Fusor arcing problems

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Luc du Plessis
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Real name: Luc du Plessis

Fusor arcing problems

Post by Luc du Plessis »

Hi everyone,

I'm Luc from the Netherlands.

I'm a high school student from Cygnus Gymnasium, Amsterdam in my senior year. Together with a classmate, we are building a fusor for our school research project.

I would like to start off with how much information is on these forums. They have helped us so much!

Unfortunately, we have some problems with our fusor.
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Our inner grid arcs with the outer chamber.

Video of the arcing problem: https://youtu.be/wdfWsjv-9f0
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We are using a 2 stage mechanical pump and an oil diffusion pump capable of getting the fusor chamber to 1 micron. Our fusor is a flowing gas fusor so it constantly needs to be pumped down. Because we are on a tight budget we couldn't afford to buy a valve to separate the chamber from the diffusion pump. In the test, we used a vacuum of around 15-20 micron.
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For our power supply, we use a 60KV Chinese precipitator supply that some people already did fusion with on the forum. It is completely submerged in transformer oil. On top of the case of the power supply, we cut a hole for the resistors.
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Here is a schematic of the wiring:
fusor_HV_2.jpg
R1 = primary voltage divider resistor (3x 200M 10W in series)
R2 = secondary resistor in parallel with the voltmeter including voltmeter impedance
R3 = 1-ohm resistor for current measuring
R4 = ballast resistor

For the feedthrough, we are using Liam David feedthrough made from quartz: viewtopic.php?f=6&t=13220
It works really well and is completely vacuum-tight.
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For the inner grid, we use 1mm tungsten wire. The diameter of the inner grid is 2.5 cm. Our vacuum chamber is 4 inch (10.16 cm). So we are using a 1:4 scale. That is recommended in the FAQ's.
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The arcing starts at 50KV. Before that, there isn't any activity. no purple glow, no arcing, nothing.

We don't know what the problem is. We think the wiring is correct and the chamber is correctly grounded. The pressure in the vacuum chamber is also low enough (15 micron). We also have deuterium and a neutron meter to test for neutrons but first, we need a plasma with just air.

Best regards,
Luc
Frank Sanns
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Re: Fusor arcing problems

Post by Frank Sanns »

More information would be helpful.

As the pressure goes down to your 15 micron reading, what happens to the current and voltage? Are there many x rays coming out of your window?

If your voltage is really 50kv then x rays should have been streaming out as you got down to that pressure. If they were not, then one of your readings for pressure, voltage, or current are not correct.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Liam David
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Re: Fusor arcing problems

Post by Liam David »

The emission that you're seeing from the grid isn't arcing. It's field emission caused by irregularities in the grid when the pressure is too low. I see the exact same thing when I put 50kV in my fusor when the pressure is below ~10 microns. Try starting at a really high pressure, say 100 microns, establish some plasma, and then slowly lower the pressure to allow the voltage to rise.

When you graduate to flowing deuterium instead of air you really want to have a valve between the diff pump and chamber. You'll be wasting a lot when you don't really need more than a few cc/min at most.

Be careful with x-rays, especially since you're putting 50kV on the grid. You should have lead in front of at least the viewport, and preferably shielding the entire side that you'll be standing on. Do not look into the viewport when you have plasma at >20kV, or 15kV even. It will pour out several rem/hr at 40-50kV. You seem to have a camera and a Geiger counter which is good.

One small recommendation with the feedthrough: Try centering the stalk within the quartz. A ceramic bushing or similar would work well. This will help reduce the chance of failure since the electric field is more uniform and not stronger in one part of the quartz.

Good work!

Edit: Frank posted his reply as I was drafting mine. I agree, double check your metering. I've found that you won't necessarily get many x-rays at 50kV when there's just field emission like you show. Not so when there's significant current flowing however.
Luc du Plessis
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Re: Fusor arcing problems

Post by Luc du Plessis »

Thank you so much Liam David!!! It was field emission. We tried again with your tip about centering the stalk within the quartz and we got our first plasma! At around 80 micron! At 2200V and 2.6mA.
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We also tried to do some higher voltage at 30KV the Geiger counter for the glassport gave about 600 μSv/h. But it didn't go through the steel. So we were safe.

Tomorrow we are going to make some neutrons with the deuterium!
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Mark Rowley
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Re: Fusor arcing problems

Post by Mark Rowley »

Excellent work!
Just a cautionary note. 80 microns is a bit high for activating the precip power supply. Try not to energize the supply above 50 microns as vacuum fluctuations can wreak havoc with the transformers (refer to the faq).

Obtaining higher voltages will be directly related to dropping your vacuum levels below 50 microns.

Mark Rowley
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Richard Hull
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Re: Fusor arcing problems

Post by Richard Hull »

Congratulations Luc du Plessis I have placed your name in the Plasma Club as the first member entering under the new rules. The key to actually doing fusion is full instrumentation to read pressure, voltage and current so that you can establish a base line for operation or, as time goes by, be able to sense problems where the instrumentation indicates issues outside your established norm.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Joe Gayo
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Re: Fusor arcing problems

Post by Joe Gayo »

Your discharge voltage will increase substantially once you switch to deuterium, but you will likely need lower pressure as well.

Congratulations on entering the plasma club!
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