New insights in the stratification of an argon positive column plasma. II. Experiments and particle simulations

Abstract

The conditions of stratification of a radio frequency positive column plasma in argon over a pressure range from 0.1 to 2 Torr are studied using experiments and particle-in-cell Monte Carlo collision (PIC-MCC) simulations. The PIC-MCC simulations include a simple chemistry model for metastable production and losses, stepwise ionization, and associative ionization. Experiments in a 1.1 cm radius positive column plasma in argon in a 13.56 MHz radio frequency field reveal well-defined standing striations between 0.05 and 1 Torr. Adding a percentage of up to 10% hydrogen in the discharge does not affect the stratification around 0.1 Torr and below. Since hydrogen is a good quencher of argon atoms in the metastable state, this indicates that the presence of metastable atoms plays no role in the stratification of the positive column at this pressure and below. At pressures above 0.3 Torr, the striations disappear with the addition of a lower percentage of hydrogen in the experiment, showing that the presence of metastable atoms is necessary for the formation of striations at sufficiently high pressure. The PIC-MCC simulations are consistent with the experimental results and with the theory based on dispersion relations, presented in a companion paper [J. P. Boeuf, L. Boufendi, M. K. Dosbolayev, S. A. Orazbayev, T. S. Ramazanov, Phys. Plasmas 31, 073508 (2024)], which demonstrates that the negativity of the thermoelectric term related to the Dufour effect in the electron energy flux is responsible for stratification even when metastable ionization is dominant. The non-linearity of metastable ionization with electron density is not responsible for stratification in the conditions considered in this paper.