BEAM TUNING FOR LOW EMITTANCE IN KEK-ATF DAMPING RING

KEK-ATF is a test facility for future linear colliders producing extremely low emittance beams. The target of the vertical emittance in the damping ring is about 1.5E-11 rad-m, 1% of the designed horizontal emittance. In an electron circular accelerator, the dominant sources of the vertical emittance are the vertical dispersion in arc sections and the x-y orbit coupling, caused by some errors. Previously, it has been reported that a small vertical emittance can be achieved by local orbit bump tuning with a good beam size monitor and a good BPM system [1][2]. We have recently developed a method to correct the vertical dispersion and the coupling by using only the BPM system. Simulations assuming a realistic misalignment, performances of monitors and correctors of the ATF damping ring predict that this method will be effective. Recent beam test showed that the vertical dispersion and the x-y orbit coupling can be made sufficiently small after the corrections. 1 MEASURES OF BEAM QUALITY The dominant sources of the vertical emittance are the vertical dispersion in arc sections and the x-y orbit coupling. We introduce here two quantities which characterize these sources of the vertical emittance. R.m.s. of the vertical dispersion in the arc sections, η arc and the x-y orbit coupling, Cxy.