The anode produces an electric field along the axis of the tube to generate plasma. Credit: CCLDAS

High School student interns at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS) celebrate having finally created a plasma generator.

“It took quite a bit of effort to get it done, they’ve been working on this for almost 3 months,” CCLDAS PI Mihaly Horanyi said proudly.

The interns managed to generate a reasonably bright plasma, the properties of which are currently being measured. Their eventual goal is to use the plasma generator and langmuir probe to measure density and temperature under various pressure and voltage conditions.

There were several challenges that the students needed to overcome, including a temperamental seal between tube components and original plastic gaskets and, ironically, a leaky leak valve. Neither of these problems were particularly difficult to solve but they both required ordering replacement parts, which always takes time.

The method used to generate plasma consists of emitting electrons and establishing an electric field (along the axis of the tube) to accelerate the electrons until they hit a passing neutral atom, with enough energy to ionize that atom. This method has a few limiting factors, however. First, in order for the average free electron to have enough energy to reliably ionize the gas, it has to have some room to accelerate before colliding with atoms, meaning that the pressure (and therefore atom density) of the gas must be low. Second, not all gasses have the same breakdown voltage at the same pressure. The students are using Argon gas because it has a much lower breakdown voltage than does Nitrogen.


The CCLDAS plasma generator. Credit: CCLDAS

To generate free electrons the interns are using a thoriated tungsten filament (red circle above & close up image below), heated until almost white hot (~900 C). Then they use the anode on the right (green circle above) to produce an electric field along the axis of the tube. Below that is a pump to keep the pressure low (blue circle above). On the left (yellow circle above) is a bank of power supplies; the black one provides power to the filament (averaging about 15 amps) and the two grey ones produce the voltage between the tube anode and cathode. Lastly, a tank of argon (purple circle above) and the leak valve supply and regulate the flow of argon.


The thoriated tungsten filament for electron emission. Credit: CCLDAS

Visit the CCLDAS blog to read more and see some really pretty pictures of glowing argon!

Posted by: Soderman/NLSI Staff
Source: http://ccldas.blogspot.com/2012/08/high-school-plasma-success.html

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