Beam Polarization Level

The equilibrium beam polarization level was measured as described in chapter 3.2. 90 buckets were filled with a current of $\approx 100$ mA. Injection was turned off and an orbit correction was performed to assure that the beam was on the reference orbit. After this correction tune and injection kickers were turned off and the current decay was observed for 30 min to 1 h. As soon as the product of Touschek lifetime and beam current started to saturate the fit of the build-up converged nicely and fit parameters could be derived.

In order to see polarization build-up the tunes first had to be adjusted; $\nu_x=.42$ was much too close to the expected spin tune $\nu=.4465$ 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. $\nu_x=.42\to .38$ and $\nu_y=.19\to .16$. As soon as these new tune settings were applied polarization build-up was observed. Figure 11 shows an example of build-up to $(60.1\pm0.3)\%$. The two dips in Figure 11 are orbit corrections that were performed during the build-up measurement and led to perturbations of the beam.

Figure 11: Polarization build-up and fit (reference orbit with temperature and insertion device bumps). The fit parameter for the characteristic build-up time is $1213\pm 6$ corresponding to an equilibrium polarization of roughly 60%. The two dips originate from perturbations to the beam induced by orbit corrections.

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 $(86.7\pm0.3)\%$.

Figure 12: Polarization build-up and fit (flat orbit). The fit parameter for the characteristic build-up time is $1751\pm 6$ corresponding to an equilibrium polarization of roughly 87%.

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.