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arXiv:1312.0547 · Physical Review A

Capture and isolation of highly charged ions in a unitary Penning trap

Samuel M. Brewer, Nicholas D. Guise, Joseph N. Tan

Published Dec 4, 2013• 21 citations
arXivHTMLPDF

Abstract

We recently used a compact Penning trap to capture and isolate highly charged ions extracted from an electron beam ion trap (EBIT) at the National Institute of Standards and Technology. Isolated charge states of highly stripped argon and neon ions with total charge $Q\ensuremath{\ge}10$, extracted at energies of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}Q$ eV, are captured in a trap with well depths of $\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}(4--12)Q$ eV. Here we discuss in detail the process to optimize velocity tuning, capture, and storage of highly charged ions in a unitary Penning trap designed to provide easy radial access for atomic or laser beams in charge exchange or spectroscopic experiments, such as those of interest for proposed studies of one-electron ions in Rydberg states or optical transitions of metastable states in multiply charged ions. Under near-optimal conditions, ions captured and isolated in such rare-earth Penning traps can be characterized by an initial energy distribution that is $\ensuremath{\approx}60$ times narrower than typically found in an EBIT. This reduction in thermal energy is obtained passively, without the application of any active cooling scheme in the ion-capture trap.

DOI
10.1103/physreva.88.063403

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We recently used a compact Penning trap to capture and isolate highly charged ions extracted from an electron beam ion trap (EBIT) at the National Institute of Standards and Technology. Isolated charge states of highly stripped argon and neon ions with total charge Q\ensuremath≥10Q\ensuremath{\ge}10Q\ensuremath≥10, extracted at energies of up to 4\ifmmode×\else\texttimes\fi103Q4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}Q4\ifmmode×\else\texttimes\fi103Q eV, are captured in a trap with well depths of \ensuremath≈(4−−12)Q\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}(4--12)Q\ensuremath≈(4−−12)Q eV. Here we discuss in detail the process to optimize velocity tuning, capture, and storage of highly charged ions in a unitary Penning trap designed to provide easy radial access for atomic or laser beams in charge exchange or spectroscopic experiments, such as those of interest for proposed studies of one-electron ions in Rydberg states or optical transitions of metastable states in multiply charged ions. Under near-optimal conditions, ions captured and isolated in such rare-earth Penning traps can be characterized by an initial energy distribution that is \ensuremath≈60\ensuremath{\approx}60\ensuremath≈60 times narrower than typically found in an EBIT. This reduction in thermal energy is obtained passively, without the application of any active cooling scheme in the ion-capture trap.
Highly-ionized atoms with special properties have been proposed for interesting applications, including potential candidates for a new generation of optical atomic clocks at the one part in 1019 level of precision, quantum information processing and tests of fundamental theory. The proposed atomic systems are largely unexplored. Recent developments at NIST are described, including the isolation of highly-ionized atoms at low energy in unitary Penning traps and the use of these traps for the precise measurement of radiative decay lifetimes (demonstrated with a forbidden transition in Kr17+), as well as for studying electron capture processes.
Abstract The ions trapped, whether in the Paul trap or in the Penning trap, usually have energies of the order of 1 eV, that is, a temperature of about 10 000 K. High temperature means high kinetic energy, and this is un favourable to most trap-based experiments for the following reasons. First, with high kinetic energy, amplitudes of ion oscillations are high and consequently the chances of ion losses are high; this results in loss of sensitivity. Second, high kinetic energy enhances both the first-order and the second-order Doppler effects; so, unless the ions are cooled, all spectroscopic experiments are adversely affected in a major way. While isolation of the ions in the trapped condition does not permit ion energy to be easily dissipated to the surroundings, for the same reason, the trapped ions can be effectively cooled and, once cooled, they can be retained cold in the trap. In recent years, most of the trap-based experiments have used ion cooling of one kind or the other. In this chapter we present an overview of various cooling methods and their applications.
The capture and confinement of a large and pure sample of anions into an ion trap is challenging due to the fragile nature of these weakly bound atomic systems. We present the results of an experimental study using ${\mathrm{Au}}^{\ensuremath{-}}$ in which anions from a keV-energy continuous beam were loaded into a cylindrical Penning trap. Experimental parameters such as stopping potential and axial trap depth were found to be crucial. The transfer of axial to radial momentum due to ion--ion collisions was identified as a main contributing factor to successful capture. A total number of up to ${10}^{7}\phantom{\rule{0.16em}{0ex}}{\mathrm{Au}}^{\ensuremath{-}}$ ions were loaded. The cooling of anions by simultaneously confined electrons was observed and seen to result in centrifugal separation of the two species. After storage times of several 10 s, electron cooling of the anions to the temperature of the cryogenic trap was achieved.
Externally, in an electron beam ion trap, generated Ar16+ ions were retrapped in a Penning trap and evaporatively cooled in their axial motion. The cooling was observed by a novel extraction technique based on the excitation of a coherent axial oscillation which yields short ion bunches of well-defined energies. The initial temperature of the ion cloud was decreased by a factor of more than 140 within 1 s, while the phase-space density of the coldest extracted ion pulses was increased by a factor of up to about 9.
A single ion at rest, unperturbed by its environment and forced into such a state for hours – once only a physicist's dream – has now been achieved by the combination of electromagnetic traps and laser technology. The Penning trap and the dynamical Paul trap developed in the 1930s and the late 1950s respectively, provide the experimenter with a unique tool to isolate a single ion from its surroundings. Tunable lasers can then be used to force the ion to fluoresce; simultaneously, as will be described, it is cooled to milli- or even micro-Kelvin temperatures. An ion driven into saturation by a sufficiently high laser intensity so that it spends half of the time in the excited state and half in the ground state, scatters roughly 108 photons per second. This leads to a high detection probability and at the same time to a reduction of the ion's kinetic energy via photon recoil.
We perform a detailed experimental study of resistive cooling of large ensembles of highly charged ions such as Ar13+{\mathrm{Ar}}^{13+}Ar13+ in a cryogenic Penning trap. Different from the measurements reported by Vogel et al. [M. Vogel et al., Phys. Rev. A 90, 043412 (2014)], we observe purely exponential cooling behavior when conditions are chosen to allow collisional thermalization of the ions. We provide evidence that in this situation resistive cooling time constants and final temperatures are independent of the initial ion energy and that the cooling time constant of a thermalized ion ensemble is identical to the single-ion cooling time constant. For sufficiently high ion number densities, our measurements show discontinuities in the spectra of motional resonances which indicate a transition of the ion ensemble to a fluidlike state when cooled to temperatures below approximately 14 K. With the final ion temperature presently being 7.5 K, ions of the highest charge states are expected to form ion crystals by mere resistive cooling, in particular not requiring the use of laser cooling.