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CRC Press eBooks

Atomic hydrogen in magnetostatic traps

Jook T. M. Walraven

Published Dec 12, 2020• 9 citations
Publisher

Abstract

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.

DOI
10.1201/9781003072973-11

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•Jan 1, 2016•0 citations
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.
Atomic hydrogen is an important model system for the investigation of the properties of ultra‐cold gases. We summarize the current experimental situation for magnetically confined hydrogen, putting emphasis on recent optical experiments with Lyman‐α radiation. As the research is aimed at gas‐phase properties, the samples are studied at densities where interatomic collisions are frequent and internal thermal equilibrium may be assumed. Interestingly, although optically thick, the samples still may be cooled with light. We discuss both Doppler cooling and light induced evaporative cooling.
We analyze the collisionless motion of atoms in magnetic traps in relation to evaporative cooling. For the example of a long Ioffe quadrupole trap we investigate both the regular and stochastic regimes of motion. We emphasize a strong influence of the regime of collisionless motion on the process of evaporative cooling. For evaporation of atoms across an axial potential barrier the stochastic motion of atoms at energies above the barrier enables three-dimensional evaporation, i.e., particles acquiring in elastic collisions a total enery higher than the barrier height E0{\mathit{E}}_{0}E0​ escape from the trap. The regular motion leads to axial evaporation: only atoms which due to collisions acquire an axial energy higher than E0{\mathit{E}}_{0}E0​ leave the trap. The rate of axial evaporation is smaller by a factor \ensuremath{\sim}\ensuremath{\eta}=E0{\mathit{E}}_{0}E0​/T\ensuremath{\gg}1, where T is the gas temperature. This has important consequences for evaporative cooling, which we discuss in relation to trapped atomic hydrogen. \textcopyright{}1996 The American Physical Society.
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 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.
Abstract This chapter provides a summary of some relevant interatomic collisional properties at low energy. Low-energy collisions allow for a fundamental understanding of the role of interactions in dilute gases and are determined by the s-wave scattering length. This is the basic interaction parameter that permits us to describe an important variety of many-body properties of dilute Bose–Einstein condensed gases, as well as of superfluid Fermi gases. This interaction parameter also allows us to describe the formation of bound pairs of atoms in the presence of a Feshbach resonance. A special section is devoted to the study of collisional effects in two dimensions. The second part of the chapter illustrates typical schemes for magnetic and optical trapping currently employed to confine neutral atoms.