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Journal of the Optical Society of America B

Energy distributions of trapped atomic hydrogen

John M. Doyle, Jon C. Sandberg, N. Masuhara, Ite A. Yu, Daniel Kleppner, Thomas J. Greytak

Published Nov 1, 1989• 32 citations
Publisher

Abstract

Progress on magnetic confinement and evaporative cooling of spin-polarized atomic hydrogen is described, and the first results are presented for the energy distribution of the trapped gas. Data are displayed for energy distributions during various stages of cooling. For the lowest temperature studied the results are in good agreement with the calculated distribution for a temperature of 800 μK.

DOI
10.1364/josab.6.002244

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Progress on magnetic confinement and evaporative cooling of spin-polarized atomic hydrogen is described, and the first results are presented for the energy distribution of the trapped gas. Data are displayed for energy distributions during various stages of cooling. For the lowest temperature studied the results are in good agreement with the calculated distribution for a temperature of 800 μK.
A gas of hydrogen atoms, confined in a static magnetic trap, has been evaporatively cooled to temperatures of a few millikelvin. The initial trap configuration held the gas at 38 mK for as long as 5 h. Evaporative cooling reduced the temperature to 3.0 mK while maintaining the central density at 7.6\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 ${\mathrm{cm}}^{\ensuremath{-}3}$. These values were determined by measurement of the rate of electronic spin relaxation and are in agreement with model calculations. Further cooling to 1 mK (inferred from the model) has been achieved. Measurements were made of the efficiency of the evaporative cooling process.
We report experiments on evaporative cooling of spin-polarized atomic hydrogen gas confined in a cryogenic magnetic trap, and present a model of the trapped gas based on a generalized truncated Boltzmann approximation for the phase-space distribution function. The model takes into account the dimension of the evaporation [three-dimensional (3D) or 1D] and the time dependence of both the depth and the shape of the confining potential. Our observations are consistent with 1D evaporative cooling. To attain maximal phase-space density, we used the model assuming 1D evaporative cooling to optimize the evaporation procedure. With this work, the low dimension of evaporation was identified as the bottleneck thus far preventing us from achieving Bose-Einstein condensation in trapped atomic hydrogen.
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.
This chapter begins with an introductory summary of basic properties of spin-polarised hydrogen. It discusses the properties of atomic quantum gases at ultra-low temperatures with atomic hydrogen as a primary example. Building on the extensive experience of various research groups with the low temperature gas phase of atomic hydrogen, many interesting aspects could be illustrated with well established experimental and theoretical results. As for many traps the single-particle motion is non-ergodic, interatomic collisions are usually essential to assure ‘sufficient ergodicity’. The chapter also discusses a number of thermodynamic properties relevant for the description of evaporative cooling using the truncated exponential energy distribution as a starting point. It considers the case of plain evaporative cooling. In forced evaporative cooling the escape threshold is lowered with decreasing temperature, thus allowing the evaporation to proceed at a desired rate.
We have confined over 5\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 atoms of hydrogen in a static magnetic trap. The atoms are loaded into the trap by precooling with a dilution refrigerator. At operating densities near 1\ifmmode\times\else\texttimes\fi{}1013{10}^{13}1013 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ the gas is observed to be electron and nuclear polarized in the uppermost hyperfine state. The long lifetime of the trapped gas (over 20 min) suggests that it is thermally decoupled from the wall and has evaporatively cooled to a temperature of about 40 mK. The residual decay of the gas density is consistent with spin relaxation induced by dipolar interactions between atoms.