Experiments with Supersonic Beams as a Source of Cold Atoms

Chapter 2
An Overview of Supersonic Beams
The atomic or molecular beam is the basis of many experiments, and the
starting point of all of the experiments described in this dissertation. As such, a basic
description of the properties of ideal gas flows is provided. An atomic or molecular
beam is generally created by allowing gas to flow through an aperture into another
chamber held at lower pressure. The flow of gas generally falls into one of two regimes,
creating either an effusive beam, or a supersonic beam.
A effusive beam can be created for almost any species, simply by allowing gas
to escape from an aperture into vacuum. This may be done with a gas reservoir, an
oven, or any number of other configurations, making the effusive beam an extremely
general technique. While they are simple to create, effusive beams do have several
drawbacks. Most notably, the temperature of the atoms in an effusive beam is the
same as the temperature in the gas reservoir. This disadvantage is particularly prob-
lematic when an experiment requires cold atoms for greater precision. Effusive beams
also generally have low beam brightnesses, which limits the number of atoms avail-
able for an experiment. The supersonic beam is somewhat more difficult to produce,
but it has several advantages over the effusive beam. In particular, the supersonic
beam is more directional, and is colder than an effusive beam, though it does have
a higher mean velocity. The brightness of supersonic beams can also greatly exceed
those produced by effusive sources.
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