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Revue de Physique Appliquée

Magnetic field profiling for low temperature H↓ confinement

Jean‐Marc Robert, Laurent Wiesenfeld

Published Jan 1, 1984• 0 citations
Publisher

Abstract

The study and the realization of a ferromagnetic system allowing to perform a magnetic compression of polarized hydrogen are presented. This system would allow to approach the critical Bose-Einstein condensation density at low temperature.
DOI
10.1051/rphysap:01984001903028100

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A gas of spin-polarized atomic hydrogen can be prepared in the upper hyperfine states and loaded into a static magnetic trap. Evaporative cooling and magnetic compression of such a gas can produce temperatures of 30 \ensuremath{\mu}K and densities of 1014{10}^{14}1014 ${\mathrm{cm}}^{\ensuremath{-}3}$. Under these conditions a Bose-Einstein condensate may form.
The density profiles for an interacting gas of atomic hydrogen in an inhomogeneous axial magnetic field are determined by extending the Gross-Pitaevskii theory to finite temperature. The profiles can be used as an identifying feature of Bose-Einstein condensation for all TTT. Radial inhomogeneities in the field are also treated.
Experimental efforts to produce a Bose Einstein condensed (BEC) gas of spin-polarized atomic hydrogen have been intensely pursued since 1980. Initial efforts studied hydrogen in contact with superfluid helium surfaces. In order to avoid the limiting wall recombination a static magnetic trap was developed which utilizes magnetic barriers to isolate atoms from the van der Wals walls. In this approach traps can be filled to about 10{sup 14} atoms/cm{sup 3} for which the critical temperature is in the region of 20-30 {mu}K. However, approaching BEC conditions, a new instability dominates the behavior. The authors have developed a new type of electromagnetic trap, a microwave (mw) trap in which the trapped atoms are in the ground spin-state so that at low temperature relaxation to a barrier state is strongly suppressed and BEC should be attainable. The shallow mw trap has recently been demonstrated experimentally for the cesium atom. Because the mw trap is so shallow it is necessary to precool the atoms for loading. The authors are constructing a hybrid trap in which the atoms are cooled in a static trap and then transferred to a mw trap where they can be further evaporatively cooled in an effort to achieve BEC.more » This trap should have density limits in the region of 10{sup 16} to 10{sup 17}/cm{sup 3} due to three-body recombination. A second, promising approach is in a 2-D geometry for which in finite sized systems BEC is allowed. Hydrogen atoms are confined to a superfluid helium surface by the absorption potential to form a 2-D gas. Although recombination is an important process on helium surfaces, recently the authors have shown that only a very small fraction of the recombination energy is coupled into the surface. With an appropriate geometry the recombination energy can be removed from the sample region to suppress heating to a negligible level. Furthermore in 2-D a magnetic field can have a maximum so that atoms can be confined to a very small area.« less
We study the decay of magnetically trapped doubly spin-polarized atomic hydrogen in the density and temperature regime of the Bose-Einstein phase transition and calculate the temperature and magnetic field dependence of all relevant relaxation rate constants ${G}_{\mathrm{dd}\ensuremath{\rightarrow}\ensuremath{\kappa}\ensuremath{\lambda}}$. At the transition temperature Tc{T}_{c}Tc​ the rate constants have a discontinuous second derivative with respect to T. Below Tc{T}_{c}Tc​ they show a pronounced decrease relative to the extrapolated above-Tc{T}_{c}Tc​ values, the observation of which is a clear signal for the experimental realization of Bose-Einstein condensation.