Improving Particle Confinement in Inertial Electrostatic Fusion for Spacecraft Power and Propulsion By Carl C. Dietrich S.B. Aeronautics and Astronautics, MIT 1999 S.M. Aeronautics and Astronautics, MIT 2003 SUBMITTED TO THE DEPARTMENT OF AERONAUTICS AND ASTRONAUTICS IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN AERONAUTICS AND ASTRONAUTICS AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY FEBRUARY 2007 ©2007 Carl C. Dietrich. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Signature of author: _______________________________________________________ Department of Aeronautics and Astronautics February 28, 2007 Certified by: _____________________________________________________________ Manuel Martinez-Sanchez Professor, Aeronautics and Astronautics Thesis Committee Chairman Certified by: _____________________________________________________________ Raymond J. Sedwick Principal Research Scientist Thesis Advisor Certified by: _____________________________________________________________ Ian Hutchinson Professor, Nuclear Science and Engineering Certified by: _____________________________________________________________ Oleg Batishchev Principal Research Scientist Accepted by: ____________________________________________________________ Jaime Peraire Professor, Aeronautics and Astronautics Chairman, Committee for Graduate Students (This page intentionally left blank) 2 Abstract Improving Particle Confinement in Inertial Electrostatic Fusion for Spacecraft Power and Propulsion By Carl C. Dietrich Fusion energy is attractive for use in future spacecraft because of improved fuel energy density and reduced radioactivity compared with fission power. Unfortunately, the most promising means of generating fusion power on the ground (Tokamak based reactors like ITER and inertial confinement reactors like NIF) require very large and heavy structures for power supplies and magnets, in the case of magnetic confinement, or capacitors and lasers in the case of inertial confinement. The mass of these reactors and support equipment is sufficiently large that no existing or planned heavy-lift vehicle could launch such a reactor, thereby necessitating in-space construction which would substantially increase the cost of the endeavor. The scaling of Inertial Electrostatic Confinement (IEC) is such that high power densities might be achievable in small, light-weight reactors, potentially enabling more rapid, lower cost development of fusion power and propulsion systems for space applications. The primary focus of the research into improving particle and energy confinement in IEC systems is based on the idea of electrostatic ion focusing in a spherically symmetric gridded IEC system. Improved ion confinement in this system is achieved by the insertion of multiple concentric grids with appropriately tailored potentials to focus ion beams away from the grid wires. In order to reduce the occurrence of charge exchange and streaming electron power losses, the system is run at high vacuum. This modification to the usual approach was conceived of by Dr. Ray Sedwick and computational modeling has been conducted by Tom McGuire using a variety of custom and commercial codes. In this thesis, a semi-analytic model of the potential structure around a multi-grid IEC device is developed. A 1-D paraxial ray ion beam envelope approximation is then used along an equatorial beamline and the assumed beam density is gradually increased until an effective beam space charge limit is reached at which point the potential fusion output is calculated. Significant use of the commercial particle-in-cell code OOPIC was made, and its ability to predict multi-grid IEC confinement properties is evaluated. An experiment was built to confirm the effectiveness of the multiple-grid structure to improve ion confinement times. It is shown that the multi-grid IEC can improve ion confinement time over the conventional, 2-grid IEC device. The PIC predicted ion bunching mode is also seen in experiment. 3 Acknowledgements There are so many people who have helped guide and support me in my (extended) time at MIT. I would like to take this opportunity to thank a few of them. First, to my parents, MaryAnn and Charles Dietrich, who gave me the amazing opportunity to study at MIT and focus on exactly what I wanted to do. Thank you for your continual support of me in all my endeavors. To the members of my thesis committee, Prof. Manuel Martinez-Sanchez, Prof. Ian Hutchinson, Dr. Oleg Batishev, and to my thesis readers, Professor Paulo Lozano and Professor Jack Kerrebrock: thank you all for your invaluable guidance and insight over the years. To Paul Bauer, thank you for your many hours of help in the lab. To my coworkers, Tom McGuire, Noah Warner, Simon Nolet, and Dr. Sam, thank you all for your friendship and support. To my wife, Anna, thank you for being part of my life, and encouraging me in all ways. I love you. Finally, I want to particularly thank my thesis advisor, Dr. Raymond Sedwick. Without Ray’s unwavering support and willingness to put up with my occasional stubbornness, this work would never have been done. Thank you for all of your help and guidance over the past six years. 4 Table of Contents Abstract............................................................................................................................... 3 Acknowledgements............................................................................................................. 4 Table of Contents................................................................................................................ 5 Table of Figures .................................................................................................................. 7 1. Introduction............................................................................................................... 10 1.1. Background....................................................................................................... 10 1.2. Rationale Behind Current Investigations.......................................................... 14 1.2.1. Ion Confinement ....................................................................................... 14 1.2.2. OOPIC Modeling...................................................................................... 18 1.3. Scaling of UHV IEC Devices ........................................................................... 19 1.3.1. Derivation ................................................................................................. 19 1.3.2. Numerical Validation of Scaling .............................................................. 20 2. Experiment Development ......................................................................................... 22 2.1. Confinement Time Detection Techniques ........................................................ 23 2.1.1. Measurement of current to the grid wires................................................. 23 2.1.2. Destructive Beam Dump........................................................................... 24 2.1.3. Capacitive Detection of Trapped Charge.................................................. 26 2.1.4. Laser Induced Fluorescence...................................................................... 27 2.2. Sources of Error ................................................................................................ 28 2.2.1. Earth’s Magnetic Field.............................................................................. 28 2.2.2. Secondary Electron Emission ................................................................... 31 2.2.3. Modeling Uncertainties............................................................................. 32 2.2.4. Equipment Limitations.............................................................................. 33 2.3. Selection of Primary Diagnostic ....................................................................... 36 2.3.1. Diagnostic Comparison............................................................................. 36 2.4. Hardware Development .................................................................................... 38 2.4.1. Vacuum Chamber Retrofitting.................................................................. 38 2.4.2. Grid Fabrication........................................................................................ 40 2.4.3. Ion Source................................................................................................. 46 2.4.4. Ion Detector.............................................................................................. 49 3. Computational Modeling for Design ........................................................................ 52 3.1. Semi-analytic potential model for a multi-grid IEC device.............................. 52 3.1.1. Derivation ................................................................................................. 52 3.1.2. Limitations and Inaccuracies .................................................................... 58 3.2. 1-D Paraxial Ray Equation Approximation...................................................... 62 3.2.1. Model Description.................................................................................... 62 3.2.2. Numerical Integration of Beam Envelope ...............................................