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CRC Press eBooks

Other Indications

Cyril Bernard Lucas

Published Nov 27, 2013• 0 citations
Publisher

Abstract

Two topics that logically follow a chapter on gases are included here, namely, the recirculation of gases used to form beams and the levitation of solids. This was originally undertaken with gas beams, but we take liberties with the chapter title by including other methods that may prove useful for atomic beam production. Details of the vacuum systems in which the beam is produced, such as pumping and differential pumping, have not, in general, been included when discussing experiments, since these tend to be closely associated with the experiment for which the beam is required. Neither have general vacuum techniques been discussed, since these are covered in the many textbooks devoted to them. We nevertheless include details of methods that have been or could be used to recycle gases used in atomic beam experiments, since these tend not to be included elsewhere. It is also not our practice to discuss atomic beam applications such as those that are covered by the review articles referred to in Section 1.2, but we make an exception for levitation by atomic beams, since we are not aware of any review that includes these. We then add other levitation techniques that might prove useful for producing beams of refractory metals.

DOI
10.1201/b16345-13

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Two topics that logically follow a chapter on gases are included here, namely, the recirculation of gases used to form beams and the levitation of solids. This was originally undertaken with gas beams, but we take liberties with the chapter title by including other methods that may prove useful for atomic beam production. Details of the vacuum systems in which the beam is produced, such as pumping and differential pumping, have not, in general, been included when discussing experiments, since these tend to be closely associated with the experiment for which the beam is required. Neither have general vacuum techniques been discussed, since these are covered in the many textbooks devoted to them. We nevertheless include details of methods that have been or could be used to recycle gases used in atomic beam experiments, since these tend not to be included elsewhere. It is also not our practice to discuss atomic beam applications such as those that are covered by the review articles referred to in Section 1.2, but we make an exception for levitation by atomic beams, since we are not aware of any review that includes these. We then add other levitation techniques that might prove useful for producing beams of refractory metals.
In Section 1.1 we introduced the subject of atomic and molecular beam formation by stating that a beam with a cosine distribution is obtained when the beam-forming impedance is a circular orice and that a narrow slit has also been employed to produce an improved beam. Most relevant theoretical papers use the convention of referring to ‘molecules’ for both atoms and molecules, but we continue to retain the consistent use of ‘atoms’ to mean both atoms and molecules unless a distinction is necessary. Following the introduction of the basic equations of the kinetic theory of gases in Section 2.2, the relatively simple expressions for the beam formed by an orice were introduced in Section 2.3. It was also mentioned in Section 1.1 that when the impedance is a single tube a much narrower collimated beam is obtained and that by employing an array of tubes a greatly improved beam intensity is obtained. In this chapter we present the theory of beam formation to include the more difcult case of ow through a tube. We only briey mention calculations that only apply when Γ ≤ 10 where Γ is the ratio of length l to diameter d (Section 2.5). These are of little interest for atomic beam formation, but are needed for calculations of Knudsen cell operation, which are outside the scope of this work. Following the discussion of the basic theory, we present in the next chapter simplications that make the design of an atomic beam and the predictions of its performance easier. However, we warn that an extensive comparison of theoretical results with experiment in Chapter 11 points to an unsatisfactory disagreement between theory and experiment for the beam halfwidth, especially in the case of gaseous rather than vapour beams and also particularly when the beam-forming impedance is a focussing capillary array.
Generally lower temperatures of evaporation make most aspects of oven design easier than those discussed in Chapter 5 and included in this group are ovens for the production of beams of alkali metals and their salts. Some special techniques are, however, necessary to handle the alkali metals because of their reactivity with moist air, which increases with atomic number Z. One solution is to produce the beam by a reaction of stable substances in the oven and we begin the discussion of the alkali metals with those cases in Section 4.2. We then consider in Section 4.3 more conventional ovens where the material to be evaporated is loaded directly into the oven. In Section 4.4 we turn to the alkali halides, which have problems due to their corrosiveness in the liquid and vapour state. Ovens where an ampoule of material has to be broken are discussed in Section 4.5. Finally in this chapter we discuss recycling ovens, where material that is not formed into a useful beam is returned to the oven.
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The experimental achievement of Bose-Einstein condensation (BEC) in 1995 has aroused a large interest among the community of the physics of ultracold atomic gases.Recently new topics have attracted the attention of this very active field of research , in particular we could mention low dimensional systems, atoms in lattices, and classical and Fermi gases.As for the case of BEC, the study of the dynamical aspects of these systems provides a very valuable information.In particular time-of-flight experiments and measurements of collective excitations have been successfully realized providing a clear signature of the BEC and the regime of interaction.Following this line, in this thesis, we want to study some dynamical aspects of these new systems nowadays avaliable due to the remarkable development of the experimental techniques.In the first chapter we present the state of the art of the field, providing the historical path and the motivation for the work developed in the next chapters.In the second chapter, we discuss some general concepts in order to introduce the formalism and notation employed throughout the thesis, as well as those topics analyzed in more detail in the further chapters.In chapter three, we explain some general techniques which are useful to deal with one dimensional systems, and analyze in detail the expansion of a one dimensional Bose gas in a guide, which differs significantly from the expansion of a BEC.In particular we are able to predict the range of parameters for which the self-similarity of the expansion is violated.Chapter four is devoted to the physics of atoms in a periodic potential.We start, as in the previous chapter, introducing some general concepts and techniques employed further.First we discuss the physics of the superfluid case in a trap, showing that under certain conditions a three dimensional gas exhibits one dimensional properties.viii In particular, static properties and the lowest collective excitations differ significantly from those of a BEC.We extend the analysis to the insulating phase showing that correlation properties depend on the on-site one-dimensional regime.These correlations can be measured in current time of flight experiments.In the fifth chapter we analyze the dynamics of classical and Fermi gases.We show in detail how expansion and collective excitations depend on the interaction regime and the trap anisotropy.In particular, the expansion can be used to distinguish between a normal Fermi gas and a superfluid one.We summarize in the last chapter the new results presented in this thesis.