Capture of highly charged ions in a pseudo-hyperbolic Paul trap

Abstract

The confinement of ions in a radio-frequency (RF) trap (also known as a Paul trap) has proven to be advantageous in many applications. In nearly all cases, singly- or few-times-ionized atoms are created in situ within the RF trap. Highly charged ions, on the other hand, are produced more efficiently in dedicated external sources; hence, the isolation of single highly charged species in an RF trap is more involved. In this work, highly charged ions produced by an electron beam ion trap/source are extracted in bunches via an ∼7 m long beamline, which is tuned to minimize the phase-space volume of the ion bunch. The charge-state-selected ion bunch is then captured in an RF trap constructed from cylindrically symmetric electrodes with pseudohyperbolic surfaces. The RF drive parameter space is surveyed both experimentally and computationally to investigate the dynamics and map out those regions favorable for ion capture. We find that an appreciable number of Ne10+ ions are captured using an RF frequency of 2.4 MHz and an amplitude range of 120 V–220 V, with an efficiency highly dependent on the RF field phase. An experimental capture efficiency of >20% was attained, with at least 500 ions being captured by the RF trap. This is slightly higher (∼135%) than that captured by a contiguous, compact Penning trap. However, in the absence of any cooling mechanism, the observed ion-storage lifetime in the RF trap is 69 ms, a factor of ∼30 shorter than in the Penning trap; potential improvements are discussed.