Extremely low vertical-emittance beam in the accelerator test facility at KEK

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than $1%$ of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than $2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\mathrm{rad}\mathrm{m}$ at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.