  Quantum electrodynamics predicts x-ray diffractions under a high-intensity
laser field via virtual charged particles, and this phenomenon is called as
vacuum diffraction (VD). In this paper, we derive a new formula to describe VD
in a head-on collision geometry of an XFEL pulse and a laser pulse. A wavefront
curvature of the XFEL pulse is newly considered in this formula. With this
formula, we also discuss the curvature effect on VD signals based on realistic
parameters at SACLA XFEL facility.
