Statistical Picture of hysteresis characteristics of transition phenomena in steady state current drive plasma

本研究的总结如下。在电流驱动等离子体中，研究了各种跃迁现象。首先，从磁滞的角度研究了约束（∝等离子体存储能量/输入功率）和电流驱动效率（∝电流^★密度^★主半径/功率）的跃迁。发现存在一个触发跃迁的功率阈值。在转变过程中，离子和电子温度升高，密度增大。由于驱动电流提高，电流驱动效率提高。并对转移概率进行了研究。驱动功率随时间变化，比约束时间长得多，从阈值功率以下到阈值功率以上的过渡特性称为前向过渡。还研究了从高于阈值的“增强电流驱动”模式到低于阈值功率的l模式的反向转换。转换概率被定义为延迟…相对于功率的变化，转换延迟的时间更长。对正反向跃迁的跃迁概率进行了研究，发现正反向跃迁之间存在一个临界差，这个差导致了迟滞。其次，研究了双向低混合电流驱动时电流驱动特性的转变。在实验中，将正向LHW和反向LHW叠加，研究了驱动电流的方向性。即正向电流由FW LHW驱动，反向电流由BW LHW驱动。实验表明，电流方向性从BW方向向FW方向转变是BW/FW功率比的函数。当功率比超过~ 0.8的临界值时，就会触发这种转变。结果表明，当BW-LHW叠加且功率比超过临界时，电流密度曲线逐渐变窄，然后电流密度曲线突然从中心峰值转变为宽峰值。这个实验没有考虑迟滞。第三，从等离子体壁相互作用的角度研究了长脉冲放电中的跃迁现象。首次发现了极低频的振荡。这被称为ULF事件。在长脉冲工作中，有几个过程决定着放电特性。输入射频功率通过射频天线前的隧穿过程从板外侧发射。与等离子体的射频耦合在很大程度上取决于该区域的密度梯度。在长脉冲放电过程中，来自等离子体的热负荷和逸出的高能电子沉积在壁面和天线上。表面温度的升高改变了等离子体组件的泵送特性。因此，在放电过程中，边缘或溶胶密度可能会发生变化。这种扰动改变了耦合的减小或反射功率的增加。在ULF事件期间发生过渡，然后终止5小时放电。热负荷和粒子扰动从环面周围的主导PFC（轨道限制器）迅速增长，最终使放电崩溃。相关的事件是热点的形成，这可能是由逸出的高能电子引起的。用红外光谱仪测量了热点的局部温度。研究了氢Balmer带和Faulcher带随表面温度的变化规律。当热点温度超过~ 2100 K的临界值时，两种发射谱线均开始非线性增加，随着温度的升高，两种发射谱线急剧增加，即使温度继续升高，两种发射谱线也会发生翻转，最后在较低温度下衰减到原来的水平。少

The summary of this research is as follows.In the current drive plasma, various transition phenomena have been investigated. Firstly, the transition for the confinement (∝ plasma stored energy/input power) and current drive efficiency (∝ current^★density^★major radius/power) is studied with view point of hysteresis. It is found that there exists a power threshold above which a transition is triggered. At the transition, both ion and electron temperature are increased and the density is also increased. Since driven current is raised, current drive efficiency is enhanced. The transition probability is also studied. The driven power is varied with a time, much longer than the confinement time, and the transition characteristics below the threshold power to above it, called forward-transition. Backward- transition is also studied from the "Enhanced current drive" mode achieved above the threshold down to the L-mode below the threshold power. The transition probability is defined as a delay … More time with which a transition is delayed with respect to a change in power. This transition probability is studied for both forward and backward transitions, and it is found that there is a critical difference between then and this difference leads to a hysteresis.Secondly, a transition in current drive property is studied for bi-directional lower hybrid current drive operation. In this experiments, the forward LHW and backward LHW are superposed and the directivity of the driven current is studied. Namely, the forward current is driven by FW LHW, but the backward current is driven by the BW LHW. The experiments show that a transition of the current directivity from the BW direction to FW direction as a function of the BW/FW power ratio. This transition is triggered when this power ratio exceeds a critical value of 〜 0.8. It is found that the current density profile becomes narrow when the BW-LHW is superposed and the power ratio exceeds above the critical one, and then the transition of the current profile changes from the centrally peaked to a broad one suddenly. A hysteresis is not taken for this experiments.Thirdly, a transition is studied in a long pulse discharge from a view points of plasma wall interaction. Oscillations at the very low frequency are found for the first time. This is called ULF events. In the long pulse operation, there are several processes which dominate discharge characteristics. The input rf power is launched from the out board side via a tunneling process in front of the rf antenna. The rf coupling to the plasma depends strongly on the density gradient in this region. During the long pulse discharge, the heat load from the plasma and escaped energetic electrons are deposited on the wall and antenna. The surface temperature rise changes the wall pumping property of the plasma facing components. Thus the edge or SOL density may vary in the discharge. Then this perturbation changes the reduction of the coupling or increment in the reflected power. A transition occurs during the ULF event, and then the five hour discharge is terminated. The heat load and particle perturbation grow rapidly from the dominant PFC (rail limiter) around the torus and finally they collapse the discharge. The related events are hot spot formation which may caused by the escaped energetic electrons. The local temperature on the hot spot is measured with the IR spectrometer. The hydrogen Balmer and Faulcher band are studied as a function of the surface temperature. It is found that both emission lines start to increase non-linearly as the hot spot temperature exceeds a critical values of 〜2100 K and they are sharply increased with increasing temperature and turns over even when the temperature still increases, and finally they decay to the previous level at the lower temperature. Less