Six Inch Cross Fusor

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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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Richard Hull
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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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Richard Hull
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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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Richard Hull
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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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Re: Six Inch Cross Fusor

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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

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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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Jim Kovalchick
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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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Re: Six Inch Cross Fusor

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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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