X.-P. Huang

Precise control of the global rotation of strongly coupled ion plasmas in a Penning trap

Rotating asymmetric electric fields have been applied to control the rotation frequency (and hence the density) of non-neutral plasmas, which are confined in Penning-type traps and have relaxed close to thermal equilibrium characterized by a global rigid-body rotation. “Infinite” confinement times and density compression were first reported for uncorrelated plasmas of ∼108 Mg+ ions with temperatures ranging from 1 K to 5×104 K (4 eV) [Huang et al., Phys. Rev. Lett. 78, 875 (1997)]. In this paper, the rotating field technique has been applied to control strongly coupled plasmas of ∼105 9Be+ ions which are laser-cooled to millikelvin temperatures so that the plasma freezes into a solid with a crystalline lattice. Here, Bragg diffraction peaks from crystals provide an accurate way of measuring the rotation frequency, and it is observed that the plasma rotation can be phase locked to the applied rotating field without any slip. In essence, these corotating plasmas have reached thermal equilibrium with the rot...

Direct observations of the structural phases of crystallized ion plasmas

Laser-cooled 9Be+ ions confined in a Penning trap were directly observed, and the images were used to characterize the structural phases of the ions. With the ions in two-dimensionally extended lattice planes, five different stable crystalline phases were observed, and the energetically favored structure could be sensitively tuned by changing the areal density of the confined ions. Qualitatively similar structural phase transitions occur or are predicted to occur in other planar single-component systems with a variety of interparticle interactions. Closed-shell structures were observed with small ion clouds that were spherical or prolate, and crystals with long-range order were observed in the centers of clouds with large numbers of ions. These experimental results are in good agreement with theoretical predictions for the strongly coupled one-component plasma. Laser-cooled 9Be+ ions confined in a Penning trap were directly observed, and the images were used to characterize the structural phases of the ions. With the ions in two-dimensionally extended lattice planes, five different stable crystalline phases were observed, and the energetically favored structure could be sensitively tuned by changing the areal density of the confined ions. Qualitatively similar structural phase transitions occur or are predicted to occur in other planar single-component systems with a variety of interparticle interactions. Closed-shell structures were observed with small ion clouds that were spherical or prolate, and crystals with long-range order were observed in the centers of clouds with large numbers of ions. These experimental results are in good agreement with theoretical predictions for the strongly coupled one-component plasma.

Crystalline order in laser-cooled, non-neutral ion plasmas

Laser-cooled trapped ions can be strongly coupled and form crystalline states. In this paper we review experimental studies that measure the spatial correlations of Be+ ion crystals formed in Penning traps. Both Bragg scattering of the cooling-laser light and spatial imaging of the laser-induced ion fluorescence are used to measure these correlations. In spherical plasmas with more than 2×105 ions, body-centered-cubic (bcc) crystals, the predicted bulk structure, are the only type of crystals observed. The orientation of the ion crystals can be phase locked to a rotating electric-field perturbation. With this “rotating wall” technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating wall technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency standards.

Doppler imaging of plasma modes in a Penning trap.

We describe a technique and present results for imaging the modes of a laser-cooled plasma of 9 Be + ions in a Penning trap. The modes are excited by sinusoidally time-varying potentials applied to the trap electrodes. They are imaged by changes in the ion resonance fluorescence produced by Doppler shifts from the coherent ion velocities of the mode. For the geometry and conditions of this experiment, the mode frequencies and eigenfunctions have been calculated analytically. A comparison between theory and experiment for some of the azimuthally symmetric modes shows good agreement.

Mode and transport studies of laser-cooled ion plasmas in a Penning trap

We describe a technique and present results for imaging the modes of a laser-cooled plasma of 9Be+ ions in a Penning trap. The modes are excited by sinusoidally time-varying potentials applied to the trap electrodes, or by static field errors. They are imaged by changes in the ion resonance fluorescence produced by Doppler shifts from the coherent ion velocities of the mode. For the geometry and conditions of this experiment, the mode frequencies and eigenfunctions have been calculated analytically. A comparison between theory and experiment for some of the azimuthally symmetric modes shows good agreement. Enhanced radial transport is observed where modes are resonant with static external perturbations, such as those caused by misaligning the trap with respect to the magnetic field. Similarly, the plasma angular momentum can be changed through the deliberate excitation of azimuthally asymmetric modes. The resultant torque can be much greater than that from the “rotating wall” perturbation, which is not mo...

Crystalline order in strongly coupled plasmas

Laser-cooled trapped ions can be strongly coupled and form crystalline states. This manuscript reviews experimental studies which measure the spatial correlations of Be+ ion crystals formed in Penning traps. Both Bragg scattering of the cooling-laser light and spatial imaging of the laser-induced ion fluorescence are used to measure these correlations. In spherical plasmas with more than 2×105 ions, body-centered-cubic (bcc) crystals, the predicted bulk structure, are the only type of crystals observed. The orientation of the ion crystals can be phase-locked to a rotating electric-field perturbation. With this “rotating wall” technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating wall technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency standards.

Structure and Control of Coulomb Crystals in a Penning Trap

We apply rotating electric fields to ion plasmas in a Penning trap to obtain phase-locked rotation about the magnetic field axis. These plasmas, containing up to 1069Be+ ions, are laser-cooled to millikelvin temperatures so that they freeze into solids. Single body-centered cubic (bcc) crystals have been observed by Bragg scattering in nearly spherical plasmas with ≳ 2 × 105 ions. The detection of the Bragg patterns is synchronized with the plasma rotation, so individual peaks are observed. With phase-locked rotation, the crystal lattice and its orientation can be stable for longer than 30 min or ∼108 rotations.

Atomic ion crystals in non-neutral plasmas

Laser-cooled ions in a trap can be strongly coupled and form crystalline states. We describe experimental studies that measure the spatial correlations of the ion crystals formed in Penning traps. Both Bragg scattering of the cooling-laser light and spatial imaging of the laser-induced ion fluorescence are used to measure these correlations. In spherical plasmas with more than 2×105 ions, body-centered-cubic (bcc) crystals, the predicted bulk structure, are the only type of crystals observed. We are able to phase-lock the orientation of the ion crystals to a rotating electric-field perturbation. With this “rotating wall” technique and stroboscopic detection, images of individual ions in a Penning trap are obtained. The rotating-wall technique also provides a precise control of the time-dilation shift due to the plasma rotation, which is important for Penning trap frequency standards.