Hello from Panama

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Johannes Toppe
Posts: 5
Joined: Thu Nov 01, 2018 8:06 pm
Real name: Johannes Toppe

Hello from Panama

Post by Johannes Toppe »

Hi guys,
My name is Johannes and I am currently working on building a demo fusor with a buddy of mine. We have a science fair coming up in a few weeks and our experiment is (probably) going to be testing the effects of varying high voltage on plasma (light intensity etc), although we do realise there is a lot of ambiguity in the question and would be grateful for any feedback. Additionally, I would like to create some simulations with software such as Matlab (I obtained a free trial), however, I am a bit lost upon as to how I should approach that as I haven't been able to find any templates that would closely resemble the demo fusor.

I was also wondering how one would obtain the temperature constant in the mean free path calculator [http://hyperphysics.phy-astr.gsu.edu/hb ... re.html#c3] in a simple demo fusor (and I was assuming that for the molecule diameter I would simply take the weighted average of Nitrogen and Oxygen given that it is air plasma though I may be mistaken). Looking forward to becoming a part of this great community!
Johannes
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Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Hello from Panama

Post by Dennis P Brown »

Hello and good luck with building your demo. As for light intensity, that will be controlled by a number of factors - system pressure, gas type, applied voltage and to a lesser extent, current and even polarity. So you need to define some of these parameters. For a typical real fusor, they tend to operate with deuterium gas of course, at about 5 microns and from 25 to 50 kV DC. These, of course, are not available for a demo fusor but are FYI.

I assume you will likely use air as your gas in your demo fusor. As for pressure, that will depend on what your vacuum pump can achieve - as such, you will need a suitable vacuum gauge that handles the pressure range/capabilities of your pump (do not use a cheap 29 inches of Hg/water type gauge. That is useless for pressures found in systems that support a plasma - under a torr is a good goal.) You will need a vacuum pump that can get below that pressure - say a 100 microns or so should work well with the voltages a neon sign transformer (NST) can provide.

As for voltage, you can use said neon sign transformer (I would strongly urge that you not use a microwave transformer - these are lethal) - NST's are fairly safe and have a range of voltages but 7 kV is common. A variac can control the x-former's output voltage. You can use diodes to create a DC polarity.

Average ion temperature is calculated from first principles and is dependent on the applied voltage and gas density. A plasma is not an equilibrium system (unlike normal air) and energy distribution between ions is not a simple calculation in reality. The site you reference is absolutely not appropriate for an ionized gas! That is for normal, unionized gases ONLY. A plasma does not really exhibit a classical temperature. For an ion of known energy, the 'temperature' is given by E = KT (where E is the ion energy in eV, and k is the Boltzmann constant.) For a plasma this simple value is not correct but for a single ion with a known energy, yields the so-called "temperature". There are programs that provide average ion energy in a plasma for a given excitation voltage and a host of on-line articles that discus this topic. You should explore this in your research to learn more on this topic for your project.

But for a very, very rough calculation of ion temperature (ignoring the mass differences), assuming the ions have a value close to your excitation voltage could be used for a first order calculation (But this is far from true. But it gives you a feel for the issue, even if far too large a valve.) Improving that calculation depends on how much work and interest you have. Certainly there are programs available for this type of calculation but not sure that is appropriate for a science fair. One does need to provide details for the bases of their program results. That is your call.

A few asides: Electron temperature and ion temperatures will differ a great deal in the plasma but light from the ions will dominate in the plasma - but do realize the ion temperature will be a good bit lower than your electron energy. The ion temperature alone will not provide you with the light emission of an ionized gas - an ionized gas is not a black body radiator but the intensity depends on quantum mechanical aspects of the system (spectral line emission, gas pressure and species type.) That is not an easy calculation - for molecules (like air), this is a very complex problem. You might consider using helium gas, instead. Helium is available from store bought balloons. Helium is a far simpler system to calculate and is well studied. Getting a light intensity from a gas can be tricky due to their non-broad band response and that most simple light meters have issues (response problems) with narrow band sources.
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