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2. N—JJ coaxial, bakeable and cryogenic vacuum feedthrough with an isolated shield useful from de to 6.5 GHz
2. N - J J Coaxial, Bakeable and Cryogenic Vacuum Feedthrough With an Isolated Shield Useful From dc to 6.5 GHz received I October 1978
An isolated (10 t i f~) bakeable (300°C) cryogenic (liquid helium temperature) N-JJ type coaxial vacuum feedthrough is described. It has a voltage standing wave ratio, VSWR ~< 1.16 from dc to 1.3 GHz, <_1.22 from 1.8 to 4.2 GHz and ~ 1.25 from 3.2 to 6.5 GHz. Vacuum feedthroughs for the GHz frequency range are used in the construction of the fast monitor system of the K E K 12 GeV proton synchrotron* and will be used in the electron storage ring 2 and superconducting electron linac 3. The very high frequency vacuum feedthrough is necessary for the measurement of the fast beam bunch instabilities in the GHz range. Since they should be able to resist intense radiation as well as be suitable for ultra-high vaccuum which necessitates baking at high temperature, epoxy resin or organic materials could not be used for insulators. Moreover, it is preferable that the outer conductor of the coaxial feedthrough be isolated from the vacuum vessel, so by floating induced noise from ground loop currents may be suppressed. Because no commercially available feedthrough fulfilled these various requirements simultaneously, an appropriate one was specially designed and made. The commercially available N - R type vacuum feedthrough whose centre pin is reverse and does not mate to ordinary N-plugs is useful only up to several times 100 MHz 4. As shown in Figure 1 the feedthrough consists of three conductors and three insulators. The conductors are made of Kovar.
l itl Ik°
..
6 Figure 1. Cross-sectional view of feedthrough showing detail of construction. 1. Coaxial shield-Kovar, 2. centre conductor--Kovar, 3. sleeve--Kovar, 4. washer--Kovar, 5-7. ceramic insulator, 8. vacuum flange, 9. argon-arc weld to the vacuum fitting, 10-12. ceramic-to-metal seal--silver solder, 13. silver solder.
The insulators are ceramic (98 % alumina: Kyocera A 473s). The N-plug with coaxial cable mates to this N-JJ vacuum feedthrough and can be connected or removed quickly both the atmospheric and vacuum sides. Because Kovar may oxidize
Vacuum/volume 29/number 1.
0042-207X/79/0101-0025502.00/0
© Pergamon Press Lid/Printed in Great Britain
during the baking procedure the surfaces were coated with about a 2 t~m thick layer of gold to ensure good electrical contact. The end of the sleeve, where it is welded, was not gold plated. The feedthrough was tested on a He leak detector and was found to have a leak rate of less than 10- ~o atm cm 3 s - 1. Kovar-ceramic seals of this type are mechanically very solid and can safely withstand baking cycles to a temperature of up to 300°C and cooling cycles to liquid helium temperature. The electrical insulation provided by the ceramic was also found to be very good: at least 10 tt f~ between the conductors. The maximum rating of the centre conductor was 10 A. The maximum voltage between the centre conductor and the shield was 5000 V dc with an N-plug. The characteristic impedance of the coaxial cable is I ~/~ b Zo = ~ ~- loge -a
(1)
where tz is the permability, = is the dielectric constant, a is the radius of the centre conductor and b is the inner radius of the outer conductor. To seal the Kovar to the ceramic a thin outer conductor was chosen. There are two kinds of matching section given by following relations :
Z o=~
1
I Z° = ~
~//~ b ~-logea_
(section A),
(2)
/p b ~l°gea-
(section B),
(3)
where ~' is the effective dielectric constant of vacuum and ceramic at the section A and a' is the decreased radius of the centre conductor 6 and c is the radius of the ceramic at the section B as shown in Figure I. The dimensions of a, a', b and c were chosen such that the characteristic impedances Zo of the two sections equalled 50 f2. The matching of the two sections of these four parameters (tolerance +0.01 ram). The final values chosen were where a = 1.50 ram, a' = 0.90 ram, b = 7.20 mm and c = 4.25 ram; in fact a was fixed by commercial availability and only a', b and c were variable. The axial length of the short slot was --~A/8,where Ais the vacuum wave length corresponding to 6.5 GHz. The very high frequency characteristics of the connector were examined by measurement of the voltage standing wave ratio, VSWR = (1 + p)/(1 -- p), where p is the reflection coefficient. The reflection coefficient was measured using a sweep oscillator HP-8620A, reflection test units HP-8471A and HP-8742A, a harmonic frequency converter HP-8411A, a network analyser HP-8410 and a polar display HP--4814A. The polar display gave the reflection coefficient. The reflection coefficients of the N-JJ at 0.01-1.3 GHz, 1.8--4.2 GHz and 3.26.5 GHz are shown in Figure 2a-c. The voltage standing wave ratio is less than 1.16 from 0.01 GHz to 1.3 GHz, less than 1.22 from 1.8 GHz to 4.2 GHz and less than 1.25 from 3.2 GHz to 6.5 GHz. In the frequency range up to 6.5 GHz the non vacuum tight N-jj which uses a teflon insulator has a reflection coefficient comparable to the N-JJ vacuum feedthrough. This N-JJ type vacuum feedthrough may be used from dc to nearly 6.5 GHz. The resulting assembly is compatible with ultra-high vacuum operation and is resistant to strong ionizing radiation. This N-JJ feedthrough has been used in the particle accelerator, a nuclear fusion device and a molecular physics apparatus. 25
Workshop notes and short contributions
Acknowledgements I would like to t h a n k P r o f S S h i b a t a for his e n c o u r a g e m e n t . I wish to t h a n k P r o f J T a n a k a for his help a n d use o f the i m p e d a n c e testing unit. A c k n o w l e d g e m e n t s are due to K y o t o C e r a m i c C o L t d for their work o n the f e e d t h r o u g h c o n s t r u c t i o n .
References Annual Report 1973-1976, National Laboratory for High Energy Physics, Japan. 2 K Kohra, Photon Factory, 2nd Symposium on Accelerator Science and Technology at INS Tokyo, Japan (1978). Y Kojima, private communication. 4 HF coaxial feedthrough, No 954-7238, NEVA, Tokyo, Japan. 5 Kyoto Ceramic Co Ltd, Kyoto, Japan. 6 H Ishimaru, Ret' scient btstrum, 49, 1978, 545.
Hajime Ishimaru, National Laboratory for Hi qh Energy Physics, Oho-machi, Tsukltba-,qttn, lbaraki-ken, 300-32, Japan
Figure 2. (a) A polar display of the reflection coefficient the improved N-JJ at 0.01 GHz-1.3 GHz, (b) a polar display at 1.8 GHz-4.2 G H z (c) a polar display at 3.2 GHz-6.5 GHz.
26
An isolated (10 t i f~) bakeable (300°C) cryogenic (liquid helium temperature) N-JJ type coaxial vacuum feedthrough is described. It has a voltage standing wave ratio, VSWR ~< 1.16 from dc to 1.3 GHz, <_1.22 from 1.8 to 4.2 GHz and ~ 1.25 from 3.2 to 6.5 GHz. Vacuum feedthroughs for the GHz frequency range are used in the construction of the fast monitor system of the K E K 12 GeV proton synchrotron* and will be used in the electron storage ring 2 and superconducting electron linac 3. The very high frequency vacuum feedthrough is necessary for the measurement of the fast beam bunch instabilities in the GHz range. Since they should be able to resist intense radiation as well as be suitable for ultra-high vaccuum which necessitates baking at high temperature, epoxy resin or organic materials could not be used for insulators. Moreover, it is preferable that the outer conductor of the coaxial feedthrough be isolated from the vacuum vessel, so by floating induced noise from ground loop currents may be suppressed. Because no commercially available feedthrough fulfilled these various requirements simultaneously, an appropriate one was specially designed and made. The commercially available N - R type vacuum feedthrough whose centre pin is reverse and does not mate to ordinary N-plugs is useful only up to several times 100 MHz 4. As shown in Figure 1 the feedthrough consists of three conductors and three insulators. The conductors are made of Kovar.
l itl Ik°
..
6 Figure 1. Cross-sectional view of feedthrough showing detail of construction. 1. Coaxial shield-Kovar, 2. centre conductor--Kovar, 3. sleeve--Kovar, 4. washer--Kovar, 5-7. ceramic insulator, 8. vacuum flange, 9. argon-arc weld to the vacuum fitting, 10-12. ceramic-to-metal seal--silver solder, 13. silver solder.
The insulators are ceramic (98 % alumina: Kyocera A 473s). The N-plug with coaxial cable mates to this N-JJ vacuum feedthrough and can be connected or removed quickly both the atmospheric and vacuum sides. Because Kovar may oxidize
Vacuum/volume 29/number 1.
0042-207X/79/0101-0025502.00/0
© Pergamon Press Lid/Printed in Great Britain
during the baking procedure the surfaces were coated with about a 2 t~m thick layer of gold to ensure good electrical contact. The end of the sleeve, where it is welded, was not gold plated. The feedthrough was tested on a He leak detector and was found to have a leak rate of less than 10- ~o atm cm 3 s - 1. Kovar-ceramic seals of this type are mechanically very solid and can safely withstand baking cycles to a temperature of up to 300°C and cooling cycles to liquid helium temperature. The electrical insulation provided by the ceramic was also found to be very good: at least 10 tt f~ between the conductors. The maximum rating of the centre conductor was 10 A. The maximum voltage between the centre conductor and the shield was 5000 V dc with an N-plug. The characteristic impedance of the coaxial cable is I ~/~ b Zo = ~ ~- loge -a
(1)
where tz is the permability, = is the dielectric constant, a is the radius of the centre conductor and b is the inner radius of the outer conductor. To seal the Kovar to the ceramic a thin outer conductor was chosen. There are two kinds of matching section given by following relations :
Z o=~
1
I Z° = ~
~//~ b ~-logea_
(section A),
(2)
/p b ~l°gea-
(section B),
(3)
where ~' is the effective dielectric constant of vacuum and ceramic at the section A and a' is the decreased radius of the centre conductor 6 and c is the radius of the ceramic at the section B as shown in Figure I. The dimensions of a, a', b and c were chosen such that the characteristic impedances Zo of the two sections equalled 50 f2. The matching of the two sections of these four parameters (tolerance +0.01 ram). The final values chosen were where a = 1.50 ram, a' = 0.90 ram, b = 7.20 mm and c = 4.25 ram; in fact a was fixed by commercial availability and only a', b and c were variable. The axial length of the short slot was --~A/8,where Ais the vacuum wave length corresponding to 6.5 GHz. The very high frequency characteristics of the connector were examined by measurement of the voltage standing wave ratio, VSWR = (1 + p)/(1 -- p), where p is the reflection coefficient. The reflection coefficient was measured using a sweep oscillator HP-8620A, reflection test units HP-8471A and HP-8742A, a harmonic frequency converter HP-8411A, a network analyser HP-8410 and a polar display HP--4814A. The polar display gave the reflection coefficient. The reflection coefficients of the N-JJ at 0.01-1.3 GHz, 1.8--4.2 GHz and 3.26.5 GHz are shown in Figure 2a-c. The voltage standing wave ratio is less than 1.16 from 0.01 GHz to 1.3 GHz, less than 1.22 from 1.8 GHz to 4.2 GHz and less than 1.25 from 3.2 GHz to 6.5 GHz. In the frequency range up to 6.5 GHz the non vacuum tight N-jj which uses a teflon insulator has a reflection coefficient comparable to the N-JJ vacuum feedthrough. This N-JJ type vacuum feedthrough may be used from dc to nearly 6.5 GHz. The resulting assembly is compatible with ultra-high vacuum operation and is resistant to strong ionizing radiation. This N-JJ feedthrough has been used in the particle accelerator, a nuclear fusion device and a molecular physics apparatus. 25
Workshop notes and short contributions
Acknowledgements I would like to t h a n k P r o f S S h i b a t a for his e n c o u r a g e m e n t . I wish to t h a n k P r o f J T a n a k a for his help a n d use o f the i m p e d a n c e testing unit. A c k n o w l e d g e m e n t s are due to K y o t o C e r a m i c C o L t d for their work o n the f e e d t h r o u g h c o n s t r u c t i o n .
References Annual Report 1973-1976, National Laboratory for High Energy Physics, Japan. 2 K Kohra, Photon Factory, 2nd Symposium on Accelerator Science and Technology at INS Tokyo, Japan (1978). Y Kojima, private communication. 4 HF coaxial feedthrough, No 954-7238, NEVA, Tokyo, Japan. 5 Kyoto Ceramic Co Ltd, Kyoto, Japan. 6 H Ishimaru, Ret' scient btstrum, 49, 1978, 545.
Hajime Ishimaru, National Laboratory for Hi qh Energy Physics, Oho-machi, Tsukltba-,qttn, lbaraki-ken, 300-32, Japan
Figure 2. (a) A polar display of the reflection coefficient the improved N-JJ at 0.01 GHz-1.3 GHz, (b) a polar display at 1.8 GHz-4.2 G H z (c) a polar display at 3.2 GHz-6.5 GHz.
26