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Trapped Antihydrogen in Its Ground State Trapped Antihydrogen in Its Ground State A thesis presented by Philip John Richerme to The Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Physics Harvard University Cambridge, Massachusetts May 2012 ©2012 - Philip John Richerme All rights reserved. Thesis advisor Author Gerald Gabrielse Philip John Richerme Trapped Antihydrogen in Its Ground State Abstract Antihydrogen atoms (H) are confined in a magnetic quadrupole trap for 15 to 1000 s - long enough to ensure that they reach their ground state. This milestone brings us closer to the long-term goal of precise spectroscopic comparisons of H and H for tests of CPT and Lorentz invariance. Realizing trapped H requires charac- terization and control of the number, geometry, and temperature of the antiproton (p) and positron (e+) plasmas from which H is formed. An improved apparatus and implementation of plasma measurement and control techniques make available 107 p and 4 × 109 e+ for H experiments - an increase of over an order of magnitude. For the first time, p are observed to be centrifugally separated from the electrons that cool them, indicating a low-temperature, high-density p plasma. Determination of the p temperature is achieved through measurement of the p evaporation rate as their con- fining well is reduced, with corrections given by a particle-in-cell plasma simulation. New applications of electron and adiabatic cooling allow for the lossless reduction in p temperature from thousands of Kelvin to 3.5 K or colder, the lowest ever reported. The sum of the 20 trials performed in 2011 in which p and e+ mix to form H in the presence of a magnetic quadrupole trap reveals a total of 105 ± 21 trapped H, or 5 ± 1 per trial on average. This result paves the way towards the large numbers of simultaneously trapped H that will be necessary for laser spectroscopy. iii Contents Title Page . i Abstract . iii Table of Contents . iv Publications . viii Acknowledgments . ix Dedication . xii 1 Introduction 1 1.1 Precision Measurements with Trapped Antihydrogen . 2 1.1.1 Tests of CPT Symmetry . 2 1.1.2 Gravitational Studies . 8 1.2 A Brief Summary of Antihydrogen Research . 9 1.3 Overview of this work . 11 2 Apparatus 14 2.1 Penning Trap . 17 2.1.1 Theory . 17 2.1.2 Cylindrical Penning Traps . 19 2.1.3 Synchrotron cooling . 27 2.1.4 BTRAP Penning Trap . 28 2.2 Ioffe Trap . 31 2.2.1 Theory . 31 2.2.2 BTRAP Ioffe Trap . 35 2.3 Penning-Ioffe Trap . 39 2.3.1 Charged Particle Trajectories . 39 2.3.2 Non-neutral Plasma Confinement . 43 2.3.3 Antihydrogen Production . 44 2.4 Field-Boosting Solenoid . 46 2.5 X-Y Translation Stage . 47 2.6 1 K Pot . 49 2.7 Scintillating Detectors . 51 iv Contents v 3 Non-neutral Plasmas in a Penning Trap 55 3.1 Plasmas in a Cylindrical Trap . 56 3.2 Plasmas in Penning Traps . 61 3.2.1 Ideal Quadrupole Traps . 61 3.2.2 Non-Ideal Penning Traps . 63 3.3 Plasma Dynamics . 64 3.4 Collision Rates . 68 3.5 Plasma Characterization . 72 3.5.1 Number . 72 3.5.2 Mode Frequencies . 77 3.6 Plasma Control . 79 3.6.1 Cylindrically Symmetric Fields . 80 3.6.2 Rotating Wall . 81 4 Standard Methods 88 4.1 Antiproton Capture . 89 4.1.1 Beam Steering . 89 4.1.2 Antiproton Slowing . 91 4.1.3 Antiproton Trapping . 92 4.1.4 Field-Boosting Solenoid . 94 4.2 Electron Loading . 98 4.3 Antiproton Accumulation . 101 4.3.1 Electrons . 102 4.3.2 Antiproton Stacking . 104 4.3.3 Rotating Wall . 106 4.3.4 Electron Pulse-out . 108 4.3.5 Magnetic Field Reduction . 108 4.3.6 Accumulation Results . 110 4.4 Positron Accumulation . 111 4.5 Particle Transfer . 114 4.6 Antihydrogen Production . 118 4.6.1 Antihydrogen Formation Processes . 118 4.6.2 Antiproton-Positron Mixing . 121 4.6.3 Detection of Antihydrogen . 123 4.6.4 Trappable Antihydrogen . 124 5 Centrifugal Separation of Antiprotons and Electrons 127 5.1 Temperature and Length Scales for Centrifugal Separation . 128 5.2 Density Distribution for Multi-Species Plasmas . 130 5.3 Demonstration by Axial Potential Reduction . 133 5.4 Demonstration by Magnetic Field Reduction . 137 5.5 Summary and Discussion . 140 Contents vi 6 Embedded Electron and Adiabatic Cooling of Antiprotons 142 6.1 Plasma Temperature Measurements . 143 6.1.1 Measurement Procedure . 143 6.1.2 Antiproton Temperature Measurements . 148 6.1.3 Space Charge Correction . 150 6.1.4 Applications and Limitations . 154 6.2 Embedded Electron Cooling . 155 6.2.1 Cooling Efficiency . 157 6.2.2 Embedded Electron Number . 159 6.3 Adiabatic Cooling . 161 6.3.1 Single-Particle Picture . 161 6.3.2 Ideal Gas Picture . 162 6.3.3 Cooling Timescales . 163 6.3.4 Demonstration of Adiabatic Cooling . 164 6.4 Summary and Discussion . 167 7 Trapped Antihydrogen: Searches and Formation Methods 170 7.1 Searches for Trapped Antihydrogen . 171 7.2 Experimental Trials . 180 7.2.1 Noise Drives . 182 7.2.2 Coherent Chirped Drives . 184 7.2.3 Slow Chirped Drives . 187 7.2.4 Chirped Drives with Positron Well Sweep . 188 8 Mirror-Trapped Antiprotons 193 8.1 Mechanism . 194 8.2 Mirror-Trapping On-Axis . 195 8.3 Mirror-Trapping Off-Axis . 197 8.4 Antiproton Energy Scales . 200 9 Trapped Antihydrogen: Event Detection and Findings 204 9.1 Event Detection . 204 9.1.1 Event Classification . 205 9.1.2 Data Thresholding . 208 9.1.3 Monte Carlo Simulation . 211 9.2 Demonstration of Trapped Antihydrogen . 214 9.2.1 Individual Trials . 215 9.2.2 Combined Signal . 218 9.2.3 Ground-State Antihydrogen . 220 9.3 Antimatter Gravity . 222 9.4 Summary and Discussion . 224 Contents vii 10 Electrode Fabrication for an Improved Penning-Ioffe Trap 226 10.1 Electrode Design Constraints . 229 10.1.1 Octupole Field Profile . 229 10.1.2 Induced Currents . 230 10.1.3 Particle Manipulation . 231 10.2 Electrode Body . 232 10.3 Electrode Support Structure . 235 10.4 Penning Trap Assembly . 236 11 Conclusion 239 Bibliography 243 Papers and Publications 1. Efficient transfer of positrons from a buffer-gas-cooled accumulator into an orthogonally oriented superconducting solenoid for antihy- drogen studies D. Comeau, A. Dror, D. W. Fitzakerley, M. C. George, E. A. Hessels, C. H. Storry, M. Weel, D. Grzonka, W. Oelert, G. Gabrielse, R. Kalra, W. S. Koltham- mer, R. McConnell, P. Richerme, A. Müllers, and J. Walz. New J. Phys. 12, 045006 (2012). 2. Trapped Antihydrogen in Its Ground State G. Gabrielse, R. Kalra, W. S. Kolthammer, R. McConnell, P. Richerme, D. Grzonka, W. Oelert, T. Sefzick, M. Zielinski, D. W. Fitzakerley, M. C. George, E. A. Hessels, C. H. Storry, M. Weel, A. Müllers, and J. Walz. Phys. Rev. Lett. 108, 113002 (2012). 3. Adiabatic Cooling of Antiprotons G. Gabrielse, W.S. Kolthammer, R. McConnell, P. Richerme, R. Kalra, E. Novitski, D. Grzonka, W. Oelert, T. Sefzick, M. Zielinski, D. Fitzakerley, M.C. George, E.A. Hessels, C.H. Storry, M. Weel, A. Müllers, and J. Walz. Phys. Rev. Lett. 105, 073002 (2011). 4. Pumped Helium System for Producing 1.2 K Positrons and Electrons J. Wrubel, G. Gabrielse, W.S. Kolthammer, P. Larochelle, R. McConnell, P. Richerme, D. Grzonka, W. Oelert, T. Sefzick, M. Zielinski, J.S. Borbely, M.C. George, E.A. Hessels, C.H. Storry, M. Weel, A. Müllers, J. Walz, and A. Speck. Nuclear Instruments and Methods in Physics Research 640, 232 (2011). 5. Centrifugal Separation of Antiprotons and.
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