Speed of sound in a complex plasma under microgravity (Vol. 43 No. 2)

image Mach cone relation linking the Mach cone angle θ with the probe particle speed v in experiments (crosses) and simulation (triangles). (a) Zoom showing the subsonic probe particle. (b) Smoothed difference between two successive frames, highlighting the Mach cone.

Complex plasma experiments under microgravity conditions in the International Space Station ISS, complementing research on Earth, are very important for the progress in this field. Complex plasmas consist of micrometre-sized highly charged particles ("microparticles") embedded in an ionized gas ("plasma"). The microparticle ensemble resembles a classical system of interacting atoms. This system can form all of the classic phases, i.e., crystalline, liquid and gaseous. It can also support the propagation of sound waves, solitons and shock waves.

In experiments carried out on Earth, gravity pulls the microparticles downwards resulting in two-dimensional structures. Under microgravity conditions it is possible to investigate big, three-dimensional systems. The PK-3 Plus laboratory provides ideal conditions for such experiments. It is installed in the Russian segment of the ISS and has been used repeatedly over the last 6 years.

We performed experiments to measure the speed of sound during two missions with cosmonauts Volkov and Skvortsov. In Figs. (a) and (b) we show how a bigger "probe" particle penetrated a cloud of smaller microparticles. At a speed several times faster than the speed of sound it excited a Mach cone. A double cone structure is discernible in this "atomistic" system. While moving through the cloud, the probe particle is decelerated to subsonic speeds. By measuring the Mach cone angle at several probe particle velocities, we determined the Mach relation. This allows us to directly measure the speed of sound and to infer the microparticle charge. In addition, the experimental results agree well with a 3D molecular-dynamics simulation, demonstrating in particular a double Mach cone structure.

Direct measurement of the speed of sound in a complex plasma under microgravity conditions
M. Schwabe, K. Jiang, S. Zhdanov, T. Hagl, P. Huber, A. V. Ivlev, A. M. Lipaev, V. I. Molotkov, V. N. Naumkin, K. R. Sütterlin, H. M. Thomas, V. E. Fortov, G. E. Morfill, A. Skvortsov and S. Volkov, EPL, 96, 55001 (2011)