  To guide the experimental design of a linear plasma device for studying the
interaction between energetic ions and Alfv\'{e}nic gap eigenmode (AGE), this
work computes AGE referring to fusion conditions in a ultra-long large plasma
cylinder ended with strong magnetic throats for axial confinement of charged
particles. It is shown that: (i) for uniform equilibrium field between the
ending throats, the dispersion relation of computed wave field agrees well with
a simple analytical model for shear Alfv\'{e}nic mode; (ii) for periodic
equilibrium field with local defect, clear AGE is formed inside spectral gap
for both low and high depths of magnetic throats, although lower depth yields
easier observation. The strongest AGE can be in order of $3.1\times10^{-4}$ to
equilibrium field, making it conveniently measurable in experiment. The AGE is
a standing wave localized around the defect which is introduced to break the
system's periodicity, and its wavelength is twice the system's period,
consistent with Bragg's law. Parameter scan reveals that the AGE remains nearly
the same when the number of magnetic ripples is reduced from $18$ to $8$,
however, there occurs an upward frequency shift when the depth of magnetic
ripples drops from $0.5$ to $0.1$, possibly due to a flute-like effect:
shrinking resonant cavity of spectral gap.
