Tyler’s Fusor Progress

Current images of fusor efforts, components, etc. Try to continuously update from your name, a current photo using edit function. Title post with your name once only. Change image and text as needed. See first posting for details.
Tyler_Johnson
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Tyler’s Fusor Progress

Post by Tyler_Johnson »

Hello!
I wanted to give a brief update on and overview of my fusor project (initially starting out as a demo fusor, then hopefully a full fusor by may). Currently, I have finished the design process of the chamber and am having it manufactured over at LDS Vacuum with the help of Josh Rain and Charles Price, who have been going above and beyond to help me throughout this entire project. I will not be machining the components myself, as I am in all honesty not experienced enough with machining to achieve the level of precision needed for this project. I will update this post with videos and pictures of the manufacturing process as it progresses over the coming few weeks.

My electrical system is just waiting on the NST to arrive. It is using a standard NST build, with a franceformer 7030 connected to ground and a bridge rectifier using microwave diodes, then off to the feed through, which will be a spark plug. I am using tungsten for my grid. My chamber is set up for upgradability, with 5 total flanges on the feedthrough end for a future deuterium backfill system and a turbo pump (side note, if anyone has a turbo, functioning or not, that they are looking to get rid of, please let me know). It is also pretty massive, with a 6”OD, .25” WD, and 8” length.

The second stage of this project, which will hopefully begin next month, will be using an X-Ray transformer (I already have an old one I am eyeing but if anyone has a spare one, let me know) and a feedthrough from MDC. As mentioned, it will also use a turbo pump and a gas backfill system for the deuterium. Neutron measurements will be made using a detector from a lab, and X-Rays will be measured for additional info using a rad alert.

The purpose of the project is to build a simultaneously inexpensive and convenient fusor that is easily replicable, for use by professors for demonstrations and experiments with students. The essential idea is that since experiential learning is vital for many students comprehension but is largely minimal in nuclear physics/engineering and Particle Physics courses at many colleges, a fusor could be highly beneficial for students to learn concepts like plasma dynamics and nuclear reactions with. As such, the entire design will be open source, with a step by step guide on constructing it and its required shielding. Additionally, the will cost below $1500 (will likely up it to 2k depending on turbo cost) (not including measuring equipment, tooling, and labor). Right now, my system is estimated to cost about $800 under the above conditions. The big hurdle will be the turbo pump, but I plan to get around their typically high cost by purchasing a used and probably broken one for cheap and fixing it with the help of the guys at LDS.

Thanks to everyone here that has helped me along this journey. I have attached some CAD illustrations of my chamber below, and will continue to update you all as things progress.
Attachments
FUSER PROJECT assembly.pdf
Illustration of the chamber
(27.52 KiB) Downloaded 137 times
Last edited by Tyler_Johnson on Tue Jan 14, 2025 9:15 pm, edited 1 time in total.
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Dennis P Brown
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Re: Tyler’s Fusor Progress

Post by Dennis P Brown »

Hope your project continues to go forward.

As for schools or Universities using fusors that is a complex issue that involves radiation (mainly x-ray) and high voltage that creates many safety problems. Schools especially are ill equipped to handle such issues. Few schools or universities will want to bother with such a device that has very limited uses; except for a high end unit (capable of activation) not much a real fusor has to offer any program.

All designs people provide here are open source - the forum here itself ;)

Rather, your design would be more useful here where people trying to build a fusor for low cost would be very interested. Posting photo's would also be useful. Many here would certainly not know how to obtain an inexpensive turbo (even non-operational ones tend to be expensive compared to other devices); however, such are also rare and every unit (needing repair) will likely have different causes and require different types of components. Not something to consider as a universal build for most people. Yet a turbo really isn't required in any case as shown by people here.

No matter; a step by step design with photo's and sources for parts (w/costs) is always welcomed.
Last edited by Dennis P Brown on Tue Jan 14, 2025 7:44 pm, edited 1 time in total.
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Rich Gorski
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

Tyler,

I don't understand how you're planning to use a bridge rectifier circuit on the NST when the NST center tap is grounded and the + terminal is also grounded. Almost all of the old NSTs are built with the center tap grounded internally to the case. Maybe I didn't understand you wording but it seems you have a problem.

Could you post a sketch of your circuit ?

Thanks,

Rich G.
Tyler_Johnson
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

I’ll have to find the copy of the diagram of the circuits that I have (the one copy I’m primarily working is currently being used to get an electrical engineer to sign off on this for the county), but essentially its just a rectifier circuit using microwave diodes. Full bridge might have been the wrong term. I specifically have been referencing this FAQ on NST’s (viewtopic.php?t=4406) and some of the design recommendations from other designs. I think the disconnect is in me not making it clear that the rectifier will also be connected back the to + terminal and the whole thing will also be grounded. I edited the original post to make it more clear :)

In terms of the use case, I plan on designing a shielding unit to maintain safety from radiation. The idea isn’t necessarily that every college rolls these things out or anything, its more of the idea that it gives colleges and professors an option for something they can use to demonstrate this stuff.
In terms of the turbo, a lot of the deal with the turbo is that while I could probably do full fusion without a turbo (there have been a few posts regarding getting a low enough pressure using only a dual stage pump as you pointed out), the process of doing so would vary depending on the pump and gets more complicated because of the size of the chamber. Repairing the turbo is how I am getting a turbo, but for the use case the idea isn’t necessarily breaking down how to find each component, since it will vary significantly depending on location and things like that. The assumption is essentially that a professor/university would have the resources to either A, just already have one (4 of the 5 of the “larger” colleges in my area have them, supplied by LDS vacuum in most cases), or acquire one at a discount and either get it repaired or have it ready to go. There’s some assumptions like that throughout, like for example with the measuring system where it is assumed that they have access to appropriate radiation monitoring equipment, or that they could have a flange manufactured or manufacture it themselves for very little labor cost. And, for people on this forum, a similar idea applies, wherein I assume that people either have the know-how or the resources to be able to get their hands on something like this OR find a way around the issue. The turbo might not end up being used, its currently the plan because it simplifies a lot of things but if we can try to figure out how to do it without one we will go with that (the guys at LDS vacuum and I are going to do a ton of testing on its minimum pressure with just the dual stage while throwing everything we can at it to give it its best chance of success). As for open source, the forum really inspired me to have this be as open source as possible. I was already going to have it be open source in the sense that the design would be included in the project and posted here, but now it also includes stuff like having some requirements for my school to essentially give pictures and design information to any colleges that might ask if I leave it to them after the fair (I might not have any space to store it myself unfortunately), making a very detailed creation guide, more cost breakdowns, a full and detailed account of each design choice and its purpose, things like that. I know that for all intents and purposes, the same thing exists here, but I want to make it as accessible and replicable as possible by having it all in one giant mega paper. Regardless, I am still finalizing all of the use case stuff, the guys at LDS have been really great for this because they have supplied the chambers/parts for chambers for fusors built at universities before as parts of research projects. They are helping to figure out how best to go about everything to make them university friendly. As an aside, LDS as a whole has been absolutely incredible throughout this, highly recommend them for anyone building these especially if they are high school/college age.
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Rich Gorski
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

Tyler,

Follow the circuit diagram in the viewtopic you mentioned (4406). It shows two diodes forming a full wave rectified circuit. Don't forget to ground the vacuum chamber as that is the current return path to ground/power supply for the plasma.

Rich G.
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Dennis P Brown
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Re: Tyler’s Fusor Progress

Post by Dennis P Brown »

Your repair of a controller will be impressive work and worth discussing in some detail. I do agree a turbo makes it easier to do fusion since one can get a cleaner system, faster.

If you 'leave' the fusor system at the school I'd strongly suggest not leaving an actual fusor level power supply with it - like your x-ray transformer. That is a huge liability issue. Simply sell that because in the long run, anyone trying to get a fusor up and running must handle that dangerous (read lethal) part of a fusor on their own. A NST, I'd think, is acceptable if separated from the fusor (no diode system broken down since that can't easily be made safe) and the x-former has a "High Voltage" warning tag's on it and the power cord is removed. These are suggestions but leaving any dangerous components is always a questionable practice because you never know who might use it and get hurt/injured and blame you later.
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Tyler_Johnson
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Dennis,
I plan to leave just the vacuum chamber to the school, as the science department is severely lacking equipment in general and a vacuum chamber would be very helpful for both our AP Chemistry program and AP Physics 1-C program. I plan to sell off the power supply, though I might end up selling it here if I get a good deal on one, as I know how hard they can be to get for cheap for someone else wanting to build a fusor, at least in my area. Trying to give back to here as much as possible given how much help this forum and the people on it have been on my own fusor journey.
In terms of the controller, I will try to provide as much detail as possible, though a lot of the repair of both the controller and a pump in general will be done by the guys over at LDS vacuum. They are already letting me sit in and teaching me about manufacturing the flanges and are assembling the chamber with me, so I will likely also be able to sit in for the repair process and can give you a general breakdown, as long as they allow it.

Rich,
In terms of the grounding, I already have a grounding wire "port" built into the chamber, which is essentially just a hole that allows me to easily connect a wire to the chamber for proper grounding. I believe the two diode symbols each represent two diodes, for a total of 4 diodes. I will go back and double check though, and I'm sure if there's any issues with the system the electrical engineer will let me know.
Thanks again to everyone for their guidance and advice! I will keep you all posted.
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

I have began work on the transformer, and I got in contact with an electrical engineer who is helping me with all of this. I just finished dismounting the SGFP, which I plan to take out tomorrow morning along with rewiring everything and capping things off. Should be a good example of how to deal with these in some types of franceformers. Mine is a 9030 PKA-2NG-1 (which manuals seem to not exist online for). I have attached pictures of the process so far. I will provide further updates hopefully in the morning. The electrical engineer thinks I may be able to put the rectifier circuit inside of the transformer body, provided it fits safely. Main reasoning to do so is that it reduces the amount of epoxying I have to do to make sure there are no exposed and uninsulated connection points, and makes maintenance easier. I am a bit worried it could pose an arcing risk though.
Attachments
PC board relay
PC board relay
The SGFI component in the module
The SGFI component in the module
After removing ground line terminal, you can cut the final support (looks identical to the first support, just on the other side) as shown to reveal the internals of the SGFP module
After removing ground line terminal, you can cut the final support (looks identical to the first support, just on the other side) as shown to reveal the internals of the SGFP module
After it is cut, you should be able to remove the hex washer on the switch and push the switch into the compartment to allow for space to cut the final connection point
After it is cut, you should be able to remove the hex washer on the switch and push the switch into the compartment to allow for space to cut the final connection point
Another closeup of what to cut
Another closeup of what to cut
Must carve out a cavity of space in the tar to unscrew the screw.
Must carve out a cavity of space in the tar to unscrew the screw.
You must first unscrew this screw to get enough clearance to cut the connection point
You must first unscrew this screw to get enough clearance to cut the connection point
Closer up of support to be cut.
Closer up of support to be cut.
Support that must be cut
Support that must be cut
NST guts with SGF module circled
NST guts with SGF module circled
Tyler_Johnson
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

At long last, the Demo Fusor has been completed! We did have some very interesting results, however, that were pretty unexpected.
To start, I have attached graphs of voltage at a set pressure vs plasma brightness/density (measured as average intensity of greyscale) and pressure at a set voltage vs plasma brightness/density (measured the same way). Do note that the pressure reading (in microns) is actually about 20-25 above the actual pressure, as the pressure gauge I could read while recording the data was determined to have this level of inaccuracy. What is interesting in these numbers is that at voltages above 16 input volts (600 volts after transformer and rectification), I was still getting plasma generation. This is in contrast to the minimum noted voltage in #2 FAQ of 3000 to get plasma. Additionally, plasma generation continued to occur all the way up to 200+ microns (not shown in the data because of difficulty sustaining those pressures long enough for measurement). I was under the impression that a demo fusor still needed around the 50 micron range max of pressure, though I could be misremembering what I read. Regardless, all extremely interesting! Really big thanks to everyone here that helped me throughout this, as well as again thanks to the guys at LDS vacuum, who were instrumental throughout this project. I look forward to upgrading this system hopefully starting next month and getting my name into the Neutron club! (Knock on wood). I will update this page with the full written out methodology, cost breakdown, procedure, and manufacturing process once I have it finished.
Attachments
IMG_5068.JPG
IMG_5171.JPG
IMG_8291.jpg
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Richard Hull
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Re: Tyler’s Fusor Progress

Post by Richard Hull »

You need to read the neon transformer FAQ. Around 5 torr you can get a discharge with lower voltages. This continues down to 1000 microns or 1 torr.
The neon transformer, in an unloaded state, will always put out its rated voltage (no glow or plasma). The moment this high voltage lights a plasma, at any gas pressure within the HV break down range, the transformer's output voltage plunges to about 500-600 volts with the current at whatever the nameplate current rating is listed.

The transformer is magnetically shunted to never deliver the nameplate voltage under any gas load. They are designed that way on purpose! At some low pressure in some plasma systems, 20,000 volts will not create a plasma! Study gas-vacuum break down ionization physics.

Let us say your vacuum system as you have it will break down to a plasma at 4000 volts. If you had a larger non-shunted transformer capable of supplying 6000 volts at 10 amps, the instant the plasma lit, the grid in your system would be vaporized nearly instantly!! The current in the plasma would try to draw nearly 60,000 watts!! The neon transformer, being magnetically shunted, will not let this happen.

Read the FAQ.

viewtopic.php?t=10333

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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Rich Gorski
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

Tyler,

Vary nice fusor chamber ready for demo use as well as doing real fusion with deuterium. I really like the huge viewport you have on it. You can easily see what's going on inside. Remember in terms of X-rays when you get to 10kV or higher the viewport will be where the most X-rays will be emitted into the environment.

Good start on plasma too. You mentioned 16 volt input for 600 volt output from the NST. Were you able to go higher in voltage or did the plasma load on the NST limit the voltage?

Rich G.

Ps. Is that a copper ground rod I see in the last photo in the background?
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Richard,
I am familiar with the mechanics of NST's. By 600 volts, I meant that was the maximum output of the transformer, not the maximum under load. We measured this based off of the input voltage. We confirmed using an oscilloscope that the step-up multiplier was fairly consistent even at low voltages, and as such could roughly calculate how much the unloaded output of the transformer was. We also subsequently tested it using a multimeter after deconstruction at that 16 volt input value because the plasma generation was unexpected. We were lighting the plasma at 600 peak no-load volts by starting the variac at the 16 volt input mark, at which point the plasma would spark up just fine. I anticipated that it could sustain a plasma at a lower voltage, as you noted, because of how the NST's work, but I did not expect it to spark at 600 volts peak.

Rich,
Thanks! I am working on getting the proper setup and funding to upgrade it (knock on wood that the county approval committee gets it together before the paperwork deadline). I was very wary of X-Rays, observing the first few ignitions only on recording while I was a distance away with the viewport facing away from me. I have a solid rad-alert for X-Ray detection that I managed to get tested for decent accuracy, so I was confident in the lack of X-Ray generation when it repeatedly only showed 30 CPM (background). And yes, that is a copper ground rod! I got it about a foot into the ground. Very overkill for the application, but I got my hands on one and figured it would be better to have an overkill ground than just a sufficient one. O
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Richard,
I re-checked my footage and notes from the testing procedure. I was correct that it was 16v input to the NST, but the 600v output is off. 600v output would be pre-rectified output, dropping to ~424v rectified output under no load. I am going to measure the actual output again likely tomorrow in order to get a good reading on it, as I did not get the actual reading on the multimeter on video. Additionally, I see what you are saying with the pressure value, I believe I was thinking about the pressure for a full fusor rather than a demo. Regardless, I will try to get more detailed and accurate readings on the actual no load output in the morning so we can know for sure. Thanks for the clarification!
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Re: Tyler’s Fusor Progress

Post by JoeBallantyne »

Tyler -

Beautiful fusor. Very nice viewport. If I were you I would make sure to get some sacrificial glass on the inside of that viewport before you crank up the voltage/current levels too much, so you don't break the viewport due to localized heating from electron/ion beams. Those large viewports aren't cheap even when purchased used off of eBay, and new, they are a very pretty penny.

Put some cheap glass in front of it on the inside.

Your wallet will thank you.

Joe.

PS. I don't know your chamber inside diameter, but when looking recently for sacrificial glass on Amazon, I remember seeing circular pieces you could get cheap that were about 6" in diameter.
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Joe,
On the price, tell me about it! The viewport was a third of the total cost of this thing. However, I got it from LDS vacuums who did some extensive testing on it for me, and let me know that under my use case, I should be good. They have experience building chambers for similar applications. The glass is quarter inch thick, so it should be pretty solid. I will likely add that additional glass though, I don't see a harm in doing so. It's a great idea that I had not considered. Thanks for the advice, and I am excited to keep this thing going!
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Liam David
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Re: Tyler’s Fusor Progress

Post by Liam David »

Nice chamber! Large viewports are indeed expensive, but nice to have.

The strength of the viewport against vacuum is not a concern: it's a standard UHV part used on a gazillion chambers. Localized heating by electron beams is an issue due to the low thermal conductivity of glass. Large temperature gradients create stress until the glass yields. We had this happen to the exact same viewport as yours on a chamber where I work. Thankfully it did not implode. I also cracked a 2.75" conflat viewport on my fusor due to a small but powerful electron beam. It can happen to you, especially as you crank up the voltage going for fusion. Also consider grid designs without openings pointing towards the viewport.

PXL_20230815_175338054.jpg


IMG_20190713_160425.jpg
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Richard Hull
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Re: Tyler’s Fusor Progress

Post by Richard Hull »

Pay attention to Liam!!! Viewports ain't cheap! Big ones are nice, but exorbitant to have to replace. Once fusion is achieved at high energies, even with a sacrificial glass or screening be prepared to have them load up with deposits towards opacity needing abrasive cleaning in some instants.

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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Rich Gorski
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

More on electron burn at viewports

The fusor configuration I have emits a strong electron beam straight down the axis of the vacuum chamber and into the center of my borosilicate viewport glass. This electron beam will easily damage and possibly crack the glass in a short time. Before I continue I want to stress that I strongly recommend the use of a sacrificial piece of glass on the inside in front of the viewport glass. One can add additional protection by using a magnetic field to deflect the electron beam away from the viewport and onto the metal wall of the vacuum chamber. I have incorporated a permanent magnet in my system to do just that as described below.

The first photo below is a -12kV run with deuterium showing the viewport lighting up white from the electron beam hitting direct center. No magnetic field is present. Note the center hot spot on the glass and the way the edge of the glass also lights up from internal reflection to the edge.
eburn-nomag.jpg

The second photo is with a strong permanent magnet placed on the outside of the vacuum chamber. Note that under the same -12kV potential the glass no longer lights up and the center hot spot is gone. No electron beams of any consequence are hitting the glass viewport. The beam is being deflected off to the side where it impacts the side of the chamber. With the magnet in place one side of the chamber gets too hot to touch (was cold without magnet) which shows that the electron beam has indeed been deflected.
eburn wmag.jpg

The third photo is a look straight down the axis of the chamber to view the plasma created. The central hot spot is not the glass fluorescing it is the plasma created at the back end of the chamber and the source of the electron beam.
eburn plasma.jpg

The last photo shows the single N52 permanent magnet(VERY STRONG) placed on top of the ISO160 chamber (rectangular item with hole). At -12kV the field created by this one magnet was enough to deflect the electron beam off to the side of the chamber near the ion gauge tube. At higher accelerating voltage I would recommend two such magnets one on top and one on the bottom of the chamber oriented with poles opposite (so they would attract each other if placed close together). That will create a vertical magnetic field twice as strong which will deflect the electron beam off to the side at much higher accelerating voltages.
eburm magphoto.jpg
For the best protection It is always a good idea to have a sacrificial glass in place to protect the expensive viewport. The addition of a magnetic field not only will help protect the viewport but could improve the view of the plasma by eliminating the fluorescence of the glass from the electron beam and eliminating any obstructive damage of the sacrificial glass. To add permanent magnets to your system it would be best to experiment with placement of the magnets for best results.

Rich G.
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Sorry for the late replies, its been a busy few days.
In terms of the viewport, duly noted. I was planning on incorporating a magnet in the system, though from the looks of the one you use I may need to get a stronger one (or two). The sacrificial glass is the plan, along with the aforementioned magnet. Honestly, the biggest reason for the viewport’s size is ease of use and observation, as the whole “selling point” on the project is its application as an open-source design for a potential teaching tool. While I know the viewport isn’t exactly a “make or break” feature, it is nice for its purpose. I plan on including instructions about installing a smaller viewport using an adapter, though.

Onto the voltage funny business, post rectified voltage was actually a little lower, at around 400 DC volts from 16 AC applied, measured using a fluke multimeter. I am specifically talking about the no-load voltage, NOT the dropped voltage once there is a load. I am sure you guys here have a lot more expertise on this matter, and I know Richard had mentioned earlier that demo fusors are capable of generating a plasma at lower voltages, but everything I have found has been around 1kv minimum to get the plasma sparked up. Granted, I have only went through about 9 or 10 designs, but still. Is this actually just normal and I missed something somewhere? I would love a better breakdown of the science if so, as the way I am interpreting ionization energy even in a very low vacuum it would require higher than this voltage to spark up the plasma. I have not gotten the chance to fully re-assemble the demo fusor, but plan on it within the next few days or week, depending on homework.

Finally, I was hoping I could pick your brains on xformers for moving on to the next stage. I know precipitator supplies are far cheaper, but I had some difficulties selling the idea of them to the school, who wants most anything high voltage to be as little “jerry rigged” as possible, in their words. I know that I need to be looking for older transformers, typically fairly massive and heavy, and to avoid the dental type and modern xformers as they aren’t made to supply continuous high voltage. Would an xformer from the mid to late 90’s work, from a full X-ray machine rather than a dental one? The one I am looking at seems smaller than those photographed in the FAQ’s. It’s a 90’s Bennet BXT-151, which comes with the controlling unit and all the fixins. I am guessing I will at the very least need to bypass the timer circuit, but that seems fairly manageable as long as the transformer itself can handle it. Am I missing something here? Thanks!
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Rich Gorski
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

Tyler,

If you’re asking about what voltage is required to ignite plasma unfortunately that is a difficult question to answer. It depends on many things and each fusor having different geometry will have somewhat different ionization characteristics. There are three main items that seem to control when a gas will ionize (ignite, breakdown)…

1. Electric field strength (geometry dependent as well as voltage dependent)

2. Gas pressure. Look up Paschen’s law which states that there exists a minimum breakdown voltage for a gas dependent on pressure and electric field strength.

3. Gas species. Each gas species has a different ionization potential.

Ionization can occur at very low voltages such as 60 or 70 volts in a neon lamp or at very high voltage such as in a fuser when the pressure is below 1 mTorr. It all depends on the combined effects of the three items above.

So, what is the process of ionization of a gas and where does the light come from? Here’s the basic process.

Let’s start with a gas at some pressure in a container along with two electrodes. This atoms of gas have an ionization potential which is defined as the energy required to remove the most loosely bound electron. Let’s say our gas is hydrogen which has an ionization potential of 13.6 electron volts. So when a potential difference is applied to the two electrodes an electric field is established which depends on the distance between electrodes and the voltage applied. This electric field can accelerate free electrons in the gas to high velocities (yes there’s always a few free electrons). These accelerated free electrons can collide with neutral gas atoms and if the electron’s energy is greater than the ionization potential (13.6eV for H) the collision can cause the atom to loose one or more additional electrons. The atom now becomes a positive ion. This ion has not yet emitted light but the release of one or more new electrons now accelerated by the electric field, can have collisions with other atoms causing them to loose electrons. Somewhat like a nuclear chain reaction.

The important thing is that the potential difference must be large enough to be able to accelerate electrons to greater than the ionization potential so that electrons can be liberated. However the voltage is not the only thing that matters in how much a free electron can be accelerated. It also depends on the density of atoms/molecules in the gas. The greater the density the more collisions a free electron will have and the harder it becomes to accelerate to the ionization energy. A very high density gas (like atmospheric pressure) will be difficult to ionize and will require large potentials to overcome the density issue. If the density is very low there simply are not enough atoms present so collision with free electrons is rare and again ignition/ionization is rare. So to create the cascade of free electrons (the chain reaction) the right combination of the three main items of electric field (applied potential), pressure (density) and species (ionization potential) all must be within the correct range for ionization to occur. This complicated process is why it is hard to give a definite answer to what voltage your particular fusor will begin ionizing. To complete the explanation the gas species also makes a difference since each has a different ionization potential. Hydrogen being the easiest to ionize at 13.6eV while helium is the hardest at 24.6 eV. It seems when the outer electron shell is filled the atom is difficult to ionize (Noble gasses) while when the outer shell is not filled (hydrogen, Lithium) the atom is easier to ionize.

About the emission of light. Once an atom has been positively ionized it is ready to absorb one of those free electrons to fill the electron shell and return back to the neutral state. In fact the ion wants to do this and become neutral again. The process of absorbing an electron is where the emission of a photon happens. The initial absorption of a free electron will put the atom into an excited state (same as raising a bound electron to a higher energy level in an atom as by the absorption of a photon). The excited atom will quickly decay back to its ground state by the emission of a characteristic photon. This is the light we see in a plasma and consist of the various wavelengths characteristic of the different energy levels of that particular gas species. For hydrogen or deuterium the strongest characteristic wavelength is the H-alpha line at 656.3 nanometers (red).

Hope that help your understanding of the ignition of plasma.

Rich G.
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

That was very helpful, I am familiar with the math behind it to an extent (Paschen’s law, ion behavior, and transition energies, that kind of thing), but had not thought of it in that way. I guess the question I am really trying to get at is “has plasma generation in a comparable demo fusor been measured to occur all the way down at -400 volts?”. The reason I ask this is because I have not been able to find one that has while scrolling through posts here about various designs, with most either not mentioning a minimum voltage requirement or having it around the 1kv mark. Again, this isn’t for sustained voltage needed to sustain the plasma, this is just for initial sparking up. What I am getting at is “is there something different about my design that is causing this difference?”, which if that is the case I would want to determine as I figure it would be of keen interest for demo fusor makers as they begin their fusor journey. I figure that with enough years of seeing posts like mine about demo fusors, someone would likely know if this is something observed before. But as you said, it is likely a number of variables likely causing this. Thanks for the insight though, I appreciate your explanation. It was very helpful!
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Richard Hull
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Re: Tyler’s Fusor Progress

Post by Richard Hull »

As I noted before, 400-600volts is a sustaining voltage once plasma is struck in a vacuum of a few torr to just under 1 torr. it is how neon signs and their transformers work. It would be tough to read an honest 400 pure DC volt ignition voltage in a large a system as you show. So....No! I have never heard of or personally experienced a 400 volt ignition of plasma in any decent size demo device.

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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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

Richard,
Ok! I am trying to come up with a way to measure it other than just calculating the number. I should note that at 15 volts input (the first point at which a tiny amount of plasma becomes visible), a fluke multimeter reads the sustained voltage as 90, which again is off from that 400+ voltage needed for sustaining. I am unsure of how linear the transformer output is (on a scope its fairly linear but I am unsure if that holds true above 10 volts). The other odd thing is both terminals of my NST are in phase with eachother. I have a voltage detector but its margin of error is about 1000 volts, as it is made for linemen and doesn’t get a ground reference. I will try to measure it based off of that and let you know, though it won’t be particularly accurate. Thanks!
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Re: Tyler’s Fusor Progress

Post by Tyler_Johnson »

An update on the voltage!

Last night, we used a Keithly SourceMeter to determine the exact voltage allowing for plasma ignition. We found it to be -383 volts that sparked a plasma, and it was a pretty visible one at that, though it helped there wasn't as much background lighting as there was in previous experiments. the supply was outputting -383 volts @ 7ma, the chamber pressure was between 70 and 150 for all experiments (fairly identical results). I am now trying to work out Paschen's law for this situation, specifically Paschen's law as it relates to Townsend's avalanche breakdown to determine what the exact cause of my demo fusor producing plasma at such a low voltage is. The current theory is the chamber to grid size ratio, the finish of each, the design of the grid, and the humidity of the air. We determined temperature, current, prior ionization, and pressure (within the range of 30 and 400+ microns) have no or negligible impact of initial ionization energy. I am having trouble with the math of Paschen's law, despite inputting and scaling all of the constants and variables to be accurate, I am getting a result of 1.26*10^23 as the breakdown voltage. I am unsure where I have gone awry. I have attached the Desmos info so that someone might be able to correct me. I already know it will be some dumb mistake, as these things often are. Thanks for all the help!
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Re: Tyler’s Fusor Progress

Post by Rich Gorski »

I'll take a guess at the problem. Kind of hard to follow your values. No identifications given such as what molecule are you using.

Anyway, the E in your equation should be the ionization potential. Your value is 3.4. No units given or molecule ID but it should be in Joules. I think your using eV. So multiply your answer by 1.6e-19.

Just a guess.

Rich G.
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