To know the exact plasma density and temperature so I can better model my process To use the plasma parameters, rather than input parameters to determine any sources of variation. Advancements where made in the Langmuir Probe theory only after assumed the plasma was changing independent of the input conditions. To describe my experimental conditions in a way that is easier to transfer to other experiments. The current drawn to the wire is a function of voltage on the wire and was shown to be related to the electron distribution function by Langmuir and Mott-Smith in a famous  Meter Base Parts Factory paper.  

Over the past 5 years the use of Langmuir Probes in industry has become widespread, the drive to understand plasma process is of great importance these days. As a way to check for faults and to diagnose the faults when they are detected.Continued development gave way to commercially available Langmuir probes that can meet accuracies of greater than 1%. This was then used the current at the plasma potential to calculate the electron density. The density and temperature of the plasma was changing by 50% from day to day. Commercially available Langmuir Probes are used to.

To ensure that the plasma is stable and repeatable.Forty years ago the Langmuir probe was hand made from glass and tungsten. Drawing a line on the electron branch, above and below the plasma potential and this line intersected at the plasma potential.  Analysing a single trace would take about 1 hour and this was done time after time to gather the data required. Usually the process of painfully re-plot the characteristic on a piece of log-linear graph paper was a necessity, to find the slope of the log of the current as a function of voltage and the inverse of this slope was equal to the electron temperature. The Langmuir Probe consisted of a small piece of wire surrounded by an insulator and placed inside the plasma.  

Langmuir Probes are now seen as a fundamental instrument for measuring plasma parameters and very few research facilities are without a commercial Langmuir Probe.  Bench power supply and an x-y plotter was usually used to attached the voltage via a resistor divider to the x and the voltage drop on a series resistor to the Y and I manually changed the voltage from -100 Volts to +50 Volts and recorded the current-voltage characteristic. In the early years the results had very high variations. Universities had workshops with machinist and glass-blowers that made a lot of basic equipment for physics laboratories. Over a short period of time Langmuir Probe measurements became available with accuracies in the region of.