New Grid

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New Grid

Postby Bob Reite » Wed Jul 12, 2017 6:38 am

I finally had a bit of spare time, and decided to build a cylindrical grid similar to Doug Coulters since I have a cylindrical chamber, in preparation for reproducing his "new mode" experiments.
newgrid1.jpg

Like Doug's grid, mine is made of graphite end pieces with tungsten wire rods. The graphite end pieces are 3/4 inch diameter, the length is 1 1/2 inches.
newgrid2.jpg

The new grid can handle 500 watts input power continuously, and 1000 watts in intermittent service.
The system is not fully "broken in" after a long idle state, so the best run so far is 320,000 N/s. I also may need a fresh supply of heavy water. Although I keep the bottle tightly capped, there is now enough air space in it that atmospheric moisture could be diluting the heavy water with ordinary water. Yes, I should break down and get a lecture bottle of D2, which would eliminate one variable in the quest for more neutrons and best Q.
Tomorrow I'm going to run the REM Ball in "average scaler mode" on a 5 minute sample time and compare it with a simultaneous bubble detector measurement. At least I'm getting enough neutrons to do that.
Last edited by Bob Reite on Wed Jul 12, 2017 7:21 am, edited 2 times in total.
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: New Grid

Postby ian_krase » Wed Jul 12, 2017 7:00 am

Nice!

I'm a little surprised, though, that you use graphite for the end caps -- is a high temperature ductile metal unsuitable?

(Do the end caps fail to radiate heat much?)
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Re: New Grid

Postby Bob Reite » Wed Jul 12, 2017 7:35 am

Tungsten end caps would be quite expensive. Graphite can take up to 1100 degrees C in a hydrogen atmosphere. Graphite was used in the anodes of some transmitting tubes back in the days of "fire in bottles".
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: New Grid

Postby Richard Hull » Fri Jul 14, 2017 6:30 pm

Looking forward to more info on this test and hope it all goes well.

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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Re: New Grid

Postby Bob Reite » Sat Jul 15, 2017 3:47 am

I did some calibration runs and the good news is that the "rough" calibration setting given in the manual for my Eberline SRM-200 with a BF3 tube REM Ball agrees within 5% of a bubble detector IF one also does a "null" run by running the fusor with HV on, but the BF3 tube out of the moderator, then subtract the "null run" count from the actual run. The null run averages a count of .47 mREM/hr. This of course is all after setting up the BF3 tube and Eberline counter as described in the FAQ on setting up a BF3 neutron detector system.

So when my bubble detector finally dies, I can be confident in the REM Ball reading, as long as I am getting at least 2.0 mREM/hr or better.
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: New Grid

Postby Richard Hull » Sat Jul 15, 2017 5:16 am

Yes, the bubble detectors do die, eventually. The rem ball along with good electronics is a great device as it is setup to read mrem rates related to human absorption. However, as Bob notes they are not very sensitive or useful for low neutron rates. 2-4 mrem/hr is what you need to warrant good accuracy and that is a lot of neutrons. Most rem balls use a very small Nancy Woods BF3 tube, but thanks to the sphere of moderator around it, it performs in a nearly idealized thermal neutron field.

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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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Re: New Grid

Postby ian_krase » Sat Jul 15, 2017 5:55 am

I guess that what I meant is that (for some reason) I anticipated that the heating would be concentrated in the wires, and that the grid end frames would be subject to less heating and would be able to heatsink better -- such that they could be made out of inconel or SS or something.
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Re: New Grid

Postby Bob Reite » Sun Jul 16, 2017 6:10 pm

The end caps also get pretty hot with sustained operation, nearly as hot as the wires.
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: New Grid

Postby ian_krase » Sun Jul 16, 2017 11:49 pm

I bet that at some point the wires get hot enough to emit extra electrons by thermionic emission for extra ionization, right?
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