VERTICAL EMITTANCE CONTROL AT BESSY

In synchrotron radiation light sources like BESSY II the reduction of the vertical emittance can increase the brilliance of the photon beam, can improve the resolution of certain monochromators, and is required for the planned production of femto second light pulses based on bunch slicing [1]. On the other hand, running the storage ring with only few bunches favours a larger vertical emittance in order to reduce the particle density and Touschek losses. Thus flexible control of this parameter is desirable. The paper describes the steps taken to accomplish this goal. The small vertical emittance is achieved by beam-based alignment, analysis of the coupled orbit response matrix in order to find suitable locations for skew quadrupole magnets, the minimisation of the vertical dispersion, and the decoupling of the transverse planes by observing and correcting the local normal modes at 63 beam position monitors (BPMs). In the single bunch mode the vertical emittance is increased by exciting an artificial difference coupling resonance with a time varying skew gradient field produced by striplines. This approach has certain advantages over other techniques used for blowing up the beam.