Turbulent transport associated with GAM dynamics near critical gradient regime; GAM intermittency

臨界勾配領域近傍のGAMダイナミクスに関連した乱流輸送

岸本 泰明; 三木 一弘*; 宮戸 直亮; Li, J. Q.*; Anderson, J.*

Kishimoto, Yasuaki; Miki, Kazuhiro*; Miyato, Naoaki; Li, J. Q.*; Anderson, J.*

トロイダルイオン温度勾配駆動乱流のグローバルジャイロ流体シミュレーションを行い、臨界勾配領域近傍での帯状流生成及びgeodesic acoustic mode(GAM)に関連した新しいタイプの輸送の間欠的ダイナミクスを発見した。間欠的なバーストは、乱流エネルギーがある閾値を超えたときに起きる、空間的に伝播するGAMの発生でトリガーされる。静的な帯状流は準周期的な間欠的バーストをいくつも経て徐々に成長し、乱流揺動は徐々に抑えられる。これは線形臨界勾配の非線形的な上方シフト、いわゆるDimitsシフトにつながる。間欠性の周期は温度勾配の増加とともに短くなる。シミュレーションで得られた間欠的ダイナミクスを定性的に記述するモデルも開発し、背景にある物理メカニズムを議論する。

We found a new type of intermittent dynamics of transport near critical gradient associated with the emission and propagation of geodesic acoustic mode (GAM) coupled with zonal flow based on our global gyro-fluid ITG turbulence simulations. We may refer the prominent dynamics to GAM intermittency. The intermittent bursts are triggered by the onset of spatially propagating GAM when the turbulent energy exceeds a critical value. The GAMs suffer a collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region as that of zonal flow. The stationary component of zonal flows gradually increases over many periods of quasi-periodic intermittent bursts and eventually quenches the turbulence, leading to a nonlinear up-shift of the linear critical gradient, i.e., the Dimits shift. The gradual increase of zonal flow results from the accumulation of non-damped residual part during the propagation of GAMs in each cycle of bursts. The period of burst becomes shorter as the temperature gradient increases. An extended minimum model including dynamical GAM damping which qualitatively describes the intermittent dynamics obtained in our simulation is presented.