Upgraded laser wire beam profile monitor

We describe in this report an upgraded version of the laser wire beam profile monitor developed at KEK to measure a low-emittance electron beam in the ATF damping ring. This monitor is based on the Compton scattering of electrons with a laser light target. A thin and intense laser beam is produced by exciting a Fabry-Perot optical cavity with a CW laser. Various improvements were made to the original setup. First, the effective laser power inside the cavity was increased by an order of magnitude, leading to a drastic increase in a signal-to-background ratio and a decrease in measurement time. Second, readout circuits were modified to separately measure beam sizes of each bunch in a multi-bunch beam. Third, two cavities were installed to measure both horizontal and vertical beam sizes. With this upgraded setup, measurements of electron beam emittance operated in a multi-bunch mode were carried out successfully.