Six Inch Cross Fusor

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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick » Thu Dec 17, 2020 9:47 pm

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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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick » Thu Dec 17, 2020 11:58 pm

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 » Fri Dec 18, 2020 4:38 am

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick » Mon Jan 18, 2021 7:30 pm

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 » Tue Jan 19, 2021 12:08 am

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 » Tue Jan 19, 2021 1:02 am

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick » Tue Jan 19, 2021 1:13 am

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 » Tue Jan 19, 2021 4:36 am

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick » Mon Jan 25, 2021 2:03 pm

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 » Mon Jan 25, 2021 4:35 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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