Laboratory experiments with complex plasmas — V. Nosenko
22 Nov 2012
Vladimir Nosenko, Ph.D., the head of “Complex plasma” laboratory
Complex plasmas are popular with experimenters. In many cases one can actually see the subject of their study with naked eyes. A plasma crystal in its two-dimensional form can have a diameter of around five centimeters and consist of thousands of micron-size particles separated by half millimeter. This makes it relatively easy to image the whole plasma crystal with a suitable video camera. Particle positions in every video frame can then be calculated using particle-tracing software (similar to tracing stars in astronomy). Particle coordinates as functions of time give the complete kinetic description of plasma crystal, just as in statistical mechanics or molecular-dynamics simulations. Given the complete kinetic description of complex plasmas, one can in principle derive any physical observable and compare it to theories.
|
Plasma lab at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The experimental setup is based on a modified GEC (Gaseous Electronics Conference) reference cell. Plasma is produced using a capacitively coupled radio-frequency discharge in argon. Pre-manufactured monodisperse particles (shown in the inset) are introduced into plasma from dispensers. Typically, the particles are dielectric, have a diameter of 3-10 m, and acquire an electric charge of ~104 elementary charges. Due to the interplay between interparticle repulsion and external electrostatic confinement, the particles organize themselves into a plasma crystal. |
There are many popular lines of research in complex plasmas. In my lecture, I will concentrate on the following promising topics:
Atomistic dynamics in liquids. Complex plasmas are especially well suited as model systems to study liquids at the level of individual “atoms”. There have been many experiments performed to measure shear viscosity, thermal conductivity, and diffusion coefficients. Several fundamental questions remain unanswered, though. The most interesting of these are rheological properties of shear flows (shear-rate dependence of viscosity), applicability of the Navier-Stokes equation at small scales, temperature dependence of thermal conductivity near melting transition, and the very existence of 2D thermal conductivity in the thermodynamic limit. Another interesting topic is the microscopic dynamics of supercooled liquids.
Linear and nonlinear waves. Linear properties of the new wave modes that arise in complex plasmas due to the presence of microparticles are well studied. However, experimental work is only beginning in such areas as nonlinear waves and oscillations and wave-particle interaction (Landau damping).
|
Dust-acoustic waves in complex plasmas. A suspension of microparticles in a direct current discharge is imaged by a high-speed camera (operating at 1000 frames per second). The dust-acoustic wave is a new wave mode where inertia is provided by “massive” microparticles and restoring force is due to electrons and ions. This is the first plasma wave mode which can be directly “seen”. |
Microparticles as tracers of plasma flows. One interesting application of complex plasmas is to study plasma flows using microparticles as tracers. So far, ion and neutral gas flows were studied using this technique. A new development is applying a rotating electric field to complex plasma (analogues to the “rotating wall” technique used in non-neutral plasmas). The physical picture of this effect turns out to be rather complicated.