Linear Accelerator Project Update

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Dennis P Brown
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Real name: Dennis Brown

Re: Linear Accelerator Project Update

Post by Dennis P Brown »

Richard, thank you for both your encouragement and comments! This project has been a constant battle but really a learning experience in more area's of applied technology (and buy cheap on ebay!) than I was ever prepared to address. Really would have been hopeless without the comments, suggestions and ideas from the many extremely talented and smart members of this forum. Really goes to show the valve of the Fusor Forum!
George Schmermund
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Re: Linear Accelerator Project Update

Post by George Schmermund »

Dennis - I think that you're missing the point about measuring the charging current on your VdG. Voltage has nothing to do with the measurement. You need to remove the HV terminal so that you can get to the top comb. Leave the terminal off. The object is to short the comb to ground through a microammeter. This can (and probably should) be an analog panel meter. They are relativity expendable. Don't use any resistors. Be cautious with the meters that have a plastic case as they tend to charge up on dry days and make the needle stick. You can check this by just running your finger across the front of the meter. If the needle moves and then sticks, breath on the plastic and the needle should return to zero when not connected up.

I'll assume that your 'off the shelf' VdG was designed to charge itself triboelectrically. If it still has the same belt and roller system you can start your current measurements with the original setup. Don't use any external charging. The polarity is inconsequential at this point. You're just trying to get an indication in order to understand the measurement.

The important thing is to remember that you only need to measure the charging current at the top comb. There is no need to try and produce any voltage at this stage. The only resistance between the comb and ground should be the meter movement. Again, making this measurement in a darkened room, without the HV terminal in place, will be very instructive.
Anything obvious in high vacuum is probably wrong.
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Dennis P Brown
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Re: Linear Accelerator Project Update

Post by Dennis P Brown »

First off, again, thanks for your posts and especially this one! You are absolutely correct and I have approached this aspect of my project totally wrong (I foolishly considered this aspect of the project as just too simple to worry about ...not exactly the best idea no matter the excuses and the best way to fail in any case...hopefully, a lesson I will not repeat any time too soon.)

Hate to say it but I never thought of directly measuring the current from the top comb directly with the hemisphere off! And I might add, this time I do deserve some sarcasm! LOL ;)

As you also correctly point out, the polarity issue is totally irrelevant at this stage. My efforts in that quest has created more issues/problems than I need right now when what is paramount, as you have posted, is an operating VdG. So I will return the device back to its correct/original configuration (something I should have done a while back ... ) and attempt to get things working properly. For something so simple I've really managed to screw it up - might make space cadet after all ...
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Dennis P Brown
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Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Linear Accelerator Project Update

Post by Dennis P Brown »

Converted the previous PowerPoint image to a PDF file. Identified some other components.
Also, I should note that the wiring system allows rapid dis-connect so all these components can be quickly removed for service. The "rat's nest" of large diameter white wires bundled together near the bottom and center of the image are common ground wires for the various HV power supplies. These in turn connect to the Rheostat's common ground point. The lone HV feed-line for the gun is not visible in this image.

A minor point: The high voltage for the gun enters the accelerator tube via a very tiny hole drilled through the side of the glass vacuum tube; this hole was then completely sealed with epoxy using minor external pressure to force the epoxy to spread away from the hole creating a wider barrier around that hole. Then while the epoxy was drying, a ultra thin needled was pressed into the epoxy to make direct connect with the metal tube that forms the end of the gun. This created a clear hole to access the gun's HV tube inside the accelerator's tube - even when it is under high vacuum! This "open" channel is not exposed to the vacuum and the metal needle can be removed/replaced as needed.

The ground for the HV deuteron gun is just the gas inlet metal capillary tube - the teflon feed line connects to this gun via a compression fitting.
Attachments
Accelerator.pdf
Accelerator Penning Gun and High Voltage System for the Electro-Static Accelerator Project
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