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Physical Review A

Computer simulation of magnetically trapped neutral atoms

A. Góngora-T, Armando Antillón, T. H. Seligman

Published Sep 1, 1990• 2 citations
Publisher

Abstract

We report the results of a computer simulation of neutral atoms trapped in a magnetic quadrupole trap formed by two coils carrying opposite currents. Effects of hyperfine structure have been neglected. We conclude from this study that approximately a quarter of the particles are initially trapped in stable orbits and the successive later loss of particles due to the nonadiabatic region is insignificant.

DOI
10.1103/physreva.42.3139

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We report the results of a computer simulation of neutral atoms trapped in a magnetic quadrupole trap formed by two coils carrying opposite currents. Effects of hyperfine structure have been neglected. We conclude from this study that approximately a quarter of the particles are initially trapped in stable orbits and the successive later loss of particles due to the nonadiabatic region is insignificant.
Methods of trapping neutral spin-polarized particles are proposed using highly nonuniform magnetic fields rotating at low frequencies, down to the resonance of the particle's orbiting motion. Such fields beget new trapping mechanisms of vast possibilities. The new resonance methods and the new traps of zero-size equilibrium orbit should give much tighter and more stable confinement than previously known. They also give the possibility of a high, resonance selectivity in magnetic separation and evaporative cooling. A general analysis of trapping with rotating fields is given.
We experimentally demonstrated the transport of cold atoms in a magnetic quadrupole trap in the nonadiabatic regime on the order of the trap period. The trap potential velocity was controlled by varying the currents of the two pairs of anti-Helmholtz coils. We efficiently transported cold atoms with two velocity regimes. For example, with a potential velocity of 18 (28 cm/s), the final velocity of a cold atom cloud is 0.6±2.7 (58.6±1.5 cm/s). The trap motion with proper velocity could expel energetic atoms from the trap, resulting in a decline in temperature of high residual velocity atoms.
A stable magnetic quadrupole trap for neutral atoms on a superconducting Nb thin-film disc is demonstrated. The quadrupole field is composed of the magnetic field that is generated by vortices on the disc introduced by cooling the disc across the transition temperature with a finite field and an oppositely directed uniform field applied after cooling. The trap is stable when all trapping processes are performed above the dendritic instability temperature T(a). When the field intensity is changed below this temperature, the quadrupole field collapses and the trap disappears. The initial vortex density decreases even when the external field is changed at a temperature T > T(a). However, the vortex density is stabilized at an equilibrium density, whereas at T < T(a), it almost completely disappears. A stable trap can be formed, even when the initial vortices are introduced through a dendritic avalanche.
Dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, is considered by applying an accurate method for solving nonlinear systems of differential equations. This method is more general than the adiabatic approximation and, thus, permits one to check the limits of the latter and also to analyze nonadiabatic regimes of motion. An unusual nonadiabatic regime is found when atoms are confined from one side of the zzz axis but are not confined from another side. The lifetime of atoms in a trap in this semiconfining regime can be sufficiently long for accomplishing experiments with a cloud of such atoms. At low temperature, the cloud is ellipsoidal, being stretched in the axial direction and moving along the zzz axis. The possibility of employing the semiconfining regime for studying the relative motion of one component through another in a binary mixture of gases is discussed.
A microscopic magnetic quadrupole trap for neutral atoms has been realized with a combination of permanent magnets, coils, and ferromagnetic pole pieces. The attainable magnetic field gradients of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0ex}{0ex}}\mathrm{G}/\mathrm{cm}$ infer a spatial extension of the ground state much smaller than the wavelength of optical transitions. The field gradient can be varied over a wide range which allows for efficient loading of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium atoms from a shallow potential by adiabatic transport and compression. During compression the observed 275-fold density increase implies a collision rate increase by a factor of 1700.