Production of Negative Hydrogen Ions in a High-Powered Helicon Plasma Source

Abstract

The production and extraction of negative hydrogen ions within plasma systems has a number of applications, the most prominent of which being the use of negative hydrogen ions in the high energy neutral beam injection systems used in the heating of plasmas in magnetically confined fusion devices such as the ITER tokamak. These applications require very high throughputs of negative ions which must be supplied by plasma sources capable of producing high densities of negative ions. There is presently a significant research interest in examining helicon sources as potential candidates for negative ion sources. Due to their high plasma density, low electron temperature, and high power efficiency, it is expected that helicon sources may offer significant advantages over existing negative ion source designs. Negative hydrogen ions are additionally understood to play a role in the detachment of plasmas in the divertor region of fusion reactors, with negative ions contributing to the molecular activated recombination process which acts as one of the mechanisms through which divertor detachment can occur. The characterisation of negative hydrogen ion production in high-density, divertor relevant plasmas is therefore also of interest in understanding divertor detachment processes.

In this thesis, we investigate the production of negative hydrogen ions and associated dynamics in the high-powered (20 kW) helicon plasma device MAGPIE developed at the Australian National University for the study of divertor relevant plasma material interactions. This investigation is performed both through simulation and direct experimental measurement.

We develop a 2D-axisymmetric fluid model of a hydrogen discharge in MAGPIE incorporating individual particle balance equations for each stable charged and ground-state neutral species, and the explicit calculation of each electron, ion, and neutral temperatures. The helicon power deposition profile is determined empirically by comparison with experimental measurements and existing full wave simulations of the antenna fields in MAGPIE. Transport is determined from classical magnetised diffusion assuming Maxwellian distributions for each species. The hydrogen chemistry is based on existing global models of hydrogen discharges with the inclusion of a number of previously overlooked reaction pathways. We observe very good agreement between simulation and experiment, although we note that the predictive capabilities of the model remain limited due to the empirical determination of the power deposition profile. Furthermore we demonstrate that observed experimental trends cannot be replicated without the inclusion of neutral depletion processes wherein neutral species are radially displaced from the central region of the discharge. Finally, we note that the presence of neutral species, in particular molecular hydrogen, is fundamental for the production mechanisms of negative hydrogen ions. The depletion and dissociation of molecular hydrogen is therefore highly deleterious for the production of negative hydrogen ions.

We also develop a Langmuir probe system and an associated probe based photodetachment system for the measurement of discharge properties including the direct measurement of negative ion densities. These systems are capable of taking both temporally and spatially resolved measurements throughout the MAGPIE chamber. To the author's knowledge this work represents the first direct measurement of negative hydrogen ion densities in a high-powered helicon source. We observe that the discharge evolves on three distinct time scales in the high-power regime. The initial breakdown and excitation of the helicon mode occurs on a rapid time scale of 100-200 microseconds with the bulk of the discharge already reaching within a factor of two or three of its steady state values. This is followed by a relatively slow axial propagation of a plasma channel as it expands into the neutral fluid on a time scale of 10-20 ms during which time the plasma density reaches its maximum value. Finally the overdense system equilibrates and the density relaxes to a lower steady-state value in the following few tens of ms. These time scales are highly dependent on fill pressure and we identify neutral dynamics as the driving factor in the latter two time scales. The initial state of the system is found to be consistent with a detached plasma and we observe that extremely high negative ion densities of ~10^18 m^-3 can be transiently produced in MAGPIE under these conditions, however these densities fall by in excess of an order of magnitude as the discharge approaches steady-state.

By combining the insights of these two investigations we demonstrate that neutral dynamics are fundamental to both the overall discharge properties and, in particular, the negative ion production in high-powered helicon devices. We discuss the implications of this and explore a variety of potential operating concepts for helicon based negative ion sources which might address the observed limitations of negative ion production in MAGPIE. We conclude that while MAGPIE would be unsuitable as a negative ion source, we have demonstrated that very high negative ion densities can be achieved in helicon devices, confirming that helicon based negative ion sources are potentially viable and should be investigated further in purpose built devices.