Previous Electron Beam Energy Measurements

In order to understand beam dynamics in the SLS Storage Ring it is of fundamental interest to know the precise beam energy. Once the energy is known further studies on beam energy stability over beam lifetime, energy shifts and non-linearities of the momentum compaction factor can be performed.

Until now the most precise beam energy measurements have been calibration measurements of the dipole fields in the bending magnets with Hall probes. These measurements have led to a presumed energy precision  [1] of $\frac{\Delta E}{E}= (1-2)\cdot 10^{-3}$.

In order to get the beam energy with a precision better than $\frac{\Delta E}{E}= 10^{-3}$ new measures have to be taken. One possibility is to measure the resonant excitation frequency at which the spins ensemble gets resonantly depolarized [2]. This technique has recently been used successfully at ELSA [3], BESSY II [4] and ALS [5]. The next sections will discuss polarization of electron beams, depolarizing effects and will explain how the polarization level of an electron storage ring can be determined.