Radio-frequency Quadrupole Trap Research Articles

On-line installation of the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap

The Cabibbo-Kobayashi-Maskawa quark mixing matrix currently does not satisfy unitarity at the 2σ-level. This could be the result of an inaccurate value of one or both of its largest matrix elements Vus and Vud. In the case of Vud, the most precise measurement is obtained from the f t-value measurements of superallowed beta-transitions between 0+ states. The accuracy of this determination can, in turn, be tested by extracting Vud in other transitions including superallowed transitions between mirror nuclei. The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the Nuclear Science Laboratory of the University of Notre Dame to perform such a determination, with the goal of shedding more light on this tension with unitarity. St. Benedict will take a radioactive ion beam produced by TwinSol, thermalize it in a large volume gas catcher, then transport it in two separate differentially-pumped volumes using a radio-frequency (RF) carpet and a radio-frequency quadrupole (RFQ) ion guide before injecting it in an RFQ trap to create cool ion bunches for injection in the measurement Paul trap. In this paper, we detail the installation of the beam preparation components of St. Benedict, and present the results of the first RIBs successfully stopped and extracted from its gas catcher.

Truncated series solution of the equation of motion of ions trapped in a radiofrequency quadrupole trap with superimposed octopolar potential

The power series method is used to solve numerically the equation of motion of ions in a radiofrequency quadrupole ion trap when a small octopolar potential is present. Every time step, the coordinates of the ion are represented by a truncated series of degree 15. The stability diagram of the trap is obtained by using the method to simulate the behavior of hundreds of ions up to a relatively large time. The nonlinear resonance lines are observed. The ion's oscillation frequencies are then studied. Their theoretical expression is derived and compared to the results of the truncated series solution.

171Yb+ 5D3/2 hyperfine state detection and F = 2 lifetime

We report on measurements of the lifetime of the state in single ions confined in a radiofrequency quadrupole trap. Experimental parameters that affect hyperfine state preparation and detection efficiency are characterized and optimized. A set of wait times for optimal sampling of the lifetime is chosen and used to measure that lifetime with high statistical sensitivity. A systematic variation in this lifetime seems to be apparent. The source of the variation was not identified, but ion number and cooling rate appear to be ruled out. A net determination is made of ms which is significantly longer than other measurements of the same quantity with four times better statistical precision than the most recent measurement. An alternate shelving scheme is proposed that would provide S–D state discrimination for Yb+ even isotopes as well as improved sensitivity for D state hyperfine discrimination in odd isotopes.

Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas

The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations, high-resolution spectroscopy of complex molecules, cold chemistry and astrochemistry. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion or a cryogenic trap environment. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH(+) molecular ions, which have been trapped and sympathetically cooled in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second--that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings--we have cooled a single molecular ion to a rotational temperature of 7.5(+0.9)(-0.7) kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest.

Upgrades at ClusterTrap and latest results

For the last decade the production of polyanionic metal clusters in the gas phase and the investigation of their size- and charge-state dependent properties was the main objective of the ClusterTrap experiment. An upgrading of the ion-trapping devices extends the range of anionic charge states: a recently installed Penning trap with a 12-T superconducting magnet provides an increased mass and thus cluster-size range, crucial to reach higher anionic charge states by means of the electron-bath method. Its cylindrical Penning trap is characterized and compared with respect to the previous asymptotically hyperbolical setup. In addition, polyanionic cluster production in a linear radio-frequency quadrupole (RFQ) trap has been initiated. Preliminary results for gold-cluster polyanion production in the new Penning trap, the RFQ trap, as well as the combined use of these traps are presented.

Sub-millikelvin spatial thermometry of a single Doppler-cooled ion in a Paul trap

We report on observations of thermal motion of a single, Doppler-cooled ion along the axis of a linear radio-frequency quadrupole trap. We show that for a harmonic potential the thermal occupation of energy levels leads to Gaussian distribution of the ion's axial position. The dependence of the spatial thermal spread on the trap potential is used for precise calibration of our imaging system's point spread function and sub-milliKelvin thermometry. We employ this technique to investigate the laser detuning dependence of the Doppler temperature.

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.

Design and fabrication of a linear Paul trap for the study of space-charge-dominated beams

The collective motion of a single-species plasma in a radio-frequency quadrupole trap is physically almost equivalent to that of a charged-particle beam propagating through a periodic magnetic lattice. This fact suggests that one can experimentally study the behavior of space-charge-dominated beams by using a compact table-top device. At Hiroshima University, a Paul trap system dedicated to beam-physics purposes has been designed and is now under construction. Controllable, pulsed radio-frequency voltages, instead of ordinary sinusoidal voltages, are applied to the trap electrodes such that collective beam dynamics in various lattice structures can be explored. The system is equipped with a laser cooler that enables us to adjust the tune depression of a Ca+ plasma. In this paper, the basic design concept of the trap and the detailed system configuration are presented.

Pushing the relative mass accuracy limit of ISOLTRAP on exotic nuclei below 10 ppb

The Penning trap mass spectrometer ISOLTRAP plays a leading role in mass spectrometry of short-lived nuclides. The recent installation of a radio-frequency quadrupole trap and a carbon cluster ion source allowed for the first time mass measurements on exotic nuclei with a relative uncertainty of δ m/ m≈1×10 −8. The status of ISOLTRAP mass spectrometry and recent highlights are presented.

Radio-frequency quadrupole trap as a tool for experimental beam physics

In recent papers [Okamoto, Hiroshima University Preprint HUBP-01/98 (1998); Okamoto and Tanaka, Nucl. Instr. Meth. A 437 (1999) 178], we proposed a novel experimental approach to investigate various collective effects in space-charge-dominated beams. It was demonstrated that either a radio-frequency quadrupole trap or a solenoidal trap could reproduce non-linear processes equivalent to those in a beam transport channel. In the present work, we study the former trap configuration in more detail, specifying a typical parameter range for beam-physics applications. A three-dimensional Maxwell-equation solver is employed to show the basic characteristics of the plasma confinement field. After deriving several useful formulae, we discuss possible trap experiments that greatly deepen our current understandings of collective beam instabilities including coherent resonances, halo formation, etc.

Effect of a Heavy Collision Partner on Ion Loss from a Radio Frequency Trap

We present a new model which represents the mechanism of loss of ions stored in a radio frequency quadrupole trap. We observed the storage time and the temperature of Mg+ ions as a function of the cooling gas (He) pressure in the presence of Kr gas, which is introduced as a heavy collision partner. Our model well reproduces the observation.

Designing an RFQ ring trap for antiproton confinement and trapping and laser cooling Mg+ ions

In the true sense of Bernie's approach to physics, an old idea (the Radiofrequency Quadrupole Trap) was taken and upgraded to be applied to a new area of physics (formation of an antihydrogen beam). During the course of the development work, new applications were identified and immediately put to use. While the collaboration is still pursuing its original goal, formation of antihydrogen by “Bernie's reaction”, collisions between positronium atoms and antiprotons, many new experiments have been found possible and are actively pursued. These include atomic and nuclear physics studies with ultra-low energy antiprotons ejected from the initial catching trap of the antihydrogen project, and the formation and study of exotic atoms and molecules in ultra-thin targets using trapped antiprotons (an extension of the work by the PS205 collaboration at CERN described elsewhere in this volume [10]). A large physics community has grown around these ideas and may even succeed in obtaining its very own antiproton source, which is a true sign of the recognition of the importance of this field. The work in this area will hopefully continue for many years to come, but we will truly miss the motivation and drive of our friend and colleague, Bernie Deutch.

Ion storage and laser-induced fluorescence measurements in an RFQ trap

A radio-frequency quadrupole (RFQ) ion trap has been constructed for laser spectroscopic measurements of isotope shifts (IS) and hyperfine structures (HPS) with a small number of ions produced from a very dilute hafnium sample placed inside the RFQ trap. The target atoms are evaporated by a Nd:YAG heating laser pulse, and then selectively ionized by resonance ionization. Through repetitive application of this procedure, hafnium ions are accumulated in the trap, and a storage time of hours has been achieved with H 2 as a buffer gas. Laser-induced fluorescence (LIF) with the 5d 6s 2 ( a 2D) ( J = 3/2) — 5d6s ( a 3D)6p ( z 4F o) ( J = 5/2) transition is observed. Through an RF phase timing coincident technique, a resolution of about 1 GHz car. Le obtained, sufficient to resolve most of the hyperfine components of the odd A isotopes Also, by continuously monitoring the LIF signal, the ion cooling time in the RFQ trap is measured as a function of buffer gas pressure, and the ion-molecule collision cross section deduced is consistent with theoretical expectations. The sensitivity of this method was tested with the IS measurement of a mass separated 173Hf source.

Dynamics of charged particles in a Paul radio-frequency quadrupole trap.

We have carried out a molecular-dynamics simulation of hundreds of ions confined in a Paul trap. The simulation includes the trapped particles' micromotion and interparticle Coulomb interactions. We have implemented a random walk in velocity to bring the secular motion to a given temperature which we have numerically measured. When the coupling \ensuremath{\Gamma} is large the ions form concentric shells which undergo a quadrupole oscillation at the rf frequency, while the ions within a shell form a 2D hexagonal lattice. Ion clouds at 5 mK show no rf heating for ${\mathit{q}}_{\mathit{z}}$\ensuremath{\lesssim}0.6, whereas rapid heating is seen for ${\mathit{q}}_{\mathit{z}}$=0.8.

Analytic potential in a linear radio-frequency quadrupole trap with cylindrical electrodes

We have analyzed a linear radio-frequency ion trap of finite length employing a four-sectored hollow cylinder enclosed between two end caps. Solutions to Laplace’s equation determine the effective potential inside the trap due to the rf trapping field, the static confining potential from dc-biased end caps, and the potential arising when a dc bias is present on the cylinder sectors. The finite length of a linear trap results in field distortion at the trap ends due to fringing effects. We have demonstrated that near the ends, the effective potential arising from the rf fields acts to propel ions out of the trap. We have further shown that the addition of a dc bias on two neighboring cylinder sectors generates an inhomogeneous field in the trap which produces a force on the ions along the trap’s long axis in a direction dependent on the sign of the bias. An exact analytic determination of the potential can be used to determine the profile of the trapped ion cloud.

Time-of-flight detection of ions ejected from a radiofrequency quadrupole trap: experimental determination of their f z secular frequency

We have studied the temporal behaviour of a very small number of ions injected into a quadrupole trap and detected by ejection. The sampling of the movement of the ions in the trap, associated with a time-of-flight detection, leads to an ion signal which gives the f z secular frequency.

New Schemes for resonant ejection in r.f. quadrupolar ion traps

In order to achieve relative ion number measurement with the best possible accuracy, a systematic investigation of ion detection by resonance ejection has been performed. The first experiment is described here. Ions stored in a radiofrequency quadrupole trap were subjected, for a short time, to a sinusoidal voltage of small amplitude, the frequency of which was variable from zero to half of the r.f. drive frequency. This pulse, of duration 10 ms, was applied to the trap electrodes in either the dipolar or quadrupolar mode. The signal formed by ions ejected out of the trap was observed at two different times which permitted differentiation of ion ejection along the z direction from combined ion ejection along the x, y and z directions. As the frequency composition of the pulse is swept through the above range, the observed ion signals show absorptions at secular frequencies ω x , ω z , doubled frequencies, and combinations of ω x and ω z . These lines show anharmonicities and space charge perturbation. The second experiment attempts to analyze the effect of an initial pulse (or tickle) of a fixed frequency with a second pulse, the frequency of which is variable as described above, in order to explore the spectrum of ion motion under the influence of this tickle.

Single, trapped Sr^+ atom: laser cooling and quantum jumps by means of the 4d^2D_5/2–5s^2S_1/2 transition

A single Sr+ atom was confined in a radio-frequency quadrupole trap and laser cooled with resonant laser light. Examination of the transition Doppler profile indicated an ion kinetic temperature of less than 500 mK. Quantum jumps into and out of the metastable 4d(2)D(5/2) level were observed by laser excitation on the electric-quadrupole-allowed transition from the ground state at 674 nm. The dark and bright periods in the strong fluorescence showed variations owing to the applied driving field on the electric-quadrupole transition. These results provide an improved determination of tau = 372 +/- 25 msec for the natural metastable-state lifetime.

Nonlinear effects in the detection of stored ions.

Ions stored in a radio-frequency quadrupole trap were subjected to pulses of a small oscillating potential. The frequency of oscillation within each pulse (dipolar or quadrupolar) was variable up to one-half of the rf drive frequency. Ions that were ejected axially from the trap were detected with an electron multiplier. Ion signal intensity was observed as a function of the small oscillating potential frequency for pulsed excitation in each of two time domains in the duty cycle. Thus it was possible to see either the ejection in the z direction, or the ejection in the x or y direction and the z direction. According to the nature of the ejection pulse, the signal shows lines at secular frequencies, at doubled secular frequencies, and other lines that can be sharp and numerous. The influence of space charge is clearly shown. The observed couplings are discussed by taking into account anharmonicity and relaxation mechanisms.

A time-of-flight method for studying the properties of an ion cloud stored in an rf trap

We report here on an experiment undertaken to obtain information on the spatial and velocity distributions of ions within a radio frequency quadrupole trap. A time-of-flight technique was used in which the progression of the rf confinement voltage was suddenly arrested, thus allowing the ions to escape axially. The measured profile of the ion time of flight is related to the ion distributions in position and velocity at the moment when the confinement voltage ceases to change (the time or phase of ejection). We have developed a general procedure for calculating the time-of-flight profile for any given spatial and velocity distribution of the ions within the trap. Although, in general, the solution involves a numerical integration, we find that, for the case of Gaussian distributions, this calculation can be carried out analytically. Shifts in the time-of-flight profile due to changes in the phase of ejection and in the mean ion energy are calculated. These results are compared with experimental measurements made on trapped Xe + ions in the presence of helium buffer gas.