A conceptual design work and a basic experimental study of a new beam profilemonitor have been performed. The monitor will be used to measure emittance ofan electron beam in the ATF damping ring at KEK, in which the transverse beamsize of about 10 micron is expected. It utilizes a CW laser and an opticalcavity, instead of a material wire, to minimize interference with an electronbeam. A laser beam with a very thin waist is realized by employing the cavityof nearly concentric mirror configuration while the intensity is amplified byadjusting the cavity length to a Fabry-Perot resonance condition. We built atest cavity to establish a method to measure important parameters such as alaser beam waist and a power enhancement factor. Three independent methods wereexamined for the measurement of the beam waist. It was found that the cavityrealized the beam waist of 20 micron with the power enhancement factor of 50.
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