Achievement of Ultralow Emittance Beam in the Accelerator Test Facility Damping Ring

For high luminosity in electron-positron linear colliders, it is essential to generate low vertical emittance beams. We report on the smallest vertical emittance achieved in single-bunch-mode operation of the Accelerator Test Facility, which satisfies the requirement of the x-band linear collider. 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 at low intensity is 4 pm at a beam energy of 1.3 GeV, which corresponds to the normalized emittance of $1.0\times {1.0}^{-8}\mathrm{m}$. It increases by a factor of 1.5 for a bunch intensity of ${10}^{10}$ electrons. The measured data agreed to the calculation of intrabeam scattering within much better than a factor of 2.

Figure 1

Vertical dispersion measured by BPMs in the damping ring, after dispersion correction. Top: May 20, 2003; bottom: November 26, 2002.

Figure 2

Current dependence of the vertical emittance: Data for the smallest emittance cases (runs B and D) are shown. The result of a SAD simulation for $0.4\%$ coupling is superimposed.

Figure 3

Current dependence of the horizontal emittance: Data for the smallest emittance cases (runs B and D) are shown. The result of a SAD simulation for $0.4\%$ coupling is superimposed.

Figure 4

Current dependence of the momentum spread: Data for the smallest (run D) and the largest emittance case (run $E^{\prime }$) are shown. ($E^{\prime }$ means that the data were taken in same condition as $E$, but on another day.) The results of SAD simulations for $0.4\%$ and $6\%$ coupling are superimposed.

Figure 5

Current dependence of the bunch length: Data are shown for the runs $D^{\prime }$, $F^{\prime }$, and $E^{\prime }$ (the symbols $D^{\prime }$, $E^{\prime }$, and $F^{\prime }$ indicate that the data were taken for the same condition as $D$, $E$, and $F$, but on another day.) The results of SAD simulations for $0.4\%$, $6\%$, and $3\%$ coupling are superimposed.