A new mechanism for pattern formation in low-pressure RF plasmas

Striations, as plasma self-organization, emerge from an ionization instability in DC discharges. Similar patterns have been reported in RF discharges, but the physical origin remained unknown. We propose a mechanism from a fluid model in which transport coefficients have been computed from a 0-D kinetic model [1]. In the quasineutral regime, the electron flux $\Gamma_e$ and the energy flux $H_e$ are expressed as a function of the plasma density gradient $\nabla n_e$ and electronic temperature gradient $\nabla T_e$ and transport coefficients $D_a, \mu_e, \chi_e \ \rm{ and }\ \kappa_e$ (e.g. for energy $H_e=\chi_e \nabla n_e+ \kappa_e \nabla T_e$). When the electron distribution is non-Maxwellian, off-diagonal terms $\chi_e$ and $\mu_e$ may be non-zero and unstable regimes may develop. Using the BOLSIG+ kinetic model at low Ar pressure, we showed that off-diagonal terms may be sufficiently negative to overcome diffusive effects, leading to an instability. This model reproduces all experimental features observed in an annular RF plasma: (1) axisymmetry is broken above a critical pressure, (2) azimutal modulations of the plasma iincreasing with pressure, (3) axisymmetry is recovered at higher pressure. [1] D\'esangles et al., \textit{Phys. Rev. Lett.} \textbf{123}, 265001 (2019)