Helicon wave propagation and plasma equilibrium in high-density hydrogen plasma in converging magnetic fields

Abstract

In this thesis, we investigate wave propagation and plasma equilibrium in MAGPIE, a helicon based linear plasma device constructed at the Australian National University, to study plasma-material interactions under divertor-relevant plasma conditions. We show that MAGPIE is capable of producing low temperature (1–8 eV) high density hydrogen plasma (2–3×10^19 m-3) with 20 kW of RF power when the confining magnetic field is converging. The original research herein described comprises: (1) Characterization of hydrogen plasma in MAGPIE, (2) Analysis of the RF compensation of double Langmuir probes, (3) Excitation, propagation and damping of helicon waves in uniform and non-uniform magnetic fields and (4) Steady-state force balance and equilibrium profiles in MAGPIE. We develop an analytical model of the physics of floating probes to describe and quantify the RF compensation of the DLP technique. Experimental validation for the model is provided. We show that (1) whenever finite sheath effects are important, overestimation of the ion density is proportional to the level of RF rectification and suggest that (2) electron temperature measurements are weakly affected. We develop a uniform plasma full wave code to describe wave propagation in MAGPIE. We show that under typical MAGPIE operating conditions, the helical antenna is not optimized to couple waves in the plasma; instead, the antenna’s azimuthal current rings excites helicon waves which propagate approximately along the whistler wave ray direction, constructively interfere on-axis and lead to the formation of an axial interference pattern. We show that helicon wave attenuation can be explained entirely through electron-ion and electron-neutral collisions. Results from a two-dimensional full wave code reveal that RF power deposition is axially non-uniform with both edge and on-axis components associated with the TG and helicon wave respectively. Finally, force balance analysis in MAGPIE using a two-fluid “Braginskii” type formalism shows that the electron fluid exists in a state of dynamic (flowing) equilibrium between the electric, pressure and thermal forces. The pressure gradient, driven by the non-uniform RF heating, accelerates the plasma into the target region to velocities close to the ion sound speed. From the measured axial plasma flux we find that the plasma column in MAGPIE can be divided into an ionizing and a recombining region. For the conditions investigated, a large fraction of the plasma created in the ionizing region is lost in the recombining region and only a small fraction reaches the end of the device. The equilibrium plasma density along the length of MAGPIE can be quantitatively explained using a 1D transport calculation which includes volumetric particle sources and magnetic compression. We show that the plasma is transported, by the electron pressure gradient, from under the antenna (0.5×10^19 m-3) into the target region where it reaches maximum density (2-3×10^19 m-3). Using the results herein presented, this thesis explores the relationship between the RF power deposition in MAGPIE, parallel plasma transport and the production of high density plasma in the target region.