Numerical simulation of the multiple core localized low shear toroidal Alfvenic eigenmodes

Wenjia Wang,Youjun Hu,Deng Zhou,Yue Ming

doi:10.1063/1.5010407

Abstract

In modern tokamak experiments, scenarios with weak central magnetic shear has been proposed. It is necessary to study the Alfvenic mode activities in such scenarios. Theoretical researches have predicted the multiplicity of core-localized toroidally induced Alfvenic eigenmodes for ε/s &gt; 1, where ε is the inverse aspect ratio and s is magnetic shear. We numerically investigate the existence of multiplicity of core-localized TAEs and mode characteristics using NOVA code in the present work. We firstly verify the existence of the multiplicity for zero beta plasma and the even mode at the forbidden zone. For finite beta plasma, the mode parities become more distinguishable, and the frequencies of odd modes are close to the upper tip of the continuum, while the frequencies of even modes are close to the lower tip of the continuum. Their frequencies are well separated by the forbidden zone. With the increasing value of ε/s, more modes with multiple radial nodes will appear, which is in agreement with theoretical prediction. The discrepancy between theoretical prediction and our numerical simulation is also discussed in the main text.

Highlights

Toroidicity-induced Alfven eigenmodes (TAEs) which are transverse low frequency electromagnetic waves that propagate along the magnetic field B exist in tokamak plasmas due to the toroidal coupling of the neighboring poloidal harmonics.[1,2] These modes may be readily destabilized by energetic particles produced by auxiliary heating and high energy alpha particles in ignited tokamak plasmas, resulting in degraded confinement.[3,4,5] the study of the TAE and their stability characteristics has attracted considerable attention
A forbidden zone appears between the even and odd modes with the even modes located at the upper half of the gap and odd modes at lower half of the gap, which is opposite to the discovery of Berk et al A recent work by Marchenko[16] took the finite pressure effect into account in the study of multiple core localized TAEs and revealed a critical pressure gradient above which the parity of modes is reversed
We have numerically studied the multiple core-localized toroidal Alfvenic eigenmodes in a tokamak with weak central magnetic shear using the NOVA code

Summary

INTRODUCTION

Toroidicity-induced Alfven eigenmodes (TAEs) which are transverse low frequency electromagnetic waves that propagate along the magnetic field B exist in tokamak plasmas due to the toroidal coupling of the neighboring poloidal harmonics.[1,2] These modes may be readily destabilized by energetic particles produced by auxiliary heating and high energy alpha particles in ignited tokamak plasmas, resulting in degraded confinement.[3,4,5] the study of the TAE and their stability characteristics has attracted considerable attention They have been under active investigations in experiments as well as in theories and simulations since their discovery.[3,6,7,8,9,10,11] In the early studies, it was found that there exists a critical pressure gradient above which no TAEs exist.[8] A late work by Fu et al[12] indicated that the critical pressure gradient is enhanced in the core region of tokamaks if the finite aspect ratio effect was taken into account.

THE EXISTENCE OF CORE-LOCALIZED TAES

PRESSURE GRADIENT EFFECT ON THE CORE-LOCALIZED TAES

SUMMARY AND CONCLUSION