Transport Properties of Charged Particles in Low Pressure Plasmas

Abstract

This thesis discusses transport phenomena of charged particles in low pressure plasmas which are of particular interest to electric propulsion systems.

Electrons of low collisionality behave nonlocally and their thermodynamic interpretation should be revisited as traditional thermodynamic concepts are based on the collision-dominated local equilibrium. The polytropic process is adapted to nonlocal electron transport during plasma expansion. A conservation relation between electron enthalpy and potential energy is derived from nonlocal electron energy probability functions and verified by previously published measurements in a laboratory helicon double layer thruster. Analysis of the experimental data shows that although the electron transport along a divergent magnetic field is an adiabatic process, it yields a polytropic index of 1.17, which is less than the classic adiabatic index of 5/3. A theoretical perspective of how nonlocal electron energy probability functions determine the polytropic index is investigated through three different bi-Maxwellian distributions. The polytropic index increases when the electron energy probability function becomes more convex and decreases when more concave. The polytropic index of 5/3 corresponds to a Heaviside distribution and is an element of a set of polytropic indices for systems governed by nonlocal particle dynamics. Considering interrelations between the solar wind and laboratory plasmas, a new scenario is hypothesized for the thermodynamic behavior of the solar wind: although the solar electrons give a polytropic index less than 5/3, their actual transport might be adiabatic.

Ion beam experiments are carried out in the Chi-Kung reactor implemented with a cylindrical plasma source (cylindrical plasma thruster) or an annular plasma source (annular plasma thruster). The cylindrical plasma thruster can be operated under a high magnetic field mode and a low magnetic field mode. In the high field mode, a bi-directional ion beam travelling in opposite directions is respectively measured in the converging and diverging parts of a magnetic nozzle, exhibiting a very different scenario from the classic one-directional nozzle flow of compressible gases. No ion beam is detected for the low field mode although an axial potential drop exists in the plasma source, for which a correlation between ion beam formation and radial plasma transport at the magnetic throat is revealed. The annular plasma thruster provides an enhanced degree of freedom in terms of electron heating by using either an outer antenna or an inner antenna. Electron transport in the annular system is characterized and compared for the two opposite antenna cases. An annular ion beam is observed downstream of the plasma source for the outer antenna case while not for the inner antenna case. It merges into a solid structure (with the central hollow filled) in the diffusion chamber and a reversed-cone wake is formed behind the inner tube.

Transport behavior of an annular plasma is greatly changed from a cylindrical plasma due to the occurrence of an inner wall boundary. Depending on the presence of ion-neutral collisions or not, collisional modeling and collisionless modeling are respectively developed to better understand radial transport of unmagnetized charged particles across annuli. The electrons are in an equilibrium state and assumed to be governed by the Boltzmann relation (equivalent to a Maxwellian equilibrium). The collisional ion transport is described by three mobility governed models: a low field electric field model, an intermediate electric field model and a high electric field model. The collisionless ion transport is studied using the Tonks and Langmuir theory and the solution is expressed in terms of the Maclaurin series approximant and Padé rational approximant. The annular modeling is applied to argon plasmas and discussed for different Paschen numbers and annular geometries. Show more