Overall System efficiency / Path to breakeven

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Retric
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Overall System efficiency / Path to breakeven

Post by Retric »

I was wondering if anyone had worked out the math what it would
take to get one of these designs to positive net energy.

“the record for any fusor was 10 12 neutrons per second by Bob
Hirsch (former DOE chairman) using Deuterium-Tritium fuel in the
late 60's. *This is roughly 30 watts of fusion power*; a record not
matched using any method of fusion until recently and very good
when you consider that the total input power was 4,000 watts. That
fuser was roughly a factor of 150 away from breaking even,
assuming the energy could be extracted with 100% efficiency.”

Note: I have yet to find a good diagram listing the exact size of his
system so I am going to assume:

A) The inner grid was 1ft in diameter. (This is a total guess based on
a few pictures but it should not change any of the math just the net
size of the system.)

B) 75% of the input energy is turned to heat inside the chamber aka
vacuum pumps and instrumentation is limited to 25% of the system’s
input energy.

C) 35% of the systems thermal energy is converted to heat. (It’s
possible to go significantly higher than this but I want safe
assumptions.)

So for 5000w of input power we get (5000 * .75 + 5000 / 150) =
3783w of useful heat and (3783w * .35) = 1324w of output power.

As you scale the device the number of reactions per unit volume
remains constant but the volime increases at D^3. However,
the losses take place over the surface area which increases at D^2.
Thus a system that is twice as wide should be twice as efferent at
producing fusion. Thus a system that is 500 times as wide would
have an input power of (5000 * 500 * 500) = 1.2GW and an output
power of (1.2 GW* .75 +1.2 GW/(150 / 500)) * .35 = 1.715GW
(note: at 40% thermal efficiency it becomes 1.96GW) so a 500 foot
wide grid should provide 500MW of net power gain. At twice that size

I understand that the mesh might need to get larger to support it’s own
weight but in a larger device it should be reasonable to attach it to the
out shell in more places to help cool it and support it. However it also
seems like the power needs for instrumentation should go down .25 *
1.2 GW for vacuum pumps and instrumentation seems excessive.

PS: If someone can give better feedback on scaling issues I would
appreciate it. I am also assuming that the grid transparency % and
the system temperature would remain constant if that’s not the case
then this numbers should be adjusted accordingly. I presume that it
would be easer to operate in “Star Mode” and it would be easer to
aline the grid on a larger system but I want cautious estimates.
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Re: Overall System efficiency / Path to breakeven

Post by Richard Hull »

This math is fast and not carried to more that one place. This is done so that others may follow the math.

1mev= 1.6x10e-13 joules.
d-t reaction @ ~18 mev per reaction X 10e12 reactions/sec = 1.8 X10e13 mev/second output.

This works out to 1.6 X10e-13 x (1.8x10e13) or ~3watt/seconds . Not 30. Still, 3 watts of neutrons is very severe to the average human animal. However at 20 feet, the solid angle of intercept is way down and one would be OK for short term exposure. They had the borated block cave walls for added safety.

To do this they used 100kv @ ~20ma in a 4 gunned system where the guns used 800 watts each. (power hogs) This is a net input minus a few minor items like pumping energy, etc., of 3200 + 2000 = 5200 watts.

Note* ion guns usually expend about one quarter of their power in a filament and the rest in the ion arc and extractor systems. Ions guns are real nasty power hogs for such a miniscule ion current realized.

Again, this was the most complex fusor that Hirsch constructed in the cave. This was never done in the simple Hirsch-Meeks design

Not real good at all.

I know Bob Hirsch and he never claims more than 10 e10 neutrons per second from his best run. (This is consistent with real devices operated similarly with d-d versus d-t seeing no more than a 100 fold increase in reaction rates for the same energy input.) Gene Meeks, Hirsch's assistant, constantly claims the higher 10e12 figure. This higher figure is often seen in the literature.

If the principal investigator is to be believed this puts the power out backwards to ~.03 watts, but the input was the same....5200 watts.

These figures are from Bob in several interviews.

One can do calcs till the cows come home and you won't see breakeven in a real device.

Hirsch's super fusor was ~8" in diameter with a 3-4 inch shell inner grid.

In my personal interviews with Hirsch both on the phone and in a couple of sit downs together, he noted that nothing scales in fusion systems except in a negative manner. He further noted that if it can't be done in a small lab, it can't be done better in an aircraft hanger. He did not used to think this way. This is why he pushed for bigger tokamaks. He said that was a real bad decision and that he would never make that mistake again. He noted that if fusion can't be done at profit in the small it will not be done at profit in the large.

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Re: Overall System efficiency / Path to breakeven

Post by Brian McDermott »

Richard,

His quote about Hirsch's fusor is taken from my website. The section is question was a direct copy-and paste from a post on the construcion forum and was based on comments by you and other members here.

The original message can be found here: viewtopic.php?f=6&t=2528#p12038

This is the math I did to arrive at the conclusion stated on my website:

10e12 (total fusions=the number of neutrons) X ~1.7X10e7 (average eV/fusion) X 1.6X10e-19 (joules/eV) = 27.2 joules or 27.2 watt-second

I have added your new comment in the perevious post to that section my web page (http://www.brian-mcdermott.com/fusionproof.htm) as well.
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Re: Overall System efficiency / Path to breakeven

Post by Alex Aitken »

I don't bilieve the fusor was that close to breakeven. I dont have the numbers off hand, but when I worked through the maths a few years ago, including the difference between D-D and D-T mixtures one very important trend came out. The fusor power does not scale with volume, only with input power. This isnt a plasma confinement mechanism as work, its a collider.
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Re: Overall System efficiency / Path to breakeven

Post by Brian McDermott »

Exactly. In principle, fusion power scales with volume in a thermal/Maxwellian plasma system like a Tokamak, hence the reason for monsters like JET and ITER.

Because it is is collisional and not thermal, if you try to scale up a fusor's volume, the losses will increase faster than the power will increase. Overunity in a fusor, if it is possible at all, will have to be done through ingenious and novel design modifications and not through scaling up.
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Re: Overall System efficiency / Path to breakeven

Post by Retric »

“This isnt a plasma confinement mechanism as work, its a collider”

Ok, I could be wrong but my understanding is that most of the energy is lost as the high velocity ions hit the inner cage. Every pass you have something like a 2% chance for an ion to hit the cage at which point some / all that kinetic energy is converted to heat and it might strip an electron from the shell.

So your average ion has ~50 paths to fuse or the energy is wasted as heat. Now if the plasma density where to increase or the probability of fusing per collision would increase then the output power goes up. That's why higher power = more fusion.

However, if you increase the size of the center cage while keeping all the other ratio's the same (aka distance from cage to outer shell ect) then you increase the distance the ion travels though the plasma. It's still making 50 trips and the probability per unit distance of fusion is the same but the distance it travels increases on average.

There is also loss from black body radiation from the plasma but that should decrease some as there is a higher chance that that radiation hits the plasma vs the cage or outer shell. It's a vary low density gas so I don't know if we should ignore this but it is still going to get better as the diameter increases.

On a side note from some of the pictures it looks like there is a higher density core inside the cage but I don't know if that's an optical illusion from looking at a sphere of plasma or something else going on. If it's a higher density gas then all the other reasons why doubling the shells diameter would increase efficiency but it might apply to this inner core vs the cage. At which point it would need to be discovered if the inner core would always operate at a fixed density and percentage of the diameter of the cage.

I am assuming that this is operating as a steady state device so that the input energy to start the system can be ignored. Clearly that energy would be directly related to the volume of the gas under containment.

PS: Definitions:
Cage = negative potential grid / mesh a8t the center of the device.
Core = higher density plasma at the center of the Cage.
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Re: Overall System efficiency / Path to breakeven

Post by Retric »

I posted something like 40 seconds after your post where I pointed out why I thought it would scale with size if you have some logic why this is not the case please respond under that post.

It was at 2005-09-06 19:00 so you can search for it but I don't want to post it here and split off the responses to the scaling idea.
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Thanks,

Post by Retric »

Ok, that's a bummer.

“He further noted that if it can't be done in a small lab, it can't be done better in an aircraft hanger.”

Now that's just silly. A little math shows that confinement losses grow as the square of the diameter but power grows at the cube of the diameter. Granted going from 1 foot to 100foot is not going to help much if your need more than an extra 10^2 boost in efficiency but larger will give some boosts.

Note when useing Google.com to check math.
1.602 × 10 ^-19 * 1000 * 1000 * 18 * 10e12 = 28.83600
1.602 × 10 ^-19 * 1000 * 1000 * 18 * 10^12 = 2.88360
1.602 × 10 ^-19 * 1000 * 1000 * 18 * 1.0e12 = 2.88360

So I think it's 3.0 is correct.

Note: If you use a lithium blanket you get to add 3.2 MeV of energy for each fusion reaction. Giving you a net of 14.1 + 3.5 + 3.2 = 20.8 MeV which is still only 3.3w. (Some of those neutrons are from D+D reactions so it's a little less than that...)

As to the size issue that seems really small. Most people are using small vacuum chambers but I was thinking going to a 3 foot vacuum chamber would let you test the value of scaling these things at little cost in terms of time The cheepest way to get something that size might be to use an old propain taink if it can take +2 atm then it should stand -1 atm.

You did use the 10^12th in an older post but if you have new info then I don't mind changing the numbers. 4Inch inner sphere > 1 foot should give an extra 3x the efficiency but that's not going to make up for the 1000 fold loss from the fixed math and revised numbers. Anyway, at 10^12 it seems reasonable to keep working on this but at 10^10th it's little more than a toy.

Posted by Richard Hull on 2005-22-02 09:40

This was the Hirsch-Meeks 1970's patent idea. It worked brilliantly with D-T, giving isotropic numbers with these gases on the order of 10e10n/sec using a 50kv supply. With more elevated voltages near 80kv, Meeks recorded 10e12 on one occassion. An image of the device is seen on my website under the Farnsworth original team photos.

We see Hirsch at his desk at APTI in DC during one of my visits with him. On his desk, between the camera and Hirsch, is the actual Hirsch-Meeks device used in the 1968 "hotel linen cart fusor" demo before the AEC. The device can be seen on the cart also in this group of images with Gene Meeks and Steve Blasing in the image.

Richad Hull “
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Re: Overall System efficiency / Path to breakeven

Post by Carl Willis »

I think we've really not experimentally explored the space charge effects in the fusor or their implications to the breakeven question. What is our simple low-current, low-circulation high pressure collider is a fundamentally different animal from the gunned high-vacuum, high-recirculation, high-current machine discussed as a candidate for possible breakeven by Robert Hirsch ("Inertial-Electrostatic Confinement of Ionized Fusion Gases", J App Phys, v. 38, no. 11, p. 4522). However, to answer your questions JKirby, that's still a good read.

If anything is responsible for real densification or confinement in the fusion "star" it is most likely space charge via the mechanism of virtual electrodes (though there are some exotic acoustic hypotheses out there too).

We do know, as a result of careful work at U. Wisconsin, that in our high-pressure self-sustaining discharge regime, most fusion comes from charged-gaseous neutral collisions and significant fusion comes from collision with ions / atoms adsorbed or embedded in the cathode itself. The star with its crossed beams is a thing of beauty, but the hobby fusors working today would probably not greatly miss its absence and replacement of the cathode with a solid metal ball, fusion-wise (might take a hit on discharge stability though). Many of the reactions occur outside the grid, throughout the "accelerating volume" and at lower energies than are available from the whole accelerating field. I'd argue that relative losses scale roughly with the number of mean-free-paths between cathode and anode, meaning that for a given grid diameter it should pay to decrease the outer shell diameter as much as physically possible. You want the ions to be moving at their highest kinetic energy for the longest possible distance and spending a relatively minor amount of path length getting to that energy.

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Re: Overall System efficiency / Path to breakeven

Post by Richard Hull »

Marvin is correct. The fusor is a collider/accelerator. This is its glory over targeted accelerators and thermal machines. It is also part of its limitation.

The actual prime reaction zone will grow little with volume more than likely. losses will be more and more horrific.

Carl puts a point on it. Small fusors seem to do about as good, in the best hands, as the modertely larger units of the universities with D-D, which is all we can study with NRC regs and small cash outlays.

The star, or for that matter, any visual appearance or depiction is worthless to determine anything scientific about the fusor. A star always indicates that your are "fusion ready" though, as it signals a clean machine and that the proper pressure range is at hand and nothing more. It is a good visual tool for the underfunded amateur.

As regards the Hirsch fusor numbers, I used to quote Gene's numbers for Hirsch's gunned system, but Bob insisted that Meeks was wrong on several occasions subsequent to my first postings. After doing fusion work and seeing the work of others like U of Il and U of W. I find that 10e10 for a d-t fusor is a reasonable figure. The best fusor Bob ever operated in the cave is shown on my website.

http://rhull.home.infionline.net/highenergy013.htm

This is at the bottom of the page with Meeks looking on. There were actually six of those 800 watt guns! That makes the figures even worse!! Gene told me the heat from those things would brand you if you touched them. This was the best there was in the Farnsworth effort.

In closing, far be it for me to stop anyone from building a 6 meter or even a 100 meter diameter fusor! On the contrary, I applaud such an effort and will sit on the edge of my chair in anticipation of the outcome. In the real world, of course, I know it won't happen. I would be stunned at a spherical 1 meter fusor in operation. Costs, of course would scale significantly with size.

The key point here reverts back to Carl's discussion. It boils down to mean free path and fuel density. Big machines mean longer MFP and thus a loss of fuel density due to the reduced pressures to bump up MFP and a concomittant loss in volumetric power density and collisonal probability.

I believe we concluded some time back that the largest simple fusor would be on the order of 10" with 6"-8" being optimum.

Gunned systems operating at grossly reduced pressures and differentially pumped could be meters in diameter, but with the reduction in power density as noted above.

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Re: Overall System efficiency / Path to breakeven

Post by Retric »

Looking at your website I was amazed at how small most of their
fusers where. I can’t help but notice their love of ion cannons. But I
keep thinking they are missing out on the size advantages. I am
going to walk though how I am thinking about this system and you
can point out why there are problems with my line of thought.

Anyway, while 800 * 6 watt guns > high-energy ions they don’t do this
vary efficiently. So I would like to step back from monetary
constraints and just think about the system.

Now let’s start with a stupidly large chamber say 1km in diameter so
we can just look at building a grid and ion source of just about any
size we want.

To start with we need a source of ions. We could use Ion guns but if
you step further from the device and use a positively charge plate vs.
the ion gun then you are going to much more efficiently generate ions
AND those ions are going to accelerate over a longer distance as
they approach the grid. A 4-inch grid is going to be 1/16 as strong at
16 inches and 1/64 as strong at 32 inches but it’s still going to
accelerate those ions. I don’t know if it’s going to produce ions of the
same energy as those ion guns but I think it would be comparable.

Now we have a stream of high-energy ions. Clearly aiming them so
they miss the grid is important but I think it’s still possible to some
extent. So we leave the grid at -100kv and the ion generating pads
at +10kv. Once the system get’s to a steady state where are the
losses coming from. Well…

A) The “hot” parts of the system are going to radiate heat (which ends
up as “lost’ energy in terms of the generation of fusion.) This occurs
over the surface area of the bodies involved and increases as the
cube of the temperature.

B) Any ion that hits the grid is going to dump kinetic energy into heat.
(Yea it was heat to start with but you get the idea.) It might also gain
an electron at which point all it’s energy is wasted and it needs to hit
one of the positive pads to gain energy back.

C) The –100kv grid is going to lose electrons which are going to
radiate out of the grid. This will increase as the temperature of the grid
increases, the charge increases, and / or the surface area of the grid
increases.

Now looking at those losses most of them depend on surface areas
but fusion is based around volume. So let’s move from a 4-inch to an
8-inch grid.

Well how far from the grid do we build the new ion source? At 32
inches the field would be 1/16 as strong and at 64 inches it’s going to
be down to 1/64 as strong but the distance it travels though that field
would be much larger so it’s kinetic energy would be much higher. I
say move it a little closer so we keep the systems temperature
constant but we could also lower the voltage. (I don’t really know
what this does to scaling but clearly as we increase the device size
the value of ion guns goes away.)

Now you stated that by increasing the grid size you would reduce the
gas pressure but I really don’t see why this is the case. Gas
pressure is limited by the temperature you can raise the grid to. But
that should remain constant as you increase the device size. The
area that the grid is heated grows just as fast as the area the grid has
to radiate that heat away. Thus the pressure * temperature limit
should remain the same.

Looking at things this way it looks like a 3-foot vacuum chamber
should be plenty to test an 8-inch grid vs. 4-inch grid. Now the
optimum amount of gas in the chamber would be higher for the 8-inch
grid but other than that it should be easy to work out the scaling
issues.

But, if you think that’s a waste of time then I am more than willing to
listen to reason. The 8 inch D-T device should still be something like
3 orders of magnitude from break even so scaling is not going go help
much but that’s not that far away. A grid that could reach a higher
temperature or some other minor changes could couple with the ~100
fold increase in efficiency from going to 300 foot chamber and get this
to work.

Note: with enough free neutrons from fusion you can probably skip
the ionizer pads.

PS: What are good numbers to gage the value of moving from D-D
neutron counts what D-T neutron counts you would end up with.
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Re: Overall System efficiency / Path to breakeven

Post by Richard Hull »

I never said making the system larger would reduce the pressure. It would DEMAND that YOU reduce the operating pressure inorder to increase mean free path or your ions would never make it more than a fraction of the way to the reaction zone in a very large machine. What one hand gives in this game the other takes away. As Don Lancaster said..."No matter how you paly the game....you can't win....because it's the only game in town....and it is rigged."

I already alluded to the numbers of D-D versus D-T in the posts above. There is a factor of about 100 between them in fusions per unit input energy in similar operational regimes. This further translates into a factor of ~1000 between them in realized energy output operated in similar devices and input energies.

Jon Rosenstiel could be getting, easily, 10e8 to 10e9 fusions in his system with a 50:50 D-T mix. Of course where would he, or anyone here, obtain the T? So we putter and sputter along on the D-D venue.

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Re: Overall System efficiency / Path to breakeven

Post by Carl Willis »

A couple things to add:

"A) The “hot” parts of the system are going to radiate heat"

The removal of heat depends on surface area (whether by convective or radiative processes, you referenced the Stefan-Boltzmann Law), but the generation of heat only depends on surface area for a few of the loss processes. You mentioned those (having to do with thermionic emission and ion bombardment at the cathode grid). Volumetric bulk heating cannot be neglected though; this is what I mentioned before. Ions that are slowed down in the fill gas due to low-energy coulombic collisions ultimately just share their kinetic energy with other gas molecules and contribute to volumetric heating of the gas through which they pass. That is a major loss mechanism in any of these self-sustaining fusors, especially ones that have a low product of gradient and mean free path.

"At 32 inches the field would be 1/16 as strong and at 64 inches it’s going to be down to 1/64 as strong but the distance it travels though that field would be much larger so it’s kinetic energy would be much higher"

This is not correct as stated. The potential--in SI units of volts or joules per coulomb--on the inner grid is an expression of how much kinetic energy a quantity of charge will have when it reaches the grid from a reference location at zero potential. It doesn't matter what path or path length is taken to get there. If you have a grid at some potential U with respect to the anode, ions of charge q entering that grid from the anode will have a kinetic energy of U * q regardless of whether the anode is 10 cm or 1 km distant from the grid. If the grid is further out, the E field is weaker. And in practice the coulombic-collisional (frictional, roughly) losses along the path are higher.

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Re: Overall System efficiency / Path to breakeven

Post by DaveC »

I am firmly convinced that virtually all the energy losses are in gas-ion to grid or wall collisions. At the high normal pressures of the standard fusor, the currents being measured from the HV supply are a lot like the current like you would measure in the HV side of a neon sign... gas ion currents.

What we need to lower these non-productive currents are a few well designed, relatively low current, ion guns that are aimed to fire their ion beams through a ported (lensed is a more descriptive term) inner electrode. Between the ion gun output end and the inner electrode ports, the ion beam is focused... as sharply as possible, at fusor center.

Beam pairs are needed so there are opposing collisions and there is probably a practical maximum around a dozen. The simple objective is to produce the maximum ion current density in the collision area with full acceleration energy, without wasting all the input energy heating up pieces of electrode here and there.

Virtually all well designed electron guns work this way... with minimal interception of the beam energy, so the ion gun is not much different. As I have pointed out in several posts over the past year or two, ionization does not require a huge amount of energy. Virtually all gases, will ionize below 30 eV. So ion currents of mA at 30 volts is a pittance of energy input. Accelerating the ions to 20 kV, is only some 20 watts per mA. This clearly presumes a high vacuum, probably in the 10^-6 T range. And the setup I am describing probably works better in a batch mode than in a continuous mode.

There should be nearly zero additional losses, beside the vacuum pump losses, and.... if you set up, pump down and turn the system over to an ion pump, pump power can also go nearly to zero. This probably is close to the minimalist power input system. Efficiency will now hinge on the collisional probably in that tiny intersection volume at the center. With a little thought it can be appreciated that now, alignment and focus will be absolutely critical.... and not trivially accomplished.

This type of system seems to have the greatest potential, not for huge neutron generation count rates, but for highest efficiency. Not much else looks promising to me.

There's a lot to discuss about beam confinment action and etc. But perhaps another post thread would be appropriate.

Dave Cooper
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Re: Overall System efficiency / Path to breakeven

Post by Alex Aitken »

Might be helpful to add that in a pure collisional system, ion gun aimed at target in a commercial D-T neutron generator, 100Watts of ion beam get you about 10^11 n/s. With D-D its about 0.5e9 for the same power.

If we consider the fusor to be an accelerator with a pure hydrogen target then the theoretical max is not much above this (factor of 4 or so).

The other thing I can think of is another disadvantage to scaling up. The bigger the fusor the more diffuse the poissor will be and so for a given energy input the less chance an ion has for undergoing fusion in a single pass.
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Re: Overall System efficiency / Path to breakeven

Post by Retric »

I am talking about the attenuation of field strength over distance. By
increasing the grid size the physical distance it takes to reduce the
field from 1/2 strength to 1/4 strength increases. I never said what it
would be the same strength though.

If your charging the grid with 100kVolt DC and increase the Amps to
keep up with losses then the "The potential" will increase as you
scale the system. But that's a meaningless value for this system.

You are creating an Ion at some distance from the grid and that's the
point where it's going to pick up its kinetic energy. My point is you
chose the location where most of these ions are created and in a
larger system you can operate at a fixed distance from the grid by
reducing the grid voltage. You get to decide what voltage to
operate the grid at AND where to create most of the ions. Think of a
grid around the sun charged up to 1 volt. It would not really change
what's going on inside the grid much but it would yank ions that
crossed the grid much stronger than the 20-100kv grids most people
around here are building. You are not going to loose ions to the gas
as moving an ion from D or T to a D or T is going to keep the ion
count unchanged. With a constant pressure the amount of
coulombic-collisional over a constant distance is constant. (Ok as
you change the amount other field attenuates over the fixed distance
the average velocity changes slightly but that's not really significant if
you go from 3/4 field strength to full field strength over 1m vs. 7/8 field
strength to fill field strength over 1m.)

Granted as you start crossing several orders of magnitude some of
the other effects should become more pronounced but I think I can
build a 3-foot chamber that will demonstrate this with out too much
trouble vs. a 6-inch chamber. I am going to skip the ion guns in favor
of a 3 (-inner, +outer, chamber wall) system and see how this all
works.

PS: Once again this is all in relation to a steady state system if it’s
been operating for a few days it’s going to end up fairly balanced. It
should balance as to the amount of gas in vs. vacuumed out, voltage
in vs. losses, heat generated vs. heat losses ect. I know most
people here are working with cycled systems where the mesh would
over heat if left on for to long ect but once again I am talking about
steady state systems.
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Re: Overall System efficiency / Path to breakeven

Post by Carl Willis »

>I am talking about the attenuation of field strength over distance. By increasing the grid size the physical distance it takes to reduce the field from 1/2 strength to 1/4 strength increases.

That is true.

>If your charging the grid with 100kVolt DC and increase the Amps to keep up with losses then the "The potential" will increase as you scale the system.

I may not understand what this means. The potential difference between this grid and the point of reference is 100,000 joules per coulomb, amperage or scale of the system notwithstanding. An uncollided deuteron originating from the point of reference and entering the grid will have a kinetic energy of 100 keV.

>My point is you chose the location where most of these ions are created and in a larger system you can operate at a fixed distance from the grid by reducing the grid voltage.

How does a reduced grid voltage help you to "operate at a fixed distance?"

>...it would yank ions that crossed the grid much stronger

The force (yank??) on a charge due to an electric field is q * E. E at a giant isolated spherical surface at a potential of 1 V is MUCH smaller than E in any practical fusor. In fact, having a smaller sphere increases E at the surface for a given potential.

>You are not going to loose ions to the gas as moving an ion from D or T to a D or T is going to keep the ion count unchanged.

Ions interact with the electrons and nuclei of atoms in the ambient gas. They act like billiard balls bouncing off of other billiard balls most of the time (with a nuclear reaction happening in like one in billions of collisions). They slow down in the process. Slow-moving ions recombine with free electrons to yield neutral atoms and give off the purplish light seen in the fusor's "star", and the neutral atoms recombine with eachother to form gas molecules. To undo this process and recreate the ions so they can be accelerated requires energy (electron binding energies, and breaking a strong covalent chemical bond). It is important to minimize the chances of collisions in which fusion is highly unfavorable because the kinetic energy of the reactants is too low. Otherwise you just excite and heat up the fill gas.

>I think I can build a 3-foot chamber that will demonstrate this with out too much trouble vs. a 6-inch chamber.

Big chambers are very expensive, but I think this plan is a good experiment. Build two systems that have the same size cathode but different sized anodes, and see which has a higher fusion rate vs input power ratio. The reason it is a good experiment is that in practical fusors ionization is not occurring uniformly at the outer wall. It could be that ionization close to the cathode (and caused by the higher electron and ion densities in that region) is more prolific in sourcing fusion-bound ions than ionization at the outer wall due to secondary electrons. This would add something to our knowledge and inform better designs.

-Carl
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Ops.

Post by Retric »

“I may not understand what this means. "

Err nope I don't know what I am talking about.”having a smaller sphere increases E at the surface for a given potential.“ Damm, that's true.

I was thinking of voltage = the average number of electrons per unit area of the grid. By increasing the Voltage (on a fixed grid) you do increase the number of electrons per unit area on the grid but I forgot that this was also dependent on the size of the grid. A larger grid = more electrons producing back pressure which means that you need to increase the voltage to get the same level electrons per unit area.

As I was thinking in the last post you don't need the same number of electrons per unit area as you just need a strong enough field to accelerate the ions from their creation point to the grid. Anyway, if you double the voltage I think you can accelerate an ion from the same distance from the grid to the grid and give it the same energy.

So you increase the amperage by X^2 and the Voltage by ~X to get the same increase in potential at the same distances. But if you move the generating mesh out more then you can lower the voltage some as not all the losses are from the ion / non ion gas colistions.

I still think it's worth looking into if only because nobody can tell me why things would be worse as you increase the system size as long as you keep the the temperature and pressure the same it's just a question of how the energy use scales to provide that temperature. So it should be a fairly simple experiment to run. Granted doubling the grid size means 8x the wattage but that might be doable if I reduce the starting voltage to low enough levels thus I would be comparing a say a 30kv 4 inch system with a 60kv 8 inch system.instead of trying to work with 100kv systems at 4 inches and 200kv at 8 inches.
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Re: Ops.

Post by DaveC »

In the conventional non-gunned fusor, the ions come from out ins the volume of the fusor. So, a lrager fusor will have proportionally more ions if all other factors are maintained.

Since the rate of ion production is field dependent, but quite non-linearly so, mean free paths (read that as internal pressure) and voltage gradients in the volume (electric field) need to be optimally set, so that as many collisions as possible can take place as far out as possible, BEFORE the ions begin their flight inward to the fusor center.

All that being maintained at the same relative ion production efficiencies, it would seem to me that large diameter sphere, could at least generate more ions.

Whether it produces more fusion, remains to be seen.

But... scaling up the diameter, and retaining the average fields, means more, not less voltage, and probably, a current boost roughly proportional to the new volume ratio to old.

Shielding and safety issues would also require some attention, since if it all works as planned, neutron and rad fluxes will be larger.


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Re: Ops.

Post by Richard Hull »

Once again, we have come full circle on an idea. It is back to put up or shut up as with so many interesting ideas espoused here.

Sounds like someone, somewhere needs to get busy on this and report back. As Franklin said, "let the experiment be done".

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: Ops.

Post by Retric »

I agree that at some point it comes down to experimentation. But, I
don’t have the resources or free time to build several of these. It
looks like you don't automatically gain efficiency from scaling the
system but it's also a little more complex than I thought so I do want
to test this out.

At this point I am going to skip the ion gun approach as it’s expensive
to buy them and does not seem to add a lot to overall efficiency. It
looks like the most efferent systems are 3 grid design but there are
still a lot of other issues.

Which leaves (off the top of my head):

Grid Shape: Probably best to use iterative testing as it should be
cheep and easy to test various designs.

Grid Size: Still up in the air it’s probably not going to change much but
I do want to look into this.

Grid Materials: As I recall energy radiates at (deg K) ^ 4 so a slightly
higher temperature material should work much better.

Gas Density: More a question of experimentation than anything but
fusion power = density ^3 so it’s a biggie but this increases the inner
grid’s temperature so it’s one of the limits.

Gas Temperature: Also a biggie but this increases the inner grid’s
temperature so it’s one of the limits.

Inner Grid Voltage: This increase gas temper and pressure so it’s
cupped with grid temperature issues.

Back to scaling issues sufficiently large systems let you use a
compound inner grid so it could have a molten conductive inner core
and a non-conductive extremely high temperature outer core. You
could also cool the grid by pumping water though it. I don’t expect I to
build anything this size but if you can scale the systems with little loss
in efficiency then these might be doable.

Now at this point I am looking for a good cheep 3’ vacuumed chamber
and a good high voltage power supply so I can start working with this
stuff. But still don’t know what type of power supply I should aim for I
mean it would be nice to get some sort of 200kv monster but I don’t
think that’s in the budget. I think an X ray owner supply would work
well but I also want to be able to regulate the voltage so it’s going to
need more than a simple on off switch.

Of course there is going to be a ton of odds and ends but I want to
know I am picking up the right stuff before I go and spend cash on e-
bay. I don't mind building a few inner grids, as that seems to be a fun
experiment.
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