Six Inch Cross Fusor

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Jim Kovalchick
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Six Inch Cross Fusor

Post by Jim Kovalchick »

Having abandoned my 2.75 CF cross fusor, I'm all in now on a six inch cross fusor. I have brought over my tungsten carbide ring grid. I'm conditioning the grid and chamber and snapped this picture at -18 kV.
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Drew Scott
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Re: Six Inch Cross Fusor

Post by Drew Scott »

What caused you to abandon your smaller one? Looks very nice! Also, did you make your Tungsten Carbide grid? Or did you acquire it somewhere?
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

My small fusor was arc limited at 38 kV. The arcs were ferocious and destroyed instruments.
.
The tungsten carbide ring is a wedding band. They cost about 11 dollars on ebay. I will probably go with a larger size than the one in the picture because the control characteristics are rough. It's too hard to keep the plasma lit at low currents.
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Re: Six Inch Cross Fusor

Post by Drew Scott »

Very cool! My wedding band actually is a Tungsten Carbide one from Amazon! It never really crossed my mind that it would be useful as an electrode.

What voltage are you attempting to use with the new one?
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

I went to the 6" cross as you well know, but now I have yet another approach. Whatever I wind up with in the end, it will most likely be the last iteration. I am cogitating with my current iteration being the plan until I move on. Will I use that 6" cross? I don't know now........

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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Mark Rowley
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Re: Six Inch Cross Fusor

Post by Mark Rowley »

Looks great! And finding a cheap tungsten ring on eBay is a phenomenal repurposing.

I wonder if you placed a small Titanium target on the wall for each one of those beams it would boost your neutron yield. Or even better a loaded Ti target.

Mark Rowley
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Mark,
I had previously tried Ti plates at the beam ends. It outgassed terribly.
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Yes focused beams destroy any loading as they create a hot spot. The grid which the Farnsworth people called a dynode allowed fusion in velocity space as Robert Hirsch noted to me back in 2000 during my interview with him. The simple fusor, like we construct, may have hot spots... (pretty grid rays), but the walls are loaded solely by fast neutrals, of which their are many and far outnumber the useful deuterons at any given instant. Doomed to slam into the hopefully cooled wall or shell of the spherical of cylindrical fusor these neutral deuterium atoms imbed themselves. Later, they can be knocked out of the lattice by bombarding electrons or high speed neutrals. If the neutrals are fast enough you might even experience a rare neutral on target fusion. Knocked out of the wall as deuterons, the huge surface area of wall loading can act as a source of deuterons that can accelerate to full fusion energy at the grid.

The above logic is why I might not use my $500 cross. #%^&<#@!&*!**.... I feel a smooth, symmetrical chamber about an equally symmetrical grid is the road to good fusion via uniform wall or shell loading in the simple fusor. I'm looking at a cylinder fusor as it is easiest to cool and allows for a "long counter" -combo-activator to lay alongside at zero range. The sphere does offer more surface area per unit dimension, but gobbles up gas and is difficult to cool with ease. Much more musing and cogitating needed.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

The tungsten carbide ring is too small and caused control problems. I may buy a larger one, but in the mean time I substituted a singled braided tungsten wire ring. The larger diameter improved my ability to control at lower currents that make conditioning easier. Symmetry is a little better this time.
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Nice! Symmetry is the thing! Increasing the size and mass of the grid assists in not having electron emissive runaway. We now know that grid transparency is a dream goal and is not the region where the bulk of all fusion takes place in the simple fusor. I remember when we were all noobs and argued that it was all about grid transparency to increase fusion and efficiency. We will never achieve anything like efficiency, energy wise. Efficiency is nothing more than a bragging right! What we can shoot for is increased fusion and, thus, neutron output for researches using them, even if it means increased losses in the grid absorption of energy. This means bigger grids with more absorptive energy capabilities and reliance on cooled, loaded symmetrical chamber walls or shells.

I hate burying thoughts like this in other than FAQs or the fusion theory forum, but this seemed a needed discussion within the thread's flow. The more we do, the more we learn within our narrow little sphere of amateur work with electrostatic fusion.

This whole cross thing began when some poor youngsters did 20-30k n/s in a 2.75 cross, and then another money strapped youngster followed suit, etc.. We might need to get back to symmetry! Crosses work, for sure, but might not be the best road as they are not perfectly symmetrical and contain sharpish nearby edges to grid.

What I see now is the smallest possible symmetrical reactor, (mean free path), with larger grids that are symmetrical about the chamber and can take the power, with directed active cooling for the entire chamber to allow for longer operation and good wall loading. As noted in another posting, the cylinder might solve a lot of these issues.

This is a rare time for this site! Frank Sanns has lamented a moribund site for some years now. Starting with the rage for crosses, more intense and active work both in the doing and in the cogitating and close observations related to results like those by Jon Rosenstiel, Joe Gayo and especially Mark Rowley, we are back in the biz of not the endless stream of "in it to win it" high schoolers, but active "doer researchers" and active theoretical deliberations of some merit, base on results.

I might add and expand on this in a FAQ in the fusion theory forum. It is important to work ideas based on observations into a theory of what "might" make a better fusor.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

First real fusion run on this fusor iteration today. Before I could get to fusion relevant voltage I had to remove the alumina from the stem. I'm not sure why I bother putting it on in the first place because I don't have the best of luck with it. The alumina tubes seem to cause a lot of sparks.

My run this afternoon was at -37 kV and 7 mA. I was able to hold these conditions with almost no adjustments for about 15 minutes. The neutron counts were not spectacular but enough to see. My Russian tube counted 1150 events per minute. We'll see if wall loading helps on subsequent runs.
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Update:
This evening's run: -38 kV 6.8 mA. A count rate of 5700 cpm was observed for the first 5 minutes after which the count rate steadily dropped to below 4000 cpm when I shut it down.
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Re: Six Inch Cross Fusor

Post by Richard Hull »

That happens all the time here as well. Wall heating and deloading. Thus, the need for cooling. My recent best work 130,000 CPM on 3He tube and Rhodium activation in my 32 deg F lab allowed for a longer run due to an ice cold start. It took longer for the fusor to heat to 105 deg C! After which the numbers declined a bit.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

I am adding a Liam David style feed through to allow boosting my voltage.

My vacuum test is okay, but I holding off on properly sealing the feedthrough end until I get a grid installed that I'm happy with.

The big disappointment is my tube fitting has a stop in it and is therefore inadequate. I don't have a lathe, so I'll be shipping it to a friend who does.
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Bob Reite
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Re: Six Inch Cross Fusor

Post by Bob Reite »

What voltage are you hoping for? That's a pretty long insulator. If you go much over 35 KV, the sharp edges of the nuts on top are going to cause corona issues.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Absolutely! It is all about radiusing and field control. Depending on sharpness, 15kv will corona in air. Corona makes ion clouds that can slowly establish a path to opposite charged surfaces and ions are conductive. Even smooth flat and especially polished flat surfaces are prone to surface currents under electrostatic stress. I have seen arcing across 2X the normal air sparking distances on clean glass.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Come on guys, give me more credit than that! This was just a vacuum test. I have plenty of experience with voltages to 40 kV. The fittings you see in the photo are for a seal only. When I apply voltage, i will add features to prevent corona.

Regarding the length, as I said, my tube fitting has a stop ring. When I get the fitting reworked, the glass will extend into the chamber from the fitting through the length of the cross end. My power supply will do at least 60 kV and overkill on the length doesn't bother me.
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Re: Six Inch Cross Fusor

Post by Richard Hull »

There is no such thing as overkill on length. The critical thing is field control and insulation properties and standoff as the HV conductor enters the all metal, grounded vessel. Once in vacuum and a conductive gas discharge environment, things ease up a bit. After a good deal of work, Liam David had success with this arrangement. Good luck with your iteration of it.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Sharing a status update

Recall that I am held up by the incompatibility of my glass tube clamp fitting because of its internal stop.

Because my lab is just a lab and not also a shop, I do not have the right equipment to modify. So, I sent it of to my brother who enjoys this type of challenge.

Yesterday he sent pictures of the fix. He elected to tool the widened opening off the flange, but he noted that the original welding of the 2.75 CF flange put it slightly off center with the throat. So, he had to take slightly more to prevent a glass tube from binding. It pays to have a detailed discussion with the shop before an expensive fitting is modified.

Here are some before and after pictures. The first shows the fitting still installed on my chamber, and you can see the inner ring that normally stops the glass tube. The second picture shows the flange side before.

Jim K
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

A nice looking piece of work. Again, all the best with the effort. Getting high voltage from air to vacuum is a big challenge. I remember my first adaptation and use of a special formula one racing spark plug to work well in fusor II back in 1997. Seems like a million years ago. It worked great up to 20kv. It is all about beating corona and arcing in air and at the feed through interface in the grounded flange.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Richard,
I agree whole heartedly with your points about voltage delivery. Through my fusor efforts, I have spent and lost more time on that than anything else. Above 25 kV or so, it is a technique driven battle. Corona, shield capacitance of conductors, potting of diodes and ballast, to use alumina or not, feedthrough ceramic outgassing, grid to stem connection techniques etc etc etc.

I try lots of different things. Some work and some do not. I learn every time. New, higher voltage levels usually mean new learning. It's all good.

Jim K
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Jim, Due to arcing I have to reconfigure my system feed through. So I am still learning. This is why I haven't done much fusion lately. I am getting #%$@#* tired of feed through issues. I have an idea based of a 2.5 inch long pure ceramic body 2.75 to 275 coupler. It means having to extend the feed through rod for the grid support past the grounded bottom 2.75 conflat. Gotta' really think that one through.

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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Bob Reite
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Re: Six Inch Cross Fusor

Post by Bob Reite »

I am also at somewhat of a standstill because my feed through now has trouble making 29 KV without arcing issues. On the vacuum side no less.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Pardon my French, but this is a bitch of a problem for all, not only once, but if here long enough, will haunt any modifications, higher voltage use or new fusor builds in future. We old boys are not immune to this continuing dilemma. One-by-one we "fess up" to this issue that one might think we should not have, based on our extensive past work here.

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Richard et al,
Sparky feed throughs are my nemeses, hence my latest endeavors. I firmly believe that it is all my fault because I expected too much from my commercial feed through.

The real problem is that mine only provided blocking insulation for too short of the stem's passage into the chamber. My insulator was built for a KF fitting. To connect it to my chamber of conflats, I used an adapter. The only trouble was my feed through's insulation only came part way through the adapter. See my pictures.

The advantage of the Liam David design is that it allows adjusting the length that the insulating sleeve inserts into the chamber. This is what I'm aiming for when my assembly is complete.
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