Electron sink on the Langmuir probe in plasma, its influence on measurements and methods to account for it

The influence of the electron sink effect on the probe on the results of measurements of the electron energy distribution functions (EEDFs), plasma potential, and electron concentration was studied. The studies were carried out for helium in a discharge with a hollow cathode at a gas pressure of <= 1 Torr for cathode-anode distances of 1 cm and 3 cm; measurements were carried out at several distances from the cathode. The distribution functions were obtained by processing the probe volt-ampere characteristics using the standard Druyvesteyn method. The form of the measured EEDFs changed with distance from the cathode. According to test calculations performed using the Monte Carlo method, this was due to the nonlocal nature of the EEDF formation under the considered conditions. Calculations also showed that the nonlocality effect manifests itself mainly in the region of the high-energy part of the EEDF, while in the low-energy region the form of the EEDF is close to local one. The latter fact was used to correct the measured EEDFs with allowance for the effect of electron sinking to the probe. In standard probe measurements, the sink (flux, flow) of electrons to the probe leads to underestimated values of the EEDFs in the region of low electron energies. The paper analyzes various known methods (formulas) for calculating the amount of sink in relation to the experimental conditions under consideration. The correction of the measured EEDFs was carried out taking into account the indicated effect. The errors in determining the macroscopic parameters of the plasma and the rates of elementary processes using the EEDFs obtained by processing the probe characteristics without taking into account the sink effect were analyzed.

Automated summary

The study investigated the influence of the electron sink effect on probe measurements of EEDFs, plasma potential, and electron concentration. It was found that the shape of the EEDFs varied with distance from the cathode, mainly due to the nonlocal nature of EEDF formation in high-energy regions.