Achievement of Ultra-Low Emittance Beam in the ATF Damping Ring

We report on the smallest vertical emittance achieved in single-bunch-mode operation of the ATF. The emittances were measured with a laser-wire beam-profile monitor installed in the damping ring. The bunch length and the momentum spread of the beam were also recorded under the same conditions. The smallest vertical rms emittance measured is 4 pm in the limit of zero current. It increases by a factor of 1.5 for a bunch intensity of 10{sup 10} electrons. There are no discrepancies between the measured data and the calculations of intra-beam scattering.