String fluid instability in a complex plasma in a direct current discharge under microgravity

Abstract

The PK-4 Laboratory is a Russian-European laboratory for studies on complex plasmas on board the International Space Station ISS. Its heart consists of a direct current plasma tube with 3 cm diameter and a total length of 85 cm. Microparticles of various sizes can be injected into the plasma. Here, we report on an instability formed in the microparticle cloud in which waves form. The plasma was driven in polarity switching mode - the DC electric field was quickly switched, and the microparticles were trapped in the center of the discharge. The polarity switching causes the ions and electrons of the plasma to stream around the microparticles, and the ions form wakes in downstream direction of the microparticles, which in turn attract other microparticles and lead to the formation of microparticle strings. Here, we report on the formation of waves in such a string fluid. We study the dispersion relation of the waves, demonstrate that they are not ordinary dust acoustic waves, and hypothesize on the excitation mechanism of the instability.