Measurement of an electron beam size with a laser wire beam profile monitor

We describe the first measurement of an electron beam size in the accelerat or test facility damping ring at KEK with a laser wire beam profile monitor . This monitor is based upon the Compton scattering process of electrons wi th a laser light target, which is produced by injecting a cw laser beam int o a Fabry-Perot optical cavity. We have observed clear signals of the Compt on scattered photons and confirmed that the observed energy spectrum as wel l as the count rate agree with the expected ones. From the measurement, we have deduced the vertical beam size sigma (b) to be 9.8 +/- 1.1 +/- 0.4 mum , where the first (second) error represents statistical (systematic) uncert ainty. Various improvements are in progress to enhance the signal-to-noise ratio, which is essential for the detailed study of the beam dynamics.