Yang, R.J. Aryshev, A. Bambade, P. Bergamaschi, M. Naito, T. Terunuma, N. Wallon, S. JACoW Publishing Geneva, Switzerland 978-3-95450-201-1 10.18429/JACoW-IBIC2018-WEPB02 English 429-433 WEPB02 photon diagnostics background detector target Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-IBIC2018-WEPB02 http://jacow.org/ibic2018/papers/wepb02.pdf To study the mechanisms of beam halo formation and its dynamics, a YAG/OTR monitor has been developed and tested at the KEK-ATF. The monitor has four ceramic Ce:YAG screens for the visualization of the beam core and beam halo and an OTR target to provide complementary measurements of beam core. A high dynamic range (DNR>10⁵) and a high resolution (<10 um) have been demonstrated through the optimization of light detection, reduction of background and suppression of scintillation saturation. Measurements using this monitor are consistent with previous results and theoretical modeling of beam halo at ATF, and have allowed further progress in the characterization of the driving mechanisms.