A HIGH-POWER LASER ABLATION ION SOURCEFOR PENNING TRAP STUDIES OF RARE ISOTOPESByDeborah Ann DaviesA DISSERTATIONSubmitted toMichigan State Universityin partial fulfillment of the requirementsfor the degree ofDOCTOR OF PHILOSOPHYDepartment of Chemistry2006ABSTRACTA HIGH-POWER LASER ABLATION ION SOURCE FOR PENNING TRAPSTUDIES OF RARE ISOTOPESByDeborah Ann DaviesA high-power laser ablation ion source has been developed, characterized and im-plemented as part of the Low Energy Beam and Ion Trap (LEBIT) experiment at theNational Superconducting Cyclotron Laboratory. LEBIT was developed to make pre-cision measurements of rare isotopes, including mass measurements with a Penningtrap. The system relies on thermalizing nuclear reaction products in a helium-filledcell and then extracting them from the gas through ion-manipulation and differen-tial pumping. Atomic ions and clusters are needed to calibrate various aspects of theentire system such as transport efficiency and the main magnetic field, in additionto understanding properties of the helium-filled gas cell. High-power laser ablationhas proven to be a successful method for producing a wide range of ions under vari-ous conditions, including atmospheric pressure. Laser ablation studies of C, Al, Ag,Au, Cu, Fe, and Zn were carried out in two separate chambers, one under vacuumconditions and one under buffer gas conditions, with the second harmonic, 532 nm,from a Q-switched Nd:YAG laser. A series of calculations were done to model theablation process and various ablation parameters of the system, and the ion trajec-tories of the ablated ions were modeled with the computer program SIMION. Manystudies were carried out under vacuum using an ion-drift system and mass analysisin a residual gas analyzer (RGA). The ablation target and laser optics were movedto the gas cell and several ablation studies were performed, including measurementsof the extraction time and the ion mobility for ions traveling through the gas cell.for AndrewiiiACKNOWLEDGMENTSThere are multiple people I would like to acknowledge for their contributionsmaking this dissertation possible. First, thank you to my advisor Dave Morrissey foryour guidance, support, and allowing me to work independently, while always beingavailable to help. I am also grateful for your patience.The NSCL has been a fantastic place to work, particularly with such an excellentstaff. Thank you especially to Jack Ottarson and Renan Fontus for your design work,help getting everything assembled, and continued interest and assistance. The facultyand staff of the NSCL are inspiring, constantly pursuing excellence.Thank you also to my second reader Paul Mantica, particularly for your encourage-ment and listening ear during my start in grad school. Also, thanks to my committeemembers Gary Blanchard and Lynmarie Posey for sharing laser wisdom, so I wouldn’thave to “reinvent the wheel.”I would also like to thank my undergraduate thesis advisor, Niva Tro, for theresearch experience, the encouragement to attend graduate school, and the advicealong the way.Thank you to my research group and fellow students. Elaine and Chandana, I’veappreciated your friendship, in addition to your willingness to listen to multiple prac-tice talks and for your help along the way. Thanks to former group member Pat Lofyfor all your help with getting started and answering my endless questions. GeorgBollen and the LEBIT group deserve a lot of thanks as well. Thanks especially toStefan, Ryan and Pete for patiently answering my questions and helping with calcu-lations. I would especially like to acknowledge my fellow graduate students for theirfriendship. Thanks Michelle for the ballet and Orchesis memories and Heather for ourconversations and fun outings. Thanks also to Jeremy, Sean, Bryan, and Jon.Thanks to my friends both here in Michigan and far away and my family forivyour encouragement and support. Thanks especially to my parents for your love andalways being there through everything. To my husband, Andrew, I would not havemade it through without your love and endless patience and encouragement. Finally,thanks to God for giving me the strength to run this race.vContents1 Introduction 11.1 Precision Mass Measurements . . . . . . . . . . . . . . . . . . . . . . 21.2 The NSCL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.1 The Gas Stopping Station and LEBIT Overview . . . . . . . . 41.3 The Gas Cell Laser Ablation System Overview . . . . . . . . . . . . . 51.3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3.2 Laser Ablation Definition and Brief History . . . . . . . . . . 71.3.3 Laser Ablation Systems at Other Facilities . . . . . . . . . . . 81.3.4 Introduction to the Laser Ablation System . . . . . . . . . . . 91.4 Organization of Dissertation . . . . . . . . . . . . . . . . . . . . . . . 102 Laser Ablation Theory and Calculations 122.1 Processes Involved in Laser Ablation . . . . . . . . . . . . . . . . . . 132.2 Evaporation Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.1 Thermal Conduction Calculation Results . . . . . . . . . . . . 172.2.2 Threshold Fluence for Ablation . . . . . . . . . . . . . . . . . 222.2.3 Surface Temperature Approximation . . . . . . . . . . . . . . 252.3 Photon Absorption Mechanism . . . . . . . . . . . . . . . . . . . . . 262.3.1 Plasma Screening . . . . . . . . . . . . . . . . . . . . . . . . . 272.4 Plasma Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.4.1 Plasma Properties . . . . . . . . . . . . . . . . . . . . . . . . 292.4.2 Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.4.3 Plume Length . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.4.4 Plume Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . 352.5 Laser Ablation Theory and Calculations: Conclusion . . . . . . . . . 363 Ion Transport Calculations 373.1 SIMION Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1.1 SIMION Calculations of Laser Ablation in the High VacuumSystem . …