Linear Traps Research Articles

Quasi-1D sedimentation of Brownian particles along optical line traps

We studied the Brownian motion of single 1 and 3 µm particles moving along optical line traps oriented in the gravity axis. The finite linear traps are formed using holographic optical tweezers which redistributes the incoming light beam into an elongated super Gaussian profile. Direct observation of the particle’s motion along the gravity axis allowed us to distinguish the non-equilibrium sedimentation of the particle, when the average velocity is 〈uz〉≠0, followed by equilibrium sedimentation when 〈uz〉=0 and the particle establishes the so-called gravitational length. We found that the bottom edge of the linear trap functions as a virtual “floor”, and subsequent Boltzmann distribution analysis of the equilibrium particle trajectories allows the estimation of the particle’s mass in the hundreds of femtograms range. In parallel, we also induced the physisorption of a macromolecule (hyaluronic acid) onto polystyrene microparticle in solution, detecting an increment of the particle’ mass of around 2 picograms. This suggests that the present technique can be used in surface chemistry, as it detects analyte absorption in solutions containing colloids by estimating the mass of both, the colloidal particle and the layer formed around it. On the other hand, we also found that when the longitudinal confinement established by the optical trap length is smaller than twice the particle gravitational length, particles remain suspended indefinitely even in the presence of a density mismatch with the medium, i.e., the notion of sedimentation disappears. Finally, we observed that the optical forces along the linear trap do not alter the Brownian motion of the particles significantly but can exert an extra pushing force on the smallest particles (1 µm) considered.

Charge exchange radiation diagnostic with gas jet target for measurement of plasma flow velocity in the linear magnetic trap

The ambipolar electrostatic potential rising along the magnetic field line from the grounded wall to the centre in the linear gas dynamic trap, rules the available suppression of axial heat and particle losses. In this paper, the visible range optical diagnostic is described using the Doppler shift of plasma emission lines for measurements of this accelerating potential drop. We used the room temperature hydrogen jet puffed directly on the line of sight as the charge exchange target for plasma ions moving in the expanding flux from the mirror towards the wall. Both bulk plasma protons and He2+ ions velocity distribution functions can be spectroscopically studied; the latter population is produced via the neutral He tracer puff into the central cell plasma. This way, potential in the centre and in the mirror area can be measured simultaneously along with the ion temperature. A reasonable accuracy of 4 ÷ 8% was achieved in observations with the frame rate of ≈1 kHz. Active acquisitions on the gas jet also provide the spatial resolution better than 5 mm in the middle plane radial coordinate because of the strong compression of the object size when projected to the centre along the magnetic flux surface. The charge exchange radiation diagnostic operates with three emission lines: H-α 656.3 nm, He-I 667.8 nm and He-I 587.6 nm. Recorded spectra are shown in the paper and examples for physical dependences are presented. The considered experimental technique can be scaled to the upgraded multi-point diagnostic for the next generation linear traps and other magnetic confinement systems.

Encouraging Results and New Ideas for Fusion in Linear Traps

Results, obtained during the last decade on linear traps in Novosibirsk, verified the original concepts and led to discovery of new ways of plasma confinement in linear traps. It turns out that the multiple-mirror confinement, once thought to be suitable for pulsed fusion only, can work in tandem with gas-dynamic traps. Its helical-mirror variant can provide additional advantages for confinement of rotating plasmas such as radial and axial plasma pinch fluxes. The specially designed gas-dynamic mirrors can be run in the high-β “diamagnetic” regime, closely resembling the field-reversed configurations both in shape and in the quality of axial confinement. These new ideas form the groundwork of the possible road for linear traps to fusion.

Diamagnetic “bubble” equilibria in linear traps

The plasma equilibrium in a linear trap at β ≈ 1 (or above the mirror-instability threshold) under the topology-conservation constraint evolves into a kind of diamagnetic “bubble.” This can take two forms: either the plasma body greatly expands in radius while containing the same magnetic flux, or, if the plasma radius is limited, the plasma distribution across flux-tubes changes, so that the same cross-section contains a greatly reduced flux. If the magnetic field of the trap is quasi-uniform around its minimum, the bubble can be made roughly cylindrical, with radius much larger than the radius of the corresponding vacuum flux-tube, and with non-paraxial ends. Then the effective mirror ratio of the diamagnetic trap becomes very large, but the cross-field transport increases. The confinement time can be found from solution of the system of equilibrium and transport equations and is shown to be τE≈τ∥τ⊥. If the cross-field confinement is not too degraded by turbulence, this estimate in principle allows construction of relatively compact fusion reactors with lengths in the range of a few tens of meters. In many ways, the described diamagnetic confinement and the corresponding reactor parameters are similar to those claimed by the field-reversed configurations.

Manipulation of micro-objects using linear traps generated by vortex axicons

We present a method of forming linear optical traps using vortex axicons. Experimental results on trapping and guiding of polystyrene microparticles with a diameter of 5 μm are discusssed. Such a linear trap is experimentally shown to possess the capturing force anisotropy.

Moment Theories of Ion Motion and Ion-Neutral Reactions in Linear Traps

Moment theories of ion motion and reaction in ideal and stretched quadrupole ion traps are extended to the case of linear ion traps. Fortran and Mathematica computer programs based on these theories are developed. They are applied to the case of O+ ions moving through an Ar buffer gas containing a small amount of a reactive neutral, N2. The rate coefficient predicted ab initio for a common set of trap parameters is 6.4±0.9×10−13 cm3/s, which is large enough that it should be measureable.

Helicoidal System for Axial Plasma Pumping in Linear Traps

A new type of axial pumping or confining system for plasma devices is proposed. It is based on E × B plasma rotation in a nonuniform helical magnetic field. In the rotating reference frame the helical ripples of the magnetic field look like traveling waves propagating along the system axis. Due to parallel plasma viscosity and/or collisions with locally trapped particles, the plasma as a whole will aquire axial momentum. Related processes are observed in tokamaks as transformations between poloidal and toroidal rotation components.

Electron cyclotron resonance near the axis of a quadrupole linear trap

The quasi-longitudinal propagation of an extraordinary electromagnetic wave in the vicinity of the electron cyclotron resonance layer in an open linear trap with a quadrupole magnetic field is studied analytically, taking into account the inhomogeneity of the magnetic field in a paraxial approximation. The ray trajectories are derived from a simplified dispersion equation, that is, nonetheless able to accurately describe the transition from finite to zero perpendicular refractive index. A criterion for an on-axis resonance point to be an attractor for the ray trajectories is formulated, which generalizes a similar criterion for axisymmetric linear traps derived in a recent paper [D. S. Bagulov and I. A. Kotelnikov, Phys. Plasmas 19, 082502 (2012)].

Viscoelastic Response of Red Blood Cells in Linear Optical Traps

The mechanical properties of biological cells can be used as a marker for individual cell's health, information not accessible by bulk measurements. For example malaria parasites are known to significantly stiffen the red blood cell (RBC) membrane, allowing identification of single infected cells based on their deformability. We have developed a non-contact method employing non-invasive optical forces, efficiently elongating RBCs within microfluidic channels to determine cell elastic properties. In this, the anisotropic beam of a single laser diode bar is used to create a line-shaped optical trap, capturing and elongating specimens along the laser axis. We perform static measurements on single RBCs to demonstrate the utility of this method. Simulations employing ray-tracing methods illustrate how the refraction of the asymmetric laser profile inherently creates antipodal stretching forces. Applying the membrane theory of thin shells enables us to determine the expected deformation based on our simulations and compare results to measured data.As the behavior of a viscoelastic material, such as the RBC's membrane, strongly depends on the timescales of applied forces, we perform frequency-dependent measurements with modulated external stimulus to determine the RBC's complex elastic moduli, properties not accessible by comparable techniques. Laser intensity, and therefore the forces responsible for cell deformation, is modulated at frequencies varying over three orders of magnitude. Cell response is recorded using a high-speed camera system, allowing us to correlate the applied external load to the phase-shifted viscoelastic behavior of the RBC and to obtain the frequency-dependent dissipation of energy during one cycle of oscillation.Employing this new technique, cells can be deformed while streaming along the line-shaped trap in flowing environment, allowing application in high-throughput systems. Use of comparably simple optics and inexpensive laser sources facilitates potential implementation in small platforms and hand-held devices.

Self-similar nonlinear dynamical solutions for one-component nonneutral plasma in a time-dependent linear focusing field

In a linear trap confining a one-component nonneutral plasma, the external focusing force is a linear function of the configuration coordinates and/or the velocity coordinates. Linear traps include the classical Paul trap and the Penning trap, as well as the newly proposed rotating-radio-frequency traps and the Mobius accelerator. This paper describes a class of self-similar nonlinear solutions of nonneutral plasma in general time-dependent linear focusing devices, with self-consistent electrostatic field. This class of nonlinear solutions includes many known solutions as special cases.

Hopping of an impurity defect in ion crystals in linear traps

Laser-cooled arrays or crystals of $^{171}\mathrm{Yb}$${}^{+}$ ions containing a single impurity, $^{172}\mathrm{Yb}$${}^{+}$ isotope, are confined in a linear radio-frequency Paul trap. Site-to-site hopping of the impurity ion, distinguished by a lack of fluorescence, is studied as a function of the $^{171}\mathrm{Yb}$${}^{+}$ laser-cooling parameters and as a function of the anisotropy of the trapping potential. Imaging of the independently resolved crystal sites permits the extraction of the impurity's hopping trajectory, from which the dwell times at a given site can be obtained as well as the spatial distribution of hopping rate and hopping destination. The onset of rapid hopping is found to occur at average thermal energies approaching a significant fraction of the Coulomb potential energy. Furthermore, the hopping rate is enhanced at trap anisotropies near the critical value for the structural phase transition to a two-dimensional zigzag phase. Finally, the hopping mobility of the impurity ion is observed to be highest near the center of the crystal, which may have an intrinsic cause related to the crystal structure and dynamics near the zigzag transition.

Feasibility Studies of Alpha-Particle Channeling in Mirror Machines

The linear magnetic trap is an attractive concept both for fusion reactors and for other plasma applications because of its relative engineering simplicity and high-beta operation. Applying the α-channeling technique to linear traps such as mirror machines can benefit this concept by efficiently redirecting α-particle energy to fuel ion heating or by otherwise sustaining plasma confinement, thus increasing the effective fusion reactivity. To identify waves suitable for α channeling, a rough optimization of the energy extraction rate with respect to the wave parameters is performed. After the optimal regime is identified, a systematic search for modes with similar parameters in mirror plasmas is performed, assuming quasi-longitudinal or quasi-transverse wave propagation. Several modes suitable for α-particle energy extraction are identified for both reactor designs and for proof-of-principle experiments.

Computer Simulation of the Gap-Tripole Ion Trap with Linear Injection, 3D Ion Accumulation, and Versatile Packet Ejection

The behavior of a completely new ion trap is shown with SIMION 7.0 simulations. The simulated trap, which was a mix of a linear and a 3D trap, was made by axially setting two ion guides with a gap between them. Each guide consisted of three rods with three symmetrically delayed radio frequency (rf) voltages (tripole). The "injected" ions were linearly contained by pulsed potentials on the entrance and exit plates. Then the three-dimensional (3D) rf field in the gap, which was created by the tripole special rod arrangement, could trap the ions when the translational energy was dampened by collisions with low-pressure nitrogen. Because the injected ions were trapped in the small gap, the trapping cycle could be repeated many times before ion ejection, so a high concentrated ion cloud could be obtained. This trapping and accumulation methodology is not possible in most conventional multipole linear traps with even number of poles. Compared with quadrupole linear trap at the same rf amplitude, tripole lost more ions due to strong charge repulsion in the ion cloud. However, tripole could catch up the ions at higher voltage. Radial and axial mass-independent ejection of the ions localized in the tripole gap was very simple, compared with conventional linear ion traps that need extra and complicated electrodes for effective axial ejection.

Convenient Synthesis of Linear Spin Traps Containing Diphenylphosphoryl Groups

Abstract Novel spin traps in which a methyl group of the tert-butyl of PBN was substituted with a diphenylphosphoryl group, were successfully synthesized. The nitrone precursors were synthesized from diphenylphosphine oxide, acetone, and appropriate amines under microwave irradiation. The obtained amines were converted to the desired nitrones using Oxone. Spin adducts of oxygen-centered radicals such as the superoxide anion radical and the hydroxyl radical, which could not be detected in adducts using PBN, were observed using these nitrones.

Design, synthesis, and modeling of novel cyclic thrombin receptor-derived peptide analogues of the Ser42-Phe-Leu-Leu-Arg46 motif sequence with fixed conformations of pharmacophoric groups: importance of a Phe/Arg/NH2 cluster for receptor activation and implications in the design of nonpeptide thrombin receptor mimetics.

The novel cyclic analogues cyclo(Phe-Leu-Leu-Arg-epsilonLys-Dap) (1) and cyclo(D-Phe-Leu-Leu-Arg-epsilonLys-Dap) (2), which differ only in the absolute conformation of Phe, have been designed and synthesized based upon the minimal peptide sequence Phe-Leu-Leu-Arg which has been found to exhibit biological activity for the thrombin receptor. Compound 1, in which all amino acids have the L-configuration, exhibited higher activity in the rat aorta relaxation and rat longitudinal muscle bioassays compared to compound 2, in which the Phe residue is in the D-configuration. This is attributed to the spatial proximity of the Phe and Arg in compound 1 which does not exist in its diastereomeric compound 2, as is depicted from a combination of NMR studies and computational analysis. Structure-activity studies (SAR) showed that the Phe and Arg side chains along with a primary amino group form an active recognition motif that is augmented by the presence of a second primary amino group in the cyclic peptide. We suggest that a comparable cyclic conformation may be responsible for the interaction of linear TRAPs with the thrombin receptor. The validity of this proposition was tested by the synthesis of four active nonpeptide thrombin receptor mimetics. Substance (S)-N-(6-guanidohexanoyl)-N'-(2-amino-3-phenylpropionyl)piperazine (3), in which the pharmacophoric phenyl, guanidino, and amino groups were incorporated onto a piperazine template, was found to be the most active compared to the other synthesized compounds which lack the amino pharmacophoric group.

Ion strings for quantum gates

Crystal structures of calcium ions have been prepared in a linear Paul trap. The trapped ions are laser-cooled by simultaneous resonant excitation near 397 nm and 866 nm. Images of the fluorescing ions are obtained with a CCD camera and show the individual ions spatially resolved. Complex crystal structures of more than 60 ions have been observed whereas smaller crystals with up to 10 ions arrange in a linear string. Measured distances between the ions in strings of different lengths are in good agreement with expected values obtained from a harmonic trap potential. The application of a calcium ion string for quantum computation is discussed. PACS: 32.80.Pj, 42.50.Vk Ion traps have been shown to provide an ideal environment for isolated quantum systems such as a single, trapped and laser-cooled atoms. Ion storage has long been applied to ultra-high precision spectroscopy and the development of frequency standards [1]. More recently, single trapped ions have been used to demonstrate and test some of the intriguing features of quantum mechanics [2, 3]. In particular, both the internal electronic state and the motional state of a trapped ion can be modified using laser light. Decoherence of internal superposition states is nearly negligible even for very long interaction times. To explore these properties, several schemes have been proposed for the preparation of non-classical states of motion in a trap [4]. Their experimental realization [2] promises further improvement for the precision of spectroscopic measurements [5, 6]. With almost perfect control of the quantum state of a single ion, attention has turned to systems of few ions with well-controlled interactions between them. Manipulations of their overall quantum state include the preparation of entangled states that have no classical counterpart. The possibility of entangling massive particles opens up many prospects for new experiments including measurements with Bell states and GHZ states [7] which would allow for new tests of quantum mechanics. Moreover, entanglement of particles will allow one to study quantum measurements such as the investigation of decoherence processes [3, 8] in more detail. A very exciting proposal is the application of linear ion traps and the collective quantum motion of a trapped string of ions for the realization of a quantum gate [9]. Quantum gates are the basic building blocks of a quantum computer and their operation relies fundamentally on the entanglement of internal degrees of freedom of the ions (electronic excitation) and their collective motion (vibrational excitation). A quantum computer works with quantum registers made up of quantum bits (qbits) which can be manipulated analogously to classical bits using gate operations. The quantum-mechanical analogue of a classical XOR gate, the so-called controlled-NOT gate operation, can be realized using a linear string of ions and a well-defined series of laser pulses to address different ions. It has been shown that a controlled-NOT gate operating on two ions is a realization of a universal quantum gate, so that in principle universal computations can be carried out using just the two-ion quantum gate and one-bit rotations [10]. The realization of these gate operations based on a string of ions would therefore be of fundamental interest and furthermore all basic algorithms could be tested using just a string of trapped ions. Many species of ions have been used for ion trapping, and strings in linear traps have been experimentally demonstrated with Be+ ions [11] and Mg+ ions [12]. However, it was shown that 40Ca+ is one of the most promising candidates for the realization of quantum gates, because of its mass, level structure, and transition frequencies and widths [13]. Furthermore, quantum gates require certain characteristics of the linear trap that have not been met in earlier experiments, in particular optical access from many directions. In this paper, we report the first trapping of strings of 40Ca+ ions in a linear trap, as a significant step towards the realization of a quantum gate. The novel trap that we use and describe in detail, has been designed especially for storing small numbers (up to a few tens) of 40Ca+ ions with the aim of producing linear strings and using them as a quantum register. The paper is organized as follows. In Sect. 1 we present the requirements for an application of ion strings to quantum computation in the particular case of 40Ca+ . We also highlight the advantages of the specific choice of this ion. In Sect. 2 the experimental setup is described and in