Six Inch Cross Fusor

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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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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Yes, you have to have a decent ceramic or thick quartz sleeve down into the chamber before allowing the HV stalk to see the gas filled vacuum. The central shaft should be much larger in diameter, (I like 1/4"), than the grid wires so that the field is all about the fine grid wires and not the stalk electrode. My current arcing issues are all external to the fusor, in air, and not in vacuo. My internal arcing on the first past fusor V was due to the small chamber size, (6 way -2.75 CF cross), and titanium ring electrode.

I currently believe my arcing issues are a permanent micro-hole path punched through the base region where the insulator base meets the metal skirt at the flange. However, I have one more card to play before a new insulator is required.

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 »

On my 2.75 cross fusor my grid arced to the side. On my 6 inch fusor it was my stem as it passed through my chamber opening. I never had trouble with my thin stem as long as I didn't use alumina. A bare stem was just fine until I had clearance problems near 40 kV. Even though I didn't have problems with a thin stem, my new feed through will have a 1/4" tube for a conductor.
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Yes, I gave up on sheathing the stem will alumina all the way to the grid back in 2005 in fusor IV. It is a point of issue right at contact with the top of the grid. The key against internal issues is get the stem shielded with porcelain or have quartz tubing well into the fusor, but run a naked, fat electrode to the grid.

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 reinstalled my newly modified ultratorr based feedthrough. I am not happy with the overall leak tightness but I was able to get good enough vacuum to make some plasma.

My first voltage tests were not satisfactory because the material I used to center the conductor sparked too much. With it removed, even with the conductor testing on the glass at the end, my slow adjustments to 22 kV were as spark free a cold start I've ever seen.

Next up 30 kV tests.
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

I am glad you seem to have straightened out the bulk of your arcing, sparking issues. I hope it will be a permanent solution for you.

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: Six Inch Cross Fusor

Post by Jim Kovalchick »

Update: smooth run today conditioning the grid at 42 kV and 2 mA. I am very happy with the control characteristics of the system. I am able to control steady current to the tenth of a mA and plasma holds at 0.8 mA without extinguishing.

My method for eliminating corona on the external side seems to be working okay for now. I coated the connection with hysol 1c epoxy. I added some teflon after that.

My neutron numbers are relatively low right now but I'm sure they will improve with loading.
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

More progress. 43 kV and 8 mA in this evenings run. My Russian tube, which read 600 cpm last weekend reached 2000 cpm. I am pretty sure my feedthrough has the ability to manage more.

I need to get a decent silver foil and try some activation. What thickness foil do people recommend?

Jim K
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Mark Rowley
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Re: Six Inch Cross Fusor

Post by Mark Rowley »

Congrats Jim!
For this I don’t think you are limited to foil. A Sterling spoon, Mercury Dime, ingot, etc will be fine. Zap it for as long as you can and quickly plop it down on a pancake.

MR
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Maybe I should dig out my atomic energy museum dime and activate that. 😁
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Richard Hull
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Re: Six Inch Cross Fusor

Post by Richard Hull »

The only reason foil is used at all is that thickness is just a waste if you are using a GM counter as it is sensitive to Betas for the most part and in metal, betas might penetrate .001-inch to get out of a silver spoon or coin which is only 92.5% Ag. Whatever you activate must be a good beta emitter for GM activation detection.

If you have a good gamma spectrometer and the element you are activating has a decent half-life and is mostly a good gamma emitter. Thick can be used to advantage up to a point depending on the energies of the gammas. Naturally, the Gamma spec will not see any Betas.

It is a matter of knowing your quarry intimately in any activation effort. All of this demands some fore-knowledge from a good isotope data table. I have posted on the ideal "Table of the Isotopes" book where useful data can be had prior to activation of any element on the planet. I sold 4 of my 8 copies of the Table of the Isotopes at HEAS 2020 in October. They were snapped up in less than a few hours during the large flea market.

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,
You are dead on about activation. Having spent a bunch of my youthful formative time doing activation analysis using the UVAR not too far from your abode, I own several booklet copies of the chart of the nuclides. I haven't yet mounted it on the wall of my South Carolina lab, but I also have a full size poster. I also spent more time with the barn book than I care to repeat (I wish I had one now though). I also have here a decent activation analysis text.

All of that was about gammas though, and as you pointed out, fledgling fusor activation can be best demonstrated by not ignoring the beta emission of activated silver. My original question about foil thickness was for this purpose, and I was hoping someone would share what thickness worked best for them. It's all about a balance of enough target mass while avoiding self shielding. From my own quick research, I'm thinking 0.3 mm will do for silver. I think I will look for that unless someone suggests a different thickness based on their experience.

Mark has a good point about just using an easily available silver object. After all, activating something like that demonstrates that neutrons can be used to identify isotopes within materials we have reason to be curious about. I may start with this just for practicality.

I am also designing a demonstration cell based on your foil wrapped GM. I have on hand a small, beta sensitive, thin window tube that I would face with silver and then encapsulate it within some hdpe beads. Ultimately, I envision extracting pulses that I would feed through a PC sound card into PRA software which has a feature of plotting counts vs time. If I get to this, I will start a new thread.

Finally, to finish off this post, I offer this picture which I ripped off from an Ortec lab experiment available multiple places on the web. It is a great chart of neutron activation sensitivities that can be used to pick out candidates for more than fledgling fusor activation studies.
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Update:
My fusor hit a new neutron milestone tonight. Steady state 12,000 cpm on my Russian tube with brief excursions during some adjustments well over 20,000.

I placed a one ounce silver coin between 1.5 inches of hdpe on each side. After a quick shutdown of the fusor lasting 10 to 15 seconds, I placed the coin directly on a 2 inch pancake GM and saw 120 cpm. Background is 30 to 40 cpm. I have some foil coming and should see better results then.

Jim K
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

30,000 cpm on my Russian tube. 42 kV 13.5 mA.

I activated a piece of 30 gage sterling silver sheet to 5 times background. It probably would have been more if I had shutdown and measured when my counts first started to die when my chamber got too hot.
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Good going Jim! You obtained about the exact same activation level that I got on silver at that exact voltage and current during and after HEAS 2020. Glad you are back doing good fusion. We just have to water cool in future, no matter what the chamber design if we are to increase the fusion numbers pushing the systems.

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 »

A little more fusor eye candy from today. -44 kV and 13.5 mA.
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

I have not been satisfied with progression of my neutron numbers. In fact, they have gone down. I replaced my grid with a beefy 304 stainless tube that proportionally better matches Jon Rosenstiel's to see what would result.

Neutron numbers stay low with the new grid until the grid gets red. For the same voltage and current, there is almost a ten fold increase in numbers once the grid is glowing hot. I assume this change is from thermionic electrons making plasma more efficiently. I get 30,000 cpm on my Russian tube, but it lasts for only about a minute, because by the time my hefty stainless grid is hot, so are the chamber walls. I think I may experiment with grids that make better use of the work function earlier.
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Re: Six Inch Cross Fusor

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I think active cooling is a must, but try going back to a spherical tungsten grid before cooling the cross. Lots of ideas rolling around in my head now with 6 different ways to go from here, (long term).

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 started planning to go back to a tungsten wire grid even before I posted yesterday. I have also been pondering using an unconventional wire spacing. The last time I used a wire grid in the cross, the beams seemed to be influenced by the chamber wall shape, and the beams did not appear focused.

As far as cooling, I have some ideas that will take some time to implement, but in the mean time I have added a second muffin fan.
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Re: Six Inch Cross Fusor

Post by Richard Hull »

Jim you are correct the cross is not a great beaming device save for along its axis. a spherical grid will tend to field line along the sharp edges in the cross arms as they are all quite close to the grid compared to the flat ends. Those edges, even if carefully rounded or smoothed/polished are still high field regions and tend to force any beaming solely due to proximity. Naturally it all works to do fusion, the cube is nice solely due to target, (flat end), proximity to the cathode grid, especially due to any target added to and protruding from the near zero distant cathode. (no need to travel down a metal tunnel to get to the more distant target, as in a cross)

The original Farnsworth all spherical system allows for tight field control better than any other form. A short cylinder with a tubular hollow grid is good as is the cube with short cathode to target distances. These latter forms are more beam on target.

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 »

Here is a picture of my latest grid. You can see where the secondary beams form.
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