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Preparation and cooling of magnesium ion crystals for sympathetic cooling of highly charged ions in a Penning trap

T. Murböck

Published Jan 1, 2017• 2 citations
Publisher

Abstract

In this work, laser-cooled ion crystals containing 1000 to 100000 singly charged magnesium ions (Mg+) were prepared in a Penning trap. The properties of the ion crystals and their structure displaying long-range ordering were analyzed by various non-destructive techniques. After creation of the Mg + ions in the form of ion bunches in an external source, the ions were injected into the Penning trap where their temperature was reduced by eight orders of magnitude within seconds using a combination of buffer gas cooling and Doppler laser cooling. The achieved temperatures in the millikelvin-regime were close to the theoretical Doppler-cooling limit and sufficiently low to induce the transition to a crystal phase exhibiting long-range ordering. The structure of these mesoscopic ion crystals is in agreement with a model describing the crystal as a set of planar shells. This allows for a derivation of properties such as the charge density or the temperature of the observed crystals. For the process of combined buffer-gas and Doppler laser cooling an analytical model has been developed, which explains the time development of the temperature and the fluorescence signal in agreement with the experimental results.

DOI
10.26083/tuprints-00006037

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•Jan 1, 1994•0 citations
In this work, laser-cooled ion crystals containing 1000 to 100000 singly charged magnesium ions (Mg+) were prepared in a Penning trap. The properties of the ion crystals and their structure displaying long-range ordering were analyzed by various non-destructive techniques. After creation of the Mg + ions in the form of ion bunches in an external source, the ions were injected into the Penning trap where their temperature was reduced by eight orders of magnitude within seconds using a combination of buffer gas cooling and Doppler laser cooling. The achieved temperatures in the millikelvin-regime were close to the theoretical Doppler-cooling limit and sufficiently low to induce the transition to a crystal phase exhibiting long-range ordering. The structure of these mesoscopic ion crystals is in agreement with a model describing the crystal as a set of planar shells. This allows for a derivation of properties such as the charge density or the temperature of the observed crystals. For the process of combined buffer-gas and Doppler laser cooling an analytical model has been developed, which explains the time development of the temperature and the fluorescence signal in agreement with the experimental results.
We have studied the cooling dynamics, formation process, and geometric structure of mesoscopic crystals of externally produced magnesium ions in a Penning trap. We present a cooling model and measurements for a combination of buffer gas cooling and laser cooling which has been found to reduce the ion kinetic energy by eight orders of magnitude from several hundreds of eV to $\ensuremath{\mu}\mathrm{eV}$ and below within seconds. With ion numbers of the order of $1\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $1\ifmmode\times\else\texttimes\fi{}{10}^{5}$, such cooling leads to the formation of ion Coulomb crystals which display a characteristic shell structure in agreement with the theory of non-neutral plasmas. We show the production and characterization of two-species ion crystals as a means of sympathetic cooling of ions lacking a suitable laser-cooling transition.
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 have studied the formation and properties of two-species ion Coulomb crystals in the Penning trap of the SpecTrap experiment. These crystals have been formed by injection of admixture ions from an external source into a previously confined and laser-cooled cloud of magnesium ions. This kind of study, performed over a range of the admixture ions’ charge-to-mass ratios, indicates the conditions for their sympathetic cooling and the formation of two-species ion crystals. This mechanism allows efficient cooling of the admixed species such as highly charged ions which do not feature suitable laser-cooling transitions, and thus make them accessible to high-resolution laser spectroscopy.
We have determined the temperature and heating rate of laser-cooled ions in a Penning trap using Doppler laser spectroscopy. Between and ions are trapped in a Penning trap and Doppler laser cooled to temperatures of a few millikelvin, where they form ion crystals. This system is an example of a strongly coupled one-component plasma. The ion temperature was measured as a function of time after turning off the laser-cooling. In the solid phase, we measured a heating rate of . Information about possible heating mechanisms was obtained directly from temperature measurements, and also from measurements of the rate of radial expansion of the ion plasma. We determined that the observed heating is due to collisions with the residual gas of our vacuum system.
A detailed investigation of ion crystals produced by laser and sympathetic cooling in a linear RF trap has been conducted.\n\nThe laser cooling methods were examined and applied to the trapped ^24Mg^(positive) ions. The crystals produced by the laser cooling were studied, including the dependence on RF voltage, end cap DC voltage, laser power and laser frequency. By manipulating the different RF voltages and endcap DC voltages, the structure phase transition of the ion crystals was observed.\n\nIn addition, the sympathetic cooling of different ion species with the laser cooled 24Mg^(positive) was carried out. In this process, the mixed Mg^(positive) and He^(positive) crystals were created andidentified, and mixed Mg^(positive) and H2^(positive) crystals were produced. The effect of an unwanted chemical reaction of Mg^(positive) and H2 was observed and minimized. After sympathetic cooling of light ion species, the sympathetic cooling of heavy molecular ions such as fullerene ions was also carried out. The efficiencies and final temperature in both cases are very different. Theoretically to interpret the results, molecular dynamics simulations of the laser cooling and sympathetic cooling were implemented. And the simulations were compared with the experimental results.\n\nIn the process of carrying out this research, the optics were rebuilt to provide reliable UV sources for the photoionization and laser cooling of Mg ions. The imaging\nsystem was reconfigured to take the images of ion crystals. New elements were added tin the ion trap to improve the ability to manipulate ions.
This thesis describes experimental work performed on laser cooled magnesium ions, confined in a quadrupole ion trap operating in the Penning or the combined mode. Information is gained about the dynamical motion of the ions. This is done by analyzing the time delays between scattered fluorescence photons in the case of small ion clouds (generally less than ten ions). For larger clouds (upwards of 100 ions), a fraction of the ions is "tagged" and a subsequent study made of the time evolution of the fluorescence. We have developed an innovative, non-invasive method for measuring ion oscillation frequencies which is based on analyzing photon-photon correlations present in the emitted ion fluorescence. This technique has been used to measure all three Penning trap frequencies for a wide range of dc voltage. The data have been fitted to theory and we have obtained a global fit of these data to one set of trap parameters. We also demonstrate that our technique enables us to measure space-charge shifted values of the trap resonances. This method has also been applied to the combined trap and we report experimental verification of the predicted trap resonances for a wide range of trapping parameters. These data fit well to theory. This is for all three oscillation frequencies for one set of trap parameters consistent with the data for the Penning trap. \nFurther experiments are reported concerned with the dynamics of larger ion clouds in a Penning trap. A pulsed laser is used to "tag" a small number of the ions in large ion clouds. This removes those ions from the cooling cycle by placing them in a metastable state. Subsequently, we have monitored the time evolution of the fluorescence. By analyzing these data we have been able to determine cloud rotation frequencies for large ensembles of ions.