Major ideas
1. Plasmas are ionized gases characterized by macroscopic parameters that include temperatures, densities, and pressure. The parameters specify statistical distributions of the positions and velocities of the particles - electrons, ions, and neutral atoms & molecules - that make up the plasma. Under some circumstances, the parameters can be different for the different particle species so that, for example, the electron temperature can be much different than the temperatures of ions and neutrals.
Measuring the parameters and gaining information about the distributions is a key step in understanding a given plasma's state prior to using such plasmas for various purposes (lighting, materials processing, fusion energy, etc.). This particular experiment focuses on characterizing a the plasma formed in the "positive column" of an electric discharge in a low-pressure gas. This positive column is closely related to the glowing regions one sees in fluorescent lamps and neon signs, but in the experiment the discharge is maintained by a DC voltage rather than an AC voltage.
2. Since plasmas contain mobile electric charge of both signs (ions and electrons), understanding a given plasma also requires knowledge of how electric and - perhaps - magnetic fields affect and are modified by the plasma. The fact that charge carriers are mobile and are part of a system with significant thermal energy has important consequences when charged bodies and surfaces with controlled electrical potential (such as the Langmuir probe) are inserted into the plasma.
Major equipment
1. Picoammeter with swept potential
2. Computer-based experiment control and data acquisition programmed using Python embedded in a Jupyter notebook.
Alternatives (in case you have already learned about the above)
3. The plasma tube (this will be featured as a prime piece of equipment in the forthcoming Paschen's Law experiment)
4. High-voltage power supply (this will be featured as a prime piece of equipment in the forthcoming Paschen's Law experiment)
(You should make a clear diagram and list in your lab notebook of all of the equipment, components, and supplies used in the experiment. Any of these is fair game for discussion during your review.)
Data analysis
-- (student's choice)
-- (student's choice)
Here is a link to the attached Jupyter notebook for you to download and read within Jupyter, answering the included questions marked Q1, Q2, etc.
Glow discharge plasmas writeup
Alternatively, here is a link to a version that is rendered on Github.
NOTE: in the Github version, you won't be able to do the last question which asks you to run the final Python code and generate a plot. If you cannot complete this before the first actual day of experiment, try to download and run your own version within Jupyter before completing this experiment and preparing your 2x2x2.
Begin with reviewing this notebook: Introduction to Jupyter Notebooks and Python (note that you can visit try.jupyter.org to work with a "live" notebook).
After becoming familiar with Jupyter and Python, proceed to reading through: Experimental Setup
Download this to your desktop and run it within Jupyter. Here is the link:
Wikipedia article
https://en.wikipedia.org/wiki/Langmuir_probe
Modeling the Langmuir Probe Current-Voltage (I-V) Characteristic:
Merlino, R. L., "Understanding Langmuir probe current-voltage characteristics", Am. J. Phys. 75, 1078-1087 (2007).
On glow-discharge plasmas and apparatus used in this experiment:
Wissel, S. A., Zwicker, A., Ross, J., and Gershman, S., "The use of dc glow discharges as undergraduate educational tools," Am. J. Phys. 81, 663-669 (2013). (This can be accessed through ComPADRE.)
IMPORTANT NOTE: OUR EXPERIMENT GROUNDS THE ANODE AND APPLIES NEGATIVE HIGH VOLTAGE TO THE CATHODE. THIS IS DIFFERENT FROM THE CONFIGURATION SHOWN IN FIGURE 1 OF THE ABOVE REFERENCE.
Yip, C.-S., and Severn, G., "Comment on 'The use of dc glow discharges as undergraduate educational tools [Am. J. Phys. 81, 663-669 (2013)]," Am, J. Phys. 83, 654-5 (2015). Be sure to read this comment on the paper by Wissel, et al.
An extensive analysis of the use of Langmuir probes by the author of a well-known book on plasma physics:
www.seas.ucla.edu/~ffchen/Publs/Chen210R.pdf
An excellent book on gas discharge physics:
Y. P. Raizer, Gas Discharge Physics (Springer-Verlag, Berlin, 1991). See pp. 167-211 on glow discharges and pp. 214-239 on glow discharge instabilities including striations. (Book available in Colorado through interlibrary loan via Prospector.) View here.
An ALPhA Immersion that motivated the development of our new experimental setup:
http://www.compadre.org/advlabs/wiki/Plasma_Physics:_Electrical_Breakdown
http://www.compadre.org/advlabs/wiki/Plasma_Physics:_Plasma_Probes
Another instructional experiment write-up with extensive background on Langmuir probes; the experiment section uses the 0A4G gas-filled triode which does NOT apply to our experimental setup: