Analysis of the MHD stability and energetic particles effects on EIC events in LHD plasma using a Landau-closure model

Abstract

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, including the effect of the acoustic modes, multiple energetic particles (EP) species, helical couplings and helically trapped EP. 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 β 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 β = 0.025 leads to a decrease of the instability growth rate. The simulations indicate that the EIC in LHD 2 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. The experimental data during the EIC bursting phase shows a complex mode structure and an inward shift of the instability, although no direct evidence of the proposed transition has been observed yet.

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

“…The present model was already used to study the stability of PM during sawtooth-like events and internal collapses [88,89,90,91], TAE [68] and EIC [92] activity in LHD, indicating reasonable agreement with the observations. T th,0 n th,0 < β tot > V A0 (keV) (10 20 m −3 ) (%) (10 6 m/s) 4 0.3 1.8 3.98 Table 1.…”

“…Such 1/2 and 1/1 AEs are in the same frequency range with respect to the 1/2 and 1/1 EIC observed in LHD discharges. The EIC are destabilized by helically trapped particles generated by the perpendicular beam in LHD discharges [43,92], although in the present model the resonance is caused by the passing EP generated by the tangential beam. These results suggest the possibility that the EIC can also be destabilized by passing EP.…”

Theoretical analysis of energetic-ion-driven resistive interchange mode stabilization strategies using a Landau closure model

“…In the case of LHD discharges with EIC events, two different EP components coexist: the passing EP particles driven by the tangential NBI and the helically trapped EP driven by the perpendicular NBI. A previous theoretical study analyzed the effect of the tangential NBI injection intensity on the EIC stability, indicating that the EIC growth rate decreases as the tangential NBI injection intensity increases [11]. The aim of this section is to identify whether this optimization trend is observed in the experimental data.…”

“…For more details regarding the derivation of the average drift velocity operator please see Ref. [11]. We also define the parallel gradient and curvature operators as…”

MHD stability of JT-60SA operation scenarios driven by passing energetic particles for a hot Maxwellian model

“…In addition, energetic particle modes (EPM) can be unstable for frequencies in the shear Alfven continua if the continuum damping is not strong enough to stabilize them [41,42,43,44,45]. Examples of EPM are the energetic-ion-driven resistive interchange mode (EIC) [46,47,48] or the fishbone oscillations [49].…”