Plasma is a gas of charged and neutral particles. In order for an ionized gas to be plasma the gas commonly satisfies three criteria. The plasma must have an overall charge close to zero at all times. Second, in order for an ionized gas to be plasma it must also exhibit collective behavior. A third criterion for plasma is that the motions of the particles must be dominated by electromagnetic forces. Therefore, the device plays an important role to collect plasma. Molybdenum disc is one part in this device.
Three important properties of plasma in experimental physics are the density of the plasma, the ion temperature, and the electron temperature. These properties can be determined by the use of a Langmuir probe. The Langmuir probe in this experiment (see fig A) consists of a 1mm diameter molybdenum disc welded to a tungsten wire. Molybdenum is used for the tip because it has a large work function. This reduces emission of electrons from the tip of the probe. The wire is insulated from the plasma and the vessel so that only the molybdenum disc is exposed to the plasma.