Production of Negative Hydrogen Ions in a High-Powered Helicon Plasma Source
Date
2018
Authors
Santoso, Jesse Soewito
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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.
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Keywords
Plasma, Helicon, Negative Ions, Hydrogen, Laser Photodetachment, Linear Plasma Device, Neutral Depletion, Fluid Model
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