Dim and bright void regimes in capacitively-coupled RF complex plasmas

TL;DR

This study experimentally identifies two regimes of voids in RF complex plasmas, characterized by emission features, and models their formation and stability through fluid simulations considering ion diffusion and electrostatic forces.

Contribution

It introduces the concept of bright and dim void regimes in RF plasmas and models their stability using a simplified fluid approach with ion diffusion.

Findings

Bright voids show plasma emission; dim voids do not.

Void transition is discontinuous with increasing power.

Model reproduces both regimes by adjusting ion diffusion and electrostatic forces.

Abstract

We demonstrate experimentally that the void in capacitively-coupled RF complex plasmas can exist in two qualitative different regimes. The "bright" void is characterized by bright plasma emission associated with the void, whereas the "dim" void possesses no detectable emission feature. The transition from the dim to the bright regime occurs with an increase of the discharge power and has a discontinuous character. The discontinuity is manifested by a kink in the void size power dependencies. We reproduce the bright void (mechanically stabilized due to the balance of ion drag and electrostatic forces) by a simplified time-averaged 1D fluid model. To reproduce the dim void, we artificially include the radial ion diffusion into the continuity equation for ions, which allows to mechanically stabilize the void boundary due to very weak electrostatic forces. The electric field at the void boundary occurs to be so small that it, in accordance with the experimental observation, causes no void-related emission feature.