Suitability of linear quadrupole ion traps for large Coulomb crystals
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Growing and studying large Coulomb crystals, composed of tens to hundreds of thousands of ions, in linear quadrupole ion traps presents new challenges for trap implementation. We consider several trap designs, first comparing the total driven micromotion amplitude as a function of location within the trapping volume; total micromotion is an important point of comparison since it can limit crystal size by transfer of radiofrequency drive energy into thermal energy. We also compare the axial component of micromotion, which leads to first-order Doppler shifts along the preferred spectroscopy axis in precision measurements on large Coulomb crystals. Finally, we compare trapping potential anharmonicity, which can induce nonlinear resonance heating by shifting normal mode frequencies onto resonance as a crystal grows. We apply a non-deforming crystal approximation for simple calculation of these anharmonicity-induced shifts, allowing a straightforward estimation of when crystal growth can lead to excitation of different nonlinear heating resonances. In the anharmonicity point of comparison, we find significant differences between the trap designs, with an original rotated-endcap trap performing better than the conventional in-line endcap trap.
KeywordsTrapping Potential Secular Frequency Normal Mode Frequency Trap Design Linear Trap
The authors gratefully thank Caroline Champenois, Eric Hudson, David Kielpinski, Joan Marler, Steven Schowalter, and Stephan Schiller for sharing their expertise in illuminating discussions. This work is sponsored by NSF Grant No. PHY-0847748 and by NSF IGERT Grant No. 0801685.