Compressional waves in complex (dusty) plasmas under microgravity conditions

Complex plasmas consist of electrons, ions and charged microparticles, with typical charge-to-mass ratios 1:10−5:10−13. The interest in these systems has grown explosively, because they can be investigated at the kinetic level (the microparticles). However, on Earth the supporting forces (against gravity) are of the same order as the electrostatic interparticle forces—and hence only strongly compressed systems can be investigated. Under microgravity conditions these “body forces” are a factor 102 smaller which allows the experimental investigation of weakly compressed three-dimensional complex plasmas. One way to study these systems is by the controlled excitation of low-frequency compressional waves. The first such experiments, conducted with the PKE-Nefedov laboratory on the International Space Station is reported. The waves were excited by modulating the voltage on the rf electrodes. By varying the modulation frequency the dispersion relation was measured. The results are compared with existing theoret...