The plasma inside a dust free void: hotter, denser, or both?

The existence of a dust-free void, as often observed in dusty plasmas with small particles, or in dusty plasmas under micro-gravity conditions, requires a maximum of the ionization inside the void. Enhanced optical emission inside the void has indeed been observed. The extra losses of plasma on the dust has to be compensated for by extra ionization, which means that the electron temperature must rise. Inside the void there is no depletion of electrons, so a rise in electron temperature is not immediately obvious. It was therefore proposed that the relatively high electron density in the void with respect to the surrounding dusty region, where the electrons are depleted, causes the higher ionization inside the void. Different observations and models have until now not given a decisive answer, however. Using a global model, we predict that a homogeneous dusty plasma without a void should have an increased electron temperature, but at a reduced electron density. A dusty plasma with a fully formed void should have both an increased electron temperature and density in the void. A fully self-consistent two-dimensional model agrees with these conclusions and also shows that the void is a complex system, which is heated by the dust on the outside, but has most of the ionization on the inside.