Beam profile

After aligning the collimator as described in sec. 2.5.2, the collimator can be used to cool the transverse velocity of the He* beam. The angles of incidence $\alpha_{0_{hor}}$ and $\alpha_{0_{vert}}$ are made as small as possible, since a smaller incoupling angle means more reflections in the collimator and thus a more effective collimation of the He* atoms.

To optimize the intensity of the He* beam with the collimator, the channeltron is put in the center of the beam and the horizontal and vertical lasers are coupled in. To make sure that the laser beams do not turn around in the collimator, we check that the laser beams leave the setup through the windows at the other side of the collimator. By letting both laser beams leave the collimator at the same side of the collimator, it can be checked if the laser beams are parallel. If the laser beams are not parallel, the angles of incidence of the pair of laser beams in that plane are not the same, and the collimator can not work optimal.

It is more difficult to make sure that the angle of incidence in the horizontal direction $\alpha_{0_{hor}}$ is the same as the angle of incidence in the vertical direction $\alpha_{0_{vert}}$. It is not possible to measure the angles accurately, since the collimator does not necessarily have to be positioned symmetric with regard to the vacuum chamber, but only to the He* beam. These two angles should be the same to get an optimal increase of He* atoms, since the detuning $-\vec k \cdot \vec v$ should be the same in both directions at every place in the collimator. We first optimized the angle of the laser beams in the horizontal direction. After finding a maximum increase of signal at a detuning of -113 MHZ, the collimator laser was locked to this frequency. We then blocked the horizontal beams, after which the vertical pair of laser beams was aligned to find the optimal angle of incidence. During this process, the position of the collimator itself was also fine tuned to obtain the largest signal increase.

When the collimator and the four laser beams are aligned, we first measure the beam profile at the end of the setup. The channeltron is moved in horizontal direction from one side to the other, and every 1.25 mm the signal is measured four times: with the laser coupled in in horizontal direction, in vertical direction, both directions at the same time, and without lasers coupled in (see fig 3.1).

Figure 3.1: Beam profile without collimator, and with 1 and 2 dimensions of the collimator working

It can be seen that the increase in the signal of the channeltron is almost the same for the vertical and the horizontal dimension. The total increase of the signal when the lasers are coupled in at both dimensions of the collimator, is about a factor two. But since only 50% of the particles detected at the channeltron are metastable Helium atoms, the number of He* atoms increases with a factor 3.

A strange thing that can be seen in fig. 3.1 is that the collimator does not give an increase in signal at the left side of the beam profile. Even if the position of the collimator was slightly changed, it was not possible to get an increase of signal at that place. An explanation could be that the wires of the MOC blocked the direct beam of particles, and the signal measured there exists completely of reflected photons.