Striations in a positive column plasma. Role of transport coefficients and stepwise ionization

The stratification of plasma positive columns is a very interesting phenomenon of instability and self-organization that has been known since Faraday and illustrates the nonlinearity and complexity of low temperature plasmas. The striations form in a variety of conditions depending on the gas,  current, pressure, tube radius, and the mechanisms leading to stratification are still not fully understood. In this presentation we show how theory, experiments and particle simulations can be combined to get a better insight in plasma stratification in rare gases. We focus on pR (pressure times radius) products below 10 torr.cm in argon. In a steady state low pressure positive column in rare gases the stratification is characterized by potential drops between striations that are directly related to the electronic excitation energy thresholds (the p, r, and s waves) and are associated with “electron bunching”. In this presentation we show that although the nonlinear, saturated stage of the instability is linked to a complex structure of the  electron probability distribution function (EEPF) that can only be described by kinetic models,  the mechanisms responsible for the instability are due to a peculiarity of the EEPF in the homogeneous regime and can be understood with a nonlocal fluid model. The non-Maxwellian nature of the EEPF in the homogeneous regime leads to a negative value of a transport coefficient proportional to the electron energy density gradient in the electron energy flux (Dufour effect). This mechanism triggers the instability not only at low pressures (pR<0.5 torr.cm), where direct electron impact ionization is dominant but even at higher pressures, where  metastable ionization prevails. The nonlinearity of metastable ionization is not responsible for stratification. In this presentation, the physical mechanisms leading to stratification are described by a nonlocal fluid model while the properties of the nonlinear stage are illustrated with Particle-In-Cell Monte Carlo Collisions simulations and experiments in the positive column of a capacitively coupled radiofrequency discharge. Adding a small percentage of a metastable quencher like H₂ in the experiments does not modify the striations below 0.5 torr.cm but tends to destroy the striations above 0.5 torr.cm, in agreement with theory and simulations.