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Fig. 3. Illustration of the approximation of the vortex volume. The calculated
vortex boundary on the transverse plane was modeled by an ellipse, which
represents the cross-section of the vortex in the transverse plane. The
calculated vortex boundary and the model ellipse have the same long axis
length and also the same width. The long axes of the calculated boundary and
the approximated shape are parallel. The volume of the vortex is approximated
by the product of the center-width w of the vortex in the laser plane
view and the projected area A of the vortex ring onto a plane
perpendicular to the laser sheet.