Homemade Cathode Ray Tube

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Jacob Shin
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Homemade Cathode Ray Tube

Post by Jacob Shin » Mon Sep 02, 2019 1:11 pm

Hello everyone. This isn't a fusor or accelerator, but the cathode ray tube (CRT) was a precursor to the atom smasher, so I thought this might fit on this forum. A CRT is basically an electron beam accelerator/electron gun, so I thought it would fit under ion guns. This project is way cheaper than a fusor or a linac, and a little bit more interesting than a demo fusor. So I made this as my final project for school based on http://www.sparkbangbuzz.com/crt/crt6.htm. I used a cheap ebay single stage vacuum pump for pumping and a flyback transformer to power the CRT. Unfortunately, the CRT didn't work quite as well as I would have hoped and I currently have no way of measuring the voltage or vacuum level for the system.

Here's my setup:
IMG_20190604_201908-min (1).jpg
IMG_20190604_212901-min (1).jpg
IMG_20190604_201540-min (1).jpg
A video of magnetic deflection: https://youtu.be/T7H3Pw8X9eo

The CRT sort of works. The electron beam makes the phosphor glow and magnetic deflection confirms that there are charged particles in the beam. However, I was never able to get a clear dot on the phosphor; instead, I always got a blob as you can see in the pictures above. This would have been the ideal result it worked: https://youtu.be/5gGddyFz3aQ. Unfortunately, since my setup never produced a distinct 'dot', I could never form any kind of images like the other guy in the video did. Is there any obvious reason why my phosphor has a 'blob' and not as focused? Could it be the vacuum level or the voltage? Would it be more beneficial for me to get a better vacuum pump, a thermocouple gauge of similar, or a high voltage metering system? Or are all these equally important? Thanks in advance for any input.

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Rich Feldman
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Re: Homemade Cathode Ray Tube

Post by Rich Feldman » Mon Sep 02, 2019 6:27 pm

Nice work there, Jacob. I like the Helmholtz coils and compass on the bench.

About 10 years ago I played with the same sort of cold-cathode CRT. Like you, following Nyle Steiner and sparkbangbuzz.

It wouldn't hurt for you to set up instruments to measure the voltage, current, and vacuum pressure.

You might be able to sharpen up your beam with quicker, easier steps.

1. Make sure the DC current doesn't have a huge amount of ripple current. The HV side of your flyback power supply needs a HV diode, and, for this application, probably a HV capacitor. The doorknob style is popular; a Leyden jar or old CRT could also serve. With capacitor comes the risk of _much_ stronger electric shocks, even after you've turned off the power and pulled the plug.

2. Does the glow discharge go out if you pump long enough? If not, give some attention to vacuum leak abatement. What happens if you put a few drops of vacuum oil on the outside of the rubber stopper three contact surfaces? Or valve off the pump and observe the discharge for signs of pressure creeping back toward atmosphere. A shorter hose might help some. When you can achieve a pressure too low to glow, then you can _intentionally_ back off on the pumping.

3. Then play with the electrode geometry and distance to fluorescent deposit. IIRC, my anode was a strip of Al foil, with a pinhole, folded over the end of narrow glass tube inside the flask.
All models are wrong; some models are useful. -- George Box

John Futter
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Re: Homemade Cathode Ray Tube

Post by John Futter » Mon Sep 02, 2019 8:03 pm

Jacob
Well done
I woulod not expect a dot at that distance without a focusing element
the simplest of which is the einzel lens a triplet of elements after your elecron source
beam blowup at low accelerating voltages is what you are suffering from the electrons are repelling each other so the beam blows up in size.
as accelerating potential rises the electrons go faster so they have less time to move aprt (blowup) so the beam tends to stay focussed longer

hence the 30kv odd used in colour crts to maintain focus over quite a distance so that each colour pixel retained true in colour whether delta dot or trinitron

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Mark Rowley
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Re: Homemade Cathode Ray Tube

Post by Mark Rowley » Mon Sep 02, 2019 11:39 pm

Hi John,
I made this one about 12 years ago. It was fairly easy to make a dot as long as the aperture was about the diameter of a pin hole. Focusing elements were not necessary but would've been nice and surely resulted in a much more tiny dot.

https://www.youtube.com/watch?v=nMOjiZR7R6U&t=

Mark Rowley

John Futter
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Re: Homemade Cathode Ray Tube

Post by John Futter » Tue Sep 03, 2019 10:37 am

Mark
Nice job
but by focus I mean less than 0.1mm in diameter at the distance your tube was working --this compared to a colour CRT colour purity

or if we go further a SEM with an electron scan dot size of 20nm diameter or less

Albert Mery
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Re: Homemade Cathode Ray Tube

Post by Albert Mery » Mon Sep 09, 2019 7:11 am

Hi Jacob, nice setup.
I have also built a similar (less cool looking) Cold-Cathode CRT with a Flyback Transformer.
I was never able to find any proper specs for homemade Helmholtz coils, though.
Would it be possible to know, roughly, what magnetic field strength is seen as reasonable for such a use?
Thanks,
Albert

Jacob Shin
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Re: Homemade Cathode Ray Tube

Post by Jacob Shin » Mon Sep 09, 2019 11:21 am

The Helmholtz Coil produced about 12 gauss and I just scaled up https://www.thingiverse.com/thing:3330563

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