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Bulletin of the American Physical Society

Electrostatic modes as a diagnostic in Penning trap experiments

Carl S. Weimer, J. J. Bollinger, David J. Wineland, Fred L. Moore

Published May 1, 1993• 0 citations
OpenAlex

Abstract

A subset of electrostatic modes of a cold cloud of electrons trapped in a Penning trap has been observed and identified using a recent theoretical model. The detection of these modes is accomplished using electronic techniques which could apply to any ion species. The modes are observed in the low density, low rotation limit of the cloud where the electrons approach a two-dimensional charged disk. We observe both axially symmetric and asymmetric drumhead modes. By measuring the frequency of these modes the shape, rotation frequency, and density of the cloud is found in a real-time non-destructive manner. In addition, it is found that radio-frequency sideband cooling compresses the cloud increasing its density. The ability to measure and control the density of a trapped ion cloud might be useful for experiments on low-temperature ion/neutral collisions or recombination rates.

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A subset of electrostatic modes of a cold cloud of electrons trapped in a Penning trap has been observed and identified using a recent theoretical model. The detection of these modes is accomplished using electronic techniques which could apply to any ion species. The modes are observed in the low density, low rotation limit of the cloud where the electrons approach a two-dimensional charged disk. We observe both axially symmetric and asymmetric drumhead modes. By measuring the frequency of these modes the shape, rotation frequency, and density of the cloud is found in a real-time non-destructive manner. In addition, it is found that radio-frequency sideband cooling compresses the cloud increasing its density. The ability to measure and control the density of a trapped ion cloud might be useful for experiments on low-temperature ion/neutral collisions or recombination rates.
We describe some recent experiments that use a new Penning trap to trap and laser‐cool more than 105 Be+ ions. In this wrok the Coulomb coupling parameter was estimated to by typically greter than 100 and in some cases greater than 300. Shell structure was difficult to observe in the ion fluorescence from these large clouds. An initial Bragg scattering experiment to obtain information on the ion correlations is described. We also briefly review our work on the electrostatic modes of a Penning trap plasma including the use fo the modes to diagnose the shape and density of a cryogenic electron plasma. Some applications of laser‐cooled plasmas in Penning traps, such as atomic clocks, are discussed.
We describe some recent experiments that use a new Penning trap to trap and laser‐cool more than 105 Be+ ions. In this wrok the Coulomb coupling parameter was estimated to by typically greter than 100 and in some cases greater than 300. Shell structure was difficult to observe in the ion fluorescence from these large clouds. An initial Bragg scattering experiment to obtain information on the ion correlations is described. We also briefly review our work on the electrostatic modes of a Penning trap plasma including the use fo the modes to diagnose the shape and density of a cryogenic electron plasma. Some applications of laser‐cooled plasmas in Penning traps, such as atomic clocks, are discussed.
Externally, in an electron beam ion trap, generated Ar16+ ions were retrapped in a Penning trap and evaporatively cooled in their axial motion. The cooling was observed by a novel extraction technique based on the excitation of a coherent axial oscillation which yields short ion bunches of well-defined energies. The initial temperature of the ion cloud was decreased by a factor of more than 140 within 1 s, while the phase-space density of the coldest extracted ion pulses was increased by a factor of up to about 9.
Two-dimensional planar ion crystals in a Penning trap are a platform for quantum information science experiments. However, the low-frequency planar modes of these crystals are not efficiently cooled by laser cooling, which can limit the utility of the drumhead modes for quantum information processing. Recently, it has been shown that nonlinear mode coupling can enhance the cooling of the low-frequency planar modes. Here, we demonstrate in numerical simulations that this coupling can be dynamically tuned by adiabatically changing the rotation frequency of the ion crystal during experiments. Furthermore, we show that this technique can, in addition, produce lower temperatures for the low-frequency planar modes via an adiabatic cooling process. This result allows cooling of the planar modes to sub-millikelvin temperatures, resulting in improved spectral resolution of the drumhead modes at experimentally relevant rotation frequencies, which is crucial for quantum information processing applications.
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.