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Physical Review Letters

Laser cooling of a stored ion beam to 1 mK

Jeff S. Hangst, M. Kristensen, J. S. Nielsen, Ove Poulsen, J. P. Schiffer, P. Shi

Published Sep 2, 1991• 115 citations
Publisher

Abstract

The interaction of laser-induced and intrabeam forces has been studied in a dense stored beam of 100-keV $^{7}\mathrm{Li}^{+}$ ions. A fraction of the ions (\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}4}$) exist in the metastable 1s2s $^{3}$S state. Using this state as a laser spectroscopic probe, we observe fast longitudinal heating in the injected, nonequilibrium distribution due to Coulomb scattering. Laser cooling of the metastable ions is ineffective during this heating period. The metastable fraction of the equilibrated beam is subsequently laser cooled to a longitudinal temperature of \ensuremath{\sim}1 mK, the lowest temperature ever reported in a stored ion beam.

DOI
10.1103/physrevlett.67.1238

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The interaction of laser-induced and intrabeam forces has been studied in a dense stored beam of 100-keV 7Li+^{7}\mathrm{Li}^{+}7Li+ ions. A fraction of the ions (\ensuremath{\sim}10\ensuremath−4{10}^{\mathrm{\ensuremath{-}}4}10\ensuremath−4) exist in the metastable 1s2s 3^{3}3S state. Using this state as a laser spectroscopic probe, we observe fast longitudinal heating in the injected, nonequilibrium distribution due to Coulomb scattering. Laser cooling of the metastable ions is ineffective during this heating period. The metastable fraction of the equilibrated beam is subsequently laser cooled to a longitudinal temperature of \ensuremath{\sim}1 mK, the lowest temperature ever reported in a stored ion beam.
A longitudinal laser cooling of ion beams at about 5% of the velocity of light has been performed at the Heidelberg Test Storage Ring with various cooling schemes employing the spontaneous force. For a 7.29-MeV 9Be+^{9}\mathrm{Be}^{+}9Be+ beam with an initial longitudinal temperature of 2700 K, the main characteristics of laser cooling in a storage ring are discussed. When undamped, the transverse betatron oscillations of the coasting ions limit the longitudinal temperature after laser cooling to typically 1 K. After damping the transverse motion by precooling the ions with an electron cooler, longitudinal temperatures of below 30 mK have been obtained in the subsequent laser cooling. In this case, the longitudinal ion-beam temperature can be understood as an equilibrium of the laser cooling rate with the heating rate due to intrabeam scattering. Moreover, single binary Coulomb collisions between the (still transversely hot) ions can cause such longitudinal velocity changes that ions are lost out of the critical capture range of the laser cooling force. In these two ways, intrabeam scattering imposes a substantial limit on the temperature or number of laser cooled ions in a storage ring. In our experiments, this process presently limits the ratio between the density-dependent Coulomb energy and the longitudinal thermal energy spread to a value on the order of 1, where liquid rather than gaseous behavior of the ion beam is expected to set in.
We report the first observation of indirect resonantly enhanced transverse laser cooling of a bunched, stored ion beam of 40 keV 24Mg+^{24}\mathrm{Mg}^{+}24Mg+. The longitudinal velocity distribution and the horizontal beam size of the laser-cooled 24Mg+^{24}\mathrm{Mg}^{+}24Mg+ ion beam with 280 nm laser light were simultaneously observed by the use of standard fluorescence-based techniques. Keeping the operation point at (2.068, 1.105), the synchrotron tune (${\ensuremath{\nu}}_{s}$) was changed from 0.0376 to 0.1299. A strong decrease in the cooled horizontal beam size and a corresponding increase in the equilibrium cooled momentum spread were observed at the expected synchrobetatron resonance coupling condition of ${\ensuremath{\nu}}_{s}=0.068$, which is the first observation of a resonantly induced coupling for enhanced indirect transverse laser cooling of an ion beam, and is an important step towards creating a crystalline ground state of the beam.
A laser cooled ion crystal containing several hundred Ca+ ions has been stored in a linear Paul trap. Cooling is provided by a red detund laser at the 4S1/2−4P1/2 resonance transition. A second laser serves for repumping of those ions which decay from the excited 4P1/2 level to the metastable 3D3/2 state. The ions can be additionally excited by a third laser to a long lived metastable 3D5/2 energy level which decouples them from the cooling laser radiation. The light pressure acting upon the laser cooled ions pushes them into the direction of the laser beam. The ions in the metastable 3D5/2 state, however, do not experience any light pressure force and diffuse to the crystal side which points towards the cooling laser. Part of the crystal then appears dark. Depending on the number of ions in the crystal and on the fraction of the excited ions, the spatial separation of the ions in the metastable state and the ions in the cooling cycle can be as large as several hundred microns.
Laser cooling of ion beams generally operates only upon the longitudinal motion of an ion beam. In order to extend this powerful dissipative force to the transverse directions, we consider introducing a Wien filter in a cooing straight section. It is theoretically shown that longitudinal cooling within a Wien filter naturally develops a transverse (mostly horizontal) cooling effect as well. Molecular dynamics simulations demonstrate that this method enables us to reach an ultralow-temperature state of a stored ion beam.
We have produced a laser-cooled ion crystal in a linear Paul trap containing several hundred Ca+{\mathrm{Ca}}^{+}Ca+ ions. A portion of the ions can be excited by an additional laser to a long-lived metastable energy level that decouples them from the cooling laser radiation. The light pressure acting upon the remaining laser-cooled ions forces them into the direction of the laser beam, while the ions in the metastable state drift to the crystal side that points toward the cooling laser. Depending on the number of ions in the crystal and on the fraction of the excited ions, the spatial separation of the ions in the metastable state and the ions in the cooling cycle can be as large as several hundred micrometers.
The study of cold and controlled molecular ions is pivotal for fundamental research in modern physics and chemistry. Investigations into cooling molecular anions, in particular, have proven to be of key consequence for the production of cold antihydrogen, the creation, and study of anionic Coulomb crystals as well as in atmospheric research and astrochemistry. The commonly used anion cooling technique via collisions with a buffer gas is limited by the temperature of the used cryogenic cooling medium. Here, we demonstrate forced evaporative cooling of anions via a laser beam with photon energies far above the photodetachment threshold of the anion. We reach runaway evaporative cooling of an anionic OH−^{-}− ensemble from an initial temperature of 370(12) K down to 2.2(8) K. This corresponds to three orders of magnitude increase in the ions' phase space density approaching the near-strong Coulomb coupling regime. A quantitative analysis of the experimental results, via a full thermodynamic model including the role of intrinsic collisional heating, represents the anion cooling dynamics without any fitting parameters. This technique can be used to cool, in principle, any anionic species below liquid helium temperature, providing a novel tool to push the frontiers of anion cooling to much lower than the state-of-the-art temperature regimes.