Langmuir probeA Langmuir probe, named after Nobel Prize winning physicist Irving Langmuir, is used to determine the ionization energy and electron temperature of a plasma. It works by inserting two wires into the plasma that are insulated on their sides from each other and the plasma. This is done so that only the tips of the wires are exposed to the plasma. The wires, typically made of tungsten, are several thousandths of an inch thick.
Mathematical analysis of this curve using calculus and physics allows the user to calculate plasma potential (Vp), floating potential (Vf), electron density (ne), ion density (ni), electron temperature (Te) and the electron energy distribution function (EEDF) or f(e).
A Langmuir probe can be purchased off the shelf for on the order of 15,000 U.S. Dollars, or they can be built by an experienced researcher and/or technician. When working at frequencies under 100 MHz, it is advisable to use blocking filters, and take necessary grounding precautions.