  The Dragon-II linear induction accelerator is able to provide triple-pulse
electron beams with an adjustable pulse spacing at the minimum of 300 ns. As a
main determinant of the image quality, the X-ray spot size is usually quoted as
an evaluation of the resolving power. The pinhole imaging method is applied to
measure the two-dimensional spatial distribution of the triple-pulse X-ray
source, by which the azimuthal asymmetry of the source can be analyzed. The
spot size of each pulse is characterized by both the full-width at half-maximum
and the LANL definition. The weighted RMS radius at different sum ratios of
photon intensity is also calculated, which directly takes into account the
contribution of every single photon within the boundary of the spot.
