Wire Resistance

26 08 2011

All photos.

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.



8 responses

26 08 2011
Raymond Rogers

Good measurements. Again I would suggest measuring the inductance; although I could estimate it from the turns, wire gauge. It’s better to have a measurement. An adequate technique is pulse the wiring with a fast pulse generator; if nothing else the calibration output of the scope. Then measure the voltage response of the system with the scope. At a time scale to capture the details. You will have to crank the gain up, because your measurements indicate that you will only have about 14mV signal with a 5V @ 50 ohm source. The inductive signal will actually be larger since that is the terminal voltage.
A real inductance meter/measurement would be better.
This provides a very small signal stimulation (50 ohm source).
It should be a R-RL waveform. If you get other waveforms check the scope grounding by attaching the probe to your ground point. This should give you a measure of the amount of signal caused by the instrumentation.


26 08 2011

Why exactly are you aiming for 2.5 kA? Is it simply the highest you think you can get with the current setup (with the nice bonus of being able to compare to the whole range of the Sydney runs)? What’s the maximum the SCR can handle?

27 08 2011
Mark Suppes

Primarily to match the Sydney runs. Also a cheap way to increase magnet strength.

26 08 2011
Rob Flickenger

If you really need to shave resistance but keep a smaller gauge (for higher turns / unit length), you could try silver wire.

27 08 2011

Are you using some sort of simple lumped element circuit to analyze this? I’ll second the first comment, what’s the inductance?

You know the capacitance of your bank, and you just measured the resistance of the components. Can we estimate the inductance from the time series data you already have? It would be neat to try and predict what the pulse should look like and compare it to what you actually get.

27 08 2011
Mark Suppes

I’m looking into inductance now.

4 09 2011
Terrifying Power « Prometheus Fusion Perfection

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

5 02 2013
Philip Williams

Hello there. Wonderful stuff you’re doing. I’m uber late to the party, but gotta mention that not only will you need to take into account the inductance (which will be by far the dominant impedance here) but you’ll also need to be aware of the skin effect, which will increase the resistive part of the impedance, quite significantly at frequencies even down into the audio) [yes, “monster cables” really do make a difference]. It was hard to tell from his google lecture, but it seems even Bussard may not have been sufficiently aware of this effect. So-called “multifiliar” wire is generally the accepted mitigation for skin effects. I haven’t looked ahead much here, so you’re probably already aware of this. If so, apologies in advance for my…can I call it prescient?…redundancy. ;)

I see you’re also looking into HT superconductors. Nice energization/decoupling via thermal switch and persistent field you made there. Cool (so to speak. *cough* I kill me.). In any event, I work with superconducting magnets, though not the high temperature sort. I assume you’re thinking of using superconductivity as an alternate way to generate the fields you need? Good idea, though I’m not sure HTS can support the sufficiently high fields without quenching. And of course, there’s the cooling. But if you run the thing at low duty cycle…might be just fine, notwithstanding the technical difficulty of getting it all running inside the vacuum chamber turned cryostat.

But my goodness, I digress. In any event, this is very nice stuff; all the best to you with it.

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