Ground terminals for HV feedthrough: chamber as return path

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ChristofferBraestrup
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Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

Hi all!

A problem that keeps popping up with high voltage feedthroughs is that they typically don't have any facility for ground connections. Take these brilliant KF40 2-pin HV (4kV maybe) feedthroughs I just got (see attached). I need both pins (they're driving a Channeltron) and ground.

So here's the question: How to best add ground, and what are the concequences of using the vacuum vessel as signal return/ground?

The easiest solution I see is to put a circlip in the vacuum fitting tube, with a braided copper wire pinned against the chamber wall underneath, but I'm unsure how well this is.

Both stainless steel and aluminium (w/oxide layer) are not brilliant conductors.

What do you think?
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Richard Hull
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Richard Hull »

Note: All vacuum lines of metal must be grounded!! The only exception is when, for some strange reason, you deliberately want an isolated section for a specific reason. As a rule, 100% of all professional vacuum couplings are so designed that electrical connection, (usually grounded), is maintained throughout the entire vacuum plumbing system.

In my system I use an 8-inch piece of classic red rubber hose connecting my mechanical pump to my all metal vacuum plumbing. The vacuum plumbing is grounded at the fusor and the turbo. The mechanical pump is grounded via the natural 3 wire power plug to the mains. For the purposes of grounding, the SS to SS, SS to Al and Al to Al clamps, nuts and bolts hooking it all together is more than adequate electrical ground connection for the common fusor.

In the end, it is tough, and one would have to go out of his/her way, to have electrically isolated, (floating), plumbing. How you ground the plumbing is your business, but I typically use the fusor as a firmly grounded section of my plumbing. Turbos and diff pumps are typically grounded via the wall outlet grounding. Like I say, it is a real costly and tedious effort to float vacuum system plumbing.

Richard Hull
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Maciek Szymanski
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Maciek Szymanski »

A side note - on the Asterix laser system in Prague it is required that all the diagnostic systems are insulated from the interaction chamber and grounded independently as required. It's usually achieved by using plastic KF centering rings and clamps or teflon extension tubes.
Lately I've built few glass discharge tubes which had KF flanges on both ends insulated by the glass itself. During the dishcarge one flange was grounded to the vacuum system and the other one was on the cathode potential. Quite dangerous solution - one usually do not expect a metal part of vacuum system to be few kV to the ground...


A5D208C6-B017-477E-96C6-78FB420AED8A.jpeg
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ChristofferBraestrup
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

Thanks for the thoughts!

I'm not at all considering floating the main vacuum chamber, it's very solidly connected to mains earth.

What I'm talking about is making electrical connections to ground/chamber wall inside the chamber.

I'm trying to mount a channel electron multiplier in a vacuum chamber, more specifically in a KF40 T. My feedthroughs, as seen in pic 1 has 2 connectors, and the channeltron needs 2 connections + ground. see attached schematic.

So in practicality, how would one go about making a ground connection inside the chamber?

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Maciek Szymanski
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Maciek Szymanski »

Usually I just drill a blind M3 hole from the vacuum side and mount an eye terminal by a small drilled screw, then solder the ground wire to the eye. Unfortunately I don't have any photograph in hand. It is easier to do it if the KF plug is thicker than standard (for the semiconductor TOF detectors I'm machining special flanges). Alternatively a threaded post may be brazed or TIG welded.
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ChristofferBraestrup
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

That sounds like the ideal option.

I think I'll begin with the internal circlip solution, as I quite like the concept of modularity and not committing hardware permanently.

The best example of a floating HV part of a vacuum system is maybe really old school electrostatic accelerators, where a Van de Graaf would elevate the ion (or electron) source to hundreds of kV,

and the "drift tube" would then be a long glass tube down to a grounded target and vacuum system.



For the small HV feedthroughs I plan to use die-cast hobby boxes drilled to slip over the two insulators, and grounded to the chassis to break the terminals out to BNC or SHV connectors.

Does anyone know, by the way, how to estimate operating voltage of straight ceramic insulators from lenght?
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Dan Knapp »

Look at the ceramaseal catalog and note the operating voltage specs for different length ceramics on their feedthroughs.
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Maciek Szymanski
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Maciek Szymanski »

This is a typical feed through flange I'm using for semiconductor detectors:


4D9DA4E5-DCDB-4BB5-9B91-55B6326B5E75.jpeg
One BNC is for bias voltage the other is for the output signal. The standard BNC usually holds up to 1kV. I've built those feed through with standard bulkhead BNC sealed with beeswax-rosin. Lately I'm using surplus bought semi hermetic Ampenhol model indicated on the drawing. The outer rubber seal holds the vacuum without problems, but I'm sealing the center conductor with beeswax-rosin or epoxy in place indicated by the arrow. The vacuum side hexagon flange of the BNC is machined to the diameter specified to fit in the KF40 flange. The flange is machined from EN AW2017 aluminum alloy.


This is a sketch of the installation of the offset photodiode particle detector in the KF40 tee:


C4CB78F0-74C9-4170-8BE9-C72B91C24C2B.jpeg
The M3 threaded stalk is used as the detector support and the vacuum side grounding point. The flange/feed through is as on the previous drawing.
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ChristofferBraestrup
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

That's a very good overview - and beautiful schematics!

I've gotten an idea that - if it works - would solve the problem in a pretty neat way. I'll post more once I've tried it tomorrow.

Long story short, the groove in a KF flange end cap / HV feedthrough is large enough to accept a braided copper "sock" - stretched out RG/58 copper braid works fine.
This is clamped against the feedthrough by the locating edge of the centering O-ring and thus makes good electrical connection. A thin wire can then be woven into the braid or soldered in a way that the ring still seals flushly - and that is a ground lead for whatever you need it for.

I plan to use 1,5 mm2 solid un-enameled copper wire to form rigid connections and mechanically support components, and then I think I'll survive my ground lead being flimsy.

Either way I'll post pictures tomorrow.
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

Here are the pictures of the finished channeltron mount!

Stretched RG213 braid fits great and is fairly stable to being pulled out. The two live wires are also mechanical mounts while the ground is merely following along. The rigidness of the channeltron is good enough that I dont think vibration is an issue.

Hope someone else can use this trick!

Oh and be VERY observant of lose copper wire bits in the braid ends - tiny free flowing copper wire fragments would wreak havoc in a running turbo!
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ArkadiuszGibes
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ArkadiuszGibes »

Where did you get from those soviet channeltrones ? Do you have some decent source of it ? :)
ChristofferBraestrup
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

They were on ebay! Complete NOS vacuum packed with serial numbered datasheet/manual! I think I bought the last 2. If they work it's very nice but who knows after 35 years storage
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Maciek Szymanski
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Maciek Szymanski »

They are in production by Baspik in Vladykalkaz:

https://baspik.com/products/keu/sem_6m/

The company has a whole range of detectors, image intensifiers and phosphor screens at good prices (I'm not related to the company - I just made some sensor quotations from them at work).
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
ChristofferBraestrup
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by ChristofferBraestrup »

Whoa! What great info! Thanks!

You wouldnt happen to also know if the VEU-6 ceramic mount is beryllia would you? I need to extract the spiral channel from one.
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Maciek Szymanski
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Re: Ground terminals for HV feedthrough: chamber as return path

Post by Maciek Szymanski »

Unfortunately I don't know the material and it is nit stated in the data sheet.
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