current lead against thermal runaways . This is important in particular when interruptions in the cooling gas flow occur. The stability in our case is achieved either by using a copper conductor with the lowest thermal conductivity compatible with the design (e.g. phosphorus deoxidized copper in the case of the 10 x 100 A and the 4 x 100 A current leads), or by increasing the current lead's thermal mass (the 6500 A current lead). The current leads would be located in the accelerator beam tunnel and they had to be fabricated of high-radiation resistant materials. The prototypes of the three current leads were constructed at the Weizmann Institute of Science workshops and extensively tested . The series production (total of 72 units) by a commercial manufacturer (Ricor Ltd., Israel) was under the supervision of the authors. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
A number of 6500 A and 4 x 100 A current leads have been in operation for extended periods of time in the magnet test facility of HERA and their performance has been flawless . 2. Design and construction 2.1 . The 6500 A current lead
The 6500 A current lead belongs to the class of current leads in which the cooling fins serve for heat exchange and as thermal mass . This current lead consists of an electrical conductor and a stainless steel tube enclosing it . The electrical conductor is a rod made of electrolytic tough pitch copper with a measured RRR of 77 . A spiral-shaped groove is cut along the conductor (see fig. 1) . This groove provides the heat exchange surface between the copper and the cooling gas flowing through. This cooling channel is 2 mm wide and 10 .5 mm deep with a pitch of 5 mm. The bulk cross section of the spiral rod is 170 mmz. We are using two Kaptoncoated copper wires for measuring the voltage drop across the lead. The wire reaching the bottom part of the current lead is wound along the inner surface of the spiral conductor and is protected by a thin (0 .2 mm) copper ribbon from possible damage by the high velocity (up to 100 m/s) flow of the helium gas. The cooling gas leaves the current lead through a ceramic high voltage insulator. The high voltage capability is necessary for handling magnet quenches . The first pair of the current lead prototypes with shorter conductor length (420 mm) had its spiral turned
1. Ben-Zvi et al. /Current leadsfor HERA
54
current lead through a standard NW16 flange. The outer diameter of the heat exchanger is 67 mm and its length is 630 mm .
2.3. The 4 x 100 A current lead The 4 X 100 A current lead consists of a stainless steel tube enclosure around four electrical conductors made of the same type of copper tube as the one used for the 10 x 100 A current leads. The conductors are supported by insulating G10 spacers . Segmented openings in the spacers are staggered by 180 ° in order to alternate the flow direction of the helium gas . The G10 spacers for both, the 10 x 100 A and the 4 x 100 A current leads, were cut to shape with laser beam . A commercially available 4 X 150 A ceramicinsulated feedthrough is used for the electrical current . The outer diameter of the heat exchanger is 29 mm and its length is 760 mm . A photograph of the 4 x 100 A current lead is shown in fig . 3 . All the welds and the demountable joints (indium seal or conflat flange connections) are vacuum tight (the leak rate is less than 10 -9 std . cm3 helium per second) and they have been pressure tested up to 26 bar. The
Fig. 1 . The 6500 A current lead spiral conductor.
on a lathe . The present current leads with their conductor length of 760 mm were manufactured on a milling machine using a rotating metal slitting saw .
2.2 . The 10 x 100 A current lead The 10 x 100 A current lead consists of two coaxial stainless steel tubes enclosing 10 conductors made of phosphorus deoxidized copper tubing . We have used a 1/4 in . diameter commercial plumbing tubing with a measured RRR of 7 .7. We prefer the use of tubes because they have more heat exchange surface area for a given conductor cross section than an equivalent rod . Fig . 2 shows a photograph of the 10 x 100 A current lead before final assembly . The conductors are being held together and properly spaced by flat, ring-shaped, insulating G10 spacers with a rectangular gas flow slot in each spacer . The slot orientation alternates by 180 ° in order to change the direction of flow of the coolant (helium gas) . Commercial 150 A ceramic-insulated copper feedthroughs are used as electric current inlets . Ten pairs of thin copper wires with Kapton insulation are provided for the measurement of the voltage drop across each current lead . The helium gas exits from the
Fig. 2 . The 10 X 100 A current lead with its enclosure removed .
I. Ben-Zvi et al. / Current leads for HERA
55
tion and enclosed in a liquid nitrogen shield . The cold helium, supplied by the refrigerator as a mixture of liquid and gas, is separated by means of a phase separator . Liquid helium from the separator is fed into the measuring vessel, where a superconducting helium level detector determines the level of the liquid helium, and stabilizes it by means of a control valve in the bypass line . The pressure in the measuring vessel is controlled by the refrigerator . The cold end heat load of the current leads is determined by the total gas mass flow necessary to keep the liquid helium level constant. The tests of the current leads were performed at several currents around the design current of 6500 A . The state of equilibrium was defined by a constant voltage drop (within 1% change in 30 min) across the current leads . The measured total refrigeration cooling load per current was Q,/I = (3 .2 t 0 .3) mW/A at 6500 A and the cooling gas mass flow rh he = 340 mg/s (for the HERA refrigerator factor f = 34 J/g). The voltage drop was 29 mV .
3.2. The 10x 100 A current lead The tests of the 10 x 100 A current leads were conducted at the Weizmann Institute . An IBM PC-AT computer was programmed to maintain certain variables at specified levels, to change parameters, to display and to store data . Fig . 5 shows the test apparatus . The vertical cryostat consists of an outer vacuum vessel, nitrogen bath with a
Fig. 3 . The 4 x 100 A current lead with its heat exchanger exposed . insulation of each conductor to ground (for the 10 x 100 A and for the 4 x 100 A current leads) was tested at 600 V in helium atmosphere and no spark or leakage current was noted . This test voltage is by a factor of three higher than the peak voltage to be expected during a quench.
cold He from refrigerator cold He to refrigerator
3. Test procedure and experimental results
3.1 . The 6500 A current lead The test of the 6500 A current leads has been executed at DESY . The test apparatus, shown in fig. 4, was connected to a 900 W helium refrigerator. All components of the cryostat containing liquid or cold helium gas are covered with a multilayer superinsula-
-
liquid Helium
Fig . 4 . The test apparatus for 6500 A current leads .
56
I. Ben-Zvi et al. / Current leadsfor HERA
duces the heat flow into the current lead and protects the current lead from a variable thermal load due to the changing liquid helium level in the cryostat . The two flows are regulated by Matheson mass flow controllers . The value of the flow regulation point may be set by the computer . The main function of the bypass flow is to change the rate at which liquid helium enters the brass cup, enabling the level control . The flow controller set point of the bypass flow is adjusted by the computer according to the error signal from the liquid helium detector in the cup . When the current lead is at equilibrium, the bypass flow becomes constant . Then we have the relation Q~=h ( mhe+lnbp)1
Fig . 5 . The test apparatus for 10 X 100 A and 4 X 100 A current leads.
radiation shield and a helium tank . The radiation shield is made of aluminum, the other parts of the cryostat are fabricated of stainless steel. The flange of the current lead is connected to the top plate of the cryostat . The bottom part of the current lead is enclosed in a brass cup with a long 1/4 in . diameter copper tube extending to the bottom of the helium tank . Pressurizing of the tank causes the liquid to reach the cup through this tube . The pressure in the cryostat is controlled by two valves ; a pressure building valve and a pressure relief valve, which in turn are controlled by the computer . The pressure of 2 .5 psi was stable within ± 0 .02 psi . The stainless steel enclosure of the current lead heat exchanger is covered with a thermal insulation . The level of the liquid in the helium tank is measured using a set of carbon composition resistors . The level of the liquid nitrogen in the liquid nitrogen bath is maintained by a liquid nitrogen controller . A platinum resistance thermometer is located on the outer surface of the current lead heat exchanger, 170 mm below the current lead flange . The brass cup contains a helium liquid detector (an Allen Bradley carbon composition resistor) which is placed 20 mm below the entrance to the heat exchanger of the current lead . The liquid is maintained at the correct level by computer control . 10 mm below the entrance to the heat exchanger the helium vapour reaching the top part of the brass cup is divided into two separate flow paths : the ?h,, (he = heat exchanger) flows through the heat exchanger, the other one, rin bp (bp = bypass), through a copper tube soldered to the thermal radiation shield and exits at the top plate of the cryostat . The helium-cooled radiation shield re-
where h is the latent heat of the liquid helium at the operating pressure. The mass flows are measured and they provide the heat leak at the cold end of the current lead . An equivalent method of measurement is to keep rit bp constant and place mhe under computer control . At equilibrium the above equation still holds . The tests of the current leads were performed at a few current levels below, at and above the designed value . The switching from one current value to another was done by computer control after the system has reached equilibrium . The state of equilibrium was defined by the change rate of the m he (for rim by constant) through the current lead, of the voltage drop across the current lead and of the temperature measured by the platinum resistance thermometer . Each of these variables was allowed a maximum change of 2% during 45 min over two successive periods of 45 min each. The results of the 10 X 100 A current lead tests are shown in fig . 6 . The optimum current is 100 A, the optimum rim he is 66 .1 mg/s, and Q,/I = (3 .9 ± 0 .3) mW/A (for the HERA refrigerator factor f = 34 J/g) . This value of Q,/I is approximately (within 5%) equal to the mean measured value for the 12 X 100 A current leads of the Isabelle project [2] . The voltage drop at the optimum current is 24 .7 mV . 3.3. The 4X 100 A current lead
Tests of the 4 X 100 A current lead were performed using the same measuring system as for the 10 x 100 A current lead . The optimum rh he was 9 mg/s and Qt/I = (2 .4 ± 0 .5) mW/A. The voltage drop at the optimum current was 37 .1 mV . The measurement of the 4 X 100 A current leads was more difficult than the 10 X 100 A leads because the thermal load which is being measured is smaller . Since it was difficult to measure the optimum current we have assigned a nominal value of 4 X 100 A based on our experience that this lead was very stable and has been
I Ben-Zoi et al. / Current leads for HERA
57
4. Conclusions
01 68
70
72
74
, 76 .
mhe
01 72
74
76
78
, 80 . m he [m 9
mg
L secI
wi A5
(F)
4 82
84
86
88
90 "'he 80 [~l
100
120
Three types of current leads were developed for the HERA accelerator . A narrow and deep continuous spiral groove was cut along a relatively long 6500 A current lead's conductor fabricated from such ductile material as electrolytic tough pitch copper. The conductor was manufactured on a milling machine using a rotating metal slitting saw . A 1/4 in. diameter commercial plumbing phosphorus deoxidized copper tubing was used for electrodes of the 10 X 100 A and 4 X 100 A current leads . We prefer the use of tubes because they have more heat exchange surface for a given conductor cross section than an equivalent rod . The stability and a good figure of merit Q,/I of these current leads prove the advantage of phosphorus deoxidized copper as a conductor material for small current leads . The tests and next operation for extended period of time showed good heat performances and high stability of the current leads .
I40
Fig. 6 . The results of the 10X100 A current lead test : total cooling power Q, vs mass flow rate m he for five values of the current (curves A-E) and Q, /I vs current I at the optimal value of rim he (curve F). operated for periods of a few hours at up to 4 X 250 A without showing any instability . The longer conductor length of this lead as compared with the 10 X 100 A current lead (for the same type of copper tube for the individual conductors) as well as the better value of Qt/I at 100 A suggest that its "optimal current" should be somewhat smaller than 4 X 100 A, but in view of the results an optimum cannot be defined. The stability and good figure of merit Q,/I of the current lead show the advantage of phosphorus deoxidized copper as the conductor material for small current leads . We feel that current leads constructed from this type of material may be operated at higher currents than the theoretical optimum. In our case we have used this possibility to make a longer conductor at the given conductor cross-section and current, resulting in an improved heat exchanger.
Acknowledgements We are grateful for the support and encouragement of Prof. Y . Eisenberg who suggested the project . The mechanical workshops at the Weizmann Institute were extremely helpful in machining the prototypes. We thank N . Stem for his computer program which helped to calculate the 6500 A current lead, J . Mersel for writing and implementing the program for current lead tests and M . Sidi for the design and construction of the specialized electronic equipment for these tests. This work was supported in part by a grant from the Israel Ministry of Science and Development.
References [1] D. Guesewell and E .U. Haebel, Proc . 3rd Int. Cryogenic Conf . (1970) p. 187. [2] W.J . Schneider and A.P. Werner, Technical Note no. 272, Brookhaven National Laboratory (1981) .