Today I returned to that model.

All resistance values in the simulation are based on real world measurements with the exception of coil inductance (code).

Starting with an estimate for coil inductance of 0.1 mH the discharge current looks like this:

The simulation’s peak of 1.5 kA is nowhere near the 2.3 kA we are getting in the real world:

OK. Maybe the value for coil inductance is off?

I played around with the value for coil inductance but the simulation would not match reality.

As a control I replaced the simulated inductor with a 1 mΩ resistor (code). Looks like this:

The simulation predicts ~1.8 kA but in reality we see 2.3kA!

Where does this discrepancy come from?

UPDATE: Reader Andrew solved the mystery:

You could try changing the ON resistance of your switch/SCR to something a bit lower than 100mOhms

.model MySwitch SW(Ron=.1 Roff=1Meg Vt=3 Vh=0)

I can’t see the part number of your SCR but 2mOhms would seem reasonable.

I didn’t notice that rather high resistance lurking in the SCR model.

Now the simulation matches reality very closely with 0.06mH coil inductance (code):

Good work Andrew and the rest of the internet brain!

I completed some nice upgrades to the coils power supply for safety and quality.

I added DIN rail terminal blocks and rearranged the parts to emphasize discharge path:

Added an internal variac:

Added terminal blocks to the trigger board:

These pins on the SCR never soldered well because they are mini-fastons. Fixed:

For safety, I added an AC switch:

I replaced the trigger’s battery with a 9V wall wart:

And added a DIN rail 0.5 A circuit breaker:

My shopmate had a TIG welder here the other day.

I took the opportunity to try welding the 3D printed metal parts I made last year:

These are intended to be coil holders, so I installed a 40 turn coil prior to welding.

Mike welding the halves together:

We used no filler rod on the theory that the infused bronze would melt and form a braze of sorts.

It worked very well:

The coil insulation didn’t survive: The coil is conductive to the casing.

I’m encouraged by the weldability. I am ordering more test parts to keep pushing this approach.

Today I tested the newly rewound polywell coils and thicker leads. Best shot was 2.36 kA.

Now we are in the right neighborhood.

Tonight I really experienced the power of the coil power supply. Whoa.

I’ve been working to increase the coil current from ~1.2kA to ~2.5kA.

Previously I discovered the coil discharge path had more DC resistance than expected.

I rewound the Polywell coils with 16 gauge wire (previously 18 gauge).

The 16 gauge DC resistance is 144 mΩ compared to 227 mΩ for 18 gauge wire.

I beefed up other wires on the coil discharge path (4 gauge):

Lets test the wiring with the dummy coil:

I took the power supply up to 100V… a small test charge…

When I fired, the noise from the coil made me flinch. It was never that loud before.

Lets turn up the power!

300V for second test.

When I fired the coil there was lightning! HOLY CRAP.  Look what happened:

The coil fucking wrapped itself around the transformer (electromagnetic forming). Then it discharged to ground:

So I haven’t measured it yet, but I think we are getting more current to the coils.

A fun and easy place to start is look at some AC going through the coil (suggested in comments).

I used the oscilloscope’s test wave generator. I’m using two 10:1 probes connected thusly:

Channel 1 is shown above channel 2 on these oscilloscope screens:

Not much distortion at  500Hz:

At 50kHz we start to see some distortion:

At 500kHz we have obvious distortion:

Not sure what this tells us, but I’m sure it tell us something!

Now lets try a more abstract approach to the problem. We have a 6cm diameter air core coil with 60 turns. I found this handy inductance calculator. I looked up the thickness of 18 AWG wire and the relative permeability of air.

Theoretically the coil has 0.56mH of inductance. Sound right?
Given inductance and frequency, you can calculate impedance…which is what we’re after. But what’s the frequency of a single pulse?

UPDATE: The rectangular function may be useful for calculating frequency.

To reach 2.5kA with a 450V power supply, the coil cannot exceed 180mΩ.

I removed the Polywell from the chamber to measure resistance of the coil alone.

The coil contributes 227mΩ.

Our 6cm diameter Polywell uses 11.31 meters of 18 AWG wire.

Based on wire resistance charts, 16 AWG wire would contribute 149mΩ.

One pin of small feedthrough: 33mΩ:

The bellows feedthrough has a tiny 1mΩ. I should use this for the coils (currently used for langmuire probe):

One leg of the power supply cable with connectors: 39mΩ. This can be improved with a shorter thicker cable.

I ordered 16 AWG magnet wire.

Did you go to great lengths to reduce the resistance of the coil path?

He responded:

Yes we try and use a thick enough wire so that the resistance is small but it cannot be too thick in order to be able to have many turns. More importantly is the contact resistance when connecting to the wires. We try to keep this low. From this perspective, a milliOhm meter is helpful.

Milliohm meters start around $200 on ebay. Too much.

Then I found this excellent tutorial on Kelvin (4-wire) resistance measurement.

Luckily I had a current sensing resistor on hand for the current reading:

I used the resistor on the right.

So current sense resistor plus two voltmeters and BINGO: milliohm meter. Results:

The coil path is 0.35Ω.

The dummy coil is 0.185Ω

As a calibration I tested the 0.001Ω resistor. Result: 0.00088888

Not bad.

Now I have excellent tool to troubleshoot resistance in the coil path. AWESOME.

I am currently repeating an experiment performed by Joe Khachan and Matthew Carr in Sydney, Australia.

Their experiment is written up here:  ”The dependence of the virtual cathode in a PolywellTM on the coil current and background gas pressure” ($1.99 pay wall)

Joe and Matt were able to delivery 2.5kA to the coil:

 The coils were driven by a pulsed current power supply that consisted of a 7.5 mF capacitor bank, which could be charged to a maximum voltage of 450 V….A maximum peak current of 2.5 kA was achieved.

We are seeing an effective resistance of ~0.45Ω compared to their 0.18Ω.

We need to lower the resistance and increase the voltage.

I started with raising the voltage. I rewired the coil power supply to use 2 capacitors in series: 0.3mF, 900V

The power supply’s transformer and rectifier only go to 600V (but I pushed them to 800V without issue)

With 800V we get 1300A. More current, but effective resistance increases to 0.61Ω.

OK fine. Lets try lowering the resistance with a dummy coil directly connected to the power supply. 45 turns 6cm diameter. The Polywell coil is same size but 15 more turns.

Here it is connected:

Now we are clearing 2.5kA with 600V!  But look at the strange pulse shape. Hmm.

I thought having the coil so close to the power supply might be a confounding factor. I added 1M of 12 gauge stranded wire to distance the coil:

Revealingly, just adding that 1 meter of wire reduced the current by almost half for same 600V:

So clearly delivering current will be a design challenge.

A note on technique. Based on comments I now ensure probes are perpendicular  to current:

Today I took a step back and measured the current going through the coils.

Previously I measured 1,200 amps going through the coils with the capacitors charged to 450V.

My setup is a little different now: There are 2 meters of cabling + feedthrough between the coils and their power supply.

For all of these shots, the capacitors were charged to 450V. The shunt resistor shows 100mV across for 100A through. Multiply the voltage by 1000 to get the current in amps.

Surprisingly I’m seeing significant variation of current for identical conditions.

The most current I saw was 1,095 amps:

But with the same conditions I saw this much lower 344 amp current:

The median current reading was around 734 amps:

Disconnected, the coil shows 0.8 ohm of resistance.

I’m rather surprised by this variation. What could be the cause?

I tried some other conditions.

With the capacitors charged to 200V, I got 560 amps of current:

I also tried charging 5 out of 10 capacitors to 450V. With an average current of 500 amps.