Finn Hammer, fusor update.

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Mark Rowley
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Re: Finn Hammer, fusor update.

Post by Mark Rowley »

Impressive Impressive Impressive!!

Activating copper to detectable levels outside of gamma spec is the bees knees! You’re deep into the mega n/s numbers.

Are you still using copper for the cylindrical electrode? And is it pure copper or some type of alloy? And if you have it, can you show a picture of how you attached the small stainless post to the cylinder?

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Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Jim, Richard, Mark
Thank you for your encouraging words

@Richard,

Vanadium. V51, 99.75%, 4.9 barns -> V52, beta emitter 3.76 minutes half life. should be doable.

Dysprosium is quite a riddle. There is so litle Dy158, 0.1% And then there are Dy160~164, each accounting for about a quarter of the volume, 130 to 800 barns, I don't quite know what to make of it, there is nothing in the coloum of "Principal means of production".....
I suppose during activation, the isotopes with the lower numbers will be stepped up to higher numbers, but still be stable, untill they reach Dy165 (through 800 barns) and more of it to Dy165m due to 2000 barns for that reaction.
The Dy165 has 139 minutes of half life, whereas Dy165m has a half life of 1.26 minutes, so due to the large crossection, and swift trip to saturation, it is the Dy165m we will see most of.
I guess this is where Camma spec. will shine, with the ability to single out the different energies from the different decay mechanisms. I am looking forward to getting my 63mm NaI(Ti) crystal online some time this coming winter.
Please correct me if I am wrong, I appreciate guidance.

@Mark

I am using a 15mmØ stainless cathode now. It is connected to the stalk with a 4mm thread. See attached picture.

grid.jpg

Cheers, Finn Hammer
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Richard Hull
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

Dy 164 is all you need to activate! It is stable, and over 28% of all elemental Dysprosium! So you are starting at the 800 and 2000 barn cross sections right out of the chute, the instant neutrons hit natural dysprosium. The only issues are the half lives of the activated elements. You have the neutrons in great abundance so you need not run for extreme periods to see something activate even to a low level with those cross sections.

The only elemental isotope of Dy to activate is the last one in the isotopic chain of the stable natural elements!! You will never change any of the percentages down a chain of a natural element regardless of the cross sections of the predecessors. In "nitpicker" math you will, of course! Let us take Dy161 with its great cross section of 600 with an atomic percent of 18.88 % the next one you will create by neutron bombardment is stable Dy162 which was 25.53% abundance. At the end of an hour you will have upped the percentage of Dy162 to 25.53000000000000000000000000000000006% there are countless quadrillions of atoms in that original 25.53 % upping that by 200million to a billion new Dy162 atoms is effectively zero increase in the atomic percent in the element of that isotope.

So when you see an isotope of any element that has a 45,000 barn cross section that will lead to another stable isotope of that element just below it in the table, just forget that beautiful cross section even exists. Always in a stable chain like that, go to the last stable element and check out its cross section and if great, see what it makes, what the radiations are, but more importantly, what is its half life?. A 5200 barn cross section that leads to a 50.6 year activation half-life is a non-starter. I discuss in great detail all of this in my paper in the FAQ on activation related to the many flies in the activation ointment for fusor users. There are all manner of nasty flies buzzing around the amateur fusioneer creating "catch 22s" when it comes to activation. Have no fear or worry, you will catch on.

Again that activation paper to be found at


viewtopic.php?f=31&t=12251&p=79744#p79744


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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Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

The aftermath.......

I forgot to turn the waterpump on for another record attempt, and that would lead to a water leak which necessitates a rethink, and probably some hardware alterations.
On the picture, you see the top water manifold, which I made from PVC, a miscalculation from my side, because when the fusor body gets really hot, then even just the orings could distort the seating face to a point where a major leak was unavoidable:

Distorted sealing face of PVC manifold.
Distorted sealing face of PVC manifold.

So I had to dismantle the fusor to do these alterations, and this is what I found:

Firstly, the grid had melted, and of course right at the points which are out of the like of sight:

20210731_133212.jpg

Notice the pristine appearance of the alloy, not a single deposit of coloured metal.

20210731_133244.jpg
20210731_133335.jpg

Another thing to notice, is the signs of arching from the stem to the (now sputtered) surface of the Macor centering plug.
This has perhaps been the cause of the intermittent flickering of the plasma which I have had at times. This is also situated at a place where I cannot see it from the grid.
I think I did an array of poor design decisions with this feedthrough.


20210731_133359.jpg

Live and learn, this is a great lesson in high voltage plasma technology, and I will manufacture a new feedthrough, which will adress the arching problem, as well as provide better cooling of the grid, although I do not want it to get too cold: It is when it shines bright red that the cube really rocks!

The end plugs looks as would be expected:

20210731_133441.jpg


Edit: @Richard. Thanks for your ever patient tuition about (in this instant) activation: I had quite forgotten about the fantazillions of molecules in a sample.
Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Jon Rosenstiel »

Finn,

Wow, a real melt-down! My condolences.

I've spent quite a bit of time messing around with alumina tubes and Macor shields and what-not and have come to the conclusion that it was all a waste of time. In the end the standard feedthrough with a bare stalk was the most trouble-free.

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Re: Finn Hammer, fusor update.

Post by Jim Kovalchick »

I no longer use alumina. It always ended up arcing. I use a stainless steel tube conductor only. If the feedthrough insulator extends far enough into the chamber, shielding the conductor further is unnecessary.
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Re: Finn Hammer, fusor update.

Post by Mark Rowley »

Finn, I concur with Jim and Jon as those are my findings as well. No alumina or ceramic grid stalk enclosure/sleeve.

Regarding the meltdown, looks like you had quite the pool of “corium” on the edge! One more question on the grid, did you make it out of a solid piece of stainless rod?

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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Jon, Jim, Mark

I am glad you all agree, because what started as a macor centering device, (the stalk is thin and flimsy) soon got sputtered and turned into a short to ground. There will be nothing like that in the new design, which will, on another note, however, employ cooling, in an attempt to process more power.
@Mark.
When I gave up on the squirrel cage grid and switched over to the tube grid, I went to the industrial scrap yard, stainless booth, and picked up what looked like suitable candidate pipe bits for grid tubes. I have a collection of tubes from 1/2" to 3/4" in diameter.

20210801_001104.jpg

So, no solid rod here, just skimming a tube inside and out, and part it off.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

I will throw a me too in there as well. I started out in 1998 with alumina insulating stalks but by 2002 they were found to create more issues than they were worth. I use a naked 1/4" diameter SS stalk now to mount my grid on and no issues.

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: Finn Hammer, fusor update.

Post by Albert Mery »

Hi, I've been following Finn's fantastic fusion work from afar and I have to say I have learnt so much from this thread. Very inspirational!

I just wanted to add to the topic of bare feedthroughs, I read in the IEC book that the researchers at Kyoto University have opted for bare metal feedthroughs in order to reduce the space taken up by thick layers of insulator present in most traditional feedthroughs, since, according to their calculations, for a classical feedtrough, around 50% of ions are lost to the stalk as opposed to the grid. Also the risk of arc-over due to sputtered material is avoided. So I suppose those are additional reasons for why you might opt for a bare metal feedthrough...

Has anyone experimented with cheap PVC as an insulator? Because I saw that one of the feedthrough designs mentioned in the book uses a fair bit of PVC which surprised me since I've read that It outgasses like hell and perhaps wouldn't be appropriate for the high temps of a fusor...

Keep up the good work Finn! Congrats on your latest activation results btw :)

Albert
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Richard Hull
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

PVC or any plastic just can't be allowed to enter a fusor and its ion/electron rich bombardment regions.

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
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Re: Finn Hammer, fusor update.

Post by Andrew Seltzman »

I also found alumina to be a poor insulator with arcing problems. Fused quartz or macor are excellent choices, with macor being particularly good as it is machinable.

Macor
Max. Temperature: 800° C
Dielectric Strength: 1,140 V/mil

Alumina
Max. Temperature: 1700° C
Dielectric Strength: 230-400 V/mil
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Re: Finn Hammer, fusor update.

Post by Dennis P Brown »

Ditto (do people know that one any more?) on a bare steel stalk as the feed-thru; had good success with that on my original large system (of which, I am FINALLY getting the last bit of work that needed to be done on my new system. Even obtained a large silver foil for my activation attempt.)

Finn, as always, great work on your design and build as well as your detailed explanations. Very useful for every one.
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Dennis, how could we ever forget?

https://m.youtube.com/watch?v=LhkwiJowIUM

Cheers to all, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

All,
It has been awfully quiet lately, so although this is a small thing, I thought I would just post a single picture of my new, and as of yet untested, feedthrough. It is right off the lathe and associated machinery, and dusty. Just could not help show it (off).

The previous kathode really was jerryrigged, yet produced respectable numbers. The cathode assy was dangling on a 2.5mm stainless rod, with no hope of getting any of the heat out to the outside. This feedthrough assembly has air cooling of the molybdenum kathode and is an attempt to place it in line with the rest of the fusors features, and will hopefully lead to even better neutron production.

a lot of work went into this one.
a lot of work went into this one.
Jeez, dust and crud really shows on photos
Jeez, dust and crud really shows on photos
Air is blown through the blue pipe.....
Air is blown through the blue pipe.....
It it works, I will give a full and documented description.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Liam David »

Wow, that looks fantastic! Looking forward to all the details.
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

A thing of great beauty for sure. Nice work. I am curious about your results with it. I have been totally successful with just packing silicone HV putty around every thing. Looks terrible. Works great. Not one air arc since I wadded in the HV silly cone putty.

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: Finn Hammer, fusor update.

Post by Finn Hammer »

@Liam
Thanks, I owe the basic design to you.
@Richard
The external flashovers can be controlled with potting, as you have done. Or with field controll like I did on the old one. Those two toroids doubled the voltage standoff from the rated 30kV to 60kV. I prefer field controll because it looks cool, and can be dismantled and modified.

In my previous experience, the fusion chamber attains it's clear see-through (lack of) atmosphere after a while
** with a red hot kathode**.
Therefore, my primary scope of investigation is to see, if a white hot kathode will be even better, and that is the reason for trying molybdenum as the material for the cathode. It should be good up to Incandesence.

However, more current is always good, and since more current means more heat, I want to investigate the effect of removing some of it. The surface area available for cooling is small so perhaps the effect is going to be neglectable. To find out, that is the reason of the experiment, but at least the cooling arrangemet will not add heat. Someone with a higher knowledge of thermodynamics than myself, is invited to suggest a better cooling medium than plain air.

In the name of collaboration,

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Matt_Gibson »

Very artistic! Definitely my style…I always shoot for form and function.
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Re: Finn Hammer, fusor update.

Post by Liam David »

I tweaked my CAD model to more closely resemble your design and did a thermal simulation. I put 100W on the (aluminum) inner cathode face and set the air in the (0.5" OD x 0.37" ID) copper tube to 20C.

Sim.PNG

Things get pretty toasty - hot enough to melt aluminum. The simulation doesn't include radiative cooling.
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Liam,

Interesting thermal image, thank you very much.

I have been wondering how the thermal "reach" of a colder stem would affect the temperature at the far side of the kathode, but your simulation shows that very well.
However, the old kathode melted on the side facing the stem, which had no more cooling than a 2.5mm stainless stalk could provide, (neglectible):

20210731_133359.jpg

From this I gather, that the old stem was heated to a point where it contributed to the kathode temperature, to form a hot spot at the stem/cathodetube junction. By cooling the stem, and reducing its cross section, I hope to reach a point, where the hottest area of the kathode is facing me, not the stalk. Both the stalk and the cathode is Molybdenum, with a thermal conductivity coefficient that is superiour to that of stainless steel.
Thermal conductivity coefficient : https://www.sciencedirect.com/topics/en ... oefficient
Copper: 386
Molybdenum: 143
Stainless: 25

The layout of the viewport is sub-optimal, because I view the kathode along the stalk axis, whereas if I viewed it perpendicular to that axis, I would have a view to the stalk. This was due to the initial desire to run a squirrel cage kathode, which did not doo well, and I am suffering from this decision now.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Peter Schmelcher »

Keep in mind a Heat Pipe for cooling, lots of energy in water phase change. A Heat Pipe transfers heat far far better than a solid copper rod.
-Peter
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Peter,
I will read up on heat pipes, I have only a vague idea about how they work.
In this first cooling attempt, I rely on air to transport the heat away, and if the compressed air carries moisture to any significant degree, the plastic pipes wil turn into conductors. Lots to ponder...

Cheers, Finn Hammer
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Liam David
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Re: Finn Hammer, fusor update.

Post by Liam David »

I'm not convinced that a heat pipe, or even an infinitely thermally conductive center conductor for that matter, would reduce the temperature of the cathode much. The heat transfer bottleneck is the cathode itself, as well as the rod that connects it to the copper, per the high gradient there in the image. In the simulation I've set the inside of the copper tube as a 20C boundary condition, which represents perfect, infinite cooling capacity. With forced air it will be much worse. Also, I simulated an aluminum cathode (239 W/mK vs 143 W/mK for molybdenum). All that said, I'm sure the cooling will prevent the copper and rest of the feedthrough from melting even when the cathode is red-hot.
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Liam,

That sounds good, since I am shooting at white hot ;-).

Cheers, Finn Hammer
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