SimIon and Fusor Simulation

[James Parkin]

Hi, guys, I've been trawling this community covertly for the past few months to help me with my fusion ideas, and I've decided to introduce myself and hopefully add to the community.

I have been in discussion with SimIon about it's products after I tried to develop a rough simulation of my concept and wished to take it further and confirm (or disprove it). I obviously did a google search and had a look at my options and SimIon stood out. I had a look at the prices realized that they were a bit steep but grabbed the trial anyway. After realizing that it would be a great way to visualize my idea, but there was no way that as a soon-to-be student, that I would have the funds necessary to risk trying my crackpot idea, I sent an e-mail off.

The result has been an enthusiastic response directly from one of the developers and one of the things he mentioned was adding a fusor like example to the trial version, so people could play, with a few settings and help to educate themselves about fusors. I have thus decided to post here to see what people think about this and if they could provide me with the best geometries for the example that would be best for the fusor community, specific dimensions would be appreciated by SimIon, and I didn't think I was experienced enough to give them, so I thought I'd consult you lot.

If anyone is interested in my 2D c++ iterative simulation source code for very basic geometries (outputs in csv) I'd be more than happy to share.

Regards

James Parkin

[Carl Willis]

Hi James,

The utility of a code like SIMION depends greatly on what you want to know. SIMION doesn't model the physics of the discharge process or the nuclear reactions that are often of interest and some of the most complicated facets of the fusor, but it can model particle trajectories in static fields quite nicely. Its space-charge features are evolving and becoming more comprehensive, but must be applied with great care. It all depends on what you want to learn.

I attached an adjustable potential array for a spherical solid shell containing a 100% transparent inner grid, if you just want to start fooling around quickly. Attached below is a gallery of various deuteron trajectories (blue) through this geometry with the grid at -50 kV. The ion starting positions, directions, and energies are essentially arbitrary, just meant to illustrate some possibilities. To be honest, I'm hard-pressed to see any great utility in SIMION for the typical fusor, but maybe you will figure out how to make this tool valuable. On the other hand, I have found it very useful in ion source design and am eager to try the new version's (8.1) Poisson space-charge solver.

-Carl

[Doug Coulter]

Yes, when I called them and asked if I could simulate a fusor to the degree I think is needed, they kindly told me "save your money, we can't do that yet". Refreshing honesty from a vendor.

[Charles Snyder]

James,

It has been several months back, but HappyJack27 was posting various polywell based particle simulations, using the multiprocessor capabilities of some of the newer, multi-processor graphics cards at:

www.talk-polywell.org

You may find some of his work interesting.

Charles

[James Parkin]

My theory, which I don't want to share at the moment because it is untested and possibly nonsense, will not involve the polywell design, it is primarily electric fields only, I have spent time looking at polywells and they are looking really promising, but I feel I have a simpler solution, or more likely no solution.

Cheers for the link though I will have a look around their forums.

James

[James Parkin]

Interesting, how long ago did you ask about it's (SimIon's) capabilities?

I do feel that, if people were to give me specific dimensions for an example for SimIon to include in it's trial version, it will:
1). Open up the community to people how can't get access to the equipment and still see ion paths.
2). Could be used educational.
3). Build intuition about the ion paths.
4). There are new updates that allow all kinds of new modeling.

Yes, it won't be able to model reactions but it may help with recirculation ideas and possibly some plasma simulation, so I'm looking at suggested inner and outer electrode designs that would be suitable and practical, i.e. not 100% transparent. I'm trying to give this community a chance to help.

James

[Carl Willis]

Hi James,

I agree with your points, and it would be kind of cool to see SIS acknowledge this technology with a fusor-based instructional of some kind.

The dimensions of projects seen on this forum range from the 2-3/4" CF size all the way up to about 10" OD. There are different methods of constructing grids (the "Rosenstiel method," the "great circle method", helical approaches, cylindrical cages, and others). Some builders have recommended certain ratios of grid size to chamber size. I believe Frank S. and Jon Rosenstiel have had comments on that. You should just search through the archives to get accustomed to the diversity as well as the time-honored favorite geometries in use. If I had to designate the classic design, it would be a 6" spherical chamber with a ~1.5" spherical grid. Richard Hull's "Fusor III" is representative.

Doug's response unfortunately doesn't shed any light on what he asked in his call or what SIMION's relevant limitations were; maybe he'll expand. EVERY model has a domain of validity that necessarily can't encompass all real situations to answer all questions.

-Carl

[Charles Snyder]

James,

You missed my point. HappyJack27 at www.talk-polywell.org was producing some interesting simulations that may help your effort. It just happened that he was modeling a Polywell configuration but the methods could be applied to other configurations just as well.

Charles

[Doug Coulter]

I asked roughly a year ago. They said they couldn't handle things like how much ionization would be happening (or being lost) due to collisions at the real gas density a fusor runs at, among other things. They also said they couldn't handle things like fast variations in space charge density should the charged particle flow get "correlated" as in any sort of pulsing that created fast changes in field gradients.

At the time I asked them if they were going to support multi-core graphics cards as well, and they said they had no immediate plans for that either, so perhaps things have changed. I got the idea that what I was asking for was so far past what you could do that even a few hundred X more computing power wouldn't bring it into reach.

For example, it's obvious in Carl's plots that collisions with neutrals aren't happening anywhere near as frequently as they really do in a fusor...He's showing an idealized case with a few ions in an otherwise perfect vacuum.

Whereas in some pictures we've made here with a cylinder grid and a long insulated pipe coming out one end, we see what just about has to be a viscous flow out of that long pipe -- when the main fusor tank pressure is well into the molecular flow region, implying we have a "compression ratio" and a heck of a lot of collisions near the "focus" -- they aren't necessarily reacting collisions - we know most aren't, due to the neutron production rate being so low (compared to what we want for power, it's still enough to measure easily and in some cases enough to be a rad hazard).

At the scale of a normal fusor (say, a 1" grid and a 6" ID tank, which numbers no one seems to be sharing with you here) there are simply too many particles to keep track of in current computers.
It's a pretty daunting programming task to get it within reason, and include charge exchange in collisions, the mutual Coulomb (space charge) effects as the particles gain and lose electrons and so on.

The fact that it can't model the actual fusion is not important at this point since the probability is so low, and the effect of a fusion is so tiny on the rest of the setup.

[James Parkin]

SimIon has improved with various gas collision modules, but I'm *NOT* trying to give you guys a perfect simulation. I'm only trying to help to give you and beginners a tool that might provide insight into how ions fly, how voltages affect fields and thus the ions and a way to play with fusor ideas before you buy all the equipment.

Of course SimIon is far from perfect where fusors are concerned but it does give a good idea about ions and allows ion-ion collisions. I'm surprised there is such a negative response to a suggestion to a free fusor example for SimIon..... I thought people would be jumping to give me geometries and dimensions.

I'm aware that it is hard to model each particle and use ion-ion collisions without a super computer, but I still feel it can be a powerful/helpful tool.

James

[James Parkin]

Carl, thank you for the advice.

I will be doing a brief bit of research and get back to all of you with the geometry that I will eventually submit to SIS and my contact there so long as there is enough support.

I was considering spherical cages, so the example will most like be of this species.

James

[Dan Tibbets]

A few links that may or may not have useful information themselves or in their bibliographies

http://ssl.mit.edu/publications/theses/ ... chCarl.pdf


http://fti.neep.wisc.edu/static/TALKS/1 ... anmeye.pdf


http://fti.neep.wisc.edu/static/TALKS/1 ... hnkipr.pdf


Pessimistic discussion of IEC devices from an energy balance perspective.

http://fsl.ne.uiuc.edu/IEC/Nevins,%20Ph ... 995%29.pdf


http://fsl.ne.uiuc.edu/IEC/Rider,%20Phy ... as1995.pdf


A more optimistic analysis.

http://www.mendeley.com/research/a-boun ... n-devices/

and

http://www.sciencedirect.com/science/ar ... 9199963945
The abstract of this article explains things clearly

Dan Tibbets

[Doug Coulter]

I think what you run into is a matter of sheer scale, like peeing into the ocean and expecting to see the level rise.

I gave you the standard dimensions, surprised no one else did. Chamber, sphere, 6" ID. Grid, three circular loops, about 1" OD, some are a little smaller, down to 3/4" or so. Grid in the center of course, and perhaps a small stalk from the chamber edge up to it -- some insulate that, some don't.

The issue is, it's not that there's a collision or two during a simulation. There are perhaps thousands per pass of a particle in some cases, and unless you can simulate it all, you can't trust the results for "any". Things get not-linear quick if for example, a bunch of neutralized ions gather in the grid center, as it appears they do from some other experiments. It's an all or nothing thing. If you can't do it at the real gas/ion density, you just can't do it, you're not simulating a fusor, but something else we don't (or can't) build.

The reason for the skepticism is experience. Many simulations have been done, then the gear built based on those predictions didn't work anything like the simulation predicted, not close, not in the same ballpark, not in the same universe. It's been going on for years -- some of it is mentioned here if you search (which would also have found you a lot of info on dimensions, gas pressures, voltages, grid geometries, you name it, it's here).

It got so bad, that most who do simulations just write papers, but never build anything, because even they know by now it's pointless. And those who build things, and spend time and money and effort making them work are rightly suspicious when they see all these simulations that seem to closely match things they've tried for real and in person -- but don't correctly predict what they actually saw when they did the real build. It's just the sorry state of the art at this point. No one's going to put you down if you do a simulation that indicates that changing this or that would improve things, then you build it and prove you were right. There's just so very little of that that's ever actually happened.

In fact, I can't think of a single case. I don't think it's impossible, just very difficult, and most aren't that willing to waste yet more time on it -- it's a "you first" kind of thing at this point.

I know my own practical lab experience has overwhelmed my own understanding so many times that I no longer depend on it so much. What seems "it's obvious how it ought to be" has been flat wrong too many times. Emergent behavior in this system is fierce and only tends to be obvious in hindsight.

Since some of us have "simulators" that run in real time, with the real stuff, warts and all, we just do that these days. If SIMION improves to the point where it can be trusted, that would be great news indeed -- but we're not there yet, or so I think. Take those numbers above (they are from Richard's fusor) and see what you get! We already know what he gets. That would be the first, necessary baby step to moving forward.

Then you can try my setup, which is a cylinder grid, 8 eqispaced .040" diameter rods 2.5" long, 7/8" OD, with graphite endcaps, fed HV from one end, the other endcap with a 3/4" hole in it, in a long 6" ID pipe, with one end open to a much larger tank -- and depth to the big tank is critical adjustment, the grid end being a little recessed working the best. It seems to work a little bit better than most of the spherical ones do, but not a lot. If your trajectories match what I see with various probes, I'll be real impressed, because I was quite surprised myself when I actually measured some things (and I have a lot more to measure before I spout any useless theories about that).

[James Parkin]

Cheers, for your time and the dimensions you've given me.

I have had a good long look at your forums and have found many a design, but I felt I wasn't experienced enough to pick a particular setup for the simulator and didn't want to make a mistake or pick an option that would be pointless to simulate, etc.

I am aware of the issues involved with fusors and their complexity and I'm not naive enough to know that I can easily simulate a fusor's operation, I'm just trying to help provide some tools that *might* help, or be a good place to start to understand fusors.

As I have never built a fusor, I thought it would be ridiculous to give schematics representative of the fusor community. Fortunately I might actually be able to rectify this in the near future as I have been accepted to study at Imperial College London, at which there is a chance I might be lucky enough to have access to a fusor.

If SimIon add the fusor example swiftly I will get back to you with the results.

Regards

James Parkin

[James Parkin]

Hi, I have spent a bit of time looking through all his simulations.

They are very intriguing and I have already, asked him about what interactions he simulates and in what way he simulates them, for example using Laplace, etc. He is actually harnessing a particle engine and using graphic card based processing, but I'm not sure how accurately he is simulating the particles till I found out exactly what he is doing, as I have seen some simulations that look good but it turns out that sometimes the physics/maths is nonsense and this has turned me into a bit of a skeptic as to where people draw the line between simulation and graphic representation of a system.

Regards

James Parkin