US3412320A - Cryostat having an effective heat exchanger for cooling its input leads and other leak paths - Google Patents

Description

3,412,320 vs HEAT EXCHANGER FOR COOL INPUT LEADS AND OTHER LEAK PATHS ING ITS Nov. 19, 1968 H. 1.. MARSHALL CRYOSTAT HAVING AN EFFECTI Filed May 16, 1966.

5 Sheets-Sheet 1 l4 mvENTbR. HARRY L. MARSHALL FIG.2

FIG. I

f TTORNEY NOV. 19, 1968 LL 3,412,320

CRYOSTAT HAVING AN EFFECTIVE HEAT EXCHANGER FOR COOLING ITS INPUT LEADS AND OTHER LEAK PATHS Filed May 16, 1966 5 Sheets-Sheet 3 h +-H n [-50 l 3 TRANSMITTER PROBE fc um MIXER f0 4, AMPLIFIER v 41 f... 5| FIG. 5

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5 Sheets-Sheet 5 POWER SUPPLY v I g 1 I r I f x CURRENT I I 37 SOURCE CURRENT SOURCE HARRY L. MARSHALL ORNEY United States Patent 3,412,320 CRYOSTAT HAVING AN EFFECTIVE HEAT EXCHANGER FOR COOLING ITS INPUT LEADS AND OTHER LEAK PATHS Harry L. Marshall, Palo Alto, Calif., assignor to Varian Associates, Palo Alto, Calif, a corporation of California Filed May 16, 1966, Ser. No. 550,382 Claims. (Cl. 324--.5)

The present invention relates in general to cryostats and, more particularly, to an improved cryostat including the provision of an effective heat exchanger using the evolved coolant for removing heat flowing into the cryostat from electrical leads, along Dewar walls and along the access port cross sectional area to devices within the cryostat. Such an improved cryostat is especially desirable for, but not limited to, superconductive magnets used with gyromagnetic resonance spectrometers as it substantially decreases the usage of liquid helium coolant, and thus reduces operating costs and prolongs the unattended operating time.

Heretofore, cryostat systems have been built wherein a portion of the evolved helium coolant gas has been channeled along helical ribbon shaped current leads, running to the device held at cryogenic temperature, by means'of a dielectric tube surrounding the leads. Others have employed low thermal conductivity plugs closing off the central helium chamber to reduce thermal conductivity along the access port cross section leading into the liquid helium chamber. While these devices operated to reduce liquid helium consumption they were not very effective inasmuch as the temperature of the exhausted coolant gas as well below the ambient temperature.

In the present invention, the numerous current leads to the superconductive solenoid or other device immersed in a cryostat have been made of thin sheet metal to provide increased surface area to enhance heat transfer to the exhaust coolant gas.

These thin leads have been positioned along the outside of the thermally insulative plug for optimizing heat transfer to the annular exhaust gas column. Moreover, the

width of the annular gas column has been reduced to increase the velocity of the exhaust coolant to a regime of turbulent flow to enhance heat transfer. In a preferred embodiment, which for most of its operating time requires very little current to flow in the leads, the leads have been made of an alloy, such as brass, which has an integrated ratio of electrical conductivity to thermal conductivity, over the cryogenic temperature range of interest, which is substantially greater than such ratio for copper, thereby substantially reducing the thermal conduction heat leak down the current leads in their non energized state. By employing the above described improvements the liquid coolant usage rate has been reduced by a factor of 2.5.

The principal object of the present invention is the provision of an improved cryostat.

One feature of the present invention is the provision of current leads to a device held at cryogenic temperatures and contained in a cryostat wherein the leads are made of thin metal and are carried about the periphery of a thermally insulative plug partially closing off a liquid coolant chamber of the cryostat, whereby the leads are placed in heat exchanging relationship to the exhaust coolant gas.

Another feature of the present invention is the same as the preceding feature wherein the plug is dimensioned relative to the inside Wall of the liquid coolant chamber to reduce the width of the annular exhaust column to a value which increases the exhaust gas velocity to a turbulent flow regime, whereby heat transfer to the exhaust gas is increased.

Another feature of the present invention is the same as of ribbon shape, are disposed about the periphery of the 3,412,320 Patented Nov. 19, 1968 ice any one or more of the preceding wherein the thin leads are made of an alloy having a ratio of electrical conductivity to thermal conductivity, as integrated over the cryogenic range of interest, which is greater than such ratio for copper.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a side elevational view of a magnet system employing features of the present invention,

FIG. 2 is an enlarged longitudinal sectional view of a portion of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 33 in the direction of the arrows,

FIG. 3a is an enlarged fragmentary view of a portion of the structure of FIG. 3 delineated by line 3A3A,

FIG. 4 is a schematic circuit diagram for the magnet of FIG. 1, and

FIG. 5 is a block circuit diagram for a gyromagnetic resonance spectrometer using the magnet system of FIG. 1.

Referring now to FIG. 1 there is shown a superconductive magnet system employing features of the present invention. The system includes a superconductive solenoid, immersed in a liquid helium bath contained within a cylindrical cryostat 2. The cryostat 2 with its internal solenoid is held in the vertical position by a magnet stand 3. The magnet stand 3 includes a centrally apertured magnet carriage member 4 holding the cryostat by means of a support flange 5 welded to the midsection of the cryostat 2. The magnet carriage is supported from a vertical shaft 6 upstanding from a base plate 7. A probe carriage assembly 3 is axially movable along the shaft 6 and includes a horizontal support plate 9 which holds a field utilization probe 11. The probe includes a vertical neck portion 12 containing the sample to be immersed in the strong field of the magnet 1. The neck portion 12 is inserted from the bottom of the cryostat 2 into an axially re-entrant bore 13 in the cryostat 2. The magnet stand 3 forms the subject matter of and is claimed in copending US. application 544,775, filed Apr. '25, 1966, and assigned to the same assignee as the present invention.

Referring now to FIGS. 2 and 3 there is shown in more detail magnet 1 with its cryostat 2. The superconductive solenoid magnet 1 is coaxially disposed of the re-entrant bore portion 13 of the cryostat 2 in a central liquid helium cryogenic fluid chamber 14. A cylindrical wall 15 of the helium chamber 14 is made of thin stainless steel tubing as of 0.020 thick sheet metal with an inside diameter of 8.250 and a length of about 48". A thin cylindrical vacuum chamber 16 envelops the liquid helium chamber 14. A liquid nitrogen chamber 17 envelops the vacuum chamber 16 and an outer vacuum chamber 18 envelops the liquid nitrogen chamber 17. The outer wall of the vacuum chamber 18 forms the outer wall of the cryostat 2. The cryogenic fluid and vacuum chambers 14-18 form a Dewar portion of the cryostat 2.

A thermally insulative cylindrical plug 19 partially closes off the upper end of the cylindrical central liquid helium chamber 14. The plug 19 is, for example, 14 long and 8.125" in diameter and includes three disk shaped transverse headers 21 as of 7 thick epoxy glass. The headers 21 are covered with a thin sheet of epoxy glass as of ,4 thick sheet, thereby forming the cylindrical side Walls 22 of the plug 19. The interior of the plug 19 is filled with a thermally insulative foam material 23 such as polyurethane foam or a glass foam.

A plurality of thin metal electrical leads 24, preferably plug 19. As used herein thin means that the conductor has a depth transverse to the direction of current transport, which is less than /2 of its other transverse extent. It is intended to encompass many different configurations including, ribbon shape, helical ribbon shape, and various thin walled hollow tubular shapes. In one embodiment of the present invention the leads 24 are ribbon shaped. Certain ones of the leads 24 carry the 15-20 amp energizing current to the magnet 1 and are, for example, metal strips 15" long, 0.010" deep and A wide. Other ones of the leads 24 carry less current and are, for example, long, 0.002" or 0.005 deep and wide.

The ribbon leads 24 are laid fiat against the exterior 22 of the plug 19 with their exposed flat sides facing the thin annular passageway 25, as of 0.063" in radial thickness, defined between the exterior wall 22 of the plug 19 and the adjacent cylindrical chamber wall 26 of the liquid helium chamber 14. Dielectric strips having side edge lip portions which overlay the edges of the ribbon leads 24 hold the leads against the plug 19.

The helium chamber 14 is filled with liquid helium to a liquid level 27 above the solenoid 1, and preferably up to the plug 19. Thus the plug extends from ambient to liquid helium temperatures. In a typical example, this requires about 15 liters of liquid helium. As the liquid helium absorbs heat, due to heat leaks into the cryostat 2, helium gas is evolved. Typical heat leaks include thermal conduction paths down the leads 24, down the wall 26, and down the plug 19. The evolved helium gaS is exhausted from the helium chamber 14 via the narrow annular passageway 25.

The radial thickness of the annular passageway 25 is preferably made sufiiciently small such that the exhausting helium gas flow is within the turbulent flow regime as contrasted with the lower velocity laminar flow regime. Turbulent flow is characterized by substantially greater heat transfer from the leads 24 and wall 26 to the annular column of exhausting helium gas. In the present case, turbulent flow was achieved when the radial thickness of the passageway 25 was reduced to less than 0.125" and with the 0.063" passageway 25 about 175 cmF/hr. of liquid helium was being exhausted through the passageway to the atmosphere.

For a cryostat wherein the electrical leads 24 are typically not carrying current during operation of the system, the leads 24 are preferably made of a material having an integrated ratio of electrical conductivity to thermal conductivity over the range of cryogenic temperature of interest which is higher than that of copper over the range of cryogenic temperatures of interest. Examples of such materials are generally metal alloys such as brass, stainless steel, Monel, and manganin, etc.

In a magnet system as aforedescribed, using brass leads 24, the consumption of liquid helium was reduced from 500 cmfi/hr. to 175 cmP/hr. as compared to a system wherein the leads 24 passed through a pair of axially spaced radiation shields closing off the central liquid helium chamber.

The plug 19 and solenoid 1 are carried on four longitudinally directed thin walled stainless steel tubes 29 connected at their upper ends to an aluminum cap 31 and at their lower ends to the solenoid 1. A radiation shield 32, as of thin copper sheet, is transversely carried on the tubes 29 inbetween the plug 19 and solenoid 1 for shielding the liquid helium from heat radiating down the chamber 14. Terminal lugs 33 are provided at the upper and lower ends of the thin leads 24. The lower lugs 33 connect to the solenoid via conventional copper leads or superconducting wire. The upper lugs 33 connect to various power supplies and protective diode circuits as more fully described below via thermally insulated copper wire.

Referring now to FIG. 4 there is shown the electrical circuit for the superconductive magnet 1. The magnet includes the solenoid winding 1 formed, for example, by

120,000 feet of superconductive NbZr wire wound into a solenoid having a bore of 1.8", and a length of about 16". A current regulated constant current source 35 is connected across the end terminals of the solenoid 1 for energizing the solenoid with about 15-20 amps of current to produce a D.C. magnetic field of about 60 kg. The solenoid 1 is segmented into several sections which are tapped out via leads to a bank of series connected diodes 36 comprising two strings of diodes with one stringof diodes connected to pass current in each of two directions .across the solenoid 1 and each segment of the solenoid. This diode bank 36 protects the solenoid and current sources against excesive voltages being developed in the circuit and forms the subject matter of and is claimed in copending US. application 543,666, filed Apr. 19, 1966, and assigned to the same assignee .as the present invention.

The various segments of the solenoid winding 1 are divided into three groups, a large central group and two smaller end groups. Separate current regulated constant current sources 37 .are connected across these groups of windings for permitting separate adjustment of the current in the three portions of the solenoid winding 1. Separate control of these currents permits cancellation of certain axial gradients in the magnetic field produced by the solenoid 1. The separate current sources form the subject matter of and are claimed in copending US. application 548,009, filed May 5, 1966, and assigned to the same assignee as the present invention.

Superconductive wires 38 are connected across the solenoid 1 in parallel with the respective current sources 35 and 37 for permitting the circulating current of the solenoid 1 to be shifted from the circuit loop portions which include the current sources to the respective parallel loop portions which include the superconducting wires 38. This is accomplished by heating the wires 38 via heaters 30 during current energization of the solenoid 1. After the magnet is fully energized with the various loop currents adjusted for optimum field homogeneity, the heaters 39 are de-energized and when the wires 38 become superconductive the current is shifted from the power supplies 35 and 37 to the wires 38 by decreasing the current supplied from the respective sources. When the sources 35 and 37 have transferred their currents to the lWlI8S 38 they are disconnected from the solenoid at points external of the diode bank 36. The heaters 39 are energized from a power supply 41 and leads connected across a voltage divider network 42.

Certain transverse field gradients are cancelled by means of separately current adjustable coil sets 43 mounted on the outside of the solenoid 1. The current is supplied to these coil sets 43 from a grounded centertapped battery 44 via two potentiometers 45 connected across the battery 44. The various current leads 24, as shown in the diagram of FIG. 4 in the region where they pass inside the liquid helium chamber 14, are made of the brass ribbon configuration to enhance heat transfer therefrom and to inhibit heat conduction into the helium chamber 14.

Referring now to FIG. 5 there is shown the electrical circuitry for observing the gyromagnetic resonance spectrum of the sample under analysis. A field modulator 47 superimposes an alternating magnetic field component Hm, at a convenient audio frequency, as of 10 kHz, on the D.C. field H over the sample volume within the probe 11. An ultra high frequency transmitter 48 applies an alternating magnetic field H to the sample at a frequency f which is displaced in frequency from the gyromagnetic resonance frequency f of the sample by the field modulation frequency fm. The U.H.F. magnetic field H is polarized at right angles to the D.C. field. Under these conditions, gyromagnetic resonance of the sample is excited at f,,, which may be on the order of 220 mHz. The excited resonance is frequency modulated having a carrier resonance component at f and Bessel function amplitude sidebands at frequency intervals separated in frequency by the field modulation frequency fm. The fm resonance signal emanating from the sample is picked up in a receiver coil, located within the probe 11, and fed to U.H.F. amplifier '50 and thence to a mixer 49. In the mixer, the resonance signal is mixed with a sample of the transmitter signal to obtain an audio-frequency resonance signal at the field modulation frequency fm. The resonance signal is then amplified by audio amplifier 51 and fed to one input of a phase sensitive detector 52 wherein it is compared with a sample of the field modulation signal to obtain a D.C. resonance output signal. The D.C. polarizing magnetic field H is scanned through the resonance spectrum of the sample under analysis by superimposing a scan field component Hs obtained from a scan generator 53, upon the polarizing field H within the sample volume. The D.C. output resonance signal from the phase sensitive detector 52 is fed to a recorder 54 for recording as a function of time or scan field intensity as obtained from the scan generator 53.

Although the superconductive magnet system of the present invention has been explained as it would be used in conjunction with a gyromagnetic resonance spectrometer, it may be used with other types of field utilization devices wherein a sample is inserted into an intense magnetic field.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A cryostat apparatus including, means forming a device to be held at cryogenic temperatures, means forming a cryostat enveloping said device means and having a chamber for containing a cryogenic fluid for cooling said device means to its cryogenic temperature, means forming a plurality of electrical leads passing into said cryostat and connecting to said device, said leads being made of thin conductor, means forming a plug partially closing off said chamber and defining a gas passageway in the space between said plug and the inside wall of said chamber and through which evolved gaseous cryogenic fluid is exhausted from said chamber for cooling the inside wall of said chamber, and wherein said leads are arranged about the periphery of said plug adjacent said gas passageway in heat exchanging relation to said exhaust gas for cooling of said leads.

2. The apparatus of claim '1 wherein said thin conductor leads are ribbon shaped.

3. The apparatus of claim 1 wherein said plug is elongated and made of a thermally insulative material, and wherein said leads are ribbon shaped and laid flat against the outside periphery of said plug.

4. The apparatus of claim 1 wherein said leads are made of an alloy material having a ratio of electrical conductivity to thermal conductivity which is higher than that ratio for copper at the cryogenic temperatures of said thin leads.

5. The apparatus of claim 1 wherein said plug is dimensioned relative to the inside wall of said chamber in the region of said leads to reduce the dimensions of the gas passageway to the extent such that the velocity of the exhausting gas in the region adjacent said leads fall Within the turbulent flow regime to enhance heat transfer from said leads to the exhaust gas.

6. The apparatus of claim 5 wherein said plug is an elongated thermally insulative cylindrical structure, and said passageway is an arcuate column having a radial thickness of less than 0.100".

7. The apparatus of claim 6 wherein said leads are made of brass.

8. The apparatus of claim 1 wherein said device means is a superconductive solenoid magnet.

9. The apparatus of claim 8 including in combination, means for immersing a sample of gyromagnetic resonance substance under analysis in the magnetic field of said solenoid, means for producing gyromagnetic resonance of the sample in the magnetic field, and means for detecting the gyromagnetic resonance of the sample to obtain a gyromagnetic resonance output.

10. The apparatus of claim 1 wherein the cryogenic fluid is a liquid, wherein said plug is elongated and made of a thermally insulative material, and wherein said plug has a longitudinal extent which extends from substantially ambient temperature to substantially the temperature of the cryogenic liquid in said chamber which is closed off by said plug.

References Cited UNITED STATES PATENTS 3,080,527 3/1963 Chester 3304 3,133,144 5/1964 Cottin-gham 335216 3,286,014 11/1966 Williams 335-416 3,349,161 10/ 1967 Latham l74--15 RUDOLPH V. ROLINEC, Primary Examiner.

M. J. LYNCH, Assistant Examiner.

Claims (1)

1. A CRYOSTAT APPARATUS INCLUDING, MEANS FORMING A DEVICE TO BE HELD AT CRYOGENIC TEMPERATURES, MEANS FORMING A CRYOSTAT ENVELOPING SAID DEVICE MEAN AND HAVING A CHAMBER FOR CONTAINING A CRYOGENIC FLUID FOR COOLING SAID DEVICE MEANS TO ITS CRYOGENIC TEMPERATURE, MEANS FORMING A PLURALITY OF ELECTRICAL LEADS PASSING INTO SAID CRYOSTAT AND CONNECTING TO SAID DEVICE, SAID LEADS BEING MADE OF THIN CONDUCTOR, MEANS FORMING A PLUG PARTIALLY CLOSING OFF SAID CHAMBER AND DEFINING A GAS PASSAGEWAY IN THE SPACE BETWEEN SAID PLUG AND THE INSIDE WALL OF SAID CHAMBER AND THROUGH WHICH EVOLVED GASEOUS CRYOGENIC FLUID IS EXHAUSTED FROM SAID CHAMBER FOR COOLING THE INSIDE WALL OF SAID CHAMBER, AND WHEREIN SAID LEADS ARE ARRANGED ABOUT THE PERIPHERY OF SAID PLUG ADJACENT SAID GAS PASSAGEWAY IN HEAT EXCHANGING RELATION TO SAID EXHAUST GAS FOR COOLING OF SAID LEADS.

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