Experiments with Supersonic Beams as a Source of Cold Atoms

a method by which beams of helium may be controlled. In the experiment described
here, specular reflection from receding single crystal surfaces is used to control the
velocity of a supersonic beam of helium atoms. The crystal is mounted on the tip
of a spinning rotor that provides the necessary crystal velocity to effectively slow the
beam. The original concept for this experiment was first proposed by Doak [41], but
the design studied had an expected slow beam flux that is significantly lower than in
the experiment described here. The work presented in this chapter is also described
in [33, 42, 43].
The slower itself is similar to the beam paddle that was developed for neutrons
[44]. For helium atoms reflecting from a linearly moving single crystal surface, the
velocity of the atoms after hitting the mirror will be
vf = −vi +2vm, (3.2)
where vf is the final velocity of the atoms, vi is the initial velocity of the incoming
beam, and vm is the velocity of the mirror. The mirror is assumed to be perpendicular
to the incoming beam. Since the typical velocity of a supersonic beam is 500 m/s,
reducing the velocity of the beam by a factor of two requires a crystal velocity of
125 m/s. While it is possible to create a device to repeatably move a mirror linearly
at these speeds (perhaps by mounting the mirror on a projectile and launching and
catching it using electromagnetic coils), it is simpler to let rotary motion approximate
linear motion and mount the crystal on the tip of a spinning rotor arm. This facilitates
the high mirror velocities required to significantly slow the beam. The introduction
of rotary motion complicates equation 3.2, and the full description of the velocity
change due to an atom reflecting from the rotor is shown in appendix A.
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