Two stage cryocooler with superconductive current lead

Abstract: A two stage cryocooler sleeve is provided having a second stage heat exchanger system capable of achieving lower temperatures than the first stage heat exchanger. A current lead (273) comprising a ceramic superconductor, has a critical temperature greater than the operating temperature of the first stage (263). The lead (273) is tapered. The broader end is thermally coupled to the first stage heat exchanger (263) and the narrow end coupled to the second stage heat exchanger (265). The tapered ceramic lead reduces the heat conduction from the first heat exchanger (263) to said second heat exchanger (265).

Description: The present invention is related to a two-stage cryocooler and current lead. In superconducting magnets current leads are used during ramp-up to energize the magnet. The leads can also be used during magnet operation to assure a constant current flowing through the winding in the presence of nonsuperconducting joints in the winding. Cryogenic current leads are presently fabricated of helium cooled metallic resistive conductors, typically with high electrical and thermal conductivity. Helium cooling is required to reduce conduction heat transfer to the superconducting magnet and dissipate the resistance heating of the leads. In many superconducting magnet systems where no loss of helium and refrigeration are essential to the economics of powering a magnet system, cryogenic current leads must either be disconnected once magnet operation has been initiated or a reliquifier must be provided to reliquify the helium used in cooling the leads. Helium recondensors and cryocoolers are preferable to reliquifiers because they keep the helium contained in a closed loop system and have good reliability. A magnet cryostat equipped with a recondensor or cryocooler permits no loss of helium for vapor or liquid cooling and therefore thermal losses of conventional leads not cooled by helium cannot be tolerated for long. Patent Abstracts of Japan, Vol. 12 number (E-629) [3081], July 5, 1988, and JP-A-63-28080 discloses a cryogenic apparatus whose purpose is to facilitate cooling of a detachable current lead by using a cooler when the current lead is inserted and a current is applied to the lead by a method wherein the respective temperature stages of the cooler are thermally connected to the insertion port of the current lead and both the current lead and the insertion port are tapered so as to contact the temperature stages thermally with the current lead main part through the insertion port when the current lead is inserted into the port. According to the invention, there is provided apparatus comprising: a two stage cryocooler sleeve having a second stage heat exchanger station capable of achieving lower temperatures than the first stage heat exchanger; and characterized in having a current lead comprising a ceramic superconductor having a critical temperature greater than the operating temperature of the first stage of said cryocooler, said ceramic superconductor being tapered with the broader end thermally coupled to said first stage heat exchanger and the narrow end coupled to said second stage heat exchanger, said tapered ceramic lead reducing heat conduction from said first heat exchanger to said second heat exchanger station, and in that said tapered ceramic superconductor spirals around the cryocooler. The invention will now be described in greater detail, by way of example, with reference to the drawings in which: Figure 1 is a graph showing the first and second stage temperature of a cryocooler as a function of the heat loads imposed on the cryocooler; Figure 2 is a graph showing the temperature distribution in resistive current leads which have an optimized length over area ratio for a given current; Figure 3 is a cutaway isometric view of the cold end of a cryocooler having tapered superconductive ceramic leads between the first and second stages; Figure 4 is a cutaway isometric view of the cold end of a cryocoolet with a tapered spiral superconductive current leads between the first and second stages according to an embodiment of the present invention; and Figure 5 is a side elevation view of the tapered spiral ceramic superconductive leads of Figure 4. Current leads in the embodiments of the present invention cannot be helium vapor cooled to reduce conduction heat transfer to the superconducting magnet and to dissipate the resistance heating of the leads since consumable cryogens are not used. The current leads used are heat stationed to the first and second stage of the cryocooler to intercept heat before it reaches the superconducting coils. In the cryocoolers used in the present invention resistive metallic conductors, such as copper, are used in the lead section from the exterior of the cryostat, which is at an ambient temperature of 300°K, to the first stage of the cryocooler which has a temperature of 50°K during operation. A resistive metallic conductor is also used in the lead section from the first stage of the cryocooler which is at 50°K to the second stage which is at 10°K. To minimize the conduction heat transfer to the heat stations by the current leads the lead aspect ratio must be optimized for a given current. Figure 1 shows the first and second stage temperature of a cryocooler used in the present invention as a function of heat loads imposed on the cryocooler. Since the resistance heating of the resistive metallic conductor is directly proportional to the length over cross sectional area, L/A, while conduction heat transfer to a lower temperature heat station is inversely proportional to L/A, there is an optimum L/A for which conduction heat transferred to the lower temperature station is at a minimum. For a resistive lead with nearly constant electrical resistivity along its length, the minimum heat transferred to the low temperature station is equal to one half the resistive heating of the lead section plus the conduction heat transferred from the high temperature station. With the aspect ratio so adjusted, the net heat transferred from the high temperature station is zero since the other half of the resistive heating balances out the conduction heat transferred from that station. The temperature profile of the current leads with optimized aspect ratio for a 50 ampere current is shown in Fig. 2. The slope of the temperature profile of the leads extending between the 10°K and 50°K heat station as it approaches the 50°K heat station is seen to be horizontal signifying that the resistive and conductive heat flows are balanced. Similarly, the slope of the temperature profile of the current leads between the 50°K heat station and ambient as the lead approaches ambient temperature is horizontal. If a high temperature ceramic superconductor is used in a lead section from the 50°K to 10°K heat station then the resistive heating in that lead section is zero and there is no optimum lead aspect ratio for that section. The ceramic superconductor lead section is made sufficiently large to carry the required current, I, and the lead length is made sufficiently long to result in acceptable conduction heat transfer to the 10°K heat station. Because of the strong decrease of the material critical current density, J c , with temperature T, the lead cross sectional area, A, must be varied inversely with temperature so that A = I J , I J c (T) with sufficient safety margin, (J c -J)/J c approximately 10 to 30 percent, where J is the actual current density in the ceramic lead and I is the current. Figure 3 shows a cold end portion of a cryocooler sleeve in an evacuated housing 260. Two straight ceramic leads 261 extending from the 50°K to 10°K stations 263 and 265, respectively, of a cryocooler sleeve with the leads tapered so that the lead has greater cross sectional area at the warmer end. The ceramic leads are heat stationed at the 50°K and 10°K heat stations 263 and 265, respectively. The high temperature section of the lead between the ambient (300°K) and the 50°K heat station comprises copper conductors having an optimized L/A to minimize the heat transferred to the 50°K station at the operating current. Generally, the leads should be metallized with silver. One method is sputtering another is using silver epoxy. The ceramic leads 261 are coated with silver loaded epoxy in the region where current conductive junctions are to be made. During processing of the ceramic, the epoxy is vaporized leaving behind a silver coating to which copper leads can be soldered. Resistive metallic conductors are soldered to the ceramic leads at the 10°K heat station using low resistivity solder, such as indium solder. The copper leads extending from the ambient are soldered to the ceramic leads in the vicinity of the 50°K heat station. The ceramic leads can be heat stationed, for example, using beryllia or alumina metallized with copper or nickel on both sides and soldered between the metallized ceramic lead and the cryocooler sleeve heat station. See copending application (RD-18522), incorporated herein by reference. Figures 4 and 5 show two tapered spiral high temperature ceramic superconductors 271 and 273 which can be formed from a single cylindrical length of ceramic superconductor such as yttrium barium copper oxide (YBa 2 Cu 3 O x ). The ceramic leads extend from the 50°K to 10°K heat station 263 and 265, respectively, and are heat stationed at the 50°K and 10°K heat stations. The ceramic leads are metallized with silver, such as by.coating them with silver loaded epoxy which during heating leaves a coating of silver behind allowing the resistive metallic conductors to be soldered to the silver coated ceramic leads at the 10°K heat station. A low resistance solder such as indium solder is preferably used. The current leads each from ambient temperature are soldered to the ceramic leads in the vicinity of the 50°K heat station. Thus, the cryocooler in the sleeve which is thermally coupled to the magnet cryostat temperature stations at 10°K, and 50°K, will experience negligible heat load from the current leads at the 10°K station, when the optimized aspect ratio resistive metallic conductors or the ceramic superconductors are used. The cooling capacity at the 10°K station is limited and the heat station receives negligible heat load from the current leads, while the lead thermal load at the 50°K heat station can be easily handled by the increased refrigeration capacity available at this temperature. Power is supplied to the magnets in the present invention by permanently connected leads supplied from a stable power supply. The power supply provides power lost due to the resistance in copper bus bars current leads and superconductor splices. To prevent arcing from occurring in case the leads become accidently disconnected or if a ceramic superconducting lead quenches, diodes are connected in the magnet to provide a continuous current path. During operation with the current leads connected and operating properly the voltage across the diodes is insufficient to cause them to conduct. If the leads current is interrupted, the voltage across the diode increases causing them to conduct. Joints made in niobium tin superconductor wire are nonsuperconductive but have a very low resistance. Using only superconductive wire and no copper bus bars, or permanently connected leads, the magnet resistance would be approximately 10 -8 ohms. The inductance of the magnet depends on magnet strength varying from 160 to 1600 henries for the embodiments shown. Once a current is established in the superconducting coils, the long time constant of the magnet circuit (thousands of years) could provide virtually persistent operation and a stable field in the magnet. The foregoing has described a cryogenic current lead which does not require direct cooling with helium vapor. The material G-10 referred to in the foregoing description is a laminated thermosetting material (comprising a continuous filament-type glass with an epoxy resin binder) identified in the ASTM specification D709-87.

Claims: Apparatus comprising: a two stage cryocooler sleeve having a second stage heat exchanger station (265) capable of achieving lower temperatures than the first stage heat exchanger (263); and characterized in having a current lead (271,273) comprising a ceramic superconductor having a critical temperature greater than the operating temperature of the first stage of said cryocooler, said ceramic superconductor being tapered with the broader end thermally coupled to said first stage heat exchanger and the narrow end coupled to said second stage heat exchanger, said tapered ceramic lead reducing heat conduction from said first heat exchanger to said second heat exchanger station, and in that said tapered ceramic superconductor (271,273) spirals around the cryocooler.

Inventors: LASKARIS EVANGELOS TRIFON, LASKARIS, EVANGELOS TRIFON
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1989
Main classification: F04B37
IPC codes: H01F6