Cooling devices

The beam of atoms defined by the nozzle and the skimmer is very divergent, and has a large spread in velocity of the atoms. All atoms with a transverse velocity larger than 0.1 % of the longitudinal velocity will not hit the channeltron, and thus cannot be used for the experiments. To reduce this loss of He* atoms from the central axis, and to reduce the spread in longitudinal velocity, we use cooling techniques to damp the transverse and longitudinal velocity.

The Zeeman slower consists of a counter-propagating laser beam which is detuned to the red to be in resonance with the mean speed of the atoms. These atoms will be slowed, and get out of resonance after a while. To keep slowing them down, a magnetic field is added which exactly compensates for the change in the Doppler shift due to the slowing. It can be shown [3] that the magnetic field should be of the form:

$$B(z) = B_0\sqrt{1 - \frac{z} {z_0}}$$ (2.2)

with $B_0 = \hbar k v_0 / \mu_B$, $v_0$ the velocity of the He* atoms entering the Zeeman slower, $z$ the location in the Zeeman slower, and $z_0$ is the total length of the Zeeman magnet. By changing the detuning of the laser it is possible to select the end velocity of the atoms.

The MOC, or Magneto-Optical Compressor, is placed behind the Zeeman slower. The slowed atoms are pushed to the center line of the MOC by use of laser light and well chosen magnetic fields. This device turns the slowed divergent atoms into a very small (less than a mm) parallel beam with well defined velocity. Since both the Zeeman slower and the MOC were not operative during my research, I will not describe them in detail (see [6]). The third cooling device is the collimator. It reduces the transverse velocity before the atoms are slowed down in the Zeeman slower.