US3389352A - Low loss microwave transmission lines across cryogenic temperature barriers - Google Patents

Description

J. KLIPHUIS 3,389,352 LOW LOSS MICROWAVE TRANSMISSION LINES ACROSS June 18, 1968 CRYOGENIC TEMPERATURE BARRIERS Filed Feb. 7, 1966 ROOM TEMPERATURE CRYOGENIC INVENTOR. JANS KLIPHU IS AT TOIRNE YS United" States Patent Filed Feb. 7, 1966, Ser. No. 525,770 6 Claims. (Cl. 333-98) This invention pertains to microwave transmission lines for coupling microwave energy across a thermal barrier. More specifically, this invention relates to a transmission line particularly adapted to couple RF energ to or from a cryogenically cooled device in such a way as to improve the thermal isolation between such cryogenically cooled device and its environment.

Gryogenically cooled devices today find numerous applications at microwave frequencies where, as a practical matter, waveguides must be used for the transmission of the' RF energy. Since waveguide consists of a continuous electrical conductor such as copper or like materials, which are 'also good conductors of heat, the coupling of RF energy through waveguide to a cryogenically cooled device presents serious thermal insulation problems. Previousetforts to reduce the heat loss due to the conductive waveguide sections have not been totally successful due primarily to the noise and/or losses contributed by the required structure. For example, a long loop of waveguide has: been employed to increase the thermal conduction path length.

Accordingly, an object of the present invention is to provide a transmission line for coupling RF energy across a thermal barrier.

Another object of the invention is to provide a microwave transmission line for coupling RF energy to or from a cryogenically cooled device with improved thermal insulation and a low added noise contribution.

Briefly, in accordance with the invention, the above objectives are obtained through the use of a choke joint having a smooth flange and a choke plate containing a conyentional quarter wave groove. The flange and choke plate are physically separated in a poor heat conducting space to provide thermal isolation whereby no conduction heat losses occur through the waveguide itself. When used with a cryogenically cooled device, such device is preferably coupled directly to the choke plate so that the insertion loss caused by the choke joint will provide only a slight noise contribution due to the reduced temperature. of the plate. Should a hermetically sealed environment be necessary, the portion of the choke joint (generally the flange in the case of a cryogenically cooled device) leading to the outer environment may readily be sealed in a conventional manner.

The manner in which the above and other objects of the invention are accomplished is more fully described below with reference to the attached drawings, in which,

FIGURE 1 is a side view in section illustrating a preferredembodiment of the invention for use with a cryogenically cooled device;

FIGURE 2 is a view along the line 22 of FIGURE 1; and

FIGURE 3 is a view along the line 3-3 of FIGURE 1.

In FIGURE 1, block represents a cryogenically cooled device which, for example, may be a parametric amplifier. The cryogenically cooled device 10 is housed in a conventional Dewar vessel 12 and the required refrigerants are provided through line 14. The construction as so far explained, and numerous equivalents thereof, are well known and detailed description thereof is unnecessary. Customarily, the Dewar vessel 12 is hermetically sealed and evacuated to thermally isolate the device 10 from its environment.

3,389,352 Patented June 18, 1968 ice The problem is to provide a transmission line for coupling microwave energy (e.g. the pump power) between cryogenic device 10 and a source outside Dewar vessel 12 whichis at room temperature. In the following description only a single transmission line is shown although obviously any number may be employed. Furthermore, no eiforthas been made to draw the parts to actual scale since the various dimensions will be appropriate for different purposes as will be obvious to those skilled in the art.

According to the invention, the transmission line includes a first waveguide section 16 connected in a suitable manner to the cryogenically cooled device 10, a choke joint 18, and a second waveguide section 20 coupled, for example, to a source of RF energy (not shown) at room temperature.

Choke joint 18 is conventional and includes a choke plate 22 suitably secured to waveguide section 16 and containing a groove 24 which is approximately one-quarter wave length deep and located one-quarter wave length (average distance) from the inside walls of the waveguide. Choke 18 further includes a flange 28 secured to waveguide section 20 and physically separated. from the choke plate 22 by a distance d. The face 29 of flange 28 may be polished to reduce RF losses to a minimum. One end of waveguide 20 may include a collar 30 which contains a conventional waveguide window 32 made of glass, quartz, or other dielectric hermetically sealed to guide 20 so as to maintain the vacuum within Dewar vessel 12. Such a window (not shown) may optionally be placed in waveguide section 16 also. The junction of guide 20 and Dewar 12 may be hermetically sealed by any conventional sealing means such as shown at 33. By way of example, the waveguides and choke joint may be .made of oxygen free copper.

Electrically, the operation of the device is known since the choke joint 18 serves in a conventional fashion to couple the RF energy from the input source to the cryogenically cooled device 10. Because of the physical spacing between the choke plate 22 and the flange 28, the two sides of the transmission line are thermally insulated from each other, thereby eliminating all conduction heat losses. In accordance with conventional theory, the distance d should be less than one-quarter wave length of the transmitted frequency, but otherwise is not critical and may be optimized for each case with a view toward the electrical and thermal characteristics of the device.

To avoid undesired effects due to the leakage of RF energy through the gap in the choke joint, a ring of RF energy absorbent material 34 may be placed around the circumference of the choke plate 22. This absorbent material may comprise an epoxy material filled with iron powder of the type manufactured by Emerson and Cummings, Inc., and sold under the trademark Eccosorb.

Generally, with the choke plate 22 and flange 28 at the same temperature, and assuming an insertion loss of about 0.2 db due to the choke joint, the system noise temperature would be increased by approximately 16 K. However, since most of the losses will occur at the choke plate 22 which is thermally connected to the cryogenically cooled device 10, the noise contribution of the choke joint will be reduced substantially in proportion to the temperature reduction. For example, if the cryogenically cooled device has an operating temperature of 30 K., the noise contribution of the choke joint will be less than 1.6 K. Similarly, absorption of the leakage RF energy by the ring 34 tends to minimize the noise contribution.

In addition to reducing heat losses (and thus the heat load on the refrigerator) to radiation losses alone the illustrated embodiment of the invention provides a number of other significant advantages. For one, it simplifies assembly of the device 10 with the refrigerating apparatus. For another, the distance between the Dewar wall and the device 10 is not relied upon for thermal isolation and can be reduced to less than one wave length thereby reducing the Dewar size, and the vacuum retention problem. Third, the noise temperature of the complete system is reduced substantially over those using prior thermal isolation techniques.

It will be apparent to those skilled in the art that the invention is not necessarily limited to cyrogenic applications. Thus, the invention would have equal utility in coupling energy between any two devices wherein a substantial temperature differential is to be maintained. Also, the invention would have utility in coupling energy to or from a device which is to be maintained at a particular temperature with respect to an environment which is subject to temperature fluctuations. In those cases where one of the two devices is to be at a temperature substantially lower than the other, the choke plate should preferably be connected to the device maintained at the lower temperature to reduce the noise contribution. Otherwise, the relaitve position or arrangement of the choke plate and flange is not material.

What is claimed is:

1. A device for coupling microwave energy between a first device at one temperature and a second device at a temperature substantially different from said one temperature, comprising means for thermally insulating said second device from said first device, a first waveguide section in a thermally conductive relationship with said first device, a second waveguide section adapted to couple energy to or from said second device, said first and second waveguide sections being spaced apart so as to interrupt the heat conduction path therebetween, and choke means positioned at the gap between said waveguide sections for reducing the amount of microwave energy radiated from said gap.

2. Apparatus for coupling microwave frequency energy between a first device at one temperature and a second device at a substantially dilferent temperature, comprising insulating means including an evacuated chamber substantially surrounding said second device, a first waveguide section coupled to said first device and extending into said evacuated chamber, a second waveguide section coupled to said second device and extending into said evacuated chamber with a substantial gap between said first and second waveguides not greater than approximately one-quarter wavelength of said microwave frequency, and choke means positioned on respective sides of said gap for reducing the amount of microwave frequency energy radiated from said gap.

3. A device according to claim 2, wherein said choke means comprises respective flanges connected to said waveguide at the ends defining said gap, at least one of said flanges including a groove for substantially reducing the impedance across said gap.

4. A device according to claim 3, wherein the temperature of said second device is intended to be less than the temperature of said first device, and wherein the flange connected to said second waveguide section contains said groove.

5. A device according to claim 3, further including means for absorbing microwave energy radiated from said gap.

6. A device according to claim 5, wherein said absorbing means comprises an energy absorbent material spaced peripherally around said gap.

References Cited UNITED STATES PATENTS 2,451,876 10/1948 Salisbury. 2,627,571 2/1953 Hiehle et al. 219-1055 2,832,045 4/ 1958 Sharpless.

HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner.

Claims (1)

1. A DEVICE FOR COUPLING MICROWAVE ENERGY BETWEEN A FIRST DEVICE AT ONE TEMPERATURE AND A SECOND DEVICE AT A TEMPERATURE SUBSTANTIALLY DIFFERENT FROM SAID ONE TEMPERATURE, COMPRISING MEANS FOR THERMALLY INSULATING SAID SECOND DEVICE FROM SAID FIRST DEVICE, A FIRST WAVEGUIDE SECTION IN A THERMALLY CONDUCTIVE RELATIONSHIP WITH SAID FIRST DEVICE, A SECOND WAVEGUIDE SECTION ADAPTED TO COUPLE ENERGY TO OR FROM SAID SECOND DEVICE, SAID FIRST AND SECOND WAVEGUIDE SECTIONS BEING SPACED APART SO AS TO INTERRUPT THE HEAT CONDUCTION PATH THEREBETWEEN, AND CHOKE MEANS POSITIONED AT THE GAP BETWEEN SAID WAVEGUIDE SECTIONS FOR REDUCING THE AMOUNT OF MICROWAVE ENERGY RADIATED FROM SAID GAP.

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