FAQ - View windows your fusor - safety first
Posted: Mon Oct 10, 2011 11:13 am
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
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