Study of interaction between energetic ions and MHD instabilities in a helical plasma

研究了大型螺旋装置(LHD)中性束注入(NBI)加热等离子体中高能离子驱动MHD模的特性及其对高能离子输运的影响。获得了以下新的重要结果：(1)观察到了两种环态诱导的Alfven本征模(TAE)。一种是核局域TAE(C-TAE)，它的带隙位置在等离子体中心附近，具有较低的磁剪切力。另一种是全局TAE，其间隙位置在小等离子体半径的外部。在几次激发中，奇偶宇称的C-Tae同时被激发。(2)当等离子体中心的转动变换超过一定值时，由于去除了Tae禁带，相应的C-Tae被转换为两个全局Alfven本征模(GaE)。在JT-60U装置的反向剪切组态中，也观察到了这种从C-TAE到GAE的模式转变。(3)在高β等离子体中，观察到频率比TAE禁带频率高8倍的高频Alfven本征模。模式与计算的剪切Alfven谱的比较表明，高频模式是螺旋度诱导的Alfven本征模(HAE)。(4)NBI产生的总高能离子中约有30-50%被快速频率啁啾的爆发Te瞬时损失。在紧凑型螺旋系统(CHS)氦加速器/力矩加速器中，由插入电极产生的外磁微扰将TEA激发成无高能离子的低密度低温等离子体。本实验是对螺旋等离子体中Alfven本征模的基础研究。这项研究表明，利用高能离子和外加磁场扰动的Alfven本征模光谱对于研究螺旋等离子体的磁流体性质是非常有用的。

Characteristics of energetic ion driven MHD modes and their effects on energetic ion transport were studied in neutral-beam-injection (NBI) heated plasmas of the Large Helical Device(LHD). The following new and important results were obtained.(1) Two types of toroidicity-induced Alfven eigenmodes (TAEs) were observed. One is a core-localized TAE(C-TAE) of which TAE gap location is near the plasma center with low magnetic shear. The other is a global TAE of which gap location is in the outer portion of the minor plasma radius. In a few shots, C-TAEs with even and odd parity were simultaneously excited.(2) When the rotational transform at the plasma center exceeds a certain value, the relevant C-TAE has been converted to two global Alfven eigenmodes (GAEs) because the TAE gap is removed. This mode transition from C-TAE to GAE is also observed in a reversed shear configuration of the JT-60U tokamak.(3) In high beta plasmas, high frequency Alfven eigenmodes of which frequency is by eight times higher than the TAE gap frequency were observed. Comparison of the modes with the calculated shear Alfven spectra suggests that the high frequency mode is a helicity-induced Alfven eigenmode (HAE).(4) About 30-50 % of total energetic ions generated by NBI is transiently lost by bursting TAEs with rapid frequency chirping. The bursting TAEs often trigger the formation of internal transport barrier.In the Compact Helical system (CHS) heliotron/torsatron, TAEs were excited by external magnetic perturbations generated by the inserted electrodes into a low density and low temperature plasma without energetic ions. This experiment was carried out as a basic study ofAlfven eigenmodes in a helical plasma.This research project has demonstrated that the Alfven eigenmode spectroscopy using energetic ions and externally applied magnetic perturbations is very useful to study MHD properties of a helical plasma.