Page 1 of 1

Tyler's Demo Fusor

Posted: Wed Feb 06, 2019 3:23 am
by Tyler Meagher
I built a small demo fusor using my rebuilt Pfeiffer Duo 1.5A vacuum pump, a pickle jar, vacuum bagging sealant (tape), a spark plug, a variac, a microwave oven transformer, and a 20K ohm ballast resistor. I put the pickle jar vacuum chamber in a clear plastic trashcan for safety. Below are some pictures of my build, and my demo fusor making plasma.


Re: Tyler's Demo Fusor

Posted: Wed Feb 06, 2019 5:15 am
by Rich Feldman
Nice spot welder. We know you were wearing eye protection. I think having a dad with good tools is as good as having a rich dad!

Nice small plasma ball. Most demo fusor projects pictured here start with a glow closely hugging the negative electrode surfaces, Nixie-tube style. That's sort of lame in my opinion. They might as well light up gas in a narrow glass tube, neon sign style, and call it a demo fusor.

How about adding a HV diode next time? Show the glowage result with both orientations. If added together, do they match the unrectified glow pattern? I would literally pull the plug before messing with any wires connected to a microwave oven transformer, whose full-blast voltage and current are well matched to killing people (with or without 20K resistor). Like pulling the plug on garbage disposal before putting your hand in to retrieve something.

Your last picture caption says "right before the plasma goes out". Does that always happen when you pump long enough? I bet you will soon add a controllable leak, and be able to let some vacuum out just fast enough that the plasma stays lit as long as the pump runs.

Re: Tyler's Demo Fusor

Posted: Wed Feb 06, 2019 7:01 pm
by Dennis P Brown
You have a very nice vacuum system. As for a thin walled glass jar, the sooner you get rid of that implosion hazard, the better. Considering your vacuum system, I'd suggest a standard 50 mm diameter vacuum tee or four-ways cross is well worth the trouble and cost. Using o-ring seals and plexi-glass end covers a very nice demo could then be created that isn't very much far away from a a good fusor grade chamber with a little more effort/work.

Creative to use paper clips for the electrode.

Agree that upgrading to a rectified current source is a worthwhile effort. Also agree that a MOT is very dangerous but so are fusor power supplies so good training if one is extremely careful. So, do be careful.

Re: Tyler's Demo Fusor

Posted: Fri Feb 08, 2019 2:53 am
by Tyler Meagher
Rich and Dennis, thank you for your comments. I do wear safety glasses when working in the shop and lab. I have a procedure I follow when I operate my demo fusor’s power supply. After each run the variac is set to zero, I turn off the power strip, and then I unplug the variac. The variac is always unplugged until the beginning of a run.

Right now the plasma goes out when the pressure reaches about 25 mtorr at full power.

I am just learning about electricity, and I don’t understand rectified output yet. I am not using high voltage diodes like the demo fusor FAQ diagram <viewtopic.php?f=29&t=4405#p27243>. I do plan on adding them. I plan on making some measurements on the output to see the difference it makes with the plasma. I will also run the test Rich suggests and report the results.


Re: Tyler's Demo Fusor

Posted: Fri Feb 08, 2019 10:10 am
by Dennis P Brown
Your fusor goes out because your vacuum system is doing better than 25 microns so there isn't enough gas in the chamber to allow conduction; the pump is doing a very good job and will be an excellent system for a fusor. As Rich pointed out, add a gas leak value to the system so that you can hold the pressure (say, 30 microns) so your plasma stays light.

Good you are wearing safety glasses but that doesn't protect your face. A thin walled glass like that is just waiting to implode. At least upgrade to a thick walled wine bottle. But the plasma will quickly weaken the glass jar and at some point, an implosion will occur with dangerous results. A inexpensive 4-way cross (50 mm diameter) makes a great fusor chamber and really is a good upgrade. Inexpensive glass drills allow one to easily cut holes in glass plates for any type of feed-thru's. Getting three thick round glass blanks that will cover the openings for a 3 or 4 way connector are inexpensive - one can be used as a window, another an electric feed-thru, and another a gas feed thru. Then a KF-50 vacuum connector for the pump line and you have a future real fusor chamber.

One can get inexpensive high voltage diodes on ebay via Chinese manufactures. Some are extremely good - both high current and high voltage at extremely low prices. Just look for the better deals. A diode based system (two diodes to create a positive and negative system or a diode bridge (less ripple)) can be easily built. Lots of details on youtube about those devices. Aside: don't add a capacitor - besides not needing it at all, they make systems far more lethal. Many diode application sights show a cap but for this type of work, they are not needed at all.

Consider building a voltage divider to read the high voltage. That way, you can 'calibrate' the variac for your system.

These are steps one can take to steadily build towards a real fusor. Your excellent vacuum system (pump, lines and gauge), a good variac and ability to make fusor grade electrodes are big advantages compared to most people that try to build fusors. You've also demonstrated the good skills required for building/operating this type of device. Your demo has shown you can create plasmas and have an excellent vacuum system. You really should upgrade to a better fusor chamber. By the way, a small 50 mm diameter T or 4-way give significantly better neutron counts compared to classical large fusor chambers for the same power.

Re: Tyler's Demo Fusor

Posted: Fri Mar 22, 2019 7:19 pm
by Tyler Meagher
I ran the experiment Rich recommended with a high voltage diode. The pictures below show the demo fusor in three different configurations. 1) No diode. 2) Diode arrow pointing away from the transformer. 3) Diode arrow pointing to the transformer. The oscilloscope traces are the high voltage output at the spark plug. On all three runs the pressure was ~50 millitorr.

I added a needle valve and a shutoff valve so I can control the pressure inside the demo fusor.

The positive voltage does not add much plasma to the picture at 50 millitorr. I also took a picture of the plasma at 100 millitorr with the positive voltage.

I do not understand why I get a plasma jet going to the glass wall. I thought all the electrons would be going to the top because it is grounded. I am going to put a metal band outside the grid to see if I get jets in more directions.

3 configurations with diode
Positive voltage at 100 millitorr

Re: Tyler's Demo Fusor

Posted: Fri Mar 22, 2019 9:28 pm
by Richard Hull
You do not have a visible "body grid" in the jar. The body grid, (second grid about the inner grid), is at positive DC potential (ground)..

Richard Hull

Re: Tyler's Demo Fusor

Posted: Sun Mar 24, 2019 9:10 pm
by Tyler Meagher
You are right. My demo fusor does not have a standard outer grid or a grounded metal shell. I have a top metal plate connected to ground. I get a poissor in the middle of the grid and a jet out one side when the pressure is low enough. I do not understand how it works. I am going to put a metal band outside the grid and connect it to ground. I will report the results.

Below are pictures of my demo fusor at different pressures. The voltage was about -600V peak.