FAQ - View windows your fusor - safety first

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Richard Hull
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FAQ - View windows your fusor - safety first

Post by Richard Hull » Mon Oct 10, 2011 3:13 pm

We have always recommended some way to view your inner grid, both in the demo and full fusing fusors. This is important due to the need to see what is going on and to avoid melting or damaging the grid due to electron runaway when too much power is made available and the electron emission of the grid starts to allow large currents to flow. This is a self feeding condition that runs away until destruction of the grid. You need to see this condition well before it does damage and back off the voltage applied. Visually, it appears as an ever reddening grid to orange heat and beyond. Only continuous visual inspection as power is applied provides a safe path to avoid ruining the grid.

Demo fusors are often built up in bell jars, dessicators, or special all glass jars or cylinders. No window is needed as the entire vacuum chamber is transparent. The voltages are often well below 10kv and the glass more than adequate to stop or severly attenuate any low energy, "soft" x-rays that might be produced at the highest voltages near 10kv.

For those building a fusing fusor or a demo fusor that will ultimately be made into a full fusion fusor, the choice of the chamber material is almost always stainless steel in the form of a sphere or cylinder. A "viewport" is needed to allow the user to observe the grid.

This viewport is invariably either some sort of kludged up window coupled to a conflat, (CF) or other home-made fitting or it is a formal, vacuum rated, viewport window made especially for vacuum work.

The homemade viewport, unless made by careful hands, is often subject to vacuum leaks or failure of ill chosen glass. Anyone working up their own viewport to save money must be aware of the pitfalls in such a project.

A professional viewport is usually a 2.75", standard conflat fitting with a custom, vacuum sealed, window of pyrex, kimax or sapphire. Prices for new windows start at about $75.00 to $100.00 and go up very fast on larger windows in larger conflat fittings. 95% of all fusor builders use the 2.75" CF windowed viewport due to cost and many other reasons to be covered below.

Note: Used viewports must be suspect as they may have a leak or other problem. They may be "pulls" due to defect and as such are almost worthless. Be carefull here.

Viewport issues of fusors operating over 20kv...........................

In a fusor operating over 20KV, x-rays will spew out of the port in a rather wide cone. This makes such a port rather dangerous, radiologically. The eye is one of the most sensitive organs in the body to radiation. Observing through the viewport with the naked eye, on axis to the viewport, is a bad thing for sure. Never do this! There are three ways you can observe the grid in a working fusor.

1. By reflection
2. By shielding glass
3. By remote camera

By reflection - A mirror or system of mirrors can be arranged such that the on axis x-ray beam is completely avoided and the image turned. The x-rays penetrate the mirror while the light image is sent off to a small viewing telescope a few feet away. Note: the x-ray beam is still dangerous going through the mirror, so design you fusor station such that a lead plate is placed behind the mirror or the beam is dumped harmlessly into the floor or the sky. This arrangement usually calls for at least a 1" to 1.25" full glass aperture.

By shielding glass - A lead glass of sufficient thickness can be placed over the viewport glass that might allow direct viewing, but be very careful here. Some folks have broken the thick face of an old television color CRT which is heavily leaded. They have then polished and shaped it to function as a viewport cover that can absorb some of the x-rays. Notice I said some of the x-rays. At voltages higher than 40kv even this solution might be dangerous.

By remote camera - This is, in many ways, an ideal solution. Tiny color and B&W video cameras with a tiny lens are easily found for as little as $30.00. Coupled to a televison monitor, you can see the interior of your fusor plastered across a large screen, if desired. This allows a large number of bystanders and visitors to also watch what is going on.
Note: The beam still goes through the small camera and a beam stop is still needed behind the camera and the position of this setup is all part of the mechanical design of the fusor station. In addition, if your fusor is producing a million or more neutrons per second, you can expect radiation damage to the CCD visual pickup device in the camera. "Blown pixels" show up as small white dots on the screen. They are not very distracting and after 5 years of use, I only have 5 blown pixels in my camera.

Advice on selecting a viewport...........................

My personal advice is to use a camera like a tiny single board camera with a tiny lens that can be easily focused. Purchase the smallest possible aperture window in a 2.75" conflat fitting. (I use a 3/4-inch window) Jam the camera lens directly aginst the glass and focus on the grid. This gives a very wide field of view on most little single board cameras. By keeping the window small, you also keep the x-ray beam limited to a small angle of divergence.

A smaller window is easier to clean once it gets coated with the ever increasing depositing materials from the grid over time during operation. (There is another FAQ that deals with cleaning viewports)

Each fusioneer will find their own path to a suitable viewport, but the above advice is sage and accumulated over the years and several fusors.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Doug Coulter
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Re: FAQ - View windows your fusor - safety first

Post by Doug Coulter » Mon Oct 10, 2011 5:50 pm

All excellent - calling like it really is.

I'd add that you can save some problems with a sacrificial piece of glass or even mica on the inside of your view port. McMaster for example sells 3/8" thick pyrex rounds in most useful sizes for this. Let something cheap and not having to be vacuum-sealed take the abuse of sputtered grid material, any stray high velocity ions and so forth. The system works. I'm on my third piece in the main fusor, the other two having turned brown and picked up some minor crazing cracks. They are a lot cheaper to replace than the window in my expensive gasketed door! And the the fact that I'm on the third one tells all you need to know. They only cut down X rays a little, but they do some good on those too.
I got a 5.5" round pyrex to fit nicely inside an 8" CF flange (6" ID) and hold it in with some little clips I made. That way it's easy to remove for access, or replacement.

I might be the guy Richard mentions as having used a CRT faceplate. Better than nothing, a real pain to polish the pebbly side of so you can see through it well, but only a last resort sort of thing. The tend to be dark glass, and cut down what you can see as well. "Real" rad-shield glass, as from a radiology room is much much better, and the thicker and more layers, the better. We managed to get some 1/2" thick from a hospital that was remodeling, and it really stops the X rays nicely, while being of good optical quality.

We're now testing some leaded plexiglass from the same source, jury is still out. All of the shield glasses you use outside the fusor, they might not respond too well to an ion bath, and are expensive.

I also built a little lead cap for a spare port, with a mirror in it to do the reflection thing. Most of the X rays go straight out and hit the lead behind the mirror, but some do scatter off the lead and the mirror and out the hole in the side. It pays to test all this sort of thing with some handheld geiger counter or other measuring device for X rays. It's a whole lot better than nothing - but I wouldn't sit there staring at it either - a geiger counter tells all. In my case, this gives me a view from another angle, handy sometimes to get a full perspective on what's going on.

The first picture is the view from my operating position. You can see the little clips holding the sacrificial pyrex piece on the inside of the (very expensive) hinged door-viewport, then the door, then the big thick chunk of X ray glass held in the wood frame. This cuts the X rays way way down, and protects my door in the bargain.
The other pic is my little lead hat on top of the tank, so I get another perspective on my grid, from above instead of end on, as I get through the main window.

We also have a 2.75" conflat saphire window. I just put a lead cap on it. The UV and X rays from that are so fierce it's not worth having, really - sunburning your eyes is no joke. Pyrex that stops most UV is a wiser choice.

The sharp-eyed will note I've also shielded the entire fusor tank. It's a good move even if it is hard work to cut all that lead, overlap and solder it well enough that it doesn't fall off. Note, feedthroughs notoriously leak X rays, and are hard to deal with easily -- I do the same thing as the mirror - make a lead box and run the wires out a hole in the side, for all but my main HV which is handled another way. In tests here, we see no significant leakage through the thick flanges, up to our max of 53 kv. Just everything else.
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Why guess when you can know? Measure!

JohnCuthbert
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Re: FAQ - View windows your fusor - safety first

Post by JohnCuthbert » Mon Oct 10, 2011 6:42 pm

Wait a minute, before you rush out and saw up an old CRT to get lead glass.
The face plate is not made from lead glass.

"The viewing section (known as screen, panel or face plate) is made of glass, typically containing a significant level of barium oxide but no lead oxide."
from
http://www.hse.gov.uk/foi/internalops/s ... ndix-4.htm
(and confirmed by a friend of mine using PXRF)
The Ba and Sr may make it opaque to Xrays ( that's the point) but it isn't leaded and unless you are in a position to check it, you shouldn't rely on it.

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Doug Coulter
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Re: FAQ - View windows your fusor - safety first

Post by Doug Coulter » Mon Oct 10, 2011 7:40 pm

Agreed John. I used it as a last resort -- it IS better than nothing, but not as good as the "real thing" in actual tests here. I didn't care then or now what it was made of, just that it was good to stop 30kv X rays in the original application, however they did it.

In no way should anyone working with fusors (or any HV in semi-vacuum) be without a way to test for their own exposure, and what their device is putting out (and preferably where it's coming out from).

That's pretty basic. Anything less puts you in the running for a Darwin award.
Why guess when you can know? Measure!

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Re: FAQ - View windows your fusor - safety first

Post by billwcf » Wed Oct 12, 2011 2:07 pm

Hi, If anyone is interested in relatively inexpensive x-ray glass, Plexiglass, or x-ray film and supplies, these guys are great to deal with. A phone call is best to find out their used inventory. -bill

[color=#FF0000]Admin note 160322: dead link to xrayatalog-dot-com has been removed. [/color]

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Re: FAQ - View windows your fusor - safety first

Post by David Geer » Thu Oct 13, 2011 3:57 am

I like your setup Doug, especially the hinged viewport door. And from Bill Fain's link for lead shielding, you're better off googling for specific x-ray shield doors, lead glass/acrylate, etc. and then doing cost comparisons. A lot of companies will do custom fabrication to include dimensions and lead concentrations.

When I get to building mine, I'll have a lead acrylate shell layered ontop of the chamber and all extrusions. Seeing as it's not very conductive, I'd even do an acrylate tubing around the HV feedthrough to cutdown on said x-ray bleedout at higher KV's.

For video viewports, I was thinking of trying a fiber optic pinhole camera system (still looking at placement and/or pressure sealing layouts). I figure a fiber cam would be a lot less affected by the x-ray scatter and have a very, very small viewport cross-section (mm/cm diameter compared to inches) that helps with the x-ray's angle of divergence.

But again, any and ALL safety precautions are good when it comes to radiated energies. A lot of great advice and good photos from Doug in this thread.
- David Geer

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Re: FAQ - View windows your fusor - safety first

Post by Justin Nichols » Mon May 19, 2014 9:04 pm

Would a fused quartz viewing window hold up? Not to block xrays, to be paired with a remote camera.

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Richard Hull
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Re: FAQ - View windows your fusor - safety first

Post by Richard Hull » Tue May 20, 2014 2:11 pm

Any view port made of anything transparent will work for remote camera viewing. Forget x-rays! They will pour freely and viciously from any view port that you use, regardless of material. As noted forever in all good posts on this subject, keep the view port tiny, 1" or less, and pointed away from any living organism!

This is all part of simple original design and implementation. Remote digital video camera viewing can easily get by with a 1/2" view port. There is virtually no need for a view port to exceed one inch in diameter. No view port will remain 100% transparent for long due to deposition and ease of cleaning is part of any decision on port material.

Borosilicate (pyrex) is the number one choice when considering expense and ease of acquisition. Quatrz is rather ideal, but a worthless over expenditure.

The original FAQ stands as the simplest and easiest solution.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Karl Froelich
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Re: FAQ - View windows your fusor - safety first

Post by Karl Froelich » Sun Jun 15, 2014 5:19 am

Richard, with webcams becoming so cheap, what of not having a viewing port at all, but relying on one or more webcams? they can just be thrown away when they're shot, and not having a viewport at all would be a significant reduction in mechanical complexity.

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Richard Hull
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Re: FAQ - View windows your fusor - safety first

Post by Richard Hull » Sun Jun 15, 2014 3:25 pm

I hope you are not suggesting putting the web cam inside the fusor. I use a tiny camera through a 1" window. You can't have stuff inside the chamber. A good think through will show why a camera can't go in a fusor.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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