[en] The proposed I-MW neutron spallation source is a rapid-cycling synchrotron (RCS) with a design intensity of 1.04 x 10¹⁴ protons. A H- beam from the linac is injected into the synchrotron via the charge exchange process. Due to the high intensity of the beam, the minimization of beam loss is one of the primary concerns. In this paper, we study a possible beam loss associated with field ionization, which includes estimates of the charge fraction and level distribution of the excited hydrogen atoms after stripping, and an estimate of lifetime of the excited hydrogen atoms in the transverse magnetic field