DIY Viewport - Photos

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Posts: 90
Joined: Fri Aug 26, 2011 10:19 pm
Real name:

DIY Viewport - Photos

Post by JakeJHecla » Sat Oct 20, 2012 8:37 pm

Hello all-
In working on my fusor, I came across issues finding a suitable viewport. To get around risking my money on eBay or shelling out nearly $200 for a new one, I decided to try my hand at creating a DIY solution. I started with a cheap 2.75" CF blank purchased on eBay for $25. I then picked up two glass disks from McMaster Carr ( one measuring 1.75" x 3/16" and the other 1" x 3/16")
I then borrowed some time on my school's lathe to complete the project. I used a boring bar to make a 1.25" hole through the piece. Next, I made a 1.75" x .25" deep cut to seat the larger glass disk in. After cleaning up and sanding the area, I used JB weld to affix the window in the cutout. Once this had dried completely, I used a minute dab of clear RTV to hold the smaller disk in place as a sacrificial viewport protector. It seems to hold vacuum quite well, and I have yet to see evidence of significant outgassing from the JB weld, even when baking out with a heat gun at ~150C. While I can't comment on the use of this design for anything more advanced than a demo fusor, it seems to work quite well in that role.

1) 2.75" CF flange on the lathe.
2) Finished VP on fusor, running about 10kV. Some fluorescence of either the glue or the borosillicate can be seen.
LatheOne.JPG (403.06 KiB) Viewed 3441 times
Window.JPG (430.18 KiB) Viewed 3441 times

User avatar
Jim Kovalchick
Posts: 438
Joined: Thu Apr 14, 2011 12:00 am
Real name:

Re: DIY Viewport - Photos

Post by Jim Kovalchick » Sat Oct 20, 2012 10:00 pm

Nice work. Is your glass borosilicate? It will hold up better to heat than regular glass. You should do fine with your JB Weld. When exposed to plasma, JB Weld seems to develop a hard coating and you will not have trouble with outgasing from it. If your window survives demo fusor conditions you will also be successful when you use D fuel. Good luck.

Posts: 28
Joined: Thu Sep 20, 2012 4:29 am
Real name:

Re: DIY Viewport - Photos

Post by kcdodd » Sun Oct 21, 2012 2:04 am

I think the way most large viewports are done is with o-rings. You just need something to hold the glass in place while at atm since vacuum will suck the window into the oring to make the seal, and a groove in the flange or something to keep the oring from collapsing. I am not sure if a small window will work as well since there wouldn't be as much pressure from the vacuum from the smaller surface area.

Posts: 209
Joined: Thu Mar 05, 2009 6:49 pm
Real name:

Re: DIY Viewport - Photos

Post by RobertTubbs » Sun Oct 21, 2012 2:14 am

Most viewports use a glass to metal seal, I've never seen one with an o-ring.

Regardless, I'm a big proponent of JB-Weld, I've used it on numerous occasions with zero evidence of leaks. I'd dare to call it the new and improved parchment sealing wax of vacuum assemblies.

Kudos on the nice VP - I'm sure I'll be seeing more of these now.


Posts: 28
Joined: Thu Sep 20, 2012 4:29 am
Real name:

Re: DIY Viewport - Photos

Post by kcdodd » Sun Oct 21, 2012 3:00 am

well, most viewports are under a foot across.

Conrad Farnsworth
Posts: 135
Joined: Thu Jul 08, 2010 1:35 am
Real name: Conrad Farnsworth

Re: DIY Viewport - Photos

Post by Conrad Farnsworth » Sun Oct 21, 2012 6:38 pm


Very well done! I used a McMaster Carr O-Ring on my demo fusor and got away just fine! I few steps behind you! I will have to give this a shot if my viewport breaks! I wasn't so sure that your choice of sacrificial glass adhesive (RTV) was a good idea, but apparently it's not outgassing. Best of luck with neutrons Jake!


User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA

Re: DIY Viewport - Photos

Post by Doug Coulter » Mon Oct 22, 2012 3:40 pm

In the spirit of TMTOWTDI (theres more than one way to do it), here's a viewport/door we made for BillF's cylinder fusor. We have an 8" flange intended to adapt to a smaller size, cut an O-ring grove where the original CF gasket seat was (nothing fancy here - just a square groove most of the depth of the O ring), used a viton ring, and our "magic sealing wax" which is ~80% pure beeswax (McMaster) and good grade oil (Diffoil-20) to grease it up a bit. The door is held on during times of no vacuum by those lift and twist pull-away little angle brackets, but can be easily removed when there's STP in the tank so you can build your ship in a bottle. There were no leaks due to this, or at least, none we could notice on a diff pump system. This system is what made our first neutrons, now in storage.
ViewDoor.jpg (62.79 KiB) Viewed 3441 times
Why guess when you can know? Measure!

Dan Tibbets
Posts: 578
Joined: Thu Apr 17, 2008 5:29 am
Real name:

Re: DIY Viewport - Photos

Post by Dan Tibbets » Wed Oct 24, 2012 3:04 am

O rings have worked well in my experience. There does seem to be a limit of ~ 1 to 1.25 inch diameter, below this and a good seal seems to be insufficient. Leakage and/or prolapse of the O ring is a problem. 1.5 inches or greater seems to work well- this would result in about 30 lbs of 'push' on the glass. Something like epoxy might work but I speculate that glass breakage may be more of a problem as the pressure and temperature changes. A small O ring would work if the glass is held down tightly with clamps/ bolts./ collars.

PS: Hot melt glue seems to work terribly. I don't know if it is out gassing or porosity, but it leaks vacuum quickly. I have had uncertain results with silicone glue. It might take a long time for the curing gasses to dissipate. It does seem to work fairly well as a sealer for threads, though I prefer epoxy, at least for metal to metal seals

Dan Tibbets

User avatar
Rich Feldman
Posts: 1308
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: DIY Viewport - Photos

Post by Rich Feldman » Wed Oct 24, 2012 4:38 am

Nice work, Jake!

Yours is not the first window fluorescence to be reported here.

While looking for windows to transmit shortwave UV (254 nm),
I found surprisingly affordable disks of fused quartz.
IIRC, some are specified to be low fluorescence.

Here's one from a dealer (not just on ebay) near me. ... 0966050438
All models are wrong; some models are useful. -- George Box

Peter Schmelcher
Posts: 173
Joined: Sat Nov 13, 2010 6:56 am
Real name: Peter Schmelcher

Re: DIY Viewport - Photos

Post by Peter Schmelcher » Wed Oct 24, 2012 9:27 am

For what it’s worth another source of glass is a surplus optics outfit
The 41mm diameter 9.5mm thick window fits nicely onto a KF40 centering ring lip (41.2mm).

Post Reply