Measurement of the phase space distribution in a complex plasma

Abstract

The development of a diagnostic technique that allows measurement of the six dimensional phase space distribution for the dust component of a plasma system is presented. In the course of the measurement and analysis processes a number of long-standing questions related to the basic properties of dusty plasmas are addressed and explained for the first time. The work described below demonstrates the importance of using a generalized form of the standard Maxwellian probability distribution function to model the velocity space portion of phase space. This generalization, the tri-normal probability distribution function, allows ellipsoidally symmetric anisotropy in velocity space; the appearance of such anisotropy has been an outstanding issue in weakly-coupled dusty plasmas for several years. The measured velocity space anisotropy is shown to be both a real effect and an effect that is too large to be accounted for as a perturbation to the standard spherically symmetric model of the velocity space. The spatially resolved measurements within the dust cloud and the new model for the velocity space are then combined to give the spatial distribution of the fluid properties of the dust cloud for the first time in a weakly-coupled system. The fluid thermodynamic and transport properties obtained through the process are discussed in detail. The analysis of these newly available transport and thermodynamic properties clearly shows that the dust component of the system is in a state of dynamic force-balanced equilibrium. The dust component of such systems has long been suspected to be in a force-balanced equilibrium; the appearance of the tri-normal distribution in velocity space unambiguously confirms the suspicion. The fact that the system is in a state of dynamic equilibrium is demonstrated by examination of the transport properties of the dust component and has not been previously demonstrated experimentally.