Measurements of Boundary Plasma in Synergy Discharges of IBW and LHCD on the HT-7 Tokamak

By applying ion Bernstein wave (IBW) heating into the lower hybrid current drive (LHCD) plasma, improved confinements have been obtained in the HT-7 tokamak. The central electron temperature was doubled and the storage energy was increased significantly. The core electron density and temperature were broadened and ther profiles near the edge were steepened. A transport barrier has been formed in the vicinity of the limlter radial location. An enhanced shear in poloidal phase velocity was found in the same region with reduction of the fluctuation levels and the coherences between fluctuations. The results suggest that the improved confinement in the IBW and LHCD plasma is at least partially due to the modification of shear in poloidal velocity and then the suppression of fluctuations and fluctuation induced fluxes via de-correlation effect.     

EAST等离子体电子温度涨落特征的初步实验研究

在EAST上使用相关电子回旋辐射(CECE)诊断系统观测到不同等离子体参数下的电子温度涨落特征,介绍了欧姆放电、L模放电及无ELM的H模放电的三种现象。在欧姆密度爬升等离子体中,电子温度涨落与电子密度之间表现出很强的相关性,即存在电子温度涨落处于较高水平的电子密度的窗口。初步分析表明,电子温度涨落变化是电子密度梯度和电子温度梯度共同影响的结果。不同辅助加热下的L模等离子体中,电子温度涨落的频谱表现出不同的行为。由于电子回旋共振加热(ECRH)的功率有限,其对电子温度的改变很小,而中性束注入(NBI)有较高的注入功率,能够明显提升电子温度,加热方式及加热功率大小引起的电子温度变化与电子温度涨落变化相关。在没有边缘局域模(ELM)的H模期间,可以观测到频率为18kHz的准相干模,其存在于归一化半径ρ=0.71～0.87较宽的径向范围内。     

HT-7超导托卡马克边界等离子体参量及其涨落的实验研究

利用径向可移动朗缪尔三探针和马赫探针对HT-7超导托卡马克边界等离子体参量及其涨落进行了空时空分辨测量.给出了欧姆放电及其与低杂波电流驱动共同作用下边界等离子体电位φ_p、电子温度T_e和电子密度n_e及其涨落的径向分布.实验表明,在限制器附近,存在一由E_r×B确定的极向旋转速度剪切层.在剪切层内,T_e和n_e分布较陡,且φ_p,T_e和n_e的相对涨落水平下降明显.这说明剪切层对边界区的等离子体涨落具有抑制作用.低杂波驱动使径向电场梯度变陡,从而使剪切程度加深但对剪切层宽度无影响.此外,测量表明等离子体环向速度马赫数M_φ存在径向梯度.环向流的这种径向梯度可能是形成径向电场所需的平均极向流的一种重要驱动源 关键词： 托卡马克 边界涨落 低杂波驱动     

Runaway Electron Beam Instability in Slide-Away Discharges in the HT-7 Tokamak

Slide-away discharges are achieved by decreasing the plasma density or ramping down the plasma current in runaway discharges in the HT-7 tokamak. In the case of plasma current ramp down, the ratio of the electron plasma frequency to the electron cyclotron frequency is higher than in the stationary pulses when the discharge goes into a slide-away regime. The instability regime is characterized by relaxations in the electron cyclotron emission due to relativistic anomalous Doppler effect which transfers energy from parallel to perpendicular motion. The triggering of relativistic anomalous Doppler effect at higher density by ramping down of plasma current may provide a alternative runaway energy control scenario.     

HT-7 装置回旋质谱探测器边界离子诊断

介绍了回旋质谱探测器的原理和设计及其在HT-7 装置欧姆放电下对边缘等离子体中离子的诊断实验。探测器安装在限制器附近, 通过一小孔引进等离子体; 设置的前置偏压使电子和离子分离, 并使离子减速; 进入腔体内部的离子在射频电场和平行磁场的作用下发生回旋共振; 通过考察收集的离子电流信号中的共振峰可得到离子的荷质比、回旋频率等参数。实验中观察到荷质比为1、0. 5、0. 3333、0 . 1819 的离子共振峰。     

HT-7装置电子回旋辐射系统接收天线改进

为了提高HT-7装置上等离子体电子回旋辐射(ECE)超外差诊断系统的极向分辨率,根据高斯光学原理,分析了诊断系统天线的改进方案。优化了天线的圆形角锥喇叭尺寸和凸透镜参数以及在装置上的相对位置,使天线接收到的等离子体ECE异常模二次谐波辐射的高斯光束束半径只有30mm左右,能量的高斯光学性质损失不大于13%。     

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为了提高HT-7装置上等离子体电子回旋辐射(ECE)超外差诊断系统的极向分辨率,根据高斯光学原理,分析了诊断系统天线的改进方案。优化了天线的圆形角锥喇叭尺寸和凸透镜参数以及在装置上的相对位置,使天线接收到的等离子体ECE异常模二次谐波辐射的高斯光束束半径只有30mm左右,能量的高斯光学性质损失不大于13%。     

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为了检测到散射信号且有较高的信噪比,使用硅雪崩二极管(APD)探测器探测汤姆逊散射光,采用电荷敏感运算放大器减小电子噪声。同时,设计了放大电路,并采取了一些抗干扰措施。在实验中,此激光汤姆逊散射系统成功地测量出HT-7等离子体的电子温度。     

Analysis of diffusive coefficient of electron heat pulse based on singular value decomposition in HT-7 tokamak

In this paper, singular value decomposition (SVD) as a filter-noise method is applied to electron cyclotron emission (ECE) diagnostic signals. The decomposed vectors contain the information about sawtooth such as the temporal vectors that show the sawtooth period and the spatial vectors that indicate the inverse radius. The propagation of electron heat pulse is investigated from electron cyclotron emission signals by using the perturbation method in HT-7 tokamak. The heat diffusivities are obtained at different densities in ohmic plasmas. The special result is that the heat diffusivity becomes larger as the heat pulse propagates outwards from the outside of the inverse radius.     

AC operation and runaway electron behaviour in HT-7 tokamak

Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition and current sustainment, is described. The behaviour of runaway electrons is analysed by four HXR detectors tangentially viewing the plasma in the equatorial plane, within energy ranges 0.3--1.2~MeV and 0.3--7~MeV, separately. High energy runaway electrons (\sim MeV) are found to circulate predominantly in the opposite direction to the plasma current, while the number of low energy runaway electrons (\sim tens to hundreds of keV) circulating along the plasma current is comparable to that in the direction opposite to the plasma current. AC operation with lower hybrid current drive (LHCD) is observed to have an additional benefit of suppressing the runaway electrons if the drop of the loop voltage is large enough.