The quasi-periodic boundary conditions in the computer code MAFIA is used to determine the effect on dispersion and on-axis interaction impedance of helical slow-wave structure parameter variations

including width

thickness and relative dielectric constant of the support rods.This is a difficult problem in theory now.The result is that

with the variations of axial width and height of the helical tape

only a slight decrease in the phase velocity

but more decrease in the interaction impedance .There is an optimum size on both axial width and height of the helical tape for the interaction impedance of a helical slow-wave structure.