Electron attachment to anionic clusters in ion traps

Ion traps are versatile tools for the investigation of gas-phase cluster ions, allowing, e.g., cluster-size selection and extended reaction times. Taking advantage of their particular storage capability of simultaneous trapping of electrons and clusters, Penning traps have been applied for the production of clusters with high negative charge states. Recently, linear radio-frequency quadrupole traps have been demonstrated to be another candidate to produce polyanionic clusters. Operation with rectangular, rather than harmonic, radio-frequency voltages provides field-free time slots for unhindered electron passage through the trap. Several aspects of electron-attachment techniques by means of Penning and radio-frequency traps are addressed and recent experimental results are presented.     

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.     

基于有限元方法的离子俘获装置模拟计算

利用有限元程序ANSYS,开展潘宁离子俘获装置的电场模拟计算。基于电场数据,结合Runge_Kutta_Fehlberg方法进行潘宁装置在多种模式下的离子俘获过程模拟工作,得到了准确的离子俘获结果。并对实际条件下具有偏离理想情况电极分布的俘获装置进行了优化计算及电场分析,同样实现了离子俘获过程的准确模拟。有限元方法用于离子俘获装置的电场计算以及后续离子俘获过程模拟流程的建立,为类似的电势阱离子俘获装置建造运行提供有效的技术支持。     

Dynamics of laser-cooled Ca+ ions in a Penning trap with a rotating wall

We have performed systematic measurements of the dynamics of laser-cooled ⁴⁰Ca⁺ ions confined in a Penning trap and driven by a rotating dipole field (‘rotating wall’). The trap used is a copy of the one used in the SPECTRAP experiment located at the HITRAP facility at GSI, Germany. The size and shape of the ion cloud has been monitored using a CCD camera to image the fluorescence light resulting from excitation by the cooling laser. We have varied the experimental conditions such as amplitude and frequency of the rotating wall drive as well as the trapping parameters. The rotating wall can be used for a radial compression of the ion cloud thus increasing the ion density in the trap. We have also observed plasma mode excitations in agreement with theoretical expectations. This work will allow us to define the optimum parameters for high compression of the ions as needed for precision spectroscopy of forbidden transitions.     

Advanced Mechanics and General Relativity,by J. Franklin

Ion Coulomb crystals, formed by atomic ions at low temperatures in radiofrequency and Penning ion traps, are structures that have remarkable properties and many applications. Images of Coulomb crystals are striking and reveal the crystal structure, which arises from a balance between the trapping forces acting on the ions and their mutual Coulomb repulsion. Applications of these structures range from frequency standards and quantum simulation through to measurement of the cross-sections of chemical reactions of ions.     

Precision Penning trap mass measurements on exotic ions: status and perspectives

Penning traps are powerful instruments for the precise and accurate mass determination of rare isotopes. At present, many Penning trap facilities installed at radioactive beam facilities provide key data for nuclear astrophysics, for the study of nuclear structure evolution far from stability, and the test of fundamental interactions. This article summarizes the present status and current limits in the field of high-precision Penning trap mass measurements on short-lived exotic ions.     

Half-open Penning trap with efficient light collection for precision laser spectroscopy of highly charged ions

We have conceived, built and operated a ’half-open’ cylindrical Penning trap for the confinement and laser spectroscopy of highly charged ions. This trap allows fluorescence detection employing a solid angle which is about one order of magnitude larger than in conventional cylindrical Penning traps. At the same time, the desired electrostatic and magnetostatic properties of a closed-endcap cylindrical Penning trap are preserved in this configuration. We give a detailed account on the design and confinement properties, a characterization of the trap and show first results of light collection with in-trap produced highly charged ions.     

Observation of three-dimensional long-range order in small ion coulomb crystals in an rf trap

Three-dimensional long-range ordered structures in smaller and near-spherically symmetric Coulomb crystals of (40)Ca(+) ions confined in a linear rf Paul trap have been observed when the number of ions exceeds approximately 1,000 ions. This result is unexpected from ground state molecular dynamics (MD) simulations, but found to be in agreement with MD simulations of metastable ion configurations. Previously, three-dimensional long-range ordered structures have only been reported in Penning traps in systems of approximately 50,000 ions or more.     

The TITAN mass measurement facility at TRIUMF-ISAC

The TITAN facility at TRIUMF-ISAC will use four ion traps with the primary goal of determining nuclear masses with high precision, particularly for short lived isotopes with lifetimes down to approximately 10 ms. The design value for the accuracy of the mass measurement is 1 ×10^???8. The four main components in the facility are an RF cooler/buncher (RFCT) receiving the incoming ion beam, an electron beam ion trap (EBIT) to breed the ions to higher charge states, a cooler Penning trap (CPET) to cool the highly charged ions, and finally the measurement Penning trap (MPET) for the precision mass determination. Additional goals for this system are laser spectroscopy on ions extracted from the RFCT and beta spectroscopy in the EBIT (in Penning trap mode) on ions that are purified using selective buffer gas cooling in the CPET. The physics motivation for the mass measurements are manifold, from unitarity tests of the CKM matrix to nuclear structure very far from the valley of stability, nuclear astrophysics and the study of halo-nuclei. As a first measurement the mass of ¹¹Li will be determined. With a lifetime of 8.7 ms and a demonstrated production rate of 4×10⁴ ions/sec at ISAC the goal for this measurement at TITAN is a relative uncertainty of 5×10^???8. This would check previous conflicting measurements and provide information for nuclear theory and models.     

TITAN: an ion trap for accurate mass measurements of ms-half-life nuclides

The introduction of Paul traps, in particular linear radio-frequency quadrupoles in the early 2000s, has revolutionized the use of ion traps for probing the properties of radioactive nuclides. It opened the path to trapping all available nuclides, independent of their chemical properties. We present an overview of direct mass measurements of short-lived nuclides using TITAN, a Penning trap mass spectrometer facility particularly suitable for precision measurements of ms-half-life nuclides.     

Suitability of linear quadrupole ion traps for large Coulomb crystals

Growing and studying large Coulomb crystals, composed of tens to hundreds of thousands of ions, in linear quadrupole ion traps presents new challenges for trap implementation. We consider several trap designs, first comparing the total driven micromotion amplitude as a function of location within the trapping volume; total micromotion is an important point of comparison since it can limit crystal size by transfer of radiofrequency drive energy into thermal energy. We also compare the axial component of micromotion, which leads to first-order Doppler shifts along the preferred spectroscopy axis in precision measurements on large Coulomb crystals. Finally, we compare trapping potential anharmonicity, which can induce nonlinear resonance heating by shifting normal mode frequencies onto resonance as a crystal grows. We apply a non-deforming crystal approximation for simple calculation of these anharmonicity-induced shifts, allowing a straightforward estimation of when crystal growth can lead to excitation of different nonlinear heating resonances. In the anharmonicity point of comparison, we find significant differences between the trap designs, with an original rotated-endcap trap performing better than the conventional in-line endcap trap.     

Observations of an ion-driven instability in non-neutral plasmas confined on magnetic surfaces

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number m=1, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the E x B flow of the electron plasma. The frequency does not, however, scale exactly with E/B, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.     

The manipulation of charged particles into and out of electromagnetic traps

The manipulation of charged particles into and out of Paul and Penning ion traps is discussed in terms of the phase space volume of a particle collection and its manipulation. It is shown that such traps can have very large usable phase space volumes but that the shape of such volumes is severely mismatched to that of a typical ion beam from a separator or storage ring. Possible injection schemes are presented as well as some aspects of ion cooling within the trap and subsequent ion extraction. Possible applications to the manipulation of antiproton beams from LEAR and 10 MeV/A highly-stripped heavy ions at GSI are discussed.     

Hot Ion Production in a Modified Penning Discharge

Ions with Maxwellian energy distributions and kinetic temperatures up to seven keV have been observed in a modified Penning discharge. Investigation of the plasma revealed two distinct spoke-like concentrations of charge, consisting respectively of ions and electrons, rotating with different velocities in the sheath between the plasma and the anode ring. Theoretical expressions are derived for the frequency of the ion and electron spoke rotation, for the ion kinetic temperature resulting from the ion spoke velocity, and for the ion heating efficiency. An extensive series of experimental measurements were made to check these theoretical expressions, and approximate agreement was obtained. It is shown that the ion kinetic temperature in the modified Penning discharge scales according to the relation Vi ~ Vani1/4/B1/2 where Va is the applied anode voltage, ni is the ion density in the sheath, and B is the magnetic field strength. The observed data demonstrate that the ion heating efficiency can be as high as several tens of percent.     

The new ClusterTrap setup

ClusterTrap has been designed to investigate properties of atomic clusters in the gas phase with particular emphasis on the dependence on the cluster size and charge state. The combination of cluster source, Penning trap and time-of-flight mass spectrometry allows a variety of experimental schemes including collision-induced dissociation, photo-dissociation, further ionization by electron impact, and electron attachment. Due to the storage capability of the trap extended-delay reaction experiments can be performed. Several recent modifications have resulted in an improved setup. In particular, an electrostatic quadrupole deflector allows the coupling of several sources or detectors to the Penning trap. Furthermore, a linear radio-frequency quadrupole trap has been added for accumulation and ion bunching and by switching the potential of a drift tube the kinetic energy of the cluster ions can be adjusted on their way towards or from the Penning trap. Recently, experiments on multiply negatively charged clusters have been resumed.     

The trap design of PENTATRAP

A novel Penning-trap tower consisting of five compensated cylindrical Penning traps is developed for the PENTATRAP mass spectrometer at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. An analytical expression for the electrostatic potential inside the trap tower is derived to calculate standard Penning-trap properties like the compensation of anharmonicities and an orthogonal geometry of the trap electrodes. Since the PENTATRAP project described in the preceding article aims for ultra high-precision mass-ratio measurements of highly charged ions up to uranium, systematic effects for highly charged ions inside the trap tower are considered for the design process as well. Finally, a limit due to remaining anharmonic shifts at large amplitudes is estimated for the resulting geometry, which is important for phase-sensitive measurements of the reduced cyclotron frequency of the ions.     

Micromotion compensation in a surface electrode trap by parametric excitation of trapped ions

We report a surface electrode trap with a relatively large trap depth (0.6–1.0?eV). The trap electrodes are formed by gold plating an alumina substrate. Calcium ions are trapped approximately 400?μm above the trap surface. We demonstrate micromotion compensation based on parametric resonance for surface electrode traps. Unlike the conventional method based on radio-frequency (rf)–photon correlation in which the wave vector of the laser beam must have a component parallel to the micromotion to be detected, the proposed method is independent of the laser propagation direction. This enables the micromotion component normal to the electrode surface to be detected without increasing the scattered light.     

Detection of parametric resonance of trapped ions for micromotion compensation

We have detected excess micromotion of trapped ions by modulating the trapping voltage. This radio-frequency (rf) modulation induces parametric resonance and excites secular motion of the trapped ions when they possess excess motion. This technique has been applied to laser-cooled ions in a linear rf trap and it provides optimum values for compensating the trapping field. We found that the technique has sensitivity equal to or greater than the conventional method for detecting excess micromotion. Because any laser propagation direction can be used, this method is expected to be applied to surface-electrode traps.     

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.