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Journal of Physics B Atomic Molecular and Optical Physics

Capture of an external anion beam into a linear Paul trap

G. Cerchiari, Stefan Erlewein, P. Yzombard, M Zimmermann, A. Kellerbauer

Published Jan 23, 2019• 6 citations
Publisher

Abstract

Abstract We describe a linear Paul trap and high-voltage platform specifically designed for the trapping of a large and pure sample of anions from an external keV-energy beam. Capture and confinement in a static configuration of DC potentials was found to occur by transfer of axial to radial ion momentum due to elastic and inelastic collisions. Stability diagrams both in transmission and capture mode were recorded. The trapping efficiency was observed to be very sensitive to the axial potentials relative to the beam energy. Several 104 oxygen anions were loaded from a 2 keV primary beam and confined for several 100 s. Identification of trapped O− ions and contaminants was achieved by selective laser photodetachment.

DOI
10.1088/1361-6455/ab0089

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Abstract We describe a linear Paul trap and high-voltage platform specifically designed for the trapping of a large and pure sample of anions from an external keV-energy beam. Capture and confinement in a static configuration of DC potentials was found to occur by transfer of axial to radial ion momentum due to elastic and inelastic collisions. Stability diagrams both in transmission and capture mode were recorded. The trapping efficiency was observed to be very sensitive to the axial potentials relative to the beam energy. Several 104 oxygen anions were loaded from a 2 keV primary beam and confined for several 100 s. Identification of trapped O− ions and contaminants was achieved by selective laser photodetachment.
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.
In order to demonstrate laser cooling on negatively charged ions, it is necessary to confine the ions in a region free of strong magnetic fields. In the course of this thesis, the pre-existing design of a linear Paul trap was modified and taken into operation. The new design possesses two additional end cap electrodes whose shape provides effective axial confinement without blocking optical access to the trapped particles. The influence of these end cap electrodes on the trapping potential was simulated and compared to experimental results. Measurements with AU- ions show that the trap allows storage of ions for multiple hours. Analysis ofthe storage time suggests that the loading process heats up the trapped ions. The lifetime τHot of the hot ions was determined to be 23(3) min. After thermalization, the lifetime increases to 75(2) min. Measurements of the ion cloud’s radius provide an estimation of the equilibrium temperature T∞ ≈5000 K. The experimentally determined time constant of the cooling process is τcool = 41:6(4) s.
We present the single-particle stability diagram for the radial motion in a linear Paul trap in a situation where the applied axial dc potential gives rise to a harmonic defocusing radial potential. Although most linear Paul trap experiments have been conducted in a regime where this approximation is reasonably valid, the effect of the axial confinement on the stability of the radial motion has not previously been analyzed. The defocusing effect in both radial directions leads to a stability diagram different from that of the two-dimensional quadrupole mass filter, and hence points toward new studies of few ion dynamics. Expressions for the effective or pseudopotentials for one and two charged particles are presented and discussed.
We report the first cooling of atomic anions by laser radiation. O^{-} ions confined in a linear Paul trap were cooled by selectively photodetaching the hottest particles. For this purpose, anions with the highest total energy were illuminated with a 532 nm laser at their maximal radial excursion. Using laser-particle interaction, we realized a both colder and denser ion cloud, achieving a more than threefold temperature reduction from 1.15 to 0.33 eV. Compared with the interaction with a dilute buffer gas, the energy-selective addressing and removal of anions resulted in lower final temperatures, yet acted 10 times faster and preserved twice as large a fraction of ions in the final state. An ensemble of cold negative ions affords the ability to sympathetically cool any other negative ion species, enabling or facilitating a broad range of fundamental studies from interstellar chemistry to antimatter gravity. The technique can be extended to any negative ion species that can be neutralized via photodetachment.
In this work, we present the design, construction, and characterization of a linear Paul trap in a blade configuration. The trap was optimized to minimize micromotion and enable the formation of linear ion chains comprising tens of ions. We discuss the design requirements for low axial micromotion and reduced capacitance and describe the simulation and optimization of the trap design parameters. The assembled trap is characterized through direct measurements of the axial and radial confinement frequencies using the tickling method. We identify deviations from the target design due to manufacturing tolerances and present a compensation procedure to calibrate and correct for these effects.