CCLDAS: August 2012

But wait! there's more! (from the highschoolers.)

So, despite the title of our last post, we youngins are here once again to celebrate having finally gotten the plasma generator in the student office to work, at least rudimentarily.

(our first plasma; isn't it cute?)

As evidenced by the above picture, and those below, we've managed to generate a reasonably bright plasma; however, it is not ideal because of the striations visible along the axis of the tube, which we will hopefully be able to correct before we try to make any measurements on the properties of the plasma. Our eventual goal will be to use the plasma generator and included langmuir probe to measure the density and temperature under various pressure and voltage conditions.

(the Langmuir probe which we use to measure plasma properties. Inside the tiny glass tube is an even smaller wire (which the camera won't focus on.)

For now however, we are happy to have overcome some of the challenges that have slowed our progress so far. Included among these set backs are the difficulty we had creating a proper seal between tube components with the 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 cost us quite a bit of working time.

Finally however, we've gotten a working set-up; so listen my children and you will hear science-y details of our setup...here (ok, it's a bit of a lame rhyme.). Anyway, the method we're using 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, hopefully with enough energy to ionize said 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, they have 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. To illustrate, we have a picture of the Paschen curves of several gasses, including atmospheric gasses and Argon, which we're using.

Taken from http://upload.wikimedia.org/wikipedia/commons/8/82/Paschen_Curves.PNG

As you can see, Argon (in red) has a much lower breakdown voltage on average than does Nitrogen, hence our use of Argon. The plot also illustrates the pressure dependence of breakdown voltage , which, for argon, has a minimum around .6 Torr cm; since our tube has a length of approximately 50cm, our ideal pressure is around 12 mTorr.

To generate free electrons we're using a thoriated tungsten filament (red circle & first close up), heated until almost white hot (~900 C). Then we use the anode on the right (green & second close up) to produce an electric field along the axis of the tube. Below that we have a pump to keep the pressure low (blue & third close up). On the left (yellow & fourth close up) we have our bank of power supplies; the black one provides power to the fillament (averaging about 15 amps) and the two grey ones produce the voltage between the tube anode and cathode. Lastly, we have a tank of argon (purple. two guesses which close up) and the replacement for the leak valve, to supply and regulate a flow of argon.