FAQ - Minimum Practical Power Requirements Needed to do Fusion

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Frank Sanns
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FAQ - Minimum Practical Power Requirements Needed to do Fusion

Post by Frank Sanns »

Elementary charge is 1.6 E-19 coulombs and an ampere is a coulomb per second. The minium current to have fusion in a targeted system is then 1.6 E-19 amperes during the second in which the collision occurs. This is assuming the same moving charge will provide the ionization and the current to move a deuterion to a resonable fusion energy of 20 KV or more.

To use the benefit of a fusor, a charge located on opposite sides of the inner grid must move which is double the elementary charge. That puts the current at 3.2 E -19 amperes using the same assumptions as above.

If you are using neutron detection as verification of fusion, you would need double this current since only half of the fusion reactions with deuteruim alone will produce a neutron (and He3). The other reaction produces a proton and H3.

These are the lowest currents to produce fusion but there are some other factors that need to be considered. Not all currents will produce ionizations or ionizations the proper distance out from the inner grid. This will add a couple of orders of magnitude more to your current. The tally now is something on the order of 1 E -16 amperes.

One mole of deuterium is 6.02 E 23 atoms per 22,400 cc. Inside a fusor the pressure is on the order of 15 E -3 torr. At this chamber pressure the number deuterium atoms are 5.3 E 14 atoms per cc. This sounds like a big number and it is but the space in between the atoms is also a big number. Estimating the nearest neighbor then is the cube root of that which is 8 E 4 atoms per cm. One over that number gives the spacing as 1 atom every 1 E-7 meters. Atomic diameters are on the order of 1 E -10 meters and nucleus diameters are on the order of 1 E -15 meters. In a fusor then, deuterons of 1 E -15 meters are being hurled at other deuterons that are 1 E -7 meters apart. The next atom would be E-7 / E -15 = E 8 times away. Scaling this to the macro world for understanding, assume that a nucleus were 1 cm in diameter (a little under half an inch). This puts its nearest neighbor at 1 million meters (450 miles) away.

Since there is 1 E 8 times more space than there are nucleii, the chance of something colliding is pretty bleak. Multiplying ampere tally of 1 E -16 times the space between crudely puts the current at 1 E -8 amperes for the miminum current to expect one fusion event per second.

This current excludes non productive collisions, near misses, grid wire collisions, collisions with other gasses in the chamber, actual fusion cross sections, ability to detect neturons or other fusion byproducts, etc.

With an ultra clean chamber of good design and a He3 detector 1 E 3 neutrons per second (isotropic) are easily detectable. Using all of the above assumptions backed up by my personal experimental evidence, the minium current to do easily verifiable fusion is on the order of 1 E -4 amperes ( 0.1 milliamperes) at 20 KV.

For somebody with no experience with a fusor AND detection equipment the number is probably 10 times higher. If somebody were just getting started, they should not really attempt fusion without a power supply capable of at least a couple of millamps current (EDIT ADDED: 1/1/12) and as Richard will point out in a couple of years from now, 20 milliamps or more of current is a more realistic recommendation for those wanting to do more serious fusion.

Frank Sanns
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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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Re: FAQ - The lowest current needed to do fusion

Post by Richard Hull »

I would like to commend Frank for this super posting!

It is important for all to remember the difference between the deuteron current required inorder to theoretically do fusion versus that required to actually detect fusion, using equipment and techniques available to the amateur.

One can often be confident that fusion is occurring at much lower current levels, but it rings quite hollow to the skeptical if you can't show that fusion is, indeed, happening.

Certainly, if you can't measure fusion, you can't hope to make improvements in a design, for there is no benchmark against which an improvement can be gauged.

No one is entered into the neutron club without acceptable proof of actually doing fusion and this means images of a setup and instrumentation capable of taking neutron measurements and data that makes sense to the old hands.

As Frank notes, a good amateur setup with a good He3 detection system would need between 500ua and 1 ma at fusion energy , (7ma;20kev), for ready detection.

Frank's post as well as my own pronouncements, above, assume a lower threshold beginner amateur voltage, (20-30kv), applied.

At very high voltages, over 80kv applied, the current needed for detection is reduced as the energy of the particles is higher and, thus, the collisional cross section is better and more deuterons will collide per unit current.

For a full discussion of how voltage and current work to do fusion see the FAQ at.......

viewtopic.php?f=14&t=6466#p42149

Richard Hull
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Re: FAQ - The lowest current needed to do fusion

Post by Wilfried Heil »

There is no smallest current for DD or DT fusion just as there is no minimal energy.

The smallest charge to do fusion would be two ions on a collisional course, or 2*1.6E-19 coulomb in an arbitrary time, so the minimum current to do fusion can be infinitely small.

If you insist on having at least one fusion event, then you have to divide this infinitesimal current by the fusion probability at whatever collisional energy you are using.

To be able to detect it is another issue. For this you will need 10^3-10^4 fusions/second and the corresponding current and voltage, like Frank has described.

The fusion efficiency rises exponentially with voltage. For example, at 50 kV and 0.1 mA you can expect 10^4 n/s, which is easy to detect with bubble counters.
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Re: FAQ - The lowest current needed to do fusion

Post by jarrod1937 »

I'm confused, one FAQ says "All fusor supplies need to be able to supply a minimum of 20 milliamperes, ( 20ma), of current. All supplies must be fully variable from zero to their full KV rating."

Yet, this one is saying somewhere around 1 ma at 20 Kv. So which is correct? There is a rather large difference there. Or am I misunderstanding something?
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Re: FAQ - The lowest current needed to do fusion

Post by Frank Sanns »

The title of this post is the LOWEST current to do fusion. In the last sentence it says "AT LEAST a couple of milliamps current".

This post was made because some people were coming on the scene trying to use ignition coils and micro amp power supplies which of course will not work for fusion. So as to not discourage them or to get them to spend big bucks for over kill power supplies, I generated this post to give them an absolute minimum requirement.

Of course 20 ma current will make it far easier to detect fusion and I would highly recommend it but it is not a requirement if your budget can not afford it.

I will amend my post to clarify. Thank you for pointing it out.

Frank Sanns
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Chris Bradley
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Re: FAQ - The lowest current needed to do fusion

Post by Chris Bradley »

Frank, does this analysis not simply, but only, reveal the *ion* current required? Whereas a power supply supplies an *electron* current.

Electrons don't fuse, but the measured current is just electrons. What is the transfer function between electron current through, and ion current within, the fusor? [I have previously posited this as x60, your analysis infers it is x1. I think it is in between those factors.]
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Re: FAQ - The lowest current needed to do fusion

Post by Rich Feldman »

Chris,
In a system circuit with 1 loop, from the electric circuit perspective, how can there be more than one value for the current?

In three dimensions: choose any closed surface between cathode and anode. The net rate of charge transfer across that surface is zero. Conduction current in the feed wire penetration has the same magnitude but opposite sign as the combined ion + electron current in space.
It's not trivial to determine how that current is partitioned between various charged species.
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Chris Bradley
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Re: FAQ - The lowest current needed to do fusion

Post by Chris Bradley »

Rich Feldman wrote:
> Chris,
> In a system circuit with 1 loop, from the electric circuit perspective, how can there be more than one value for the current?
Recirculation.

It is the total charge that needs to remain neutral, not the total flux. If you have ions reciprocating, then electrons may be flowing through, maintaining the charge. However, whereas the ions may be 'hanging about', the electrons would zip through as there is no potential well to hold them.

Consider a wire an example of 'perfectly trapped' ions. Electrons flow, but ions don't move out of the wire. However, something more lossy like an electrophoresis experiment, a flow of electrons is continuous (and may be quite high) whilst the ions slowly migrate.

The charge in the sample space must remain balanced - so says Gauss. But the flux of + and - need not be balanced. So long as all the - in equals all the - out, there is no need for any + motion at all.
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Re: FAQ - The lowest current needed to do fusion

Post by Frank Sanns »

The fusor is a plasma not a solid. All bets are off for comparing neutralization in a solid with neutralization in a rarified plasma. What happens in nanometers in a solid can happen in meters in a fusor.

My original post was to help guide newcomers to size of their power supply. That was the purpose of the post and I think it serves it well. Feel free to conduct your conversations here or better yet, in threads that are focused on your specific topic.

There were innumerable pieces of information that were deliberately left out for simplicity back in 2007 and I think they should remain out now for the same reasons. Unless somebody else some driving objection, my original post will stand as it was intended.

Frank Sanns
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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Re: FAQ - The lowest current needed to do fusion

Post by Dan Tibbets »

To expand on what Chris Bradly said, I have the impression that you (Rich) are thinking about typical current in a wire. The electrons are the only mobile charge carrier, and thus exclusively carries the current (ignoring holes and other complications). But in a plasma (ionized gas) the electrons and ions are both mobile, so either can carry current . The ions move in the opposite direction compared to the electrons, but by convention this is also accounted for in solids. Except the ion velocity in solids is zero in the lab reference frame, but the total amount of curent would remain the same. They move in opposite directions, but they also have opposite charges, so the net effect is the same (whatever it takes to satisfy the potential difference.

Having high Z ions (more than one charge (like Boron +5)), different speeds of ions compared to electrons with the same KE, etc. and things become more interesting. Then throw in considerations of recirculation of the electrons and/ or ions, charge exchange reactions, recombination reactions, sputtering, secondary ionizations, and relative lifetimes of the electrons and ions before loss or fusion leads to very interesting results. Then throw in segregation of the charges (potential well) and things become very very interesting (and difficult to predict with simulations) .

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Re: FAQ - The lowest current needed to do fusion

Post by ibogaine »

Most of the discussion appears to center around electrical current, but what if I were building a Farnsworth reactor with exposed electrodes, using only the difference in voltage (potential) to drive ions around? From your experience what is the minimum difference between positive and negative electrodes with which D-D fusion reactions would reliably and detectably appear?
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Re: FAQ - The lowest current needed to do fusion

Post by Carl Willis »

I like what Frank just said about this FAQ's purpose being to help newcomers size up power supplies, and his emphasis on keeping the ensuing thread simple and practical.

Frank's FAQ on this topic and Richard's FAQ say some different things about desirable voltages and currents, as Jerrod pointed out. Frank's emphasis was more on minimum practical power requirements (~0.1 mA / 20 kV) assuming an excellent neutron detector. Richard's point of view was more towards the "ideal" power supply (for painless progress and richly-satisfying neutron yields, look for ~20 mA and up to 80 kV.) A key understanding that should inform Jerrod and anyone else confused about this apparent discrepancy is that, in theory, there is NO voltage / current threshold for fusion; these FAQs are talking about PRACTICAL capabilities of the power supply, with implicit assumptions about what's involved in detecting neutrons. FAQs on this site are almost always written by single individuals and promoted a posteriori to the canon of FAQhood wisdom by administrators based on perceived utility. Readers can expect stylistic, point-of-view, and emphasis variations as well as the occasional factual contradiction. It's just like the four gospels of the New Testament (with the exception being that we go back and edit for clarity and accuracy).

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Re: FAQ - The lowest current needed to do fusion

Post by Rich Feldman »

Dan DT wrote:
> To expand on what Chris Bradly said, I have the impression that you (Rich) are thinking about typical current in a wire. The electrons are the only mobile charge carrier, and thus exclusively carries the current (ignoring holes and other complications). But in a plasma (ionized gas) the electrons and ions are both mobile, so either can carry current . The ions move in the opposite direction compared to the electrons ...>
> Dan Tibbets

With apologies to Frank for academic nit-picking on a FAQ thread:

That's exactly my point, Dan. Suppose the inner-grid feed current is -1 mA. That means 1 mA worth of electrons (6.2E+15 per second) are drifting along the wire from HV PS toward grid.
Now if we consider a closed surface that encloses the inner grid, and don't count the feed wire current, there is 1 mA worth of net charge crossing that surface in the inward direction. We can't easily determine the partition between X mA worth of electrons and negative ions crossing that shell outward, and (1-X) mA of positive ions crossing the shell inward -- they both contribute.

I think Chris wants to count some allegedly much larger flux of ions that are "recirculating", with equal numbers crossing inward and outward in a given time interval. So what? I could draw a spherical surface in water of neutral pH and figure the astronomical absolute current of both H3O+ and OH- ions "recirculating" across that shell due to thermal motion. In slightly acid water (pH 6), 99% of the mobile ions are positive, and the thermal "recirculation" current density would be even higher -- with no practical consequence except to satisfy academic curiosity.
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Re: FAQ - The lowest current needed to do fusion

Post by Dan Tibbets »

Yes, they both contribute. The net current is the sum of the two. The point about recirculation is that if an ion recirculates 10 times, then it is only contributing 1/10th of the net current between the anode and cathode (which means the electrons are contributing 9/10ths of the current- they enter and leave the reaction space 10 times as fast). Remember that the opposing charges have to almost match each other or the voltage goes through the roof. It gets more convoluted as you consider neutral ionization by energetic electrons, Z, etc, but that is my understanding of the basics.

A weak attempt to return to the topic. The required current to do and detect fusion is highly dependent on the recirculation current/ net throughput current. Thus all of the schemes to greatly change this ratio, which is very difficult, if not impossible, to do - especially when both species are considered. This ties directly into any attempt to improve Q.

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Richard Hull
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Re: FAQ - The lowest current needed to do fusion

Post by Richard Hull »

I just posted an FAQ on this issue and hope to avoid this totally different discussion topic in this FAQ discussion. check out......

viewtopic.php?f=14&t=6957#p42640

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Re: FAQ - Minimum Practical Power Requirements Needed to do Fusion

Post by Doug Coulter »

I'd just point out that yes, there is a theoretical ion current and energy to get to detectable fusion with a given detector - BUT!

Without an ion gun setup or other additions, you can't just run an arbitrary volt/ma set of numbers and have a fusor keep itself lit very well, if at all. At some given pressure and geometry, putting on a certain voltage will get you a certain current related to all the re-ionization losses, Paschen's law and other issues, and if you try to lower the current by lowering either the voltage or gas pressure, you're going to find that it "goes out" and goes to zero current - you can't achieve the full continuum here in a "simple" fusor.

In other words, the gas pressure required to get ANY current to flow at 20kv might just be enough to draw significantly more current than desired once it lights off - and doing anything you can at the point that cuts current down to sub-ma just makes it go out. Anyone who has run a fusor through the parameter space knows this well.
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Richard Hull
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Re: FAQ - Minimum Practical Power Requirements Needed to do Fusion

Post by Richard Hull »

All of what you touch upon were covered in the very old FAQ in the operation forum.

viewtopic.php?f=24&t=3111&hilit=FAQ#p12621

Here, I give tips on actual, real time, fusor operation, provided all needed appliances are in place.

The delicate spider monkey routine and artice of fiddling with the voltage, gas flow, vacuum and current interactions are laid out in fusor operation. The simple fusor is operated over a very narrow and often ragged zone between runaway arc discharge and simple glow mode. Holding it in this sweet spot while doing fusion is the art of operation.

Once mastered it is second nature, but in learning mode, grids are often melted and power supplies or their fuses or diodes are blown.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: FAQ - The lowest current needed to do fusion

Post by jarrod1937 »

Wow, well thanks for all of the discussions in response to my question. That certainly cleared it up for me. Essentially the main post in this thread is describing the theoretical requirements needed to reach levels of actual fusion... but the rates may be so low that really sensitive equipment and statistical models may be required to properly judge if fusion is actually taking place. Whereas extremely higher ma ratings may be required for my readily testable fusion (aka easier neutron detection with cheaper equipment).
However, as has been stated here, you can get quite deep in the rabbit hole, so to speak, with coming up with some figures and taking other variables into account. But, for me, the basic info is all I need, for now at least.
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