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Ultra High Voltage Insulator to Metal Bonds Per NEC VP Interview

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LutzH

Fri Nov 13 2009, 08:58PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Posts: 136

Reference HV insulator to metal bonds I recently had the privilage to talk with Dr. Gregory Norton the VP of NEC (National Electrostatics Corp) reference HV particle accelerator tube construction. These tubes are essentialy a series of dish shaped metal electrodes, seperated by HV insulating rings. In this application the HV insulation envelope is pushed to the absolute limit. They put these in SF6 (Sulfur Hexafluoride) gas at 80 PSI. SF6 insulates 3-4 times better than air, even at normal pressure.

NEC has tried many materials, and combinations, for building HV accelerator tubes. Dr. Norton was kind enough to give me about 15min of his time last week on the subject, so now I want to share part of what he told me:

NEC uses 2 basic styles of construction, they make both a high, and a low gradiant design. The interesting part of the conversation was concerning polymers, and HV. I asked him if they have ever tried to use any types of Polymers as insulators? His reply was that they did, but they all failed in one way or another.

He mentioned that one company suceeded with a low gradiant HV design for an ion implanter, using a cast low vapor pressure material, probably a filled HV epoxy material. The problem with these is if there are any bubbles in the metal to polymer bond area, these will lead to eventual failure via an internal conductive carbon track, which to make matters worse is often invisible to a external visual inspection.

The one place a polymer is used in this ultra HV area, is in the accelerator tubes made by NEC's main competitor High Voltage Engineering in the Netherlands. They use dish shaped metal electrodes, with a center hole for the ion beam, PVA glued to glass insulating rings to make their tubes. The glue is PVA (Polyvinyl Acetate) Elmer's Glue!!! I was like; You have got to be kidding, so my children have been using the best ultra high voltage glue at school for all of their little projects!!! I am sure that they use a more refined form of PVA, but chemicaly it is basicaly school childrens glue, I even remember the taste, but thats another story.

I asked Dr. Norton about why do they not use Torr Seal? or some other high vacuum epoxy, his reply was that the carbon tracking problem presents, along with delamination of the joint and that this has of course been tried. So PVA is still the glue which is used for ultra HV metal to insulator bonding.

I just wanted to pass on this information because I thought that it may be useful to folks in other ways also. I can think of many times where I have had to glue something to do with HV involving metal, and insulators etc. Now for some applications the solution could be dirt cheap and available 24-7 at the local 7-11 store :) PVA works with metal to glass and ceramic bonding, but which plastics if any that it will bond to I do not know.

Take Care.....Lutz

PS: This info is making me look even harder at Polycarbonate for bobbins ect, since it is clear. A carbon track could be seen upon visual inspection, which would be very nice for forensic purposes if something goes wrong. Maybe this is part of the reason Polycarbonate has been recently used by Lawrence Livermore in HV pulse applications, where before Teflon dominated.

]acceltb1.pdf[/file]
]acelltb2.pdf[/file]

jpsmith123

Sat Nov 14 2009, 06:31AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Those accelerator tubes are probably extremely expensive...I would guess at least several thousand dollars each.
That's one reason that the accelerator I hope to build some day will be pulsed.

Anyway, as far as polycarbonate for HV goes, I believe it has been used for spark gap housings for many years. However, I have one paper from LLNL where they used it for a spark gap housing, and they had some problems with crazing as a result of machining it, IIRC.

Patrick

Sat Nov 14 2009, 07:14AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Yes, i too have had stress crazing problems after machining, (but iirc) machining speed, rpm sfm, feed rate are chosen as for low heat on milled surfaces. reduces the crazing greatly, I suspect UV light effects PC harshly too, and for a spark gap maybe this plays a destructive role too.

also i have had better results with makralon (cell cast i think) so manufacturing and or production may make some PC better then others. i think, maybe?

-Patrick

jpsmith123

Sat Nov 14 2009, 01:38PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

What about acrylic, does that machine any better than polycarbonate, or do you get crazing with that too?

Herr Zapp

Sat Nov 14 2009, 06:38PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

JP -

Acrylic and polycarbonate have similar machinability, but polycarb is "tougher" and does not have the tendency to "chip" quite as badly as acrylic does.

Cutting tools with "zero" rake angle work best; standard metal-cutting drill bits have far too steep a rake angle, and tend to self-feed so agressively that they frequently crack both acrylic and polycarbonate, especially at the instant where the drill bit first penetrates the far side of the workpiece. You can modify the cutting edge of conventional drill bits with a small stone in a Dremel type rotary tool to create a 0-degree rake angle.

Both polycarbonate and acrylic are susceptable to solvent-induced stress cracking and surface crazing, especially if the part is under mechanical stress. DO NOT expose these plastics to any hydrocarbon solvents! You can use soapy water or milk for a lubricant when cutting or drilling; use a slow-enough speed so the cutting tool cuts, and does not melt the material.

Regards,
Herr Zapp

Patrick

Sun Nov 15 2009, 02:37AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Yes, I second Herr Zapp's statement, and his milk idea too.
Aromatic hydrocarbons and alcohols are quite harsh on acrylic and PC, if i must then i use VM&P Naptha.

-Patrick

jpsmith123

Sun Nov 15 2009, 07:00AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well the 12" x 12" x 0.75" acrylic sheet I ordered for my vacuum chamber is to be drilled by the vendor (I paid a few dollars extra to get a 1" hole drilled through the center of the piece), so I'll report on that when I receive it (hopefully sometime next week).

The main reason I asked about machining these plastics is because, sometime in the future (once I get this transformer built to my satisfaction, which may be never), I intend to try to build a trigatron spark gap, similar to a model #SG-101M, made by R.E. Beverly, which has a polycarbonate body.

BTW, does anyone know how well "Ultem" (polyetherimide) can be machined compared to polycarbonate and acrylic?

Herr Zapp

Sun Nov 15 2009, 06:50PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

JP -

Ultem has excellent machinability, and I don't believe it has nearly the susceptability to solvent-induced stress-cracking that acrylic or polycarbonate have.

Brian Basura built a beautiful tragatron several years ago, using an aluminum body with HDPE liner, and polycarbonate end-plate with brass electrodes. This gap gave excellent performance in his quarter-shrinking system.

See:

and

Regards,
Herr Zapp

jpsmith123

Sun Nov 15 2009, 11:19PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well that looks like some nice machine work there.

For applications like his, i.e., "quarter shrinking", he probably doesn't need to worry very much about the inductance or the rise time, he just needs a robust and reliable switch.

Since I want to minimize the inductance and the rise time, I'm interested in something like this:

wylie

Mon Nov 16 2009, 05:54PM

Registered Member #882 Joined: Sat Jul 07 2007, 04:32AM
Location:
Posts: 103

Sweet link Herr! Here was my favorite part: "Looking at the shadows cast by the explosion..."

Good lord!

Gotta second the "nice machine work" sentiment too. Those brass electrodes are a thing of beauty.

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