Convective Power Loss Measurements in a Field Reversed Configuration with Rotating Magnetic Field Current Drive Paul Melnik A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2014 Reading Committee: Alan L. Hoffman, Chair Richard D. Milroy Brian A. Nelson Program Authorized to Offer Degree: Department of Aeronautics and Astronautics ©Copyright 2014 Paul Melnik University of Washington Abstract Convective Power Loss Measurements in a Field Reversed Configuration with Rotating Magnetic Field Current Drive Paul Melnik Chair of the Supervisory Committee: Professor Alan L. Hoffman Department of Aeronautics and Astronautics The Translation, Confinement, and Sustainment Upgrade (TCSU) experiment achieves direct formation and sustainment of a field reversed configuration (FRC) plasma through rotating magnetic fields (RMF). The pre-ionized gas necessary for FRC formation is supplied by a magnetized cascade arc source that has been developed for TCSU. To ensure ideal FRC performance, the condition of the vacuum chamber prior to RMF start- up has been characterized with the use of a fast response ion gauge. A circuit capable of gating the puff valves with initial high voltage for quick response and then indefinite operational voltage was also designed. A fully translatable combination Langmuir / Mach probe was also built to measure the electron temperature, electron density, and ion velocity of the FRC. These measurements were also successfully completed in the FRC exhaust jets allowing for an accurate analysis of the FRC power loss through convection. TABLE OF CONTENTS Page Chapter I: Introduction to plasma confinement and the TCSU experiment .......................1 1.1 Introduction..............................................................................................................1 1.2 The field reversed configuration ..............................................................................3 1.2.1 FRC introduction ...........................................................................................3 1.2.2 FRTP formation .............................................................................................5 1.2.3 RMF formation and current drive..................................................................6 1.3 The Translation, Confinement, and Sustainment Upgrade Experiment ..................7 Chapter II: Fast response ion gauge (FIG)........................................................................14 2.1 FIG design and operation.......................................................................................14 2.2 TCSU puff valve analysis ......................................................................................18 Chapter III: Magnetized cascade arc source .....................................................................23 3.1 Introduction............................................................................................................23 3.2 Puff valve driver circuit .........................................................................................31 Chapter IV: Combination probe design and theory ..........................................................34 4.1 Combination probe design .....................................................................................34 4.2 Double Langmuir probe theory..............................................................................40 Chapter V: Combination probe on TCSU.........................................................................49 5.1 Probe installation on TCSU ...................................................................................49 5.2 Probe operation in TCSU.......................................................................................53 Chapter VI: Combination probe results ............................................................................62 6.1 FRC parameters for even and odd parity RMF operation......................................62 6.2 Combination probe data for the 122 kHz even-parity FRC ...................................67 6.2.1 FRC midplane results..........................................................................................67 6.2.2 Results from the north exhaust jet (even-parity FRC).................................81 6.2.3 Results from the south exhaust jet (even-parity FRC).................................93 6.2.4 Additional analysis of the 122 kHz even-parity FRC................................101 6.3 Combination probe data for the 107 kHz odd-parity FRC...................................110 6.3.1 FRC midplane results.................................................................................110 6.3.2 Results from the south exhaust jet (odd-parity FRC) ................................115 Chapter VII: Summary and conclusions.........................................................................120 Bibliography ....................................................................................................................123 ACKNOWLEDGEMENTS I would first like to thank my wife Jane Kuramoto Melnik and my mother Louise Melnik for their continued support during my studies. This dissertation would not have been possible without the guidance and patience of my graduate advisor Prof. Alan Hoffman, and Prof. Richard Milroy whose insight has been invaluable. My sincerest thanks goes out to the entire RPPL staff whose knowledge and expertise made the TCSU experiment a success and this dissertation a reality. To George Vlases for helping understand the most complex problems and to Bob Brooks who can solve any challenge no matter how daunting. Houyang Guo and James Grossnickle for helping me operate the machine. To Kennith Miller for helping simplify difficult engineering designs. Terry Dehart and Dan Lotz without whom the combination probe would not have been possible. Jon Hayward, Scott Kimball, Matthew Fishburn, and Mike Dellinger are all experts at what they do and kept the TCSU experiment running. I would also like to thank Dennis Peterson and Dzung Tran at the Guggenheim machine shop who can make complicated designs a reality and are quick to spot potential improvements. And a special thanks goes out to George Votroubek, Chris Deards, Kiyong Lee, Aydin Tankut, and Katherine Velas who have made RPPL the great place that it was. 1 Chapter I: Introduction to Plasma Confinement and the TCSU Experiment 1.1 Introduction Thermonuclear fusion will potentially be the Earth’s energy source of the future. Fusion of the hydrogen isotopes, deuterium and tritium, has the lowest ignition temperature which means it requires the lowest amount of input energy for a reaction to occur. Hydrogen in the most abundant element in the universe and it is very plentiful on Earth so the typical fusion reactants will always be available and in almost limitless quantities. For a D-T fusion reaction to occur a deuterium ion must collide with a tritium ion with enough energy to overcome their electrostatic repulsion. The P-P solar fusion chain is the process that powers all of the stars in the universe, including the Sun. The appreciable reaction cross-section in the Sun’s core allows for nuclear fusion to occur because an extremely high density of high temperature ions is confined for a long time by gravity. In a terrestrial based fusion experiment we do not have the luxury of gravity to confine the high temperature reactants so magnetic fields are used for steady state confinement. The most common magnetic confinement fusion concept is the tokamak [1], in which the high temperature, ionized gas called plasma is confined in a torus shape around a central conductor. The plasma in a tokamak is stabilized by a large toroidal magnetic field generated by magnet coils that surround the torus shaped vacuum chamber. The central conductor is a solenoid that is essentially the backbone of the magnetic configuration. The change of magnetic flux in the center solenoid induces a toroidal electric field via Faraday’s law which drives the plasma current. The plasma current is responsible for the generation of a poloidal magnetic field that confines the plasma and is necessary for an equilibrium in which the plasma pressure is balanced by the magnetic 2 forces. The poloidal field is also used to counteract the particle drifts inherent in toroidal plasma geometry. The doubly connected geometry of a tokamak is difficult to design and build and may not be the ideal confinement geometry of fusion plasma. To this end, a range of Innovative Confinement Concepts (ICC) such as the field reversed configuration have been explored. 3 1.2 The Field Reversed Configuration 1.2.1 FRC Introduction The simply connected plasma confinement concept known as the field reversed configuration (FRC) consists of an elongated azimuthal current ring confined by an axial magnetic field [2,3]. The cylindrical geometry of the FRC (r,θ,z coordinates) is a major advantage over other magnetic confinement concepts in that there is no vacuum boundary or magnetic field coil linking the hole of the plasma torus. This offers a beneficial reduction in the engineering complexity and constructions costs of the vacuum chamber and magnetic coils. The FRC also has a natural divertor so any particles lost are incorporated into the exhaust jets streaming away from the FRC at both ends. The effect of the natural divertor is advantageous is many ways; simply because the divertor plates can be located far from the FRC or, in a more ambitious,