Alfv\'{e}nic gap eigenmode in a linear plasma with ending magnetic throats

Abstract

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.