Langmuir probe PIC dynamic simulation of collisional plasma*

The low-temperature plasma physics has plenty of different applications from purely scientific ones like the study of chemical reactions in flowing afterglow to industrial applications like magnetron sputtering. All of them have in common that their success relies heavily on diagnostic of plasma properties. One of the most common and oldest methods is the probe diagnostic using the Langmuir probe. The theory for Langmuir probe measurement is very well described for collisionless plasma but with rising pressure the collisions with neutral background affects the probe results and the theoretical description is difficult. Also there are cases when it is required to perform the measurement in the shortest time possible due to time resolution of studied phenomenon or when there is a need to limit the probe exposure to plasma to prevent its damage by, for example, flow of high energy particles. Those dynamic properties of the probe are almost impossible to describe analytically and the experimental approach would provide only limited information. The particle simulation is well suited for this task since it provides very detailed information about the surroundings of the probe when its potential is changed.  It provides the probe current same as the experiment would do, but it also gives the time evolution of potential near probe, sheath size and charge enclosed in sheath. Such information is crucial to obtain the time constant specific for plasma reaction to the probe potential change and determine the lowest time needed for the measurement to give reliable results. 

We present the results of various particle simulations, which we used to obtain the plasma stabilization time constant after the probe potential change. The simulations were performed at different pressures to obtain the pressure dependence of the stabilization time constant.