Alexey's fusor progress

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Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

I finally got a new PEM cell delivered. It works very efficiently, with a voltage of 1.7V, current of 1A, deuterium yield of 8.3 ml/min. I purposely lowered the voltage to keep the gas from foaming the oil in the gasholder. However, there is an unpleasant moment, when the electrolyser is running, water rises up the hydrogen tube. I don't know where it comes from, I only pour water into the oxygen side of the cell. I had to make a drip trap to keep the water out of the desiccant cartridge.

Dennis mentioned that the bare contacts can cause corona problems. To avoid this I wrapped all the bare wires with duct tape and then covered them with a thick layer of plasticine. This proved to be a good insulator - breakdown is about 10kV/mm. And it seems to have worked, I stopped hearing the buzzing, I used to think it was coming from the transformer. Thanks again!

I also bought a leaded rubber apron to protect against x-ray radiation. It's old and heavy, about 4kg, equivalent lead thickness 0.3-0.4mm, blocks all the x-ray coming from the fusor. I just need to make a tinfoil hat and my wife will kick me out of the house))))
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

Slate is a good shielding material - just use the density ratio to determine equivalent thickness require. It is cheap, safe, a non-conductor, easy to cut and drill. Just a suggestion.

As for the foil hat, well, just glad I wasn't drinking hot coffee when reading that part ;)
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Last weekend my fusor produced its first neutrons! Working with deuterium plasma turned out to be more difficult than with air plasma. The pressure range in my chamber was from 8-12 microns. Even with hydrogen it was easier, I attribute this to the fact that hydrogen was not dry enough in my first experiments. Deuterium, on the other hand, I dried and purified from air residues very carefully. By the way, I have improved the cooling system, now the chamber does not get hotter than 40 degrees when running for 5 minutes at about 120 watts. I glued the waterblock to the fuser using TR-922 heat conductive glue. I did 7 experiments of 3-5 minutes each. The optimum pressure was 10 microns, voltage 25-30 kV, and current 2-4 mA. To measure the voltage I additionally connected an oscilloscope, according to it the real voltage was 10% higher than what my voltmeter showed. As a neutron detector I used a professional radiometer MKS-01R with a scintillation detection unit based on lithium-6 isotope, which is installed in the moderator unit and covered with cadmium from outside. It is an old device bought at a flea market, but I have tested its performance on an AmBe neutron source, but the detection efficiency is unknown to me. The radiometer allows you to measure the neutron flux per cm2 (fluence) for any measurement time. This greatly simplifies the calculation of the isotropic neutron yield. To do this, simply consider the area of a sphere with radius equal to the distance from the center of the fuser to the detector and subtract background neutrons. The background neutron count rate was 0.12 n/sec*cm2. Since I do not know the efficiency of the detector, I do not multiply the measurement result by a correction factor, so the actual yield may be slightly higher than I assume. In the table I give all the data measured during the experiments. The maximum isotropic neutron yield was recorded in the very first run and amounted to 9.3E4. In subsequent runs it was significantly lower (1.0-3.0E4), I do not know what this is due to, perhaps the heating of the chamber caused the release of gases from the rubber or vacuum grease. To test the radiometer, I changed the deuterium to air and ignited the plasma at the same voltage. At first the result disappointed me, as the radiometer was still registering neutrons. However, after 2 minutes the count rate dropped by a factor of 2, and further decreased until the 7th minute when the radiometer started to show only the background value. It seems that deuterium is well adsorbed by the chamber walls and is released for quite a long time even after air purging.
I made a short video showing the operation of my fusor https://youtu.be/VJyRAfCIhSI. You can see that there are often small discharges in the chamber, this does not allow me to increase the voltage above 30-35 kV. The breakdown seems to occur at the point of contact between the metal rod and the glass. In spite of some difficulties I am very glad that the fuser is finally working. Of course, the neutron yield is very low compared to most fusor builders, but for me it is already a great achievement. Based on this result, I can work further on improving the design.
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

Next step, record the performance with and without the moderator; that is the 'gold standard' for neutron proof. Especially for lower signals.
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Richard Hull
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Re: Alexey's fusor progress

Post by Richard Hull »

I agree with Dennis and have always preferred this method with any claim of fusion especially first pass efforts with unknown detectors and low per minute counts at lower voltages and currents.

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
Alexey Khrushchev
Posts: 118
Joined: Thu Jul 06, 2023 3:36 am
Real name: Alexey Khrushchev
Location: Moscow

Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Yes, I will definitely try to measure neutron flux without a moderator. I also want to do experiments on indium activation. The low neutron flux leaves little chance of success, but I will still try. As a moderator I want to use two 5 cm thick pieces of paraffin with an indium plate in between.
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Looks like I managed to screw up my neutron counter..... Last time I leaned the detection unit against the fusor chamber and when I ignited the plasma there was a breakdown and the detector readings became meaningless. I rebooted it and it seems to be working, but now it's reacting to X-rays. I suspect that the pulse filtering system (I have a neutron scintillation detector) has stopped working. I doubt I can fix it. Now I will have to find an alternative way to measure neutron flux. I have a small B10 tube, but I don't know how to make it work.

In order to reduce the amount of arc breakdown at high voltage I replaced the 5mm steel tube with a 12mm rod. On one side I drilled a hole to hold the tungsten rings and on the other side I threaded M5 threads. I also formed the rod into a cone that tapers towards the rings. It was interesting to do this, but the idea didn't seem to work, the breakdowns at low pressure and voltages of about 40 kV remained. I think the discharge moves along the glass surface from the electrode to the chamber wall. I want to install a glass with a drilled bottom as a barrier, maybe that will improve the situation.
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Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

I noticed that the pressure in the chamber pulses with an amplitude of 5 microns, for example I set the pressure to 10 microns, after 1 minute it increased to 15 and then dropped to 5 again. What can this be related to? I have two assumptions: I need to change the oil in the diffusion pump; or the deuterium flow is too high and the pump is running at its limit.
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

Is the power running? If so, likely out gassing. If not, possibly a slow leak or more likely a virtual leak. But guessing here due to lack of details (like any valves open/operated or deuterium flowing or adjusted. High vac running?)
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Pressure pulsation occurs both when high voltage is applied and without it. The diffusion pump is on. Perhaps the problem is in the deuterium feed tap. Vacuum oil has gotten into it as a result of a small accident. I cleaned it, but it is possible that traces of oil got into the needle seal.
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

That would act as a temporary 'virtual' leak (a process called 'burping', for literal meaning), so maybe that is the issue.
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Thanks for the clarification! That pulsing is really annoying. I'll try flushing the valve with hexane.
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Today I added to my collection of SNM-18 neutron tubes (4 atm helium-3). Its price was $3, I couldn't pass it up. Also in my collection I have a tiny SNM-11 (boron-10), SNM-14 (boron-10), SNM-56 (helium-3).
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Richard Hull
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Re: Alexey's fusor progress

Post by Richard Hull »

Great catch at $3! There is little better than a decent sized, 4ATM, 3He neutron detection tube. You will be able to catch a mere whisper of fusion with this tube! All the best on your effort.

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
Alexey Khrushchev
Posts: 118
Joined: Thu Jul 06, 2023 3:36 am
Real name: Alexey Khrushchev
Location: Moscow

Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Thanks Richard! Yes, sometimes the flea market has some interesting items at a very attractive price. Unfortunately I do not have a ludlum dosimeter to run this tube. Sometimes these dosimeters are found on the secondary market, but their price starts at $1000. I wonder if there is another way to run this tube?
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Re: Alexey's fusor progress

Post by Richard Hull »

You can make your own preamp (critical component) and the variable 2000 volt capable bias supply The US made tubes of some size and 4ATM bias around 1200-1600 volts. A linear simple pulse amp of variable gain can be plugged into a cheap digital readout counter.

A number of posts in the radiation forum talk about a homemade Preamp using the ready to use Cremat CR-110 preamp IC.

viewtopic.php?t=12649

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
Alexey Khrushchev
Posts: 118
Joined: Thu Jul 06, 2023 3:36 am
Real name: Alexey Khrushchev
Location: Moscow

Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Thank you very much! This is very interesting information! However, I am afraid that I will have problems with buying Cremat CR-110. I'm almost sure that they will not ship it to Moscow....
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

A neutron detector for perfectionists. 10 SNM-18 tubes in one unit. Has anyone used such devices?
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Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

Just ran my fusor and noticed an interesting feature. At low deuterium flow rate the plasma behaves stably, but if at the same pressure increase the deuterium flow rate by 3-5 times it becomes very difficult to ignite the plasma. I increase the voltage to 30 kV at 20 micron pressure and immediately an arc forms, no plasma formation. I never realized that the flow rate of the gas is so important. It is not quite clear to me what the reason for this phenomenon is. Perhaps at high gas velocity the kinetic energy of the molecules is higher than the attraction energy of the ions to the cathode?
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Liam David
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Re: Alexey's fusor progress

Post by Liam David »

Here's what I suspect is happening: It's more difficult to break down pure deuterium than deuterium mixed with contaminants (due to ionization energies and secondary electron coefficients). At higher flow rates, the contaminants are flushed out faster, hence your difficulties in igniting the plasma. As to why there's an arc, I think that either the pressure is higher than you think, or you have sharp edges somewhere.

"Perhaps at high gas velocity the kinetic energy of the molecules is higher than the attraction energy of the ions to the cathode?" is not true. Consider the thermal speed and compare that to even a 1eV deuteron.
Alexey Khrushchev
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

The arc in my fusor forms at pressures below 10 microns at a deuterium feed rate of about 1 ml/min. At higher pressures (30-100 microns) there has never been an arc, only the cathode gets white hot. However if I increase the deuterium rate to 5 ml/min I cannot ignite the plasma even at 20 microns, at voltages above 30 kV an arc is formed. The fusor behaves as if the pressure is much lower. It is not very clear to me how the washout of contaminants can contribute to this effect. It should be the other way around, contaminants should improve ionization compared to pure deuterium.
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

You are discovering something we all experience for a fusor when operated at near the correct conditions; it is very sensitive to voltage, pressure, flow rate, and cathode/anode arrangements. Certainly a clean system is essential. I struggled the first times I tried operating with my first fusor. With time (and better builds, high vac precondition and a plasma burn) I can now run my fusor for an arbitrarily long time (over 30 minutes) and even unmonitored for a few minutes without arcing.

I do not run above 22 kV (through 26 - 40 ma range for current) due to my transformer limits and about 5 microns. After conditioning, I close then barely open the gate valve such that the system barely holds a set pressure. Then I pre-set my deuterium flow rate to my know setting (for me, 5.5 turns of the valve). Then I slowly increase the voltage and closely read the current meter - it reads zero but at some point, climbs rapidly (through no visible arc.) Then I fine tune the feed valve (turning it slightly down) to control the current meter so I can reach my full operating voltage. This takes a few iterations. Finally, all parameters are set and the system is almost stable. After a few minutes of up/down current changes (and using the flow valve to correct) I get a very stable setting. This procedure works perfectly and I get a nice plasma with no arcing every time and it is highly stable for at least half an hour. Issue do develop as the chamber heats up, through.

Its generally occurs that for many beginners with new systems it is a challenge to get a stable plasma - as you are obviously are experiencing. Hence, they need to determine their best parameter space and be certain their system is clean.
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Liam David
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Re: Alexey's fusor progress

Post by Liam David »

I see, I was under the impression that the arc formed at higher pressures only.

I still think the difficulty in breaking down is a "contamination" issue. You describe 5 mL/min as being impossible to break down in the 10-20 mtorr range compared to 1 mL/min which is easy to break down in the 10-20 mtorr range. Same pressure + higher purity = more difficult breakdown. That checks out.

The arcing is a little more perplexing. Important discerning question: where on the stalk/cathode is the arcing occurring? It's rare to see arcs at <10 mtorr as they are high-pressure, high-current phenomena. At those pressures you'd typically see electron emission from triple junctions or sharp (microscopic) features causing transient current events, not so much a sustained arc.
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Re: Alexey's fusor progress

Post by Alexey Khrushchev »

I'd like to clarify what I'm calling an arc. It is a brief discharge lasting a fraction of a second, like a lightning strike inside the chamber. I think the ballast resistor trips and breaks the arc. I can't tell exactly where the discharge is coming from, I disassembled the chamber and found metallization of the insulator (glass) where the rod comes out. This is a triple point (metal-insulator-vacuum), so it is logical that breakdowns occur there.

About impurities. It seems to me that at low flow rate gas has higher current conductivity and therefore plasma is formed at lower voltage and behaves stably. When the flow rate is high, the gas resistance somehow increases, the charge accumulates on the cathode, and then the breakdown occurs.
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Dennis P Brown
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Re: Alexey's fusor progress

Post by Dennis P Brown »

For any given voltage conduction depends on gas pressure. Flow rate has no effect.

While I'll agree that distance between the cathode & anode, as well as point (corona issues) will strongly affect gas break down, the issue I find very likely to cause such a problem is contamination. I've seen 'flash over' in my system if I rush the start up. But as the plasma slowly develops, my system becomes very stable. Yet the conditions my fusor is stable NEVER works at the start. That indicates that trace water vapor (walls) and or other trace contaminates are being removed via the discharge.

Metal sputtering always occur in a fusor to some extent and my ceramic stalk is no exception (nor my chamber & windows.) But this has not been an issue for my fusor. I will, of course, lose some power up the stalk but after a while, I use acetone to clean it from time to time.
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