  The aim of this study is to perform a theoretical analysis of the
magnetohydrodynamic (MHD) stability and energetic particle effects on a LHD
equilibria, calculated during a discharge where energetic-ion-driven resistive
interchange mode (EIC) events were triggered. We use the reduced MHD equations
to describe the linear evolution of the poloidal flux and the toroidal
component of the vorticity in a full 3D system, coupled with equations of
density and parallel velocity moments for the energetic particles species. We
add the Landau damping and resonant destabilization effects using a closure
relation. The simulations suggest that the helically trapped EP driven by the
perpendicular neutral beam injector (NBI) further destabilizes the 1/1 MHD-like
mode located at the plasma periphery (r/a = 0.88). If the beta of the EP driven
by the perpendicular NBI is larger than 0.0025 a 1/1 EIC with a frequency
around 3 kHz is destabilized. If the effect of the passing EP driven by the
tangential NBI is included on the model, any enhancement of the injection
intensity of the tangential NBI below beta=0.025 leads to a decrease of the
instability growth rate. The simulations indicate that the perpendicular NBI EP
is the main driver of the EIC events, as it was observed in the experiment. If
the effect of the helical couplings are added in the model, an 11/13 EIC is
destabilized with a frequency around 9 kHz, inward shifted (r/a = 0.81)
compared to the 1/1 EIC. Thus, one possible explanation for the EIC frequency
chirping down from 9 to 3 kHz is a transition between the 11/13 to the 1/1 EIC
due to a weakening of the destabilizing effect of the high n modes, caused by a
decrease of the EP drive due to a loss of helically trapped EP or a change in
the EP distribution function after the EIC burst.
