Brütal neutron counts from tetrode fusor at higher voltage

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Carl Willis
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Brütal neutron counts from tetrode fusor at higher voltage

Post by Carl Willis » Sat Nov 24, 2012 7:40 am

You read that right: brütal with a ü.

Earlier thread about this project is here:
viewtopic.php?f=6&t=3231#p20281

Tonight, the tetrode fusor got tested at 65 kV, 11.5 mA (748 W), and 33 mtorr, thanks to its new oil-insulated / cooled socket. The chamber is directly water-cooled by copper tubing coils. One of the photos below shows the neutron detector meter reading 4000 CPS under those operating conditions. Back-of-the-envelope calculations for this detector suggest that to be in the ballpark of 5 million n / s.

In low-voltage, air-insulated testing, the maximum voltage I reached was about 25 kV and the maximum current at that voltage, about 6 mA (150 W). The neutron count rate there was a whopping 7 CPS. I also had enough data to extrapolate a relationship for neutron yield at higher voltages and currents. That relationship predicted a count rate from today of 1340 CPS. I ended up a full factor of three higher. That's obviously a good thing for me, and a not-so-good thing for the model--at least this far beyond the range of physical observations that informed it.

I don't yet have automated data collection on the new assembly. I am replacing my horsecrap FuTek DAQ unit with a proper LabJack U6 acquired from Doug Coulter, and my LabVIEW interface for the LabJack isn't finished yet. When that is in business, I can expand and refine my earlier models for performance.

Another observation: the x-ray exposure rate in contact with my 2-3/4" viewport at 65 kV / 11.5 mA was 30-40 roentgen / hr. Typical x-ray artifacts (speckling) can be seen in the viewport photo. In the new configuration of this chamber, the viewport points toward the ceiling and can only be observed by mirror or camera. That's probably a good thing, too!

A quick word about some of the photos. Some show the oil socket, which is of acrylic construction. The 100-kV silicone noodle cable passes through a 3/4" compression fitting on a 2-3/4" ConFlat flange, which is itself face-sealed to the acrylic with a buna o-ring. Oil in the container is a mix of 5W-30 automotive oil and the rest of my Walgreens USP-grade mineral oil before they raised the price on this bowels-loosening commodity. The VTVM is set to read 15V per 75kV, via the Ross HV divider. The current meter is 25 mA full-scale on the setting shown. The Baratron pressure indicator needs re-zeroing and 3.4 mtorr must be subtracted from its reading as shown.

Good times. I can't wait to get this baby worked up to the power supply's max rating of 100 kV, at which point it would seem that no "metal umlaut" superlative will adequately describe the performance.

-Carl
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Chris Bradley
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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Chris Bradley » Sat Nov 24, 2012 9:50 am

I wonder how important having a regular geometry helps in fusion rate?

Doug has reported similarly that he swears by accurate geometry.

I don't see a particular obvious reason for that, in regards the process of the fusion reactions. But perhaps it is significant in *avoiding* the 'unhelpful' discharge processes that may also go on, and therefore deliver a 'cleaner' and more controllable set of discharge conditions so that the discharge can be held at higher volts.

Not sure how you're going to shield well at 100kV. Distance, and remote operation?

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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Richard Hester » Sat Nov 24, 2012 10:48 am

If you don't mind something perhaps intended for a constipated horse, I've picked up gallon quantities of mineral oil on e-pay. My intended application is a combination moderator/fast neutron scintillator, but I suspect the oil would insulate just fine.

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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by JakeJHecla » Sun Nov 25, 2012 8:46 am

Carl- Have you formed a hypothesis as to why the neutron numbers are so "brütal"? If I recall correctly, this is very near the all time (amateur) record set by Jon Rosensteil (sp?) a few years ago! In addition, I notice the chamber pressure seems to be quite high in comparison to most successful fusors. Is this something that has been observed before? If so, do you have a hypothesis as to why this particular machine thrives in this pressure regime?

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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Carl Willis » Sun Nov 25, 2012 4:31 pm

Hi Jake,

Jon's record is 1.2E+07/sec. at 80 kV / 21 mA. Right now, I'm in the vicinity of 0.5E+07/sec at 65 kV / 11.5 mA. If I ran at the same current, it's likely I'd be seeing about 75% of his record. The additional 15 kV might put me beyond his record, but I'm not sure. I don't think there's any doubt that I can beat his record, and with lower power, at 100 kV.

So this device is a good performer, but it is not other-worldly in its efficiency. It is more efficient than my earlier fusors by a noticeable margin, and indeed more efficient than a lot of other fusors described here. I have some ideas about why that is, although they're not exactly revolutionary. First of all, this system is very effectively cooled. The grids are massively heat-sunk to conical copper sheet supports that go to the socket. At 750 watts in, there is NO color on that grid. That means there is insignificant thermionic emission. Electrons emitted at the grid represent a major inefficiency in fusors. So there is that.

The higher pressure has to do with the scaling laws that apply to gas discharges. This system has a larger grid relative to the chamber than most, so inter-electrode distance is relatively small. A certain number of gas particles need to occupy this space in order for the discharge to be able to sustain a certain current. When the space gets smaller, the density of gas must increase to maintain that number. In essence that is why the pressures are high. Also, it's a small fusor to begin with, fitting inside a 4" pipe.

-Carl
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Steven Sesselmann
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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Steven Sesselmann » Sun Nov 25, 2012 7:39 pm

Carl,

These are impressive numbers...

Can I ask you, how many minutes run time did you do today, and how far from the fusor are you standing when you are operating it?

What are you measuring 40 rem at the viewport with?

This run would put you next to Jon on the fusion reactors by energy quotient list.

PS: What is the internal architecture of that tetrode, how does the grid look?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Jon Rosenstiel » Mon Nov 26, 2012 1:24 am

Awesome results Carl, keep pouring the coal to her and you have a pretty good chance of breaking the record. I'll clean and polish the trophy!

For a round-about comparison my recently upgraded / recalibrated Ludlum 12-4 (had the original BF3 tube replaced with an He3 tube) recorded the following from my fusor...
30 kV @ 10 mA: 3.0E+05 n/s
40 kV @ 10 mA: 1.0E+06 n/s
50 kV @ 10 mA: 2.1E+06 n/s
60 kV @ 10 mA: 3.2E+06 n/s
70 kV @ 10 mA: 5.0E+06 n/s

Jon Rosenstiel

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Carl Willis
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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Carl Willis » Mon Nov 26, 2012 5:47 am

Even brütaler numbers from tonight: 15000 CPS at 74kV / 8.8mA / 651W. Just shy of 2E+07 n / s by my calculation.

I am having my doubts about one issue in particular, x-ray pileup. Quadrupling the count rate with 10 kV added is suspicious. My moderator-removal experiments on the detector were done at low voltages where I feel it is safe to handle the tube near the socket, but there is the significant possibility that at these higher voltages, x-rays are causing pileup at the detector. My means of answering that question will be to check the pulse-height spectrum from the tube and determine if it is still shaped appropriately for neutron-capture events. Adjustment of the LLD threshold may be necessary, or alternatively, a lead sleeve for the tube and its moderator. I also want to check the neutron yield on a secondary instrument that is actually dosimetric in response, the remball we have at work. I'll hope to bring that home tomorrow night.

Steven, you asked about the dose rates and I attached two photos from today, showing dose rate at the viewport (now partially shielded with a lead tube), with 32 R / hr; and near the socket assembly, showing about 2 R / hr. The instrument is a thin-walled ion chamber. My runs so far have been brief, not lasting more than about 5 minutes, during which time I have been mostly standing about 5 feet away.

I thought I had an earlier photo of the tetrode grid assembly. If not, I'll make a photo of the spare tomorrow.

-Carl
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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Carl Willis » Mon Nov 26, 2012 7:25 am

Update: I can confirm there is a problem with x-ray pileup at voltages above about 60 kV. I have made some tests with the MCA and verified this problem exists and I can also solve the issue with a piece of sheet lead between the detector and the socket assembly. I'll try to get a more accurate neutron number tomorrow after a more permanent effort to shield the detector.

So all bets are off until that can be fixed.

-Carl
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Re: Brütal neutron counts from tetrode fusor at higher voltage

Post by Doug Coulter » Mon Nov 26, 2012 4:06 pm

Great work, as usual, Carl. Once you get your instrumentation sorted out (and good of you to take that care) - I'd love to see what happens if you play a little with having your two grids at different potentials (in either direction). Focus, anyone?

Looks like you've got a great setup for activating silver there too - and there is enough experience to get pretty decent neutron numbers from 5 min runs doing that, which kind of sidesteps pileup and EMI kinds of issues with other instruments.

Very roughly, with a 5 min run, and a 2" sq piece of silver sheet .01" thick and 2" pancake geiger, you get (approximately) 1100 cpm per million neuts/second at the instant of fusor turnoff. This is not super hard to measure and back-extrapolate if the silver activation counter was already on, and shows the X rays (so you can see on the graph the exact instant of fusor turn-off). Using a log plot, you can simply back extrapolate a straight line to that instant. We use a counter that returns counts once a second - then multiply by 60 to get the cpm number.

Due to variations in moderators and gear, this won't get you 1% class results, but you'll never be off an order of magnitude either. When my counters and the silver disagree, I know which one to trust.
Why guess when you can know? Measure!

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