Demo Fusor Suggestions

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Johannes Toppe
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Demo Fusor Suggestions

Post by Johannes Toppe » Sun Jan 06, 2019 2:11 am

Hi all,
We are now in the process of building the demo fusor for our science fair project and are only lacking a few materials that we are going to source in a few days. Our chamber is going to be a stainless steel salad bowl of diameter 40cm (15.75in) and of height 16.5 cm (6.5in). As pictured below, we also have an inner grid jig 3d printed from (https://www.thingiverse.com/thing:11643) but we are going to scale that up so that it will be in a 5:1 ratio (scaled up to 3in diameter). Our plan is to put a slab of 1/2 in acrylic to cover the salad bowl (facing upwards) and using rubber to seal it. to seal it. Our vacuum inlines and inner grid will be inserted through the acrylic so we will have a hanging inner grid. One of our doubts was whether we would add an outer grid inside or use the salad bowl as a live outer grid with a current running through the whole thing (but wouldn't having the outer grid touching the salad bowl induce a current in that too?). Lastly, we drafted our research idea to either be the effects of varying high voltage on plasma light intensity (we have a variac) or investigating how varying the voltage (because a flowing current determines the strength of the magnetic field and voltage determines magnitude) affects the plasma being confined - so essentially investigating how the magnetic field created by the wires confine the plasma. Any and all feedback would be greatly appreciated!
20190105_180534.jpg
2 MOTs and hv probe
20190105_180604.jpg
full wave diode bridge rectifieer
Attachments
20190105_180639.jpg
have yet to clamp shut the last tube (vinyl)
20190105_180628.jpg

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Dennis P Brown
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Re: Demo Fusor Suggestions

Post by Dennis P Brown » Sun Jan 06, 2019 10:53 am

I do not see why you have such large (or any) capacitors? Besides those can make a fairly safe power supply into a lethal supply. Caps will provide no usefulness in the demo fusor - real fusor systems do not use capacitors, either. I would urge you to remove those, at the least.

I cannot identify the type of transformer you have. Is it a microwave type? If so, I would suggest that not be used since it can develop lethal voltages/currents. The output wires look like they handle a high current. If so, even a few hundred volts can be lethal. Obtaining an inexpensive neon sign transformer is still a better and far safer approach.

The current from the grid to your ground will not generate a magnetic field that will constrain your plasma. You will just obtain a general discharge that will fill the entire volume. To obtain a confinement field one must create a plasma discharge along a narrow axis. That is not easy to do. Generally, an external field is applied but that requires a very different geometry than generally found in fusors. Look up mirror plasma machines for details of this complex application.

I am not familiar with the use of 3D printing for electric grids for high temperature applications - the grid will glow extremely hot so it might melt/vaporize your printed grid. Most people simply bend a high melting point wire into a rough grid shape.

Cai Arcos
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Re: Demo Fusor Suggestions

Post by Cai Arcos » Sun Jan 06, 2019 6:53 pm

As Dennis has correctly pointed out, NST are a better solution that MOTs for two reasons. The first one being safety. While a shock from a Neon Sign Transformer can cause great harm, its current limiting still allows you to have a chance to survive. MOTs, in the other hand, are not that kind. Rated normally at 2kV 500 mA they are always lethal. The second one being the electrical characteristics of each one. As stated previously, Microwave Transformers have an output of around 2kV, while NSTs are normally rated for around 7-8 kV. According to the FAQs I have read on these forums, it is a good idea to have a voltage range of around 0-10kV.

On the hand, if you are in the same situation that me and thus have no access to a NST, you can still work effectively with MOTs, but extremely carefully. First of all, those capacitors/diodes could be used to make a voltage quadrupler, as to have a wider and better operating range. Again, as Dennis has pointed out, be VERY careful with these kind of capacitors. Due to the high output current of the transformer, you would still have a manageable output capability.

Lucklily, there are some good news. If I'm correct, those capacitors are from a Microwave Oven. Those normally have a high value bleeder resistor that makes sure that the capacitor gets discharged. This is normally indicated in the capacitor itself, and can be easily checked with a common ohmmeter. You can use this information along with the capacitance value to calculate the approximate time it would take for them to discharge, them multiply that by three (¡at least! safety first).

Finally, I would recommend submerging the whole multiplier/rectifier circuit in oil, to make a better insulation.

Best regards:
Cai Arcos

Johannes Toppe
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Re: Demo Fusor Suggestions

Post by Johannes Toppe » Thu Jan 10, 2019 10:46 pm

Thanks a lot for the replies.
We have finally received all the materials we ordered and plan to finish construction by Saturday. I forgot to mention that, yes, we are submerging the diodes in mineral oil for insulation. We are using MOTs (salvaged from microwave ovens) because, unfortunately, they are the only thing available to us, however we will take necessary safety precautions - and the capacitors are just also salvaged from the microwave but not meant for use. The 3d printing is to create the jig that we will mould the wire around, just a minor thing. We are just wondering if it were ok to (somehow) use the actual salad bowl as a sort of outer grid itself, with the current going around that, or just having a separate inner grid inside it (but if the inner grid were to touch the salad bowl wouldn't that induce a current in that)? And finally, we are leaning towards the light intensity experiment due to time constraints, but out of curiosity, do the magnetic field from the wires have any effect whatsoever on the plasma (not confinement wise, but perhaps where plasma density is - although this is more qualitative)?
Your feedback is invaluable to us and thank you again.
- Johannes Toppe

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Richard Hull
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Re: Demo Fusor Suggestions

Post by Richard Hull » Thu Jan 10, 2019 11:14 pm

The wires have absolutely no magnetic properties that impact in anyway, the plasma.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Demo Fusor Suggestions

Post by Dennis P Brown » Thu Jan 10, 2019 11:45 pm

If you are going to uses a MOT (I do not think that is either a good idea or safe for one with your experience level) than I would urge you to place it as well as the caps (if you do use them) under oil. The output wire's insulation rating absolutely MUST be able to handle the voltage you plan on using whether 1000, 2000 or 5000 volts or whatever voltage you have.

Have you considered building a 10-15 kV supply using a 70's style automotive transformer? They are cheap and easy to obtain. The drive electronics are simple, easy to obtain, are a great science project in of itself, and the unit is ultra safe.

Yes, ground the 'salad' bowl and use it as the anode.

Also, one does not build a diode bridge for a two cap voltage multiplier. If you are not using a multiplier, again, do not use caps at all - they are not needed nor used in fusors. Adding a mega-ohm resistor on the output of the bridge under oil could be a life saver but will still allow a plasma to be created in the demo 'fusor'.

It is critical you understand grounding issues and be absolutely certain you have valid grounds on the fusor case and the MOT (the positive side of the diode bridge/MOT - don't float the bridge.) Remember and remind yourself always when the MOT is hot, just one mistake and you are dead - no second chances.

I built a 75 kV monster voltage multiplier based on a MOT. I abandoned it because it was simply far too dangerous to operate - it too was under oil. I've worked with many fusor supplies and I am highly experienced with HV but I recognized my limitations. I started in high school using NST's and it certainly was a good choice.

Johannes Toppe
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Re: Demo Fusor Suggestions

Post by Johannes Toppe » Tue Jan 15, 2019 1:30 am

Hi all.
Science fair coming up this Thursday so we are running tight on schedule, but all materials have been acquired and we're well into the building. We tried searching for neon sign transformers and unfortunately, to our dismay, found none available. We are planning to submerge both MOTs (salvaged from microwave ovens) and the full wave rectifier in oil, but were wondering if we should also ballast one MOT or if putting them all under (mineral) oil would be enough. Only concern would be is if turning one into an inductive ballast would end up not providing enough voltage for the system (around 2kV). I attached a rough sketch of how we are planning to do it - with the power all coming from the variac which will be plugged into the wall socket (grounded). We have our 1cm thick acrylic slab for our changer on top of our salad bowl where both the gas and high voltage line will go through. Our last concern was the ratio of the inner grid to the outer grid (Paschens law), with our being an 8:1 ratio (outer grid diameter 40cm and inner grid diameter 5cm). Would simply turning up the voltage high enough compensate for that? Our inner grid will be a simple blender ball as outlined in this post viewtopic.php?t=8649

Having to order and scrounge everything certainly put us behind schedule, but we're working overclock to get us ready on time. We will certainly not overlook safety.
Thanks
Johannes

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Richard Hull
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Re: Demo Fusor Suggestions

Post by Richard Hull » Tue Jan 15, 2019 5:32 am

All the best on getting things done well on the crash project.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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