A Compton polarimeter has been installed in Hall A at Jefferson Laboratory. This letter reports on the first electron beam polarization measurements performed during the HAPPEX experiment at an electron energy of 3.3 GeV and an average current of 40 muA. The heart of this device is a Fabry-Perot cavity which increased the luminosity for Compton scattering in the interaction region so much that a 1.4% statistical accuracy could be obtained within one hour, with a 3.3% total error.

Supported by DOE Contract DE-AC05-84ER40150.

Authors: Maud Baylac, E. Burtin, C. Cavata, S. Esco er, B. Frois, D. Lhuillier, F. Marie, J. Martino, D. Neyret, T. Pussieux, P.Y. Bertin, Kees de Jager, J. Mitchell

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