In order to see polarization build-up the tunes first had to be adjusted; was much too close to the expected spin tune so that the beam was expected to polarize only to low levels. Indeed, the polarization build-up was not noticed in a first experiment at this tune setting. The tunes were set back to ``old'' values used before injection optimization in December. and . As soon as these new tune settings were applied polarization build-up was observed. Figure 11 shows an example of build-up to . The two dips in Figure 11 are orbit corrections that were performed during the build-up measurement and led to perturbations of the beam.
In order to reach higher values of polarization another run was performed with a differently corrected orbit and with vertical corrector settings for a flat orbit. This build-up is shown in figure 12; the fit shows polarization build-up of .
The build-up measurements show that equilibrium polarization levels close to the Sokolov-Ternov level can be reached. This is in good agreement with the assumption that the depolarizing effects in the SLS storage ring are small.
The verification of high polarization levels in the SLS storage ring is crucial for the following energy calibration measurements. Only high polarization levels of the beam allow noticeable depolarization which leads to significant changes in the Touschek scattering rate and therefore to noticeable signals. For the energy calibration measurements a decent polarization build-up always has to be observed in advance. Once such a build-up has occurred the outcome of the depolarization measurement becomes significant for energy measurement.