Rebuilding my fusor.

[Dennis P Brown]

I use a mirror as well.

[William_Estlick]

I have finally chased down the vacuum leak. After a lot of troubleshooting, I found both valves have failed.

Now there is a hole in my vacuum chamber, and I have to figure out how to connect my vacuum pump to it.

[Dennis P Brown]

Simple ball valves are generally not vacuum grade. Obtain proper vacuum valves. Trying to mate an o-ring to a curved surface is not something that will hold vacuum very well. Flat mating surfaces are required. This is why vacuum chambers, if round, use extensions (welded) that are flat. Due to thermal expansion issues, epoxies and similar sealant's will not 'fix' this issue. Maybe use the top (flat) plate for your vacuum connection. Also, a far bigger access hole would vastly improve your pumps ability to achieve the required vacuum.

[Rich Gorski]

William,

Dennis is right. Ball valves maybe water tight but generally not good for vacuum. You need a valve that has either an o-ring seal or a metal to metal seal that can be tightened down. You can't tighten the seal on a ball valve. As far as your vacuum connector on the chamber, you need a bulkhead connector that uses an o-ring. As Dennis said these are for flat surfaces not curved although you might get away with it if the radius of curvature is large enough and the o-ring thick enough so that the o-ring can deform to the radius of curvature. I have done this with success creating electrical feedthroughs for ion sources through the side wall of a cylindrical chamber. I'm not sure of the leak rate but my pumping speed was high enough (turbo) to get the system into the low 10-5 Torr region.

Have you thought of using the flat bottom of your chamber with a o-ring bulkhead connector as in a bell jar system?

Rich G.

[Ryan Ginter]

I have a cooking pot-type vacuum chamber like yours that I use for non-fusor related projects. I modified the chamber by sealing off the ball valves with JB Weld from inside and then replaced the glass lid with polycarbonate.

I drilled a hole about 1/3 of the radius in through the lid and used that to mount a KF-25 flange. The flange has a viton gasket that seals the inner surface.

Here is an Image of the setup.

As you can see the chamber is able to drop down to 10 microns, though it should be stated that I use a rather large vacuum pump (Leybold D-16E). I don't know that a plastic lid would be suitable for a fusor, issues would likely arise from the ion beams.

I would go with Rich's suggestion. Use an KF-25 adapter like the one on my setup and drill a hole into the flat bottom of your chamber.

[Rich Feldman]

Nice pic, Ryan. (Ryan was talking about the other Rich.) How big is the hole in chamber lid?
Did you consider a KF-25 bulkhead clamp, where the KF o-ring directly seals to flat chamber surface?
https://www.htcvacuum.com/en-global/vac ... head-clamp

I used one on a plastic bell-jar baseplate in support of a science workshop program many years ago.
Bulkhead need to be thick enough for internal threading of blind holes, or through-holes that are capped on vacuum side.
3/8 inch worked for me.

As for flat lids on wide vacuum pots, as discussed in this thread:
They can be plenty thick enough for strength, and still experience substantial deflection under vacuum.
How big of a gap do you guys get in the center, when touching a straight edge onto lid of evacuated chamber?
Like floor joists for wide rooms, design for stiffness (acceptably small deflection) can require much more depth than design for strength.
It's easy to find formulas for deflection of round flat plate with free edge and uniformly distributed load.

[edit] The flat bottoms of metal pots deflect inward under vacuum, perhaps more than plastic lids. The bottoms are typically much thinner, but elastic modulus of metals is generally much higher than that of hard plastics. Deflection formula for clamped edge would get closer to reality than the formula for free edge.

[Ryan Ginter]

I believe the hole was 1" diameter (project was from a few years ago). I didn't consider the use of a bulk head clamp. The chamber walls weren't thick enough for internal threads, and I wasn't a fan of attempting to prevent leaks from through holes.

The lid on my pot vacuum chamber is half-inch polycarbonate. Its been a while since I last had it out, but I want to say the deflection in the center is roughly 1/8".

[Rich Gorski]

This is the kind of baseplate feed through I was thinking about. You can find it at LDSvacuum.com (link below) for around $200. A KF25 version, kind of expensive but there are no screws into the baseplate and the plate can be quite thin. Maybe there's something similar elsewhere for less money.

baseplate fdthu.jpg (14.44 KiB) Viewed 1334 times

https://www.ldsvacuumshopper.com/nwto1bab.html

Rich G.

[Ryan Ginter]

You can find Chinese made versions of this on eBay for $50. Quality is the same. In general, you don't lose any performance by choosing cheaply made KF fittings.

Actually, after looking I'm not seeing any listed at the moment.
https://bmotiontech.com/products/bmotio ... yn0702lb2M
This is basically what you're looking for. Not sure if this particular site is reputable though.

[JoeBallantyne]

Bmotiontech makes good stuff. I've bought a few different parts from them. I wish they would make a one piece 1/4 inch female flare to KF16 adapter out of 304 or 316 stainless.

Joe.

[Richard Hull]

A point of note:

This great info on a reliable chi-com source of vacuum goods. Such info should have been added to the "trading post - resources" listing as a reply with URL.
It would also have been great in the vacuum forum. I created the resources listing for others to add info of proven, reliable sources of fusor related goods. This should prove a great benefit to newbies who are bewildered as to how to obtain much needed parts and supplies.

Richard Hull

[William_Estlick]

I am replacing the valve assembly with a simple barb connector and using silicone as the gasket.
If it leaks, it will be an easy fix.

[William_Estlick]

My lab has also grown and I have added another workbench.

I think I am going to work on the viewing port next while I am waiting for one more to complete the electrical.

[William_Estlick]

It has also been suggested that I gat an x-ray dosimeter. I can't imagine this one will be very accurate and it won't measure energy levels, but it will be fun to play with.

[Dennis P Brown]

While a dosimeter is certainly a useful safety device, properly measuring the x-ray threat and following proper procedures is how 99% of fusor people tend to operate. Doing a proper test of the x-ray threat is your first and most important procedure to master. And yes, those devices do not measure a very serious energy range of possible x-rays (below 10 keV.) However, that does not prevent one from getting an idea of the threat and I would expect, your use of Pb shielding is (depending on your voltage) likely sufficient.

[Cinar Kagan]

If I calculated it right 1/16” = 1.5mm pb is enough to shield 30 keV rays.

Cinar Kagan

[William_Estlick]

Thank you for the calculations, Cinar. If it was not for the guidence from this forum, I would have run this experiment at 30kv without any lead shielding.

[William_Estlick]

A mirror mount made of foam board and duct tape.

[William_Estlick]

Just cutting an x in the lid, I could fold over the triangles to create a window for the mirror.

[Ryan Ginter]

That turned out quite nice. It's good to know you won't be at risk of unnecessary exposure now. I imagine you may potentially handle the lid quite often? If that's the case, you may want to throw a coat of paint over it.

Be mindful of the sharp edge next to what I presume is an HV wire. You should roll it over to reduce the electric field strength.

[William_Estlick]

Thanks, Ryan.
I plan on wrapping the inside with a rubber sheet becouse of I am concerned about the high voltage arcing over to the grounded shielding.
I am also not sure how the insulation of the hv wire is going to handle 30kv.
We'll see.

[Dennis P Brown]

A rubber sheet will do nothing relative to 30 kV. Your wire should be rated for 30 kV. Ryan's suggestions should work fine if your wire is properly rated. Definitely paint all exposed lead surfaces; always use soap and water to clean your hands after handling lead. The entire internal metal case should be grounded - otherwise, you have an extreme electrocution hazard. Then the Pb is fine if the case is grounded. The HV connection points are the most likely places to short to ground.

[William_Estlick]

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The shroud will cover the hole and I'll view the plasma using a camera on my laptop.

"Any sugestions for the color of paint for the shielding? I'm thinking that 1950s industrial green."

[William_Estlick]

After chasing leaks for an hour, I am able to bring the vacuum down to 2225 microns. When I turn the pump off and close the valve, I still have a very, very slow leak. I think it might be the gasket between the chamber and glass.

I'm going to break it all down to install new cathodes and wiring preparing for 30kv and hopefully find the last leak.

After looking up the rating for the red hv wire, it is 50kv. I don't know what the white hv wire is rated for as it is not labeled, but the insulation is thicker and the conductor is of a larger guage.

[Richard Hull]

I find it abysmally easy to understand why you can't get even a halfway decent vacuum. Others have already advised. I choose not to waste my time in furtherance of their prior efforts and advice. 10 pages of wheel spinning effort.

Richard Hull