Wave Phenomena in a Stratified Complex Plasma

PK-4 is a plasma facility that uses a dc discharge inside a glass tube for experiments on complex (dusty) plasmas in microgravity onboard the International Space Station. It was launched in October 2014. During development, several models of PK-4 were built. The experiments described here took place in the engineering model of PK-4. During a test run, we injected particles with different diameters. They formed a stratified complex plasma where an instability generated waves inside the strata. The analysis of the video data is shown and the type and the cause of the instability are discussed here.

I. Introduction

Pk-4 is a plasma facility onboard the International Space Station (ISS) [1]. PK stands for (German) plasma crystal. With its launch in 2014, PK-4 continues the successful history of complex (dusty) plasma experiment setups onboard the ISS since 2001 (PKE-Nefedov, PK-3 Plus) [2]. But unlike the former facilities, PK-4 uses mainly a dc discharge to generate a plasma inside a glass tube. Microparticles injected into this plasma acquire a negative charge due to the higher mobility of the electrons. Through a (screened) Coulomb force the particles interact with each other and form structures that can be visualized by video cameras. Depending on the plasma parameters, the complex plasma can be set to any state of matter: solid (crystalline), liquid, or gaseous, having one big advantage: in the video images, the dynamics of the system can be observed on the basis of individual particles, which is hardly possible with atoms in real matter. Furthermore, the tubular design of PK-4 is ideal for the study of complex plasmas in the liquid state—a unique experimental access to fluid dynamics at kinetic level of individual particles.