Electron polarization measurement using a Fabry-Perot cavity at HERA
  A new Compton longitudinal polarimeter currently under construction for HERA is presented. The key component of the polarimeter is a Fabry-Perot cavity located around the electron beam pipe. With such an optical cavity, a continuous laser power equivalent to 5 kW, much higher than those commercially available, can be achieved, leading to one backscattered photon per bunch crossing. This ``few-photon mode'' will allow a very precise determination of the calorimeter response with little systematic uncertainty. The electron polarization measurement at the per mill level is expected. 
Zhiqing Zhang (LAL-Orsay)