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An easily made demountable electrical feedthrough for use in superfluid helium
An easily made demountable electrical feedthrough for use in superfluid helium G . J. B u t t e r w o r t h
The need frequently arises in the low temperature laboratory for a demountable seal that will allow the passag e of an electrical lead from one section of a cryostat to another while remaining leak tight at the lowest temperature. Though a variety of glass-metal seals is available commercially not all of these are found to be impervious to superfluid helium and, moreover, it is not always possible to obtain one of suitable size. Epoxy-metal seals of the kind described here, however, are easily made in any size from readily available materials and have proved dependable in service at liquid helium temperatures. This type of seal was developed for use in a cryostat in which it was necessary to pass a wire carrying radio frequency signals from a 30 MHz oscillator immersed in a helium bath into an evacuated chamber. There, in addition to its remaining leak tight, the feedthrough was required to maintain alignment of the central conducting wire while introducing minimal stray capacitance, and also allow easy disassembly. Fig.1 shows a cross sectional view of the complete feedthrough. The seal consists essentially of part A, which is machined from copper, and the copper tube T bonded into the epoxy resin casting C. Following the principle of the Housekeeper seal, the upper cylindrical part of A is finely tapered to a feather edge which is moulded into the block C. This arrangement leads to a relatively strain-free bond capable of withstanding a fair degree of thermal shock and repeated thermal cycling. The central tube T carrying the rf lead (in this case a stainless steel capillary of diameter 0.020 in) consists of a suitable piece of copper refrigerator tubing. The stability of the joint between C and T depends on the thermal expansion coefficient of the epoxy resin exceeding that of copper such that on cooling the e p o x y metal interface is subjected to a radial compression. For high reliability the epoxy casting should therefore have a fairly large diameter relative to that of the tube T and should consist of a material having adequate tensile strength at low temperature. The length of tube protruding from C is kept fairly long so that the central conductor can be soldered into T, with the help of a heat shunt placed at S, without risk of damage to the epoxy-metal bond. Below the seal the central wire forms with the tube E a coaxial RF line to the sample chamber, the feedthrough being soldered into the flange F using Wood's metal. The purpose of the recess in the bottom surface of A is to reduce any creepage of soldering flux into the feedthrough. To fabricate the feedthrough the copper parts are first degreased, etched in concentrated NH4OH solution or nitric acid, then rinsed in distilled water and dried in hot air. Whilst it is not essential, the etching treatment is found to produce a stronger and more reliable epoxy bond. Casting of the epoxy plug is readily performed with the aid of the PTFE mould shown in Fig.2. With the tube T set in position the mould is tidied to the appropriate level with The author is in the Mathematics and Physics Building, University of Sussex, Falmer, Brighton BN1 9QH, Sussex. Received 27 October 1974.
40
A ~
I cm Fig.1 Cross-sectional view of the feedthrough shown in place at the end of a coaxial rf lin
.
e
i
Fig.2
The PTFE mould used in making the epoxy seal
epoxy. Part A is then put in place, taking care to avoid air bubbles at the feather edge, and the seal is cured. The use of Stycast 2850 FT epoxy resin and catalyst no 11 (manufactured by Emerson and Cuming Inc, Canton, Massachusetts, USA) is recommended here since this forms a tough solid having a thermal expansion coefficient comparable to that of copper. To obtain a reliable joint the epoxy should be cured at an elevated temperature in the region of 75°C. The resulting seals have been found to remain leak-tight to superfluid helium following many thermal cycles between room and liquid helium temperatures. Provided they are not overheated they can be unsoldered and reused several times.
CRYOGENICS.JANUARY 1975
The need frequently arises in the low temperature laboratory for a demountable seal that will allow the passag e of an electrical lead from one section of a cryostat to another while remaining leak tight at the lowest temperature. Though a variety of glass-metal seals is available commercially not all of these are found to be impervious to superfluid helium and, moreover, it is not always possible to obtain one of suitable size. Epoxy-metal seals of the kind described here, however, are easily made in any size from readily available materials and have proved dependable in service at liquid helium temperatures. This type of seal was developed for use in a cryostat in which it was necessary to pass a wire carrying radio frequency signals from a 30 MHz oscillator immersed in a helium bath into an evacuated chamber. There, in addition to its remaining leak tight, the feedthrough was required to maintain alignment of the central conducting wire while introducing minimal stray capacitance, and also allow easy disassembly. Fig.1 shows a cross sectional view of the complete feedthrough. The seal consists essentially of part A, which is machined from copper, and the copper tube T bonded into the epoxy resin casting C. Following the principle of the Housekeeper seal, the upper cylindrical part of A is finely tapered to a feather edge which is moulded into the block C. This arrangement leads to a relatively strain-free bond capable of withstanding a fair degree of thermal shock and repeated thermal cycling. The central tube T carrying the rf lead (in this case a stainless steel capillary of diameter 0.020 in) consists of a suitable piece of copper refrigerator tubing. The stability of the joint between C and T depends on the thermal expansion coefficient of the epoxy resin exceeding that of copper such that on cooling the e p o x y metal interface is subjected to a radial compression. For high reliability the epoxy casting should therefore have a fairly large diameter relative to that of the tube T and should consist of a material having adequate tensile strength at low temperature. The length of tube protruding from C is kept fairly long so that the central conductor can be soldered into T, with the help of a heat shunt placed at S, without risk of damage to the epoxy-metal bond. Below the seal the central wire forms with the tube E a coaxial RF line to the sample chamber, the feedthrough being soldered into the flange F using Wood's metal. The purpose of the recess in the bottom surface of A is to reduce any creepage of soldering flux into the feedthrough. To fabricate the feedthrough the copper parts are first degreased, etched in concentrated NH4OH solution or nitric acid, then rinsed in distilled water and dried in hot air. Whilst it is not essential, the etching treatment is found to produce a stronger and more reliable epoxy bond. Casting of the epoxy plug is readily performed with the aid of the PTFE mould shown in Fig.2. With the tube T set in position the mould is tidied to the appropriate level with The author is in the Mathematics and Physics Building, University of Sussex, Falmer, Brighton BN1 9QH, Sussex. Received 27 October 1974.
40
A ~
I cm Fig.1 Cross-sectional view of the feedthrough shown in place at the end of a coaxial rf lin
.
e
i
Fig.2
The PTFE mould used in making the epoxy seal
epoxy. Part A is then put in place, taking care to avoid air bubbles at the feather edge, and the seal is cured. The use of Stycast 2850 FT epoxy resin and catalyst no 11 (manufactured by Emerson and Cuming Inc, Canton, Massachusetts, USA) is recommended here since this forms a tough solid having a thermal expansion coefficient comparable to that of copper. To obtain a reliable joint the epoxy should be cured at an elevated temperature in the region of 75°C. The resulting seals have been found to remain leak-tight to superfluid helium following many thermal cycles between room and liquid helium temperatures. Provided they are not overheated they can be unsoldered and reused several times.
CRYOGENICS.JANUARY 1975