Calculating Resistance for a Fusor

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Joshua Guertler
Posts: 50
Joined: Fri Apr 21, 2017 2:59 pm
Real name: Joshua Guertler

Calculating Resistance for a Fusor

Post by Joshua Guertler » Thu Jul 26, 2018 9:00 pm

Greetings fusor community,

In the recent couple of days, I have been looking to potentially set up an experimental high voltage system that uses an electrostatic precipitator-based power supply to convert wall-socket power to 20-30 kVDC at about 6-10 mA - or so it claims.

In order to evaluate whether or not a proper amount of current is being drawn for fusion to be achieved (and in order to gauge its level if it is not), I will need to design a ammeter for this high voltage and low-current range. Also, due to the lack of open transformers in the power supply, I will be unable to use Mr. Hull's design for an ammeter depicted in the FAQs.

In my own attempt to design a ammeter and a series of equations that would be able to accurately measure this, I was wondering how I could also calculate the resistance of a fusor under a certain vacuum pressure with deuterium present. I know that such an equation should include aspects of the plasma conductivity equation, however, I am not certain what else would be entailed. Thank you.

Sincerely, Joshua Guertler

Posts: 494
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Calculating Resistance for a Fusor

Post by ian_krase » Fri Jul 27, 2018 5:19 am

First, a fusor (or any other DC plasma discharge including an electrical arc) has a nonlinear non-monotonic current-voltage response that makes the use of "resistance" fairly meaningless.

We say that a gas discharge tube goes from "insulating" to "almost a dead short" when it ignites, but really it's nonlinear or so-called "negative resistance" meaning that for some parts of the gas discharge regime adding more voltage leads to less current.

(This is part of why ballasts are useful, both on fusors and on things like light bulbs.)

Second, calculating this is fairly difficult given the geometry of a fusor and the endless possible complications.

While I do not know the exact details of your power supply I think that it should be possible, in practice, to set up an ammeter on the low side of the supply.

You also may want to check out the ammeter shown in this link: ... neugen.htm (German language)

Thomas Rapp made a very simple current measuring device with a capacitor and a neon lamp which blinks to indicate current, and can safely reach high voltages.

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