Polarization Measurements

If beam lifetime is limited by Touschek scattering, changes in the level of polarization can be correlated with the Touschek scattering rate. Touschek scattering has a polarization dependent cross section [4]: Increasing beam polarization leads to a smaller Touschek cross section and therefore to smaller loss rates. On the other hand, a sudden decrease in beam polarization (due to a resonant depolarization for example) will lead to a rise of Touschek scattering losses. Recent experiments [3] have shown that beam lifetime in the SLS is Touschek dominated when using a single bunch current of $0.5\--1.5$ mA. By filling only 90 buckets we remained at a low total beam current which guaranteed for low gas pressure and therefore small elastic scattering.

The product of Touschek lifetime and beam current is generally given by the decay constant: $\tau = -\dot{I}^{-1} \cdot I$. A change of $\tau$ can be identified with a possible polarization build-up. Furthermore, measurement of the equilibrium polarization build-up time allows the determination of the equilibrium degree of polarization: After observing the exponential build-up process $\tau_{eff}$ can be fitted. According to eq. 3 the equilibrium polarization level $P_{eff}$ is obtained.

A fit of such a build-up is depicted in fig. 1. Several measurements [1] have shown that high polarization levels can be reached in the SLS storage ring with flat orbit correction and tunes $\nu_x=20.38$, $\nu_y=8.19$, $\nu_{ST}=5.45$ (see eq. 4). After performing vertical beam based alignment (BBA) [5] levels of polarization close to the Sokolov-Ternov level were reached (see fig. 2).

Figure 1: First observed polarization build-up and fit (flat orbit). The fit parameter for the characteristic build-up time is $(1261\pm 12)$ s corresponding to an equilibrium polarization of 63%. In following measurements polarization values of up to 87% were observed.

Figure 2: Polarization build-up and fit (flat orbit and vertical BBA). The fit parameter for the characteristic build-up time is $(1837\pm 1)$ s corresponding to an equilibrium polarization of 91%.