Coax for vacuum applications

[Arch]

Hi,
Can anyone suggest where i can get the 50 ohm coax for UH-vacuum
applications?
thanks in advance.

 

[Steve Noll]

Hi,
Can anyone suggest where i can get the 50 ohm coax for UH-vacuum
applications?
thanks in advance.

I'd consider semirigid coax (Google "semirigid coax")
Solid wire center conductor, solid Teflon dielectric, solid copper
tube for the shield. May be low enough outgasing for all but EHV.
0.141" od is common, but there are smaller and larger diameters.
Might be an arcing issue at the ends of small diameters?
Pasternack has some, but 50-foot minimum.

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[no_one]

what power, what frequency?

 

[Bret Cannon]

Try MDC Vacuum,
http://www.mdcvacuum.com/urd/uniface.urd/ecf0070w.display?6.1.e.3
This is bakeable to 260°C and claims it is designed for HV and UHV.

Bret Cannon

 

[Roy L. Fuchs]

I'd consider semirigid coax (Google "semirigid coax")
Solid wire center conductor, solid Teflon dielectric, solid copper
tube for the shield. May be low enough outgasing for all but EHV.
0.141" od is common, but there are smaller and larger diameters.
Might be an arcing issue at the ends of small diameters?
Pasternack has some, but 50-foot minimum.

The slightly less rigid, tinned braid outer sheath versions are out
there as well.

 

[Arch]

Thanks people.
The signal is not more than few mVolts and the bandwidth is 10MHz.
I saw at MDC but they don't sell less than 30ft. I need only may be
10ft and yes inexpensive the better.

 

[[email protected]]

Try MDC Vacuum,
http://www.mdcvacuum.com/urd/uniface.urd/ecf0070w.display?6.1.e.3
This is bakeable to 260°C and claims it is designed for HV and UHV.

Bret Cannon

Looks promising. Ultra-high vacuum is all about pumping rates and
baking, so I''d think about making my own connections by stretching the
centre wire between UH -coaxial feedthroughs (tricky enough - you need
parts with glass-to-metal seals on the centre pin)
and mechnicallly locating a thoroughly rigid copper outer around it
(assembled in reverse order).

Not as conveninet as the store-bought cable, but it might pump down
faster.

 

[Arch]

Thanks Bill,
If i don't get something inexpensive store - made , i will go the route
you told me to.
I dont think i get the glass-to-metal seals for the center conductor
part? Can we buy such wires with glass- insulator on them?

 

[no_one]

you need the glass to metal seal at the vacuum chamber interface to keep the
atmosphere out and vacuum in you chamber. Usually a vacuum rated
feedthrough at the chamber wall is used. you also need to consider
contamination from the coax if it has not been vacuum baked previously.

 

[Rich Grise]

Thanks Bill,
If i don't get something inexpensive store - made , i will go the route
you told me to.
I dont think i get the glass-to-metal seals for the center conductor
part? Can we buy such wires with glass- insulator on them?

Soitenly! ;-)
http://www.google.com/search?hl=en&q="vacuum+feedthrough"

Cheers!
Rich

 

[Arch]

Thanks guys,
I have some feedthrough for ultra high vacuum so i am planning to use
them.
So to make a coax, i will take some stainless steel wire (prferably
stranded). Now, before i put the copper around for EMI , i need a
insulator?
Are there any ultra-high vacuum insulators which won't break at
cryogenic temperatures. Teflon may be the right choice?

 

[[email protected]]

Are there any ultra-high vacuum insulators which won't break at
cryogenic temperatures.

How about vacum?

As in an empty void instead of an insulator.

At your voltage level it should be fine. You will have to adjust the
geometry and velocity factor slightly compared to popular dielectrics,
but that may not matter either.

 

[Winfield Hill]

Arch wrote...

Are there any ultra-high vacuum insulators which won't break
at cryogenic temperatures. Teflon may be the right choice?

We use "coax" shielded cable that's made with ceramic disks
spaced every so often. No problem for bakeout or ultra-high
vacuum. It's not 50 ohms, and it has a higher capacitance
than any coax you're used to in air.