US2676198A - Electrical connector - Google Patents

Description

April 20, 1954 R. G- PICARD ELECTRICAL CONNECTOR Filed Aug. 31, 1950 Bnnentor ROBERT E. PICARD Patented Apr. 20, 1954 ELECTRICAL CONNECTOR Robert G. Picard, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 31, 1950, Serial No. 182,475

2 Claims.

The present invention relates to electrical connectors and more particularly to vacuum-tight connectors for high power, high frequency currents.

It has been found necessary, on occasions, to supply high frequency, high power electrical energy from an external source to a utilization device which is enclosed in a chamber and under extremely low pressure. An example of a situation in which such circumstances prevail is in the process of evaporating certain high melting point metals, such as platinum, in a vacuum whereby a very thin coating of the evaporated metal is deposited on objects placed in the vacuum chamben Such a process is useful in the preparation of certain specimens for observation in an electron microscope. Some substances cannot be viewed directly in an electron microscope due to their opacity to the electron beam. If the surface of such a substance is to be studied under an electron microscope, a collodion replica of the surface is first prepared. Since collodion is almost completely transparent to the electron beam, means must be provided whereby the contours of the surface of the collodion can be rendered discernible. This need is fulfilled by the above metal-evaporation process, and is known as shadow-casting. The metal being evaporated in the above-mentioned process must be heated in an extremely low-pressured atmosphere. convenient method of so heating the metal is by induction heating. It has been found that platinum is a very desirable metal to use in this process. However, platinum has a very high melting point temperature, and consequently a large quantity of energy is required to carry out the process. Because the size of the charge of platinum is small, very high frequency current is needed to produce inductive coupling between the metal and the induction coil.

Due to these requirements of high power, high frequency energy and the low-pressure atmosphere, problems arose as to adequate means of conducting the energy into the chamber.

Since high frequency electrical energy is most efficiently carried by coaxially disposed conductors, a suitable connector should also be arranged as coaxial conductors. Since the space between the conductors must be sealed, a suitable material for effecting the seal should have the properties of being a good high frequency insulator and at the same time be able to withstand the very low pressure on the inner surface without giving ofl gaseous emanations into the chamber.

It is, therefore, an object of the present invention to provide an improved connector for high frequency electrical currents.

Another object of the present invention is to provide improved means for feeding high frequency, high power electrical energy into an evacuated enclosure.

In accomplishing these and other objects, there has been provided a coaxial connector in which the space between the conductors is sealed with a relatively thick plug made of polytetrafluorethylene (known by the trade name, and hereinafter referred to as Teflon). This material exhibits the desired electrical properties and stands up well in extremely low pressured atmospheres. The material also has sufficient mechanical strength to support the inner conductor.

A better understanding of the present invention may be had from the following detailed description when read in connection with the accompanying drawing in which:

Figure l is a view, partly in cross-section showing one form of the invention, and

Figure 2 is an enlarged view, partly in crosssection of a portion of the apparatus shown in Figure 1 and showing the details of the structure.

Referring more particularly to the drawing, there is shown a portion of the wall 2 of a vacuum chamber in which an opening 4 is provided for the insertion of the electrical connector. A collar 6, which serves as the outer conductor of a coaxial pair of conductors, is inserted in the aperture. The collar includes a flanged portion 1 secured to the outer wall of the chamber by a plurality of screws 8. The inner conductor H] of the coaxial pair of conductors is supported in the space inside the outer conductor by an insulating washer I2 of annular shape. The washer is made of Teflon (polytetrafluorethylene). The washer has an outer diameter such that it fits fairly snugly within the hollow outer conductor 6. The inner conductor extends through a hole IS in the center of the washer. The washer is made relatively thick in order to rigidly support the inner conductor It in place in the assembly. It should be noted that washers made of other insulating materials were tried, but they failed under test. For example, natural rubber and various synthetic rubbers failed to withstand the intense high frequency electric field set up between the conductors, and phenolic resins such as Bakelite failed in the presence of the reduced pressure.

ince Tefion" is a relatively inelastic, incompressible substance, the Teflon washer does not provide an air tight seal in the opening. Further, if a tight, pressed fit instead of the previously specified fairly snug fit, were made, gas leaks would soon develop due to the fact that Teflon is subject to cold flow. Therefore, the washer is sealed to the inner and outer conductors by a suitable vacuum sealing material I4 which is capable of wetting the surface of both the Teflon and the metal of the conductors, and has a low vapor pressure. The fairly snug fit which i not tight pressed nor air tight, between washer and conductors insures that cold flow will not occur. If cold flow occurs, the sealing material would be ruptured and leaks would develop. A material suitable for this purpose is known commercially as Picein Wax. Another such substance is Apiezon W. These materials are compounded of a blend of long chain hydrocarbons. They have been and are supplied in this country by the James G. Biddle Company of Philadelphia, Penna. Their physical properties are fully described in Bulletin 43 published by the James G. Biddle Company. A thin metal retaining ring it is positioned in and soldered I? to the inner surface of the outer conductor to provide suiflcient mechanical strength to the assembly to withstand the difference in pressure on the opposite sides of the washer 12. In the assembly, the ring [6 is positioned and soldered before the Teflon washer is inserted since the heat required in the soldering process would have deleterious efiect on the Teflon. A second thin metal retaining ring I8 i positioned snugly against the opposite side of the washer from the first ring E5. The second ring is held in place b a plurality of small screws or pins 20. Both of the rings are made as thin as possible to maintain the insulating distance between the conductors. The exposed sides of the rings are preferably rounded as shown in the drawing to prevent a break-down of the electric field between the conductors.

The sealing material is capable of wetting the metal of retaining rings 16 and 18 in the same manner as it wets the metal of the conductors. The sealing material is, therefore, applied as shown at 4 to make the joints between the conductors, retaining rings and washer vacuumtight.

To obviate the necessity of providing additional vacuum seals, the inner conductor I0, is made of solid stock, rather than the usual hollow material.

A seal between the flange portion I of the collar or outer conductor and the wall 2 of the vacuum chamber is effected by a compressible annular gasket 22 made of any suitable material.

In order to prevent the seal around the inner conductor from being broken by strain exerted thereon when the apparatus is being connected or used, a hole is drilled through the assembly and a tight-fitting Teflon rod 24 is inserted therein, passing through the inner conductor, the Teflon washer, and the outer conductor or collar.

A flexible connecting terminal 26 is secured to the vacuum chamber end of the inner conductor to reduce the probability of exerting excessive strain on the seal while attempting to connect the assembly to a utilization device inside the chamber.

In a working model of an assembly constructed in accordance with the present invention, the connector successfully carried a kilowatt of electrical energy at 60 megacycles While the absolute pressure inside the vacuum chamber was on the order of 10- mm. of mercury.

Thus it may be seen that there has been provided a simple, inexpensive, yet eilicient coaxial connector for conducting high-power, high frequency electrical energy into an evacuated chamher.

What is claimed is:

1. An assembly for feeding high power, high frequency electrical energy through an aperture in the wall of a vacuum chamber comprising a pair of coaxial conductors disposed within said aperture, the outer conductor of said coaxial pair being secured to said wall, a polytetrofluorethylene insulating washer, the inner conductor of said coaxial pair supported within said outer conductor by said insulating washer, said washer having an inner and outer diameter capable of providing a fairly snug fit between said coaxial conductors, said fit having a clearance capable of leaking air, a sealing substance having a low vapor pressure and having the ability to wet said coaxial conductors and said insulating washer, said sealing substance being disposed about fitted portions of said washer and said inner conductor for providing therebetween a vacuum-tight seal, and retaining means for said washer, said retaining means being disposed to hold said washer in a predetermined position with respect to said outer conductor.

2. An assembly for feeding high power, high frequency electrical energy through an aperture in the wall of a vacuum chamber comprising a pair of coaxial conductors disposed within said aperture, the outer conductor of said coaxial pair being secured to said wall, a polytetrofluorethylene insulating washer, the inner conductor of said coaxial pair being supported within said outer conductor by said insulating washer, said washer having an inner and outer diameter capable of providing a fairly snug fit between said coaxial conductors, said fit having a clearance capable of leaking air, a pair of retaining rings, said retaining rings being disposed to hold said washer in a predetermined position with respect to said outer conductor, and a sealing substance having a low vapor pressure and having the ability to wet said coaxial conductors, said retaining rings and said insulating washer, said sealing substance being disposed about the fitted portions of said washer, said retaining rings and said conductors for providing therebetween a vacuumtight seal.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,089,688 Clark Aug. 10, 1937 2,103,081 Krefft Dec. 21, 1937 2,389,454 Picard Nov. 20, 1945 2,436,284 Bondon Feb. 17, 1948 2,454,625 Bondon Nov. 23, 1948 2,498,589 Steinke Feb. 21, 1950 2,512,604 Bierwirth June 27, 1950 2,556,642 Bird June 12, 1951

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