Emma - Fusor Number 1. First deuterium run

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Emma Black
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Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Whist I have a few threads going with the fusor, as the first deuterium was run though it this week, though it was about time for a dedicated thread. Huge thanks to everyone on this site for all the help. Looking forward to lots more experiments in the future.

Power Supply - 110v to 33kv oversized potential transformer, rectified and controlled via variac.

Pumps – Edwards Diffstak 700 diffusion pump & Edwards E2M18 two stage vane pump.

Chamber and valves - 4.5” / 2.75 Conflat cross, manual valve for diffusion pump throttle & sapphire precision leak valve for gas inlet. Deuterium generated via the Horizon PEM kit and held in a 60ml syringe. Grid uses a tungsten carbine ring.

Metering – voltage via high voltage probe and meter. Current via nichrome wire shunt, calibrated manually. Low pressure via Edwards Penning gauge. “High” pressure via Edwards Pirani gauge.

Neutron detection via Ludlum 2363 with prescilla probe. CPM/graphs calculated via Andrew Seltzman’s standalone program.

Run details:
First Run with Deuterium
Duration 10 minutes
Voltage 29KV
Current 11ma
Pressure 8 microns
Recorded CPM (With Andrews code): 37.5CPM

Using the Ludlum over several hours the background count seems stable at 5-6 CPM. After several previous runs this seems unaffected when in proper position next to the fusor and operating on hydrogen only.
The 37.5CPM is potentially low and maybe not sufficient to decisively prove fusion is taking place yet, but this was the very first run with deuterium.
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Jim Kovalchick
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Re: Emma - Fusor Number 1. First deuterium run

Post by Jim Kovalchick »

Nice work Emma!
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Richard Hull
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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

Emma, A first first rate report. Just the thing we are looking for.

Now: A proton recoil detector is not a great detector for fast neutrons at amateur levels specially at 29kv which is just starting to "get there".
I used this same type of Bicron detector in 1998-99 to detect and claim fusion at 30kv back then. I know its terrible 0.5% to 1% efficiency as well as its gamma reject capacity.

As such, coupled with your claim of a net 10X increase in numbers over a careful background count, I am going to place you in the neutron club. I would hope you can get that voltage and pressure up a bit in future as long as you are strapped to the Prescila probe. I rapidly graduated to the BF3 tube in an Eberline PNC-1 and got far better results, (higher numbers over background, 100X). Finally with the 3He 4 atm. I can get reliable neutron detection at 15kv. (30,000 X over background at 40kv)

You squeaked into the neutron club just like I did using proton recoil detection in a good probe made for a much higher flux of fast neutrons.

Richard Hull
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

I’ve got this Ludlum and can detect neutrons at that voltage, so I’d also agree that you’re seeing them. Congrats!

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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

Congrats! No doubt you're doing fusion. Welcome to the club.

When you push the voltage higher and counts start to increase, do be aware that Andrew's program will lose accuracy due to how the meter accumulates and calculates the dose. You'll want to calculate TIER from the dose directly. Also, the hammer is somewhat sensitive to x-rays. At much higher voltages (>50kV) my unshielded Prescilla probe gave 1-2 orders of magnitude too many counts. Probably not an issue for you yet, but something to keep in mind if you run at a higher voltage in your current configuration. I've managed to push those 30 kV feedthroughs to 70 kV in air with a toroid, so you have lots of headroom. Use low-profile socket head screws to get that extra little bit, and keep things clean.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Amazing I’m so pleased- thank you Richard!

Upgrading the neutron detection is definitely on the list of projects. I want to learn more about the different setups, as my knowledge of detection tubes, amplifier modules, signal shaping etc is very limited and I’m quite keen to get some hands on experience. Plus being able to experiment with moderators and how they effect the counts is really interesting.

Liam, that’s a very good point - sounds like I have some interesting maths to do to when the counts hopefully start to improve. Before I added the torrid you could literally start to hear what I assume was corona discharge. Good to know that you can push these things. Adding a voltage multiplier is also on the future list as well as some cooling, this thing gets warm!

Also I think there may be a minor leak on the deuterium inlet side so think some air is still getting in diluting the deuterium, so plenty of immediate work to do.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Jim Kovalchick »

Pushing a feedthrough is not without risk. I pushed one designed for 30 kV in the 40 kV range and ruined it. A small blow through in the ceramic at the throat of the connection with the chamber made the feedthrough permanently unusable. It's a crap shoot. You can try it but don't be surprised when it fails.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Congratulations on measuring neutrons and building a very good system. I also get neutron detection at 30 kV but yes, a 3He detector should do better. Jim raises a good point - those feed-thru's have a rating level that should not be pushed. I isolate my feed-thru by mounting it on a large glass disk that is used as the vacuum flange for my chamber.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Yes fair point - I do have a couple of spares but would rather not blow one. Wondering how difficult it would be to make something custom from scratch. A friend of mine is excellent at pottery with a kiln etc. Initial idea would be to 3D print the shape of the ceramic needed, make a mould and then slip cast (liquid clay) the ceramic, before finishing, firing and glazing etc.

Anyway. This morning think I have fixed the little leak, todays short run was a bit better:

Voltage & current - 32kv 10ma
Pressure 9 microns
Average CPM= 64.5 over 6.75 min
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

I also push my 30kV insulator to 70kV using a toroid on top and a half toroid over the bolts.

What mR/hr values are you seeing on your Ludlum?

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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

When pushing the 30 kV feedthroughs much beyond their rating, cleanliness is paramount. That includes a lack of any metal scrapes or deposits, as well as loose dust. A bakeout can also help tremendously. For those not wanting to go the acid cleaning route, I've found that 30 min - 1 hr in a heated ultrasonic bath works to remove all metal from the insulator. Add Alconox or some other strong cleaner, and after a rinse, it's UHV ready.

Rolling your own feedthrough is definitely a possibility, and I'd certainly follow along with interest. Just be sure that the parts are non-porous and don't have any residual binders, or anything other than ceramic, for that matter.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

Does current (increasing heating) come into consideration when pushing a feed through way beyond its rating? I’ve pushed a 20kV to 54kV and a 30kV to 70kV without any problems, but I also don’t get much more than 10mA at these higher voltages.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

I haven't had issues up to 20 mA. The plasma heating load certainly goes up, but these feedthroughs are rated to something like 350 or 450 C.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Matt - I'm getting 2-3 mR/hr and the hammer is 34cm from the centre of the fusor.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

That’s pretty good for 30kV from that distance. I have mine around that distance and get around 8mR/hr pushing 70kV (8mA).

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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

Your cathode doesn't look very symmetric within the chamber. Fixing that might help improve rates a little, although with that geometry it might not make that big of a difference. With 2-3 mrem/hr and 34 cm, I'm calculating 2.3-3.4e5 n/s. Not too shabby at that voltage/current.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Yep I know. It's off by a few degrees although it looks worse in the photo than it is. I can only think I must have knocked it during a clean, but didn't initially notice. With the chamber now getting down to 3.5x10-7 mbar I'm reluctant to open it again for the moment.

Feel I should know this but out of pure laziness in asking, what is the equation used for calculating the n/s in this situation? Edit: I should have looked at the FAQ's...it's been a long day.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Luca Aldridge »

Emma,
Well done. The build looks professional and something I’d aspire to. Good luck on improving your flux!
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Thanks Luca, you will get there soon :)

I've been running it few times a day for the past few days now and it seems to be improving by 5-10% better counts each time.

There still must be either a leak or lot of off gassing happening though, as when left overnight, it takes a while for the current to drop at the higher pressures as the deuterium takes over inside. The plasma is an interesting whiteish colour during this stage. This seems to be getting quicker each time as I assume the chamber gets cleaner.

Hopefully I can have a go at some simple activations in the next few weeks.

The raw data from the Ludlum for a short run:

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Re: Emma - Fusor Number 1. First deuterium run

Post by Luca Aldridge »

That’s good, I guess it’s all down to practice.
Data looks good!
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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Is the plasma white or rather a faint blue white? Deuterium tends to be red or a light blue in my experience.

Doubt you have a leak if the system is improving with time; that tends to indicate out gassing and that issue is being reduced by pumping and plasma interactions.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Yeah it was a blue white glow and was decreasing with each run.

Another one of the rubbish diodes has blown anyway. So while I wait for a better quality replacement to arrive, I opened the chamber to sort the cathode alignment.

Have a look at the colours in the steel, being a small chamber with welds, the heat is very intense.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Sputtering of the cathode is normal with a fusor. In mine, the window was so coated it appeared as a high quality mirror. Needless to say, I added a protective glass plate to stop that. Cleaning off the window wasn't fun nor good for the window.

Those diodes are, at best, rated to 20 kV but likely that is optimistic and considering that the voltage rms is a good bit higher then what we measure on a voltmeter, even using two in series (40 kV) isn't really much insurance.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

I'm not even sure how you would go about cleaning that!

Yep I used 3 of the 20kv ones in series but they still failed in the high 30's. The failure took me a minute to figure out what was going on.

All running fine and the plasma suddenly died. No current. As I increased the pressure in the chamber, the plasma lit up again. Then current suddenly spiked massively and would probably have popped a fuse etc had I not had my hand on the variac to immediately turn it down. The power supply is now outputting AC only and is only able to support plasma at higher than normal pressures.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

I might have missed it, but are you using a ballast resistor? They will save your diodes when current spikes like that. Something like this is a good one:

https://www.ebay.com/itm/114714898756?m ... media=COPY

Also, try to control current via pressure control. Run at lower pressures and very gradually increase to increase current.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Yep there are two 50k's in parallel, normally its remarkably controllable. Those sapphire leak valves give you such fine control over the pressure.

However with some failed diodes its power supply was putting out AC only, it was behaving exactly like a fluorescent tube. Nothing for ages then it suddenly lit up with more current than normal and the pressure jumped at the same time.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Sorry for the diode fails - again. I keep a NST on hand to test my diodes - very easy and fool proof to determine their status.

What Matt suggested is how I control my fusor. I get it to high vac (under 10^-4 torr), then throttle the turbo (or diffusion pump) to it is almost closed position (but the system does not experience a pressure rise.)

I then set my variac to about 10 to 15 kV. After this I slowly increase gas pressure via the leak value (For my system this is always four turns. But everyone's system is different but should be consistent once you set up your system.) When I see current flowing, I bump up my voltage up slowly. As I do that, I notice my current increasing. I stop increasing my voltage when I'm at max. current for my x-former. If below 25 kV, I lower my deuterium pressure by slightly turning the leak value down.

The current falls.

I then bump up the voltage a few kV. I repeat this process till get to full voltage (30 - 31 kV) and normal max current (30 ma.) Since my system is clean this tends to hold steady; however, after it has been exposed to air, I'll often get current spikes. Again, like you one has to be fast on the variac.

Aside: there is zero reason (and a lot of reasons not to) clean your fusor. Those metal deposits will do no harm and even are helpful to act as a "getter" on the surfaces. The window, however, is an other matter. Acetone and a soft lint free rag should work. If stubborn, I use a very fine steel wool (but that can scratch so be careful.)
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

What’s the peak current rating on your diodes? 25kohm ballast might be low enough to allow too much current during a “short”.

I bet your system is still burning off “junk”. I’d keep voltage low and let it do some cleaning runs where you let the cathode heat up, and remain hot, for longish runs. Eventually you’ll see just the faint reddish glow of deuterium.

I cleaned my viewport with a dremel, some 50k grit powder, and a felt bit. It did a great job in not much time.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Two 50 k's in parallel? That is a problem - two resistors in parallel operate such that most all current runs through the resister with the slightly lower resistance value - so using two doesn't buy you any reduce load on the resisters - one is already carrying 99+% of the current anyway. Those resisters need to be in series giving you 100 k-ohms. As you are seeing, 50 k-ohms is just too low.

Matt is likely correct that your system isn't clean enough, yet. Running the plasma at lower voltage (with some deuterium) is a good idea.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

Two resistors in parallel is not a problem... the current will flow in inverse proportion to their resistances. In fact, there is even a limited degree of thermal regulation. The lower resistance will dissipate more power (V^2/R), thus heating up, increasing in resistance, and so reducing the fraction of current it carries.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Not an electrical engineer - simply learned from one that identical resisters in parallel do not carry similar currents. Rather one gets the majority of the load. Maybe not - never did the experiment myself but if you did, then that is the case.

Regardless, 50 k-ohms has proven inadequate (obviously the resisters in series have failed to protect in runaway conditions.) so it would appear they need greater resistance so placing the resisters in series is the better idea in order to protect her transformer and diodes.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

I think it’s diodes in parallel that don’t share load equally.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Yeah I think i'll add this 100k monster while I'm diving into the oil tank, just to be on the safe side, as clearly 50k is not limiting the current enough. The old diodes said 2amps but take that with a huge pinch of salt.

Correction from earlier - It was two 100K's in parallel 50k total.

New ones are microwave type, but from a proper supplier with full spec sheet etc. 12kv @ 500ma. I will use many.

Anyway original issue it turns out was entirely due to my ineptitude. A usb cable had physically fallen and brushed past the feedthrough on its way down. Did not notice until I had a look today. There are tiny burns on the cable and the camera and power adaptor are both dead. The resistor value issue still applies though.

The fusor itself is fine and after a pump out I'm getting down to 2x10-7mbar now, the lowest its ever gone.

Denis - my previous operating procedure was pretty much identical.
Let it hit below 10^-5torr. Throttle back the pump. I have also my moved my deep pressure gauge between the fusor and the pump, so you have an idea of how much gas is leaving the chamber. The other gauge will stay put. Then slowly build the voltage and pressures adding the deuterium, while watching the current.

I was more interested in how oddly a chamber filled with low pressure deuterium behaves, when exposed to HV AC current. Nothing for ages then brightly flashes to life in a fraction of a second and acts like a dead short.

Image
Last edited by Emma Black on Thu Sep 08, 2022 2:54 pm, edited 1 time in total.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

2A peak rated diodes would definitely struggle with a short, especially if you have a capacitor in there.

Take a look at the 2CL2FM diodes. They are rated for 20kV, 100mA, and can run up to around 35kHz. They are able to take up to a 10A spike (non repetitive).

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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Thanks Matt - the ones I have picked up are 12kv 500ma and can deal with 30A surge. A local store had these for £18.50 ($21) for a 100 pack.

https://www.tme.eu/en/details/by12-dio/ ... ctor/by12/
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

That’s a solid deal right there!

Edit- Btw, sorry for your loss, the Queen was a truly classy woman and great leader!

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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Thanks Matt, yeah I know. I only met her the once very briefly but she came across as genuinely nice with a very quick sense of humour.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

Diodes in parallel are indeed a no-no.

Resistors of identical value100k @5w, in parallel, share all currents perfectly 50:50. Their total would be 50k @10watts. With 10ma flowing through this combo would subtract 500 volts from the supply voltage to the fusor. The 10 watts would be double what a 10ma would demand (5W) and would be nice.

Two 25k 5w resistors in series would do the same thing yielding a 50k @10watts. Again, this combo would do the same thing as the above example.

Resistors of any equal value but of a fixed wattage will have their wattages in either series or parallel combination sum.

A100k resistor would drop 1000volts from the supply at a fusor current of 10ma. The wattage would be 10 watts demand so a 20 watt resistor would be better.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Dennis P Brown »

Well, that settles the matter on parallel resisters.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Though I had replied already, thanks for the clarity Richard Is there any real benefit for large wattage resistors for use in fusors i.e. 100W+ ?

Have also been getting ready for some activation attempts - lots of HDPE, from Amazon cutting boards. I was going to use wax, as I have a massive bag but the fire and melting risk is not worth the hassle.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

Wattage only comes in handy with dead short conditions. 25 to 50 watts will handle most anything up to a longish short like 3 seconds or more. (time to reach the kill switch.)

Most fusor supplies are whimpy. 20 ma is a death point at full tilt. Monster supplies that will handle 60kv @100ma will need the higher wattage ballasting as the supply is quite happy vaporizing a central grid in pressure bumps or electron runaway conditions.

Compute a good resistor value that you can tolerate under a load of say 30ma to a running fusor. Let us say 50K . Now figure the power rating.
P = I^2 x R .030 X.030 = .0009 X 50,000 = 45 watts Double that to 100 watts or so. Now, few power supplies will supply 30ma at full voltage. Still be prepared for near short conditions. Most likely 20ma would be more normal as max current before the supply blows up. .0004 X50,000 = 20 watts and a 50 watts would be OK.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

The higher wattage resistors are probably larger/longer and might handle higher voltages than smaller ones. That might be a reason to go for one of those.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Bob Reite »

During an arcing event, figure the impedance is close to zero, even though the duration is short, so you need at least a 100 watt resistor. You can find 225 watt wirewound resistors at hamfests. I plan to bring some extras to sell to HEAS.
The more reactive the materials, the more spectacular the failures.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

That's great Bob! I dare not think how many fusors have benefitted and been brought to life within a budget via the hamfest route. Hamfests are hit-and-miss related to fusor construction, but very viable. The HEAS flea market is the only place where you can see, touch and feel what is for sale and also be talking to a knowledgeable person selling his fusion related wares. E-bay is far more vast and has most anything you might need, but remote at best, with sellers ranging from idiots to the very helpful, but rarely knowledgeable in your area of need.

Fortunately, most arriving and staying on fusor.net know how to use real tools in their hands to assemble materials and build things. What they don't know, they pick up through self-directed learning, reading, seeing and then the doing. The materials needed range for hardware store found materials to the most complicated of scientific materials and instruments. As always, the true go-getters, go and get.

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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

I'm off to a hamfest next month to take a look for useful stuff, never been before we shall see what crops up.

Anyway - rebuilt the bridge using 6x4 12kv diodes, installed a new 225w resistor and we are back in business. Pressure and current is nice and stable again. Neutron counts are around 0.6-1.0 mrem/h, so some I think some reconditioning needed.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

I did not appreciate how much the weather can play a part in HV systems. Today we have thunderstorms and the humidity in the workshop is crazy. All sorts of discharge sounds coming from cables and connectors.

The video (for sound only) was taken at around 30KV (with the fusor disconnected) via a iphone recording on top of my potential transformer, sound is mostly from the transformers HV connectors.

Image
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Emma Black
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Update
Somewhat predictably my feedthrough has started arcing at voltages over 30kv. Out of the other two I though I had, one arcs even worse and the other wont hold a vacuum.

Its failed where the ceramic goes through the conflat, as seems to be common with other posts on here about failed feedthroughs. Running things in high humidity can't have helped at all. I now realise what may have been obvious from the previous post and that the discharge noise was being caused by condensation on the cables, due to the humidity and low temps.

Fixes & Changes
I've added a dehumidifier & heater to the workshop and re-routed the HV cables. They are now all sitting in PVC pipes filled with oil and wrapped in some shielding tape. Also spent some time dealing with all the sharp points, now no more audible corona. I've also changed the oil - was previously using fully synthetic motor oil, now switched to proper transformer oil (it was on offer on amazon)

New cable routing:
Image

So figure it's time to build a new better feedthrough.

Thinking of building something close to the excellent post by Liam David around building a custom feedthrough using quartz and compression fittings. I could just use borosilicate glass and heat to seal. Making my own ceramic is also an option, but the main issue I think is going to be the material shrinkage 10%+ after firing and I'm not sure how I could get that to seal. Maybe something like this:
Image

Or get another 30KV feed through and modify it to cope with higher voltages, perhaps by adding an "oil jacket" to the air side of the ceramic. Either way I'm currently playing around with Ansys Maxwell to experiment with some simulations.

A few of the posts around very high voltage feedthroughs discuss two stage biased designs (I'm not thinking of building this) maybe a silly question but how does this work?
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

That’s too bad. I’ve got three of these feed throughs and have been able to way exceed their voltage ratings. I do use some 99% isopropyl alcohol on them before use…

Were you getting your feed through really hot? I haven’t pushed much beyond 700w and I have a fan blowing across, so maybe that’s a thought?

-Matt
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Richard Hull
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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

I have a large box fan blowing across my insulator so that any small corona ozone/air ions that might cause or support an air arc via rising heat columns are blown laterally away from the insulator. Corona in air can be subtle and unseen even in the dark. It doesn't take much in a rising heat column to establish an air arc. The fan also helps cool the fusor, too. Many tricks out there when pushing the limits of materials.

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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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Retired now...Doing only what I want and not what I should...every day is a saturday.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

Could you post a picture of the insulator failure, and the one that arcs worse? My personal sample size is one, but even with a toroid, the failure mode is typically arc-over on the air side. Getting the voltage high enough to puncture the insulator, or arc along the vacuum insulator surface, would typically require an oil socket. Unless something in your commercial part is much different from mine, I don't see how anything other than contamination, or perhaps a problem with the stalk, can cause a problem.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Richard - I did see mention of you using a fan to help blow away the corona, this is on this list to implement! It's perhaps worse in this setup being all inside a box. Also I could hear the corona, but even in the dark could not see it. I tried a "bat detector" to amplify any ultrasonic sound, which I think is similar to how commercial corona detectors work. It worked slightly better than the mark 1.0 ear but not a massive improvement.

I think Liam you are quite correct and its not indeed shorting through the ceramic but forming an arc all the way around the end due to contamination. The reason I thought it did was because when it flashed over the ceramic lit up brightly like a lightbulb, it's fine on the air side. Also both of the other used ebay feedthroughs leak it turns out. The feedthrough and stalk are absolutely filthy now.

I risked a very non-definitive test in air with the feedthrough on the bench and the arc indeed forms around the end of the ceramic not through it. Perhaps I was jumping to conclusions too soon. Maybe, after a deep clean and a stalk re-design it may actually be ok.

Matt - I was running it fairly warm for short periods of time - the cooling has become quite an issue.

Image
Image
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Feed though has been cleaned by carefully using 1500 grit paper followed by an ultrasonic bath, with water based solvent to clean any sanding residue. Finally finishing with IPA and wipes to remove anything else. The feedthrough itself seems ok, yay!

As for the second part, posting as a failure for what not to do.

For the stalk re-design I should probably have actually done some simulating or indeed any calculations rather than throwing something together to experiment with. More thought less rush etc..... This sparks excessively in one place, even after a long cleaning run at low voltages. Issues seem to include:
-Glass is very thin & close to the stalk, meaning its insulating value is likely near zero.
-Stalk diameter is too large and the junction with the glass creates a horrible area of high field.


Image

Damage to the glass - post removal

Image
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Liam David
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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

Yep, that would be a triple junction problem. The glass can also trap contaminants at the metal surface, shielding them from effective cleaning by the plasma. It'll do much better without the glass.

As you suggest, there's an ideal diameter for your 2.75 conflat port. Too small and the high theta curvature (r, theta, z coords) enhances the field. Too large, and proximity to the wall increases it again. This ideal region is actually pretty flat, so for the often dirty, crooked, or otherwise nonideal stalks in most fusors, its diameter never really becomes the limiting factor. For an ID of 3.65 cm, which is pretty typical for 2.75 conflats, the ideal diameter comes out to 1.35 cm. This diameter gets modified by the presence of a dielectric like feedthrough ceramic, but it remains analytically solvable.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Richard Hull »

You want all the high field in the system at the central, grid, ring, sphere, etc. You have a lovely, smoothly radiused and polished ring!
The large diameter stalk would be just the thing, if, you had more clearance around it and the feed through insulator. I bet a sharp edged thin cylinder would cure many issues as long as you haven't "tracked". permanently. the insulator. It is forever all about low field control along the stalk merging to extremely high field conditions at your central toy, whatever that is.

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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Liam David
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Re: Emma - Fusor Number 1. First deuterium run

Post by Liam David »

A high field at the cathode doesn't help, generally speaking. Quite the opposite, in fact. I'd argue it hurts the performance in most systems here. It also has little to nothing to do with where the plasma will form. My first suggestion would be to increase the cathode length several times and decrease its diameter somewhat, but the one you have now is nicely made and it would be worth collecting data on it.
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Re: Emma - Fusor Number 1. First deuterium run

Post by Matt_Gibson »

Is that a tungsten wedding ring?
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Re: Emma - Fusor Number 1. First deuterium run

Post by Emma Black »

Friday update time

The cathode has been remade. I've downsized the stalk diameter by 2mm and pulled the glass right back, so its now just being used to keep everything central. I cut the glass using a simple tube cutter then melted the ends to smooth it out and get rid of any dust. Have only done the first couple of re-conditioning runs but so far no arcs. I'm picking up a new lathe soon so it should be a lot easier to make my own stuff going forward, be fun to get back into machining things.

I've also had to replace one of my PM1029B panel meters as the previous arc issue seems to have destroyed it. It now only displays nonsense segments. It's now in a metal box and the cables shielded.

Matt- yep from Amazon. I cut a hole in the side to thread the stalk on using a little diamond bit.

Before cleaning:
Image

Grid is now much better aligned to.
Image
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