The structure of the complex plasma boundary

Abstract

In recent experiments on the International Space Station, a void or dust-free region was often observed in the central region of a complex plasma. A characteristic feature of the void-complex plasma boundary is a region, several inter-particle distances thick, where the complex plasma is compressed. Typically, ε = 0.6 is the ratio of mean particle separation at the boundary to the bulk region. A collisionless model is developed, under the assumption of an electrostatic double layer at the boundary. However, the model showed no compression with ε = 6.2, and the boundary width approximately the ion absorption length, λi,ab ≈ 1 cm, due to absorption by the micro-spheres. A sufficient condition for compression is obtained which shows that, if the ion density gradient is sufficiently greater than the electron density gradient, the boundary becomes compressed. The collisionless model was modified by including ion-neutral collisions, of mean free path λin, in the complex plasma. It was found that if λin ≪ λi,ab, the ion density gradient is greatly increased. The degree of compression is significantly reduced to ε = 1.2. Despite the boundary not being compressed, the boundary width is in good agreement with experimental observations.