  As part of the positron source R&D for future $e^+-e^-$ colliders and Compton
based compact light sources, a high finesse non-planar four-mirror Fabry-Perot
cavity has recently been installed at the ATF (KEK, Tsukuba, Japan). The first
measurements of the gamma ray flux produced with a such cavity using a pulsed
laser is presented here. We demonstrate the production of a flux of 2.7 $\pm$
0.2 gamma rays per bunch crossing ($\sim3\times10^6$ gammas per second) during
the commissioning.
