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Journal of Modern Optics

The Temperature of Buffer-gas Colled Ions in a Paul Trap

M.D.N. Lunney, Fritz Buchinger, R.B. Moore

Published Feb 1, 1992• 41 citations
Publisher

Abstract

A measurement of the temperature of a typical ion cloud in a Paul trap with buffer gas cooling has been made using time-of-flight of the extracted ion cloud. A temperature of 0·3 ± 0·1 eV was obtained for 20 00039K ions in a trap of potential well depth 60 eV with helium buffer gas at 3 × 10−2 Pa. This value is intermediate by a factor of about 10 in each direction between that of other published works using different observational methods on Paul traps operated under similar conditions. Explanations for the discrepancies are suggested. In any case, the present work indicates that sub-electronvolt temperatures can be achieved, at least for small trap loadings. The result considerably eases the design of transport systems for such ion clouds and greatly enhances the possible uses of a Paul trap collection device for highly sensitive precise experiments.

DOI
10.1080/09500349214550341

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A measurement of the temperature of a typical ion cloud in a Paul trap with buffer gas cooling has been made using time-of-flight of the extracted ion cloud. A temperature of 0·3 ± 0·1 eV was obtained for 20 00039K ions in a trap of potential well depth 60 eV with helium buffer gas at 3 × 10−2 Pa. This value is intermediate by a factor of about 10 in each direction between that of other published works using different observational methods on Paul traps operated under similar conditions. Explanations for the discrepancies are suggested. In any case, the present work indicates that sub-electronvolt temperatures can be achieved, at least for small trap loadings. The result considerably eases the design of transport systems for such ion clouds and greatly enhances the possible uses of a Paul trap collection device for highly sensitive precise experiments.
Abstract Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.
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.
We describe and characterize an experimental apparatus that has been used to study interactions between ultracold lithium atoms and ytterbium ions. The preparation of ultracold clouds of Li atoms is described as well as their subsequent transport and overlap with Yb+{\mathrm{Yb}}^{+}Yb+ ions trapped in a Paul trap. We show how the kinetic energy of the ion after interacting with the atoms can be obtained by laser spectroscopy. We analyze the dynamics of the buffer-gas-cooled ion after releasing the atoms, which indicates that background heating, due to electric-field noise, limits attainable buffer gas cooling temperatures. This effect can be mitigated by increasing the density of the Li gas in order to improve its cooling power. Imperfections in the Paul trap lead to so-called excess micromotion, which poses another limitation to the buffer gas cooling. We describe in detail how we measure and subsequently minimize excess micromotion in our setup. We measure the effect of excess micromotion on attainable ion temperatures after buffer gas cooling and compare this to molecular dynamics simulations, which describe the observed data very well.
A single ion immersed in a neutral buffer gas is studied. An analytical model is developed that gives a complete description of the dynamics and steady-state properties of the ions. An extension of this model, using techniques employed in the mathematics of economics and finance, is used to explain the recent observation of non-Maxwellian statistics for these systems. Taken together, these results offer an explanation of the long-standing issues associated with sympathetic cooling of an ion by a neutral buffer gas.
Buffer gas cooling is the most effective and practical method to cool ions in ion trap.The kind and quantity of buffer gas are the key technologies in mercury ion microwave frequency standard experiments.Buffer gas made of helium,neon or argon was studied to cool trapped mercury ions(~(199)Hg~+) in a linear ion trap by introducing a resistance term in Mathieu's equation.It's found that the decay time of motion of ~(199)Hg~+ in argon gas is the shortest,and the frequency shift of the clock transition(40.5 GHz) is minimum when the pressure of helium is 10~(-5) Torr or the pressure of neon gas is 2.4×10~(-5) Torr.Neon is the most suitable buffer gas among helium,neon and argon,considering the decay time-constant of motion of ~(199)Hg~+ in buffer gases and ~(199)Hg~+ clock transition shift's sensitivity to the change of the pressure of buffer gas.