MEDIUM AND LONG TERM STABILITY

In this regime high mechanical stability is needed to achieve stability on the sub-micron level:

• Stabilization of tunnel, cooling water temperature and digital BPM electronics [5] to $\approx\pm$0.1$^\circ$and the experimental hall to $\approx\pm$1.0$^\circ$.
• Minimization of thermal gradients by discrete photon absorbers and water-cooled vacuum chambers.
• Mechanical decoupling of BPMs with bellows, stiff BPM supports with low temperature coefficients (Invar [6], Carbon Fiber [16]) and/or monitoring of BPM positions [13].
• Monitoring of girder positions [7].
• Full energy injection and stabilization of the beam current to $\approx$0.1 % (``top-up'' operation).

``Top-up'' operation guarantees a constant electron beam current and thus a constant heat load on all accelerator components. It also removes the current dependence of BPM readings under the condition that the bunch pattern is kept constant [5]. Figure 6 depicts the horizontal mechanical offset of a BPM located in an arc of the SLS storage ring with respect to the adjacent quadrupole in the case of beam accumulation, ``top-up'' and decaying beam operation at 2.4 GeV. During accumulation and decaying beam operation BPM movements of up to 5 $\mu$m are observed. The position does not change during ``top-up'' operation at 200 mA after the thermal equilibrium is reached ($\approx$1.5 h). APS [29], SLS [30] and very recently SPring-8 [31] are running ``top-up'' as preferred mode during user operation.

Figure 6: Horizontal mechanical offset of a BPM located in an arc of the SLS storage ring with respect to the adjacent quadrupole in the case of beam accumulation, ``top-up'' and decaying beam operation [30].

It is a difficult task to guarantee sub-micron long-term stability. But since beam lines can be realigned or recalibrated between measurements campaigns which require short and medium term sub-micron stability this seems acceptable.