Diagnostics of collisional flowing plasma using flat Langmuir probe*

A continuum flat Langmuir probe has been developed to diagnose collisional flowing plasma ignited in atmospheric conditions. At atmospheric pressure, collisions between neutral and charged particles in plasma dominate since the ion-neutral mean free path is now exceedingly small compared to the sheath thickness and probe characteristic size. Collisions with atoms and molecules will serve as a friction force, slowing the charged particles as they accelerate towards the probe surface, affecting current collection. Therefore, conventional Langmuir probes does not work effectively. Hence the idea of using flat probes has been conceived. Moreover, placing the probe directly in the downstream zone causes extra filamental discharges and changes the plasma properties. Therefore, to characterize the jet plasma without changing its original nature, a half cylindrical vacuum chamber has been built and a sample of plasma plume is taken from atmosphere into the 100mbar vacuum chamber where the flat probe is placed. Experiments with various Langmuir probe setups were carried out. The IV curve was extracted using a double copper-cladded flat plate configuration with a 5mm separation and the plasma parameters were extracted using a continuum flowing plasma theory. The number density of electrons in plasma is calculated to be 5.3×10¹⁶ m^-3, and the electron temperature inferred from the curve is around 6.1 eV. The Druyvesteyn electron energy distribution function shows there are enough high-energy electrons in the plasma to trigger various chemical processes, which is important in plasma processing.