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Physical Review A

Loading of a continuous anion beam into a Penning trap with a view to laser cooling

G. Cerchiari, Stefan Erlewein, C. M. König, A. Kellerbauer

Published Aug 14, 2018• 3 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.98.021402

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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 Au\ensuremath−{\mathrm{Au}}^{\ensuremath{-}}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 107Au\ensuremath−{10}^{7}\phantom{\rule{0.16em}{0ex}}{\mathrm{Au}}^{\ensuremath{-}}107Au\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.
We discuss the possibility of using optical dipole forces for Sisyphus cooling of ions stored in a Penning trap by addressing the specific case of the molecular cooling candidate ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$. Using a GPU accelerated code for Penning trap simulations, which we extended to include the molecule-light interaction, we show that this scheme can decrease the time required for cooling by an order of magnitude with respect to Doppler cooling. In our simulation we found that a reduction of the axial anion temperature from 10K10\phantom{\rule{0.28em}{0ex}}\mathrm{K}10K to 50mK50\phantom{\rule{0.28em}{0ex}}\mathrm{mK}50mK in around 10s10\phantom{\rule{0.28em}{0ex}}\mathrm{s}10s is possible. The temperature of the radial degrees of freedom was seen to thermalize to 150mK150\phantom{\rule{0.28em}{0ex}}\mathrm{mK}150mK. Based on the laser-cooled ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$, a study on the sympathetic cooling of anions with masses 1--50 nucleon was performed, covering relevant candidates for investigations of chemical anion reactions at ultracold temperatures as well as for antimatter studies.
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.
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.
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.
We have performed laser cooling of Mg+{}^{+}+ ions confined in a Penning trap. The externally produced ions were captured in flight, stored, and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single-ion level together with the electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number, and ion temperature. Evidence for the formation of ion crystals has been observed. These investigations are an important prerequisite for the sympathetic cooling of simultaneously stored highly charged ions and the precision laser spectroscopy of forbidden transitions in these.
We report the first demonstration of the axialization of laser cooled ions in a Penning trap. Axialization involves the application of a small radial quadrupole drive which couples the cyclotron and magnetron motions. This enhances the laser cooling, allowing tighter confinement of the ions to the central axis of the trap than is otherwise possible. Using an intensified charge-coupled device (ICCD) camera we have imaged the axialization process for the first time. For a single ion, we recorded a large decrease of the magnetron amplitude corresponding to a reduction in ion temperature of approximately 2 orders of magnitude to an upper limit of order 10 mK. We have discovered dynamics specific to the laser cooled system which depend critically on the axial drive frequency and amplitude.