New Ion Trap for Frequency Standard Applications*

We have designed and built a nwel linear ion trap which permits storage of a large number of ions with reduced susceptibility to the second order Doppler effect caused by the BF conjbing fields. This new trap should store about 20 times the number of ions as a conventional RF trap with no corresponding increase in second order Doppler shift from the confining fleld. Other comparisons to etandard RF ion traps will be made. Introduction We have designed and constructed a hybrid RF/DC linear ion trap for use as a frequency standard. This new trap has about 20 times the storage volume as a conventional RF trap with hyperbolic electrodes with no corresponding increase in second order Doppler shift from the micro-motion induced by the RF trapping fields. Alternatively, if loaded with 2.10~ ions, the Doppler shift from the trapping fields is reduced by a factor of 10 below comparably loaded hyperbolic traps. Second Order Doppler Shift for Ions in an RF Trap Figure 1 shows a conventional RF ion trap along with the applied voltages. The trapping forces are generated by the driven motion of the ions (at frequency R) in the inhomogeneous electric field created by the trap electrodes[l]. Ions are trapped around the node point of the oscillating electric field at the center of the trap for certain trap voltages, ionic masses, etc. The motion in each of three directions for a single ion in an RF trap is characterized by two frequencies , the fast driving frequency Cl and a slower secular frequency o. An exact solution to the equations of motion shows that frequencies k 0 k w, k = 2, 3, ... are also present. However, in the limit o/n << 1 the w and R f w frequencies dominate and the kinetic energy of a particle, averaged over one cycle of R, separates into the kinetic energy of the secular motion and the kinetic energy of the driven motion. The average kinetic energy is transferred from the secular to the driven motion and back while the sum remains constant just as a harmonic oscillator transfers energy from kinetic to potential and back. The second order Doppler shift for a small and/or hot ion cloud, where interactions between ions are negligible is: 'This work represents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract sponsored by the National Aeronautics and Space Administration, Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE DEC 1988 2. REPORT TYPE 3. DATES COVERED 00-00-1988 to 00-00-1988 4. TITLE AND SUBTITLE New Ion Trap for Frequency Standard Applications 5a. CONTRACT NUMBER