Experimental simulation of high temperature plasma confinement using dimensionally similar low temperature plasma

更好地理解高温等离子体中的输运势垒形成等输运现象是实现聚变燃烧等离子体的关键问题。然而，这种现象非常复杂，认识还处于初级阶段。主要原因是难以在高时间和空间分辨率下测量各种等离子体参数和湍流波动。为了解决这一难题，本项目拟采用“量纲相似”的低温等离子体模拟高温等离子体中的输运，其中归一化碰撞频率、等离子体环面β等几个无量纲参数与高温等离子体中的输运相同，但归一化离子陀螺半径用电子温度计算。在低温等离子体中，Langmuir探针可用于测量平衡等离子体参数、波动和各种波动之间的相关性，具有高的时间和空间分辨率。本实验在小型氦回旋加速器（CHS）上进行，主要有两个研究目的：(1)通过发射2.45GHz电子回旋波（ECWs），在低环形场条件下产生并加热低温和密度等离子体；(2)测量低温等离子体中的静电波动，阐明等离子体湍流输输。(1)：在175 ~ 1050g环面场范围内，通过2.45GHz ECW发射，成功并常规地产生了超过普通模式（o模式）截止密度的超密等离子体。在这些密度过大的等离子体中，MHD（碰撞）、平台和1/v状态的碰撞在等离子体小半径的相当一部分范围内实现。这种等离子体的产生和加热被认为是由发射的电子伯恩斯坦波（ebw）转化而来的。用ECW功率调制技术测量的功率吸收分布集中在x模截止层和上层杂化层以外的内部区域。这一结果与模式转换的EBW加热一致。(2)：当发射微波功率超过一定阈值时，电子密度分布图从通常的抛物线分布图明显过渡到具有陡峭梯度的极峰分布图。这表明内部粒子输运屏障的形成。作为一项新的试验，我们试图通过将ECW功率从极低（~1kW）提高到高功率（~20kW）来加热等离子体。在加热功率很低的初始阶段，湍流粒子通量很低，但随着WCW功率的增加，湍流粒子通量迅速向强湍流状态增加。然而，湍流粒子通量在一定水平上立即饱和。随着湍流粒子通量的增加，径向电场的波动明显增大，表明产生了剪切极向流。详细的分析正在进行中。与此同时，我们还研究了高温等离子体在CHS和大型螺旋装置LHD中形成的边缘输运势垒特性。这些结果支持了该项目的进展。少

Better understanding of transport phenomena such as transport barrier formation in high temperature plasma is a key issue for realization of fusion-burn plasma. However, the phenomena are very complex and the understanding is still preliminary. The main reason is difficulty to measure various plasma parameters and turbulent fluctuations with high time and spatial resolutions. To resolve this difficulty, this project aims at simulating transport in high temperature plasma using "dimensionally similar" low temperature plasma where several dimensionless parameters such as normalized collision frequency, plasma toroidal beta and so on are the same as those in high temperature plasma, except normalized ion gyro radius evaluated with electron temperature. In the low temperature plasma, Langmuir probes can be applied for measurements of equilibrium plasma parameters, fluctuations and correlations among various fluctuations with high time and spatial resolutions. This experiment was carried ou … More t in a small sized heliotron/torsatronl device CHS, having two main research objectives : (1) to produce and heat low temperature and density plasmas at low toroidal field condition by launching 2.45GHz electron cyclotron waves (ECWs), and (2) to measure electrostatic fluctuations in this low temperature plasma and clarify turbulent plasma transport. (1) : Plasma production was performed by 2.45GHz ECW launching in the range of the toroidal field from 175 G to 1050 G Over-dense plasmas exceeding the cut-off density for ordinary mode (O-mode) were successfully and routinely generated. In these over-dense plasmas, collisionality regimes of MHD (collisional), plateau and 1/v-regimes were realized over a substantial fraction of a plasma minor radius. This plasma production and heating is thought to be done by electron Bernstein waves (EBWs) converted from launched ECWs. Power absorption profile measured with ECW power modulation technique is concentrated in interior region beyond X-mode cutoff and upper-hybrid layers. This result is consistent with mode-converted EBW heating. (2): When launching microwave power exceeds a certain threshold, electron density profile exhibits a clear transition to a very peaked profile having steep gradient from a usual parabolic one. This indicates formation of internal particle transport barrier. As a new trial, we attempted to heat up a plasma by ramping ECW power from very low(~1kW) to high power (~20kW). In the initial phase with very low heating power, turbulent particle flux is very low but quickly increases toward strongly turbulent state with the increase in WCW power. However, turbulent particle flux immediately saturates at a certain level. With the increase in turbulent particle flux, fluctuations of radial electric field grow clearly, suggesting generation of sheared poloidal flow. Detailed analysis is under way.In parallel to these researches, we also studied characteristics of edge transport barrier formed in high temperature plasmas in CHS and the large helical device LHD. These results support the progress of this project. Less