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Cooling The Stored Ions

Pradip Kumar Ghosh

Published Nov 23, 1995• 0 citations
Publisher

Abstract

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.

DOI
10.1093/oso/9780198539957.003.0005

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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 limitations to the achievement of low kinetic energies for laser cooling of single ions confined in electromagnetic traps are discussed. Sideband cooling of an ion in an rf (Paul) trap is reexamined including the effects of finite laser bandwidth and the energy of the rf micromotion. The micromotion is the oscillatory motion of the ion at the same frequency as the rf voltage applied to the trap electrodes. Sideband cooling of ions in a Penning trap is examined for the first time. In both cases, cooling to the zero-point energy of the ion in the trap should be possible and a method for verifying this condition is suggested. The implications for high-resolution, high-accuracy spectroscopy are investigated. Under certain conditions, the uncertainty in the second-order Doppler shift may be significantly less than 1 part in 1018{10}^{18}1018. .AE
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.
Abstract One of the most remarkable achievements of laser control is the manipulation of atomic particles, their velocities, positions, etc., which is referred to as laser cooling and laser trapping . These two effects are interrelated and usually prove effective when used jointly. That this is so one can see by considering as an example the laser cooling of trapped ions and the laser trapping of laser-cooled neutral atoms (Fig. 5.1). The sequences of processes in these two situations are directly opposite. Ions can easily be trapped in an electromagnetic trap of large depth (Paul 1990) and then be cooled for a long time by laser radiation (sideband cooling) (Wineland and Dehmelt 1975; Dehmelt 1990). Neutral atoms are first slowed down and cooled by laser radiation down to millikelvin temperatures, and only then confined in an optical trap. Trapped atoms can further be cooled to microkelvin temperatures. The present chapter and Chapter 6 consider the cooling and trapping of neutral atoms, the present chapter concentrating on their cooling, and Chapter 6 on trapping. But it would be reasonable first to familiarize the reader with the history of the early ideas in this field that eventually led to the advent of a new domain of physics—the physics of ultracold atoms and molecules (Chapters 7 and 8).
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.
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.
Radiation pressure from lasers has been used to cool and compress 9Be+ ions stored in a combination of static electric and magnetic fields (Penning trap) to tem eratures less than 10 mK and densities greater than lo7 cm-q in a magnetic field of 1.4 T. A technique called sympathetic cooling can be used to transfer this cooling and compression to other ion species. ions sympathetically cooled by laser cooled 9Be+ ions is given. source vi? sympathetic cooling is also discussed. A n example of lg8Hg+ The possibility of making an ultracold positron Ion traps have been used in low energy atomic physics experiments for a period of roughly 30 years.1*2) years, however, the technique of laser cooling and compression has greatly increased the potential use of stored ions in a number of applications. One example is time and frequency standards. With the technique of laser cooling and compression, ion temperatures less than 10 mK, densities a factor of 10 less than the Brillouin limit (defined below), and essentially indefinite confinement times have been obtained. 3,4) Unfortunately, elementary charged particles (electrons, positrons, protons, etc.) can not be directly laser During the past 10