Here's a kick-off question; why have a vacuum for fusion?

Theorys about fusion devices of all types, plasma math, that sort of thing
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This is for your theories of how fusors, colliding beam, tokamaks, Q devices, you name it, actually work, or your predictions of how to improve them. We of course hope this won't devolve into endless gassing with no real testing.

Here's a kick-off question; why have a vacuum for fusion?

Postby chrismb » Mon Aug 30, 2010 4:28 pm

So I'm trying, here, to get the 'theory' section going with a, somewhat, historic question to stimulate a debate: Can you 'do' electric-fusion at atmospheric pressure, and if not, then why not?

[My glossary of 'electric-fusion' is any situation where ions gain fusible energy by acceleration in an electric field, rather than thermal energy in an ensemble of ions.]

As a naïve construction, imagine we just shrink a regular Farnsworth fusor by a linear 100x, but otherwise do all the same things as would be done in a vacuum. Why would the outcome differ to its vacuum brother with a 10^6 bigger volume at 10^-6 density?

There are plenty of plasmas at atmospheric pressure. Here is an alternative; what happens if you leak deuterium out of a fine metal tip held to a substantially negative voltage such that a coronal discharge forms? Deuterium ions formed at the edge of the corona would accelerate back to the tip, causing much the same situation as in an evacuated fusor.

I mentioned that this was a historic point and I say this because of a chap called Lidsky. Lawrence Lidsky was an assistant director of MIT fusion centre when he published an article in 1983 in which he blasted fusion research. One of the main gripes he used to deliver to his students [apparently] was the appauling power density of fusion. This is something I have commented on several times vis-a-vis the Sun, on fusor.net. Those who say they seek to reproduce the Sun are misleading at best because the Sun produces, volume-per-volume, 1/1000th of the heat output of your own warm, mammalian body. The modern tokamak collossi are figured to run at about 0.5W/cc. Lidsky's point was that the size and, thus, complexity to produce useful power outputs will never, ever, compare with fission when you can breed fuel from U238 and thorium.

Prior to Lidsky's 'unappreciated' output of critique [for which he was quietly removed from his post] he discussed the pathetic power densities available to controlled fusion with his students and it lead him to make the statement (reported as) "If there's a leaf left on a tree to burn, you won't want to build this" for each fusion method available. I believe one of his research aims was to look for ways to run fusion experiments at higher pressures.

The issue of power density is one I am adversely sensitive to, also. As you may or may not know, I have filed a patent for a 'new approach' to electric fusion which involves the active recovery of scattered ions. More on this later, but the point is that the device I have patented will, I calculate, also have a pretty miserable power output. In my case, I calculate that a device some 2m across would generate about 1kW. Volume-per-volume, it's probably much as tokamaks are, but you can see the issue - if we want it to power houses then you might conceive of burying one under each house, but is that really more satisfactory than simply sticking solar panels all over the roof space?

So my maiden post, here, asks a few questions of the audience..;
a) can we make an atmospheric-pressure fusion device?
b) if not, why, and are we expecting from [vacuum-based] terrestrial fusion to get beyond a 0.5W/cc limit?
c) was Lidsky right? Is it better to burn what we've got before wasting time on fusion power?
d) why go to the trouble of inventing fusion power if it is going to take up more space that a load of solar panels of the same power output/space?


...and make statements;
e) I have an idea for electric fusion that I'll be running though here on this forum, in due course, and the maths seems to work out to suggest there might be something in it, but I don't really see it as useful energy at any moment for several generations to come because all fusion looks a bit 'thin' on the energy-producing front due to the minuscule specific power outputs, so I'm in this just to see if I can't beat these generations of scientists to a better approach than the still-unproven magnetic confinement idea.
f) it's looking like the best means to make use of fusion power is to round up 2x10^30 kg of H2, put it all in a 400,000 mile radius bubble 90 million miles away, and suck up the incandescent heat off of it!
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby johnf » Tue Aug 31, 2010 4:05 am

Chris
as you postulate there is no reason, things just have to get smaller
We have at work got field emission to work at STP by keeping the distances between electrodes very small, so far a diodes and triodes working at around 10 volts.
The idea has merit but the scaling hence acceleration space poses its own problems.

The sun of course has cured this through its own gravity keeping things close enough to provide what we all live on ie squeezed up and hot helps heaps(size of this helps also).
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby Doug Coulter » Tue Aug 31, 2010 9:16 am

Ah, you guys on the other side of the pond get the jump on me -- wonderful :D

Of course, the logical limit of "going dense" is solids and beyond, and in my view may have merit -- see my smart target ideas for just one example. I think there are a lot more possibilities than that, this is just the one that got my attention -- and in fact, was at least partly inspired by something I think John does -- ion implanting, where it turns out they go deeper into some lattice orientations than others -- showing the effect, which in that case is maybe not wanted -- but confirms the possibility of ion channeling in solids. The question there is do you lose too much in scattering vs the increased probability of interaction. In other words, by the time you get a hit, has the incoming particle lost too much energy to sill do good? I've already thought about how to deal with lattice damage that all that implies. Here is a cheezy wikipedia entry that mentions channeling, and I've been talking to chemists and crystallographers about how we might make such a lattice that has the fuel atoms in the right places to try this. What you want is a molecular framwork that makes the channels, then the fuel atoms in the middle of each one. It turns out to be a bit of a challenge to do with monovalent fuel atoms from a chemists point of view, but maybe not impossible. Here, you get "effective" density in lieu of actual per-cc density, but I don't see that this matters too much.

The laser boys are working that at some level, but I think they are missing something important, or I am (who knows?). A long time back there was an interesting article in SciAm about doing interesting things acoustically. The basic idea was like this -- suppose you had an explosion in the middle of a regular solid (a sphere is the simple case). This would produce a certain pattern of shock on the surface sometime later, with reflections back and forth and so on. Why not instead put a time-reversed shock wave into the surface and have it propagate and concentrate in the middle instead? I think this falls apart at some energy density where you can only make the shock "so loud and no louder" but the concentration idea seems a good one. Of course, "between us girls", we know the laser boys don't really have an interest in fusion power as such, they are part of the nuclear weapon stewardship program, and trying to find ways to do nuclear tests without violating some treaties.
Their green spin is mostly that -- spin. They sure do get some fun toys to work with out of that.

I think to explore these further we'll have to find or work out some scaling laws -- to better than first order. My own work has been more with improving the percent of accelerated particles that do interact per pass, worrying about any recirculation later. It would seem interaction probability would go up faster than linear with density, as each incoming particle has more other ones to maybe interact with.

But I'd bet there's another side of that story -- above some density, they start hitting each other on the way in, obliquely and so on, at low center-mass energies (and making for more thermalization, which issue I know Cris relates to). And if you are talking about ions I believe the space charge repulsion builds up quicker then linear (heck I know it does, I'm just too lazy to go find the math right now) so you wind up having to overcome that with more input energy. Or do something like Chris's cool recirculating beam-collider-re-gatherer. One must never forget that mean free path means something utterly different for particles with charge on them than it does for a neutral gas.

Chad is now working with what would be a pretty tiny fusor, and with a little more work there we will get a data point. He's just getting going on a good vacuum system, and will need a lot of grid work etc to really become a contributer there, but at least lives close enough that we can meet FF and his fusor is tiny after all -- so it goes in the back of a car. He was here last weekend, and we got a good increment of progress on his device, and got some old b10 tubes working for us both. He is trying to do this in a 2.75" CF cross...we ran Paschen's law, but due to not screening off the side arms for field control, got it wrong that time -- Chad, you listening? Fix that with some SS screen over those long paths! If that is done, he should be running at much higher pressure (about 1 mbar) than a normal fusor and that would be instructive. We may have to get him setup in a grid that is more in proportion to the conditions -- right now it's far bigger than the normal size ratio most fusors would do best at, but...it's not a normal fusor either, and some other scaling laws may affect what's best there. He is just going to have to try some things and find out. To make a grid the same ratio as I use in the big guy, it's going to be really tiny, have to be thin conductors, and need a lot of precision to make. My guess is that he's going to need a bit of help with that one -- and that with the tiny grid, getting heat out of it is going to be interesting -- not much surface area to radiate it from easily. But see the key word -- guess.

For sure, we are quite fortunate the sun is such a lousy fusion device. Else, boom, and we never existed.

I think at some point worrying about energy density goes to a scaling problem (actually, a bunch of them depending on which detail you're concentrated on at the moment). To simplify my learning process, what I've been concentrating on in Q, gain, whatever you want to call it -- interaction probability for a given input energy, feeling that once we've got that -- the rest is one of those nasty "exercises for the student". Or not, but if you don't have Q, why bother with having a lot of whatever? The old saw "I'm losing money on each one, but making it up on volume?" Of course, in that search for Q, one always has in the mind what it would take to get big with it -- any engineer will work like that to make the trade-offs possible to hold in the head while thinking about other details. So maybe you don't go down paths you can't see a way to scale, but you leave that for a later checklist and another design rev.

I have run the back of envelope calcs for a silly idea already (with the help of a guy who really has the math at some point in the past). Could one not envision building a device that fired single fuel atoms at one another with such precision as to get a 100% fusion rate? The answer is, yes, it's theoretically possible if the atoms are reasonably cold at the time, nothing in uncertainty theory, or just plain classical physics prevents this if the optics are large enough to smooth out individual atom vibrations in them for example, and in this case with only two particles and fine aiming means the absolute minimum of space charge issues -- they repel one another but only straight on. With that interaction rate -- you don't need as many. Now extend that to still a single file, but one following the others -- bunching on the smallest level of just one per "bunch". Now your scaling limit is how fast you can have bunches follow one another.
As soon as you try to have multiple per bunch, you run into a lot of other issues, whether they are soluble or not, I dunno -- could go either way. Really, a lot of the problem of electric fusion boils down to this model someplace at the bottom of things. Do you try for more per bunch and deal with in-bunch repulsion, maybe with pre-distorting optics? Do you just try the "Shiva" kind of thing? I've never liked that latter too much as it seems you get a lot of particles interacting with low CM energies, but maybe at some point it becomes synergistic and confining? Doesn't "feel right" to me, though.

My own take on density issues is this, at least in re ions. Since it appears you want it, but can't easily get it in a simple rig due to space charge and other issues, why not gather a bunch of ions into a more diffuse space at first, bunch them up in groups, then and only then fire them all at some focus. By the time they get to the inner focus, their trajectories are set, the energy is already in them, and there's nothing else in there yet to repel them back out. This divides the space charge issues up into parts that can be managed separately. While diffuse, you can collect them around say, an outer grid, and bounce them back and forth between the wires with RF, kind of like an ion trap or mass spectrometer does (I'm in the process of learning that math). In my case, I'm visualizing cylinder grids (because that's what works best otherwise here) end on, and seeing orbits in figure 8 shapes between each rod pair -- all this is done at fairly low energies so far, and the cloud spends most of it's time in one of the big loops in each figure 8, passing through a sort of lousy focus as it goes through the wires. Then at some point, you fire a pulse into the center grid, and you've got all these pre-aligned ions already going the right way and from almost point sources to focus down into the middle. Just a thought, but it's the one I'm backing with money and time just now -- gearing up to try that one, because whether it works or not, I am sure to learn a heck of a lot, not just "head knowledge" but in the all important (to me) feel for how things work empirically.

I've found that once I get that, my own brain is about the world's best simulator of what would happen if I tried this or that -- but not before getting that feel. I've learned a lot over the past few years, but that's just taught me I need a little more yet. My hit rate on predicting what will happen if I try this or that is improving, but I don't feel like I'm "there" yet.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby Doug Coulter » Tue Aug 31, 2010 12:30 pm

On some further reflection, it looks to me like you go through a bad place scaling down, before you potentially get to a good place, and I have a tiny bit of data backing that up. We tried various sizes here -- easy to do with cylinder fusors, and that's what we saw. By putting a grounded pipe around various sized grids to try smaller dimensions. It then needed more pressure to "light off" as you'd expect, but the current density to hold voltage at the higher pressure went up fast -- so fast that at volts it would light off at (and high enough to potentially make fusion), it became a huge heating problem and tried to devolve into a low voltage arc as the metal of the grid itself started to become part of the plasma equations. The old surface area to volume thing? Dunno, here it seemed to be more than countered by other effects. At some point, a dense gas seems too close to the density of whatever conductor you make a grid out of (using the term density very loosely here, stuck-together-ness might be better) then there's not this free discrimination where you can whack the gas without whacking the grid.

I am however, well aware that non-obvious things happen at less than macroscopic scales (what most call nano scale) a lot of times -- they are always obvious in hindsight though.

Here as we have gone to yet lower pressures, with help to keep the ion/neutral ratios up decently high -- it gets better, not worse. We ran last weekend and got roughly 3 m/neuts/sec
at roughly the same power input as Richard's last HEAS run. So, call it in the region of 3x the Q -- and output. Seems some of the forces working against us go up at least as square law, maybe more, and so higher density doesn't work out for charged particles so well.

I do note that in most running, a Faraday probe in my tank has several hundred volts negative across a 100 meg ohm load. When I diddle with that using a second grid elsewhere in the tank, I get some very interesting behavior -- there's more to come on that, but there are timing issues not yet understood (it takes longer than it should) and I find that almost any thing that perturbs the main fusor results in a large burst of activity either going out of, or back into (not sure which yet) that nice stable dynamic equilibrium with a negative net charge on the plasma. I am still taking data on that one, in preparation to trying the "ion trap/buncher" thing. Here's just a taste of it.

And Chris -- get that patent up here so we can take it to pieces. I personally think that we won't actually do that at all -- if anything, the nits we pick will simply improve everyone's understanding of it all and make it yet more likely to work out. I really think you had a slick idea with that one -- now to make it real!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby johnf » Tue Aug 31, 2010 5:02 pm

Doug
we use channeling to increase our resolution in RBS measurements. We sent our He or h beam through a saphire crystal to collimate the beam into an atomically thin ribbon beam. This allows extremely accurite depth profiling of samples but takes quite some time to XYZ align the saphire.
In semiconductor making ion implantation is done off substrate axis so that chanelling does not occur and the implant depth is accurite. If you did manage to send the ions down a channel they could end up anywhere even right through the wafer and into the support stage.
Following are a few simulations on ion implant into various things at various energies
D2_Pd.pdf
(22.47 KiB) Downloaded 615 times
Trim_Y_80KeV_SS.pdf
(38.77 KiB) Downloaded 532 times
D2_Pd_2.pdf
(12.57 KiB) Downloaded 526 times


As can be seen in the above especially on the Pd target a good amount of D2 is close to the surface. so for this you would use a 30kV conditioning period of D2 ions then you could ramp up the beam to the operation voltage and be sure of some early hits in the Pd of trapped D2 atoms
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby Doug Coulter » Thu Sep 02, 2010 1:07 pm

Am I correct in supposing that what these show is a spreading out with depth into the target? (except for the deepest penetrating projectiles that didn't scatter off as much) That is what I would expect in the "normal" cases. I am wondering though if with a correctly structured target lattice, you'd still see as much of that. There is the channeling mentioned on the Wiki page (though I don't consider anything there as gospel). The crux of my idea is to have those channels, but with fuel atoms centered inside higher Z things, so scattering would be into channel center.

Or perhaps, with really fine resolution, what you would see would still be this spreading, but it would be from channel to channel at the atomic level, and while in a channel, centering in that channel? Only a few types of crystal structure would be expected to show that, of course. Obviously for the fusion case, only the behavior in the first part of penetration would matter much, as after that the projectiles have lost too much energy to the lattice to make fusion, even on a direct hit with a fuel nucleus.

Although it is kind of a lousy example, and at the wrong scale, imagine loading up a bunch of carbon nanotubes with H in them in the middle and shooting at that end-on. If you could do this with high-Z atoms instead of carbon, and get the fuel centered in each channel, wouldn't scattering of a "near miss" beam particle tend to push it towards channel center? Bad misses would of course scatter wildly around, as your pdf's appear to show, but improving the odds is all this idea needs to work -- not perfection.

Of course, any such approach would quickly ruin the target lattice, and you'd have to step and repeat or some such other dodge -- you might get a little more life annealing the already hit areas before trying again on them? Or simply ion-etching off a new surface for each shot?
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby johnf » Thu Sep 02, 2010 3:48 pm

Doug
Ion iplantation tends to put the ions into the interstitial spaces between the target atoms. True the target atoms are hit and sometimes displaced so the the ion takes up the position of the target atom and the target atom gets displaced into another interstitial site, we refer to these as damage cascades as there can be several bounces, displacements as the ion looses energy.
Most materials are polycrystaline in nature (except true glasses) and these crystals are not aligned with each other with size order bing from a few nm to um.
To make an implanted material change one has to anneal it ie raise the temp to a point where the bonds are jiggling hard enough to break with the extra strain imparted by the interstitial ions to become part of a new crystal lattice that combines the ions with the host target atoms. before annealing the added ions form an amorphous layer at the implant depth causing strain on the target polycrystaline structure.
When channeling is done to fine up a proton beam for instance a single crystal is used. In semiconductors the base wafer is also single crystaline and the implant layer becomes amorphous so that normal solid state theory as to Fermi levels etc does not work. annealing reforms the crystal structure so that the device will work as a semiconductor.
I did the D into Pd as the interstitial places in the Pd are perfect for trapping D and H atoms (Ti is also very good) so the damage cascades are different from the SS being implanted where more internal bouncing jiggling =damage occurs.
The final D2_2 graph shows the ion energies dispertion and the vertical line shows the eV level at which higher energy will start sputtering target atoms from the surface. This fact is used in commercial semiconductor implants where just before implant the beam is decelerated to below the sputtering point to preserve wafer surface layers from damage.
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby Doug Coulter » Thu Sep 02, 2010 8:45 pm

John, I agree with you all the way on how ion implanting works (and I'm wanting to learn enough to someday do it well here for other reasons -- and thanks in advance) but I think we're talking alongside one another here, probably my fault for not being clear enough.

Let us assume I want to take a beam (think shotgun blast -- random, flat, or gaussian intensity distribution doesn't matter much) and hit a lot of little bulls-eyes with it -- I want to do this by putting little funnels over them all, so that the shot what would have gone between the targets and missed between them gets funneled into the targets instead. That's the essence of what I'm calling a "smart target" here. I plan to make this target with chemistry and crystallography, as that would be the only way to get perfect single crystals of what I want -- high Z funnels with low Z fusion fuel at the focus of each one. The stuff from ion implantation comes later -- that's the shotgun beam in this case we'll use to hit the targets with - using the funnels to help us hit what we want -- to concentrate a random beam distribution into tiny pixels, if you will.

You can see how firing a shotgun at a bunch of golf balls about 1/3 mile apart means mostly missing (big waste of powder and shot) -- that's the problem with most fusion designs that involve beam-on-target. Even a solid D target would have about a 20k^2 to one ratio of misses to hits, based on sizes of the atoms vs nuclei. It only gets worse with D (or whatever) implanted in some metal at random, particularly as you say, if the metal were polycrystalline and random orientation.

So assuming the Wiki is correct (and if you don't think so -- say!) this is more or less possible "in theory" -- along some axes of a perfect crystal (of certain atomic arrangements, not any xtal would do here, the Wiki example is silicon) there is a seeming funnel effect for incoming beam ions, which go down the "holes" rather than hit atoms of the matrix so often and lose energy randomly, which is why they get farther in there when things are aligned just so...

Sure, they wind up as interstitials -- no argument there, but what I'm interested in is the behavior during only the very first few atomic layers of the target, while they are still energetic enough to cause fusion on collisions with the targets held in these little funnels. Where they wind up if they miss isn't important to this idea.

If we could use the beam to fill the targets (make them in the first place) -- no argument with that either, but I think that with what I know at the moment, that's going to be quite the challenge, as I think you're also saying above. You just do too much damage to the lattice, and things wind up in more or less random places in there -- some substituting for the original lattice atoms, some in the interstices. Even after annealing, I'd think you'd have kind of a stressed/warped lattice arrangement if there was a lot of doping -- chemistry and crystallography say I should be able to have a pretty high percent of fuel in the funnel holes compared to doping semiconductor limits without completely ruining the lattice -- because chemical bonds will hold it all together in a desired regular arrangement for me.

Chemistry should be able to fabricate the targets better and easier in this case -- or so I think at the moment, probably not knowing enough yet to really think well.

This thought arose as a way out of difficulties with another idea, the explaining of which might make things more clear.

My first idea was to not accelerate ions that weren't going to hit - select the misses out pre-acceleration. Only put energy in ones already starting in the right places and already going the right way -- they are "cheap" when they are just low energy ions, so that's where you do the selection process.

One possible way would be to have what amounts to a shadow mask in an old CRT, for example -- just only allow through into the rest of the optics the ones already starting from, and going, your way, so as not to waste energy accelerating ones sure to miss (the huge bulk of them at the size ratios of atoms to nuclei). In essence, make an inside out ion microscope, focusing an image of a shadow mask that could be made, down to the inter-nucleus spacing on the target. Of course, there are some serious limitations with that -- initial alignment for one (solvable but hard, with a single particle at a time beam to "find" the lattice phase), the size ratio of the little holes to their spacing which is tiny -- you couldn't make one with enough pixels at a manageable size. And scalability -- you couldn't do this over more than about a couple hundred on a side due to even zero-temperature motions in the target lattice. Even if you could make a magic shadow mask of little rifle barrels out of some nanotech, the numbers just don't quite roll out as you'd like for that -- a stack of carbon nanotubes would have a very much too large hole size to spacing ratio for example. And a raster scanned beam can't be switched on and off quick enough to dispense with all this if you try to gate the ions before they are going fast...which is the whole point of the exercise -- to not waste energy on those missing (or going to miss) the target nuclei.

A smart target would solve most all of this, as it would all jiggle together locally, and not need more than a "shotgun" beam, instead of a ton of tiny aimed "rifles". I'm not actively pushing on this idea just now, other than doing background research while I do the fusor thing, which is turning out better than I had any reason to hope/plan for just now -- the game's afoot there for sure, but this is kind of a backstop, and a concept I've been thinking about since I was in high school in the '60s. Sure, you ruin the part of the target you hit -- so put it on cassette tape and step and repeat, for example, now you don't have to have super-good alignment of the beam and optics -- the target itself does that, it's "smart". I'm thinking that you'd get most of the good out of this with some tens (at most) atomic layers of target. Anything that gets past that has already probably lost too much energy to be useful anyway.

Hopefully, either "aha" or "that's not even wrong" happened just above ;)
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby johnf » Thu Sep 02, 2010 10:06 pm

Okay Doug
I see what you are tryinig to do
Difficult but!!

I did a bit of work implanting He into Al a couple of years back and formed high pressure bubbles of He below the Al surface
6ximplanted.tif

In this pic you can see the bubbles here and there

so I upped the dose to above 50% atomic concentration of He
and this pic shows total delamination of the Al at the implant depth
8bximplanted.tif


and of course I had to go and check the layer thickness

8cximplanted.tif


so a target could be made Loaded with high pressure D and a high eV value d beam run into it, bashing the trapped D atoms in the bubble. worst case is that you raise the pressure of the bubble too far and it bursts.
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Re: Here's a kick-off question; why have a vacuum for fusion

Postby chrismb » Fri Sep 03, 2010 9:04 am

John - nice thread, great stuff! (Are you sure there aren't any vacancies for 'playful experimenters' at your lab!?)

On the Paschen thing. That is definitely one answer to the thread question. As you come down that curve, we're looking for a spot which allows maximum accelerations without electrical breakdown, and that region lies below the Paschen minimum rather than above it. That is, probably, *the* answer to the question, but I still wonder if there aren't actually some high density 'mini-scopic' sizes that would also work out, if only you could make the kit small enough.

I would still be interested to know what happens if you burn deuterium from the tip of a very fine needle, and then apply a big negative voltage to that needle.
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