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CRC Press eBooks

Theory of Collimated Atomic Beam Formation

Cyril Bernard Lucas

Published Nov 27, 2013• 0 citations
Publisher

Abstract

If a beam is to be produced from a material that is solid or liquid at laboratory temperature, then it must be converted to a vapour in an oven before it passes through the beam-forming impedance. In the reviews mentioned in Section 1.2, the approach has usually been to give details of numerous designs. In this chapter, we discuss the general design principles, and then the next two chapters become more of a reference source for the evaporation of different substances. We thus include the experience of a large number of experimenters who have needed to heat a liquid or solid substance to form it into an atomic or molecular beam. If the material can be evaporated and raised to a temperature to give the required input pressure at the input to the impedance at a temperature less than about 1500 K, it is usual to describe the heating device as an oven. At higher temperatures, the device is usually called a furnace, but the distinction is not really necessary. Hence we will avoid the term except for thermal dissociation of a molecular species, discussed in Chapter 6, when very high temperatures are usually needed, so a furnace is a more appropriate term. For convenience we will also term a completely different approach, namely, the evaporation of the material by direct electron beam heating of the surface (Section 3.2.5) as an oven. ‘Crucible’ is used by many authors interchangeably with ‘oven,’ but it often implies a container for the material to be evaporated that is placed within a heated envelope.

DOI
10.1201/b16345-9

Related papers

Designing an Atomic Beam

Cyril Bernard Lucas

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.

CRC Press eBooks•Nov 27, 2013•0 citations

Concluding Observations

Cyril Bernard Lucas

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.

CRC Press eBooks•Nov 27, 2013•0 citations

Atomic oven with rapid thermal response for atom experiments

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Atomic oven generating controllable atomic beam flux plays a fundamental role in quantum gas experiments. Here, we report a new heater design that can heat up an high temperature atomic oven with fast thermal response. The new heater shows a heating rate improved by 7.65 times comparing to that of the conventional resistive heater while the crucible temperature can heated up to 1200K. With this oven, we generated a collimated ytterbium beam with flux exceeding $10^{14} \text{ atoms/s}$ at 823 K. We believe that our design offers a promising solution for shortening experimental dead time and improve the experiment efficiency in cold atom researches.

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Production of Beams of Dissociated Atoms and Other Radicals

Cyril Bernard Lucas

When a gas or vapour at a higher pressure passes through an impedance to the ow into a region of lower pressure, then in general a beam is formed. Whether the beam is composed mainly of atoms or of molecules, the beam-forming process is the same, and without loss of generality; we will mainly use the term ‘atomic beam’ to mean either an atomic or a molecular beam. We justify this preference since atomic beams are much more common than molecular beams. However, we note that many authors use ‘molecular beam’ to cover atomic beams as well and ‘particle’ is also used. It is also more usual to refer to gas ow through a bounded space such as a tube as ‘molecular ow,’ and ‘molecular beam epitaxy’ (MBE) is a well-established expression.

CRC Press eBooks•Nov 27, 2013•0 citations

Other Indications

Cyril Bernard Lucas

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.

CRC Press eBooks•Nov 27, 2013•0 citations

References

Cyril Bernard Lucas

In Chapter 8, the formal theory of the production of an atomic beam by a beamforming impedance when it is a single tube has been presented. In Section 9.2 we provide easily applied relationships that allow the atomic beam properties to be predicted for any atom in practical circumstances. The limitations are that Γ should be sufciently large and that the gas pressure should be sufciently low that the beam shape is cusp shaped as shown in Figure 1.3 rather than approaching the cosine distribution of Figure 1.1. These limitations, which are normally achieved in practice for a well-collimated beam, mean that the equations are insensitive to tube end effects. Hence the gas density in the entrance plane of the tube ZP is assumed to be the same as that of the ambient gas in VP so that no allowance is made for the pressure drop due to the gas ow in the tube. However, the gas density in the tube exit plane ZV is not assumed to be the same as the zero of V0, since we are following Zugenmaier’s (1966) approach that is discussed in Section 8.5.4. The theoretical tube end effects are discussed in Section 8.5.3. Further discussion of them is deferred to Section 12.4, since their existence could be a source of a disagreement between experiment and theories that do not take them adequately into account. For simplicity the shape of the distribution is also expressed in terms of the halfwidth. If the detailed shape of the distribution is required, this must also be calculated.

CRC Press eBooks•Nov 27, 2013•0 citations
If a beam is to be produced from a material that is solid or liquid at laboratory temperature, then it must be converted to a vapour in an oven before it passes through the beam-forming impedance. In the reviews mentioned in Section 1.2, the approach has usually been to give details of numerous designs. In this chapter, we discuss the general design principles, and then the next two chapters become more of a reference source for the evaporation of different substances. We thus include the experience of a large number of experimenters who have needed to heat a liquid or solid substance to form it into an atomic or molecular beam. If the material can be evaporated and raised to a temperature to give the required input pressure at the input to the impedance at a temperature less than about 1500 K, it is usual to describe the heating device as an oven. At higher temperatures, the device is usually called a furnace, but the distinction is not really necessary. Hence we will avoid the term except for thermal dissociation of a molecular species, discussed in Chapter 6, when very high temperatures are usually needed, so a furnace is a more appropriate term. For convenience we will also term a completely different approach, namely, the evaporation of the material by direct electron beam heating of the surface (Section 3.2.5) as an oven. ‘Crucible’ is used by many authors interchangeably with ‘oven,’ but it often implies a container for the material to be evaporated that is placed within a heated envelope.
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.
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.
Atomic oven generating controllable atomic beam flux plays a fundamental role in quantum gas experiments. Here, we report a new heater design that can heat up an high temperature atomic oven with fast thermal response. The new heater shows a heating rate improved by 7.65 times comparing to that of the conventional resistive heater while the crucible temperature can heated up to 1200K. With this oven, we generated a collimated ytterbium beam with flux exceeding 1014 atoms/s10^{14} \text{ atoms/s}1014 atoms/s at 823 K. We believe that our design offers a promising solution for shortening experimental dead time and improve the experiment efficiency in cold atom researches.
When a gas or vapour at a higher pressure passes through an impedance to the ow into a region of lower pressure, then in general a beam is formed. Whether the beam is composed mainly of atoms or of molecules, the beam-forming process is the same, and without loss of generality; we will mainly use the term ‘atomic beam’ to mean either an atomic or a molecular beam. We justify this preference since atomic beams are much more common than molecular beams. However, we note that many authors use ‘molecular beam’ to cover atomic beams as well and ‘particle’ is also used. It is also more usual to refer to gas ow through a bounded space such as a tube as ‘molecular ow,’ and ‘molecular beam epitaxy’ (MBE) is a well-established expression.
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 Chapter 8, the formal theory of the production of an atomic beam by a beamforming impedance when it is a single tube has been presented. In Section 9.2 we provide easily applied relationships that allow the atomic beam properties to be predicted for any atom in practical circumstances. The limitations are that Γ should be sufciently large and that the gas pressure should be sufciently low that the beam shape is cusp shaped as shown in Figure 1.3 rather than approaching the cosine distribution of Figure 1.1. These limitations, which are normally achieved in practice for a well-collimated beam, mean that the equations are insensitive to tube end effects. Hence the gas density in the entrance plane of the tube ZP is assumed to be the same as that of the ambient gas in VP so that no allowance is made for the pressure drop due to the gas ow in the tube. However, the gas density in the tube exit plane ZV is not assumed to be the same as the zero of V0, since we are following Zugenmaier’s (1966) approach that is discussed in Section 8.5.4. The theoretical tube end effects are discussed in Section 8.5.3. Further discussion of them is deferred to Section 12.4, since their existence could be a source of a disagreement between experiment and theories that do not take them adequately into account. For simplicity the shape of the distribution is also expressed in terms of the halfwidth. If the detailed shape of the distribution is required, this must also be calculated.