Trapped positrons for antihydrogen

Positrons from a 12 mCi²²Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e⁺/s is observed for 32 h in this initial demonstration. More than 1.6×10⁴ positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 10⁵ antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.     

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 10^{6 9}Be⁺ 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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trapping positrons for low energy antihydrogen production

A method of trapping large numbers of positrons at liquid helium temperatures in a 6 Tesla magnetic field is described. Positrons from a sodium-22 source are moderated to low energies with a tungsten reflection moderator. A Penning trap with hyperbolic electrodes holds the positrons in a magnetron (EXB) orbit. The positrons are then cooled via coupling to a tuned circuit that is in resonance with the axial oscillation of the positrons. At this point, many slow positrons are permanently trapped in the Penning trap. The positrons are centered in the trap by applying a radio-frequency field at a frequency near the sum of the axial and magnetron frequencies. This method promises to produce 10⁶ trapped positrons at a density of 10⁷ to 10⁸ per cm³. Such densities of positrons would be useful in producing antihydrogen in combination with existing antiproton plasmas.     

Precise spectroscopy for fundamental physics

We have applied experimental techniques that were developed for use in atomic frequency standards andclocks to investigations of local Lorentz invariance, the linearity of quantum mechanics, andanomalous long-range spin-dependent forces. These experiments used a hyperfine transition in⁹Be⁺ ions in a Penning trap. Recently, we have studied hyperfine transitions in¹⁹⁹Hg⁺ ions in a linear rf trap. Hg⁺ ions might be used for similar investigations in the future.Work of the National Institute of Standards andTechnology. Not subject to US copyright.     

Extremely cold positrons for antihydrogen production

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10⁻¹⁷ Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi²²Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×10⁶e⁺/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.     

Highly charged ion Coulomb crystallization in mixed strongly coupled plasmas

The investigation of highly charged ion Coulomb crystallization in mixed strongly coupled plasmas is of interest in many areas: white dwarf astrophysical plasmas are believed to have very similar thermodynamic properties, cold highly charged ions can be used as an object for high precision laser spectroscopy of fine and hyperfine transitions in the visible due to the absence of Doppler broadening and, an entirely new area of research is the potential application to highly charged ion based quantum computing schemes. We report the formation of such plasmas in a cryogenic Penning trap. These plasmas consisting of many species including Be⁺ and Xe⁴⁴⁺ or Be⁺ and Xe¹⁵⁺ ions, are formed at a temperature of less than 4 K. The temperatures were obtained by applying a laser based sympathetic cooling scheme. The determination of the temperature and density from the laser resonance width and the fluorescence imaging of the Be⁺ clouds, respectively, yields a Coulomb coupling constant for the centrifugally separated Xe plasma high enough for crystallization. A molecular dynamics code, developed just for this purpose, was run to clarify the understanding of these plasmas and it was possible to show consistency between experiment and simulation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Loading of large ion Coulomb crystals into a linear Paul trap incorporating an optical cavity

We report on the loading of large ion Coulomb crystals into a linear Paul trap incorporating a high-finesse optical cavity (?～3000). We show that, even though the 3-mm diameter dielectric cavity mirrors are placed between the trap electrodes and separated by only 12 mm, it is possible to produce in situ ion Coulomb crystals containing more than 10⁵ calcium ions of various isotopes and with lengths of up to several millimeters along the cavity axis. We show that the number of ions inside the cavity mode is, in principle, high enough to achieve strong collective coupling between the ion Coulomb crystal and the cavity field. The results thus represent an important step towards ion trap based Cavity Quantum ElectroDynamics (CQED) experiments using cold ion Coulomb crystals.     

GBAR

The GBAR project aims to perform the first test of the Equivalence Principle with antimatter by measuring the free fall of ultra-cold antihydrogen atoms. The objective is to measure the gravitational acceleration to better than a percent in a first stage, with a long term perspective to reach a much higher precision using gravitational quantum states of antihydrogen. The production of ~20 μK atoms proceeds via sympathetic cooling of $\mathrm{\overline{H}^+}$ ions by Be^?+? ions. $\mathrm{\overline{H}^+}$ ions are produced via a two-step process, involving the interaction of bursts of 10⁷ slow antiprotons from the AD (or ELENA upgrade) at CERN with a dense positronium cloud. In order to produce enough positronium, it is necessary to realize an intense source of slow positrons, a few 10⁸ per second. This is done with a small electron linear accelerator. A few 10¹⁰ positrons are accumulated every cycle in a Penning–Malmberg trap before they are ejected onto a positron-to-positronium converter. The overall scheme of the experiment is described and the status of the installation of the prototype positron source at Saclay is shown. The accumulation scheme of positrons is given, and positronium formation results are presented. The estimated performance and efficiency of the various steps of the experiment are given.     

Concept of a 6-T Penning trap at liquid-He temperature for the accumulation of positrons

High densities of ultra cold positrons are required for applications such as positronium production, scattering processes with atoms, surface analysis, cooling of highly charged ions and antihydrogen production. At the University of Aarhus, Denmark, an accelerator based slow positron source delivers about 5 × 10⁴ positrons within a 10 ns bunch at a repetition rate of 10 Hz. The energy spread is below 1 eV and the beam diameter is about 1 mm. The positron bunches shall be injected into a 6-T Penning trap at the temperature of liquid helium. The bunches can be captured at nearly 100% efficiency by a fast time variation of the trap potential. The cyclotron motion cools down by synchrotron radiation with a time constant of 80 ms. The axial motion can be cooled by coupling to the radial motion or by resistive cooling in a tuned circuit. By stacking of 100 pulses about 5 × 10⁶ positrons can be accumulated within 10 s. After this time most of the positrons have cooled down sufficiently that the trapping cycle can be started again. At the anticipated accumulation rate a positron plasma at the space charge limit should be obtainable within 1 h. This revised version was published online in July 2006 with corrections to the Cover Date.     

Developments at the SLOWRI facility at RIKEN: precision optical spectroscopy of 7,9,10,11Be+ ions

Precision optical spectroscopy of radioactive Be isotopes produced in projectile fragmentation has been performed for the first time at the prototype SLOWRI facility of RIKEN RI-Beam Factory. The ground state hyperfine constants of ⁷Be⁺ and ¹¹Be⁺ were determined with relative accuracies of 6 × 10^?7 and 3 × 10^?8, respectively, by laser-microwave double resonance spectroscopy of laser-cooled ions in a trap. The optical transition energies from the ground S-state to the excited P-state of Be isotope ions were also measured to determine the nuclear charge radii from the isotope shifts. Development of the universal slow RI-beam facility??SLOWRI??based on the rf-carpet ion guide technique is progressing at RIKEN RI-beam factory. An additional capability of providing parasitic slow RI-beams from the projectile fragment separator BigRIPS is also discussed.     

Using GPU parallelization to perform realistic simulations of the LPCTrap experiments

We performed classical molecular dynamics (MD) simulations in order to search the conditions for efficient sympathetic cooling of highly charged ions (HCIs) in a linear Paul trap. Small two-component ion Coulomb crystals consisting of laser-cooled ions and HCIs were characterized by the results of the MD simulations. We found that the spatial distribution is determined by not only the charge-to-mass ratio but also the space charge effect. Moreover, the simulation results suggest that the temperature of HCIs do not necessarily decrease with increasing the number of laser-cooled ions in the cases of linear ion crystals. We also determined the cooling limit of sympathetically cooled ¹⁶⁵Ho¹⁴⁺ ions in small linear ion Coulomb crystals. The present results show that sub-milli-Kelvin temperatures of at least 10 Ho¹⁴⁺ ions will be achieved by sympathetic cooling with a single laser-cooled Be⁺.     

Elaborate treatment of the inelastic collisions of positrons with alkali atoms and alkaline-earth positive ions using a restricted coupled-static approximation

Summary We present the results of a very elaborate investigation of the inelastic collisions of positrons with the ground states of the first four alkali atoms (³Li,¹¹Na,¹⁹K and³⁸Rb) and their electronically resemblant alkaline-earth positive ions,i.e. ⁴Be⁺,¹²Mg⁺,²⁰Ca⁺ and³⁹Sr⁺, respectively, using a restricted coupled-static approximation in which the reactance matrices are adjusted to be symmetrical. Our interest is concentrated on the calculation of the total elastic and positronium formation cross-sections and their behaviour in the low-energy region (below 10 eV for the atoms) or the regions above the positronium formation thresholds (when the ions are considered). Our results are the most accurate cross-sections expected from the approximation employed. They reveal, within their limits, the most important characteristics of these cross-sections and provide us with a satisfactory comparison between the main features of positron-atom and positron-ion collisions. Present address.     

A high-precision segmented Paul trap with minimized micromotion for an optical multiple-ion clock

We present a new setup to sympathetically cool ¹¹⁵In⁺ ions with ¹⁷²Yb⁺ for optical clock spectroscopy. A first prototype ion trap made of glass-reinforced thermoset laminates was built, based on a design that minimizes axial micromotion and offers full control of the ion dynamics in all three dimensions. We detail the trap manufacturing process and the characterization of micromotion in this trap. A calibration of the photon-correlation spectroscopy technique demonstrates a resolution of 1.1 nm in motional amplitude of our measurements. With this method, we demonstrate a sensitivity to systematic clock shifts due to excess micromotion of $|(\Updelta\nu/\nu)_{\rm mm}|=7.7\times10^{-20}$ along the direction of the spectroscopy laser beam. Owing to our on-board filter electronics on the ion trap chips, no rf phase shifts could be resolved at this level. We measured rf fields over a range of 400 μm along the ion trap axis and demonstrated a region of 70 μm where an optical frequency standard with a fractional inaccuracy of ≤1 × 10^?18 due to micromotion can be operated.     

A 750-mW,continuous-wave,solid-state laser source at 313 nm for cooling and manipulating trapped <Superscript>9</Superscript>Be<Superscript>+</Superscript> ions

We present a solid-state laser system that generates 750 mW of continuous-wave, single-frequency output at 313 nm. Sum-frequency generation with fiber lasers at 1550 and 1051 nm produces up to 2 W at 626 nm. This visible light is then converted to ultraviolet by cavity-enhanced second-harmonic generation. The laser output can be tuned over a 495-GHz range, which includes the ⁹Be⁺ laser cooling and repumping transitions. This is the first report of a narrow-linewidth laser system with sufficient power to perform fault-tolerant quantum-gate operations with trapped ⁹Be⁺ ions by use of stimulated Raman transitions.     

Storage of an electron plasma in a sextupole radial antihydrogen trap

This work reports for the first time experimental data obtained with electrons stored in a Penning–Malmberg trap surrounded by a sextupole radial magnetic field. This trap geometry is one of the candidates for trapping antihydrogen atoms in the place where they are produced starting from cold antiprotons and positrons or positronium. The measurements show that electron plasmas with parameters matching the range used for positrons and electrons in the antihydrogen experiments (number of particles ranging from few 10⁶ up to several 10⁷ and densities of the order of 10⁸–10⁹ cm⁻³, radius of the order of 1–2 mm) can be transported with 100% efficiency in a trap region that simultaneously confines completely the charged particles and the neutral antihydrogen in the radial plane. Inside this trap plasma storage times of the order of several tens of seconds up to some hundreds of seconds are measured. The plasma storage times are consistent with those needed for antihydrogen production; however the increase of the plasma temperature due to the expansion is not negligible; the consequences of this effect on the antihydrogen trapping are outlined.     

A thin wire ion trap to study ion–atom collisions built within a Fabry–Perot cavity

We report on the implementation of a thin wire Paul trap with tungsten wire electrodes for trapping ions. The ion trap geometry, though compact, allows large optical access enabling a moderate finesse Fabry–Perot cavity to be built along the ion trap axis. The design allows a vapor-loaded magneto-optical trap of alkali atoms to be overlapped with trapped atomic or molecular ions. The construction and design of the trap are discussed, and its operating parameters are determined, both experimentally and numerically, for Rb⁺. The macromotion frequencies of the ion trap for ⁸⁵Rb⁺ are determined to be f _r  = 43 kHz for the radial and f _z  = 54 kHz for the axial frequencies, for the experimentally determined optimal operating parameters. The destructive off axis ion extraction and detection by ion counting is demonstrated. Finally, evidence for the stabilization and cooling of trapped ions, due to ion–atom interactions, is presented by studying the ion-atom mixture as a function of interaction time. The utility and flexibility of the whole apparatus, for a variety of atomic physics experiments, are discussed in conclusion.     

Trapping of ions from high energy sources into a radiofrequency ion trap

An attempt is described to confine ions, created externally and accelerated to some energy, in an rf quadrupole trap. 4 keV Ba⁺ ions were stopped on a Ni foil, placed in an aperture of one trap electrode. The Ba then was evaporated from the heated foil and ionized by electron impact. At background pressure of about 10^–5 mbar of various light buffer gases (He, H₂, N₂), the trap was filled once with 10⁵ ions, at a minimum primary ion number of 10¹⁰. The storage time was 10 min. From the data obtained the possibility of spectroseopic experiments on rare isotopes, created with accelerators or nuclear reactors, is discussed.     

Positron lifetimes and distributions in the infinite-layer compound SrCuO2 and related materials

We have calculated distributions and lifetimes of positrons in the infinite-layer compound SrCuO₂ and those trapped at possible point defects therein. In the delocalized state, positrons show their density maxima at interstitial sites in the Sr planes and have a significant overlap also with Cu and O atoms. The corresponding positron lifetime is 149 ps. It has been revealed that the Sr vacancy strongly localizes positrons with the binding energy of 2.8 eV and the lifetime of 238 ps, while the O vacancy does not trap positrons. Calculations are also performed on related materials Sr₂Cu₄O₆ and Sr₄Cu₆O₁₀, which are characterized by one-dimensional networks of edge-sharing CuO₄ squares. Positrons are predominantly distributed between these networks in these materials and their corresponding lifetimes are 170–171 ps.     

Ion crystals in a linear Paul trap

Large ion crystals containing as many as 10⁵ laser-cooled 24Mg⁺ ions arranged in 10 cylindrical shells around a central string have been observed in a linear Paul trap. For smaller crystals consisting of approximately 3500 ions, the varying linear charge densities along the trap axis give rise to well-defined transition regions between the different shell-structures. These structures and the transition regions as a function of linear density are in good agreement with molecular dynamics simulations. The micromotion of the ions in such smaller crystals is also investigated through comparison with MD-simulations. Finally, the degree and progression of ordering in the plasma have been observed for various temperatures and compared with MD-simulations. Good qualitative agreement is again obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laser cooling of a small number of Be+ ions in a penning trap

Be⁺ ions stored in a Penning trap were cooled by a laser beam perpendicular to the magnetic field. The cooled ions are strongly coupled and phase transitions of up to 100 ions were observed. In experiments with only a few ions stored in the trap, a stepwise decrease in fluorescence intensity was observed. All steps are of the same size and so every step is attributed to a single ion. The discrete changes in fluorescence occurred more frequently when the background pressure was increased, caused by collisions between stored ions and background neutral molecules.