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

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Production of Negative Hydrogen Ions in a High-Powered Helicon Plasma Source Production of Negative Hydrogen Ions in a High-Powered Helicon Plasma Source Jesse Soewito Santoso A thesis submitted for the degree of Doctor of Philosophy of The Australian National University Plasma Research Laboratory Research School of Physics and Engineering The Australian National University July, 2018 This thesis is an account of research undertaken between January 2014 and July 2018 at the Research School of Physics and Engineering, ANU College of Physical and Mathematical Sciences, The Australian National University, Canberra, Australia. Except where acknowledged in the customary manner, the material presented in this thesis is, to the best of my knowledge, original and has not been submitted in whole or part for a degree in any university. Signed: Jesse Soewito Santoso Date: December 3, 2018 This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968 Signed: Jesse Soewito Santoso Date: December 3, 2018 \I believe that the human spirit is indomitable. If you endeavor to achieve, it will happen given enough resolve. It may not be immediate, and often your greater dreams is something you will not achieve within your own lifetime. The effort you put forth to anything transcends yourself, for there is no futility even in death." | Monty Oum (1981-2015) Dedicated to the family I picked up along the way. ABSTRACT The production and extraction of negative hydrogen ions within plasma sys- tems 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 neg- ative 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 characterisa- tion 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 as- sociated dynamics in the high-powered (20 kW) helicon plasma device MAGPIE developed at the Australian National University for the study of divertor rele- vant 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 MAG- PIE 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 deter- mined 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 hydro- gen 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 dis- placed from the central region of the discharge. Finally, we note that the presence of neutral species, in particular molecular hydrogen, is fundamental for the pro- duction 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 µs 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 ∼ 1018 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 dis- cuss the implications of this and explore a variety of potential operating concepts for helicon based negative ion sources which might address the observed limita- tions 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 he- licon based negative ion sources are potentially viable and should be investigated further in purpose built devices. ACKNOWLEDGEMENTS This thesis would not have been possible without the academic, technical, and personal support of countless colleagues and friends. First and foremost, I would like to thank my supervisor, Cormac Corr, whose instruction, understanding, and support have been invaluable over these past four and a half years. Whilst always happy to provide advice, he has offered almost complete autonomy to me in deciding the direction which my research has taken, allowing me to develop my abilities as a researcher, often { and potentially most valuably { through misadventure. His mentorship and friendship will be appreci- ated for many years to come. The staff of the Plasma Research Laboratory have been unreserved in their willingness to share their knowledge and experience, which has been instrumental in the progression of my research. In particular, I would like to thank Boyd Blackwell, Clive Michael, and Michael Blacksell. I would also like to thank Amy Shumack, Mark Gwynneth, Tom Kitchen, and Luke Materne, for their contributions to the technical aspects of this work. The friendship and camaraderie of my fellow students has also been invalu- able throughout this process. In particular it is necessary to acknowledge the contributions of Stuart Nulty, who assisted during the development of the pho- todetachment technique, and visiting student Hannah Willett who contributed both to my understanding of plasma detachment and to the development of the Langmuir probe techniques for high-power operation. I would also like to thank Sam Cousens, Alex Thorman, Matt Thompson, and Romana Lester. This work has built upon the enormous contributions of previous students from PRL. It is therefore necessary to acknowledge the giants upon whose shoulders I now stand: Cameron Samuell and Juan Caneses, whose early works on the charac- terisation of hydrogen plasmas in MAGPIE are referenced extensively throughout this thesis. During my research I have also had the great privilege to be involved in the education of physics students at the ANU. I am therefore grateful to Cormac Corr, Paul Francis, Craig Savage, Ben Buchler, C´edricSimenel, John Debs, and Edie Sevick, for extending me these opportunities over the years. I would also like to acknowledge the many wonderful students that have made teaching and mentoring such an enjoyable experience. Outside of academia, I would like to acknowledge the family that I have picked up along the way, mostly through my stay at Ursula Hall, who have made my time in Canberra so thoroughly enjoyable and worthwhile: Tom, Carmen, Marcus, Dylan, Victor, Suzy, Jeshka, Alex, Jo, Rowan, Jonah, Jackson, Michelle, Ebe, Samuel, and James, as well as countless others. I would also like to acknowledge the continued friendship of my brothers (and sister) in arms
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