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

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

RF compensation of double Langmuir probes: modelling and experiment

An analytical model describing the physics of driven floating probes has been developed to model the RF compensation of the double langmuir probe (DLP) technique. The model is based on the theory of the RF self-bias effect as described in Braithwaite's work [1], which we extend to include time-resolved behaviour. The main contribution of this work is to allow quantitative determination of the intrinsic RF compensation of a DLP in a given RF discharge. Using these ideas, we discuss the design of RF compensated DLPs. Experimental validation for these ideas is presented and the effects of RF rectification on DLP measurements are discussed. Experimental results using RF rectified DLPs indicate that (1) whenever sheath thickness effects are important overestimation of the ion density is proportional to the level of RF rectification and suggest that (2) the electron temperature measurement is only weakly affected

Initial characterization of Argon plasmas in the "MAGnetized Plasma Interaction Experiment" (MAGPIE)

RF magnetic fluctuation probes and symmetric double Langmuir probes have been utilized to characterize Argon helicon plasma in a converging magnetic field. We observe that when enough RF power is supplied an Ar II blue core mode is formed suggestive of t

Design and characterization of the Magnetized Plasma Interaction Experiment (MAGPIE): A new source for plasma-material interaction studies

The Magnetized Plasma Interaction Experiment (MAGPIE) is a versatile helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University. Various material targets can be introduced to study a range of plasma-material interaction phenomena. Initially, with up to 2.1kW of RF at 13.56MHz, argon (1018-1019m3) and hydrogen (up to 1019m3 at 20kW) plasma with electron temperature 3-5eV was produced in magnetic fields up to 0.19T. For high mirror ratio we observe the formation of a bright blue core in argon above a threshold RF power of 0.8kW. Magnetic probe measurements show a clear m=+1 wave field, with wavelength smaller than or comparable to the antenna length above and below this threshold, respectively. Spectroscopic studies indicate ion temperatures <1eV, azimuthal flow speeds of 1kms1 and axial flow near the ion sound speed

Wave modelling in a cylindrical non-uniform helicon discharge

A radio frequency (RF) field solver based on Maxwell's equations and a cold plasma dielectric tensor is employed to describe wave phenomena observed in a cylindrical nonuniform helicon discharge

B2. 5-Eirene modeling of radial transport in the MAGPIE linear plasma device

Radial transport in helicon heated hydrogen plasmas in the MAGnetized Plasma Interaction Experiment (MAGPIE) is studied with the B2.5-Eirene (SOLPS5.0) code. Radial distributions of plasma density, temperature and ambipolar potential are computed for several magnetic field configurations and compared to double Langmuir probe measurements. Evidence for an unmagnetized ion population is seen in the requirement for a convective pinch term in the continuity equation in order to fit the centrally peaked density profile data. The measured slightly hollow electron temperature profiles are reproduced with combinations of on-axis and edge heating which can be interpreted as helicon and Trivelpiece–Gould wave absorption, respectively. Pressure gradient driven radial charged particle diffusion is chosen to describe the diffusive particle flux since the hollowness of the temperature profiles assists the establishment of on-axis density peaking.This work was sponsored by the Office of Fusion Energy Sciences of the US Department of Energy at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy under Contract No. DE-AC05- 00OR22725