Disruption mitigation using laser ablation of high-Z impurities in HL-1M tokamak

A preliminary experiment triggering a plasma current quench by laser ablation of high-Z impurities has been performed in the HL-1M tokamak. The injection of impurities with higher electric charges into tokamak plasmas can increase the radiation cooling of the plasma. Resistive, highly radiating plasma formed prior to the thermal quench can dissipate both the thermal and magnetic energies, which is possibly a simple and potential approach to reducing significantly the plasma thermal energy and magnetic energy before a disruption thereby a safe plasma termination is obtained.     

Disruption Mitigation Using Laser Ablation of High-Z Impurities in HL-1M Tokamak

Major plasma disruptions are considered to be a serious problem for development of tokamak fusion reactors. The preliminary experiment results in HL-1M presented here describe the methods of amelioration of plasma current quench in major disruptions using laser ablation of high-Z impurities, which support the design of next generation large tokomaks like ITER. Using injection of impurity with higher electric charge can produce resistive highly radiating plasma and increase the radiation cooling of plasma to make a safe termination of the disruption. It can be possibly a simple and potential approach to decrease significantly the plasma thermal energy and magnetic energy before a disruption then obtain a safe plasma termination. The magnetic energy can be dissipated by impurity radiation .     

用激光吹气注入高Z杂质使HL-1M托卡马克放电安全终止的研究

采用高Z杂质注入HL-1M等离子体中, 触发等离子体电流衰竭的实验已经施行.用激光吹气注入高Z杂质能够增加辐射冷却，等离子体在-3ms时间内迅速冷却而且在电流终止之前电子温度损失约80%.实验证明：它是一种使得大型聚变实验装置上在放电破裂之前显著减少等离子体中热能而且安全终止放电的简单、快速和有效的途径. 关键词： 高Z杂质 破裂 等离子体终止 激光吹气     

Characteristic of Disruption on HL-2A Tokamak

The tokamak disruption is a dramatic event in which the plasma confinement is suddenly destroyed. Detailed experimental studies of disruptions have been made in many machines. During disruption, the plasma current and plasma thermal energy content collapse in an uncontrollable way, thereby applying mechanical forces and heat loads onto the vacuum vessel components. For that reason, the disruptions in a tokamak must be investigated and the physical processes leading to and occurring at the disruption need to be understood.     

POTENTIAL SAFE TERMINATION BY LASER ABLATION OF HIGH Z IMPURITY IN THE HL-1M TOKAMAK

A preliminary experiment for triggering a plasma current quench by high Z impurities has been performed on the HL-1 M tokamak. Using injection of impurity with the higher charge of the nuclei allows us to increase the radiation cooling. It can be a simple and potential approach for decreasing significantly the plasma thermal energy before a disruption and for safe termination of the plasma.     

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HL-2A装置MHD不稳定性实时预测破裂系统采用了一种简单有效的方法来预测MHD不稳定性导致的等离子体大破裂。利用Mirnov线圈探测MHD信号,根据信号的振幅或频率特点设定计算方法,来预测等离子体破裂先兆,然后用激光吹气注入杂质来缓解等离子体破裂。研究结果表明,该系统能够实时预测破裂先兆,按量注入杂质后,可达到破裂缓解目的。     

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HL-2A和HL-1M装置采用了激光吹气注入高Z杂质来缓减大破裂中的等离子体电流衰竭,并给出了初步实验结果。在HL-2A装置上建立了利用MHD扰动的参量预报放电破裂先兆的报警系统,研制了MHD实时检测与处理系统,实现了放电破裂先兆的预报、快速触发激光吹气、形成阻性高辐射等离子体、消耗热能和磁能,缓减大破裂。实验证明,这是一种使得大型聚变实验装置在放电破裂之前显著减少等离子体中热能和磁能,而且能安全终止放电的简单、快速和有效的途径。     

黑腔中等离子体相互作用的流体力学现象观测

激光间接驱动惯性约束聚变实验中,黑腔内情况复杂,在激光烧蚀和辐射烧蚀等的驱动下,光斑区、冕区、纯辐射烧蚀区、射流区的多种等离子体以不同规律运动.发展了X光双能段窄能带的时间分辨成像方法,用以观测黑腔内多种等离子体的运动情况.在真空黑腔中观测到清晰的射流,分析了射流产生机制及其速度;在黑腔中充气,能有效消除射流和抑制冕区等离子体运动,但两种物质界面处可能会出现流体力学不稳定性等现象,分析了界面处的压力平衡关系和密度陡变情况.     

CT-6B托卡马克装置反常电子热导测量

在CT-6B托卡马克装置上,使用软X射线探测器阵列,测量了磁流体内破裂引起的温度锯齿扰动。经对扰动信号的幅度、周期、相位等分析,计算出不同半径处的平均电子热导系数。实验结果比新经典理论值大一个数量级,证明了CT-6B装置等离子体电子热导的反常性。实验表明,磁流体扰动明显加速能量输运,电子热导随扰动幅度增加而增加。 关键词：     

小孔等离子体运动实验方法

在间接驱动惯性约束聚变的黑腔中,辐射烧蚀的高Z等离子体的流体力学运动过程对激光注入黑腔的效率、辐射场均匀性和通过诊断口的黑腔辐射温度诊断都有显著影响。为研究诊断口在黑腔辐射场中的等离子体缩口过程,用激光产生X光辐射加热低Z泡沫填充的金黑腔诊断口,以激光辐照钛平面靶产生的2～5 keV高能段窄能区X光作为背光源,用X光分幅相机获得了源靶和小孔靶两种靶型的小孔等离子体运动过程图像,研究了X光烧蚀的小孔等离子体的流体力学运动过程,探索了定量测量小孔等离子体面密度的空间分布与时间演化过程的实验诊断方法,初步给出小孔等离子体的面密度。     

Suppression of runaway electrons by resonant magnetic perturbations in TEXTOR disruptions

The generation of runaway electrons in the international fusion experiment ITER disruptions can lead to severe damage at plasma facing components. Massive gas injection might inhibit the generation process, but the amount of gas needed can affect, e.g., vacuum systems. Alternatively, magnetic perturbations can suppress runaway generation by increasing the loss rate. In TEXTOR disruptions runaway losses were enhanced by the application of resonant magnetic perturbations with toroidal mode number n=1 and n=2. The disruptions are initiated by fast injection of about 3x10{21} argon atoms, which leads to a reliable generation of runaway electrons. At sufficiently high perturbation levels a reduction of the runaway current, a shortening of the current plateau, and the suppression of high energetic runaways are observed. These findings indicate the suppression of the runaway avalanche during disruptions.     

Laser Blow-Off Injected Impurity in HL-2A Tokamak

Transient perturbation methods are most appropriate to study particle transport in tokamaks. Two most commonly used techniques of impurity injection are laser blow-off and gas puffing. Short bursts of impurities, injected using the laser blow-off injection technique, are among other transient perturbation methods, undoubtedly best suited to study impurity transport The injection time and the amount of injected material can be controlled in order to study a certain phase of the discharge with a minimum perturbation of the plasma parameters. Furthermore, the source is of very short duration and thus provides an experimentally more direct measure of impurity transport.     

Opacity calculations for high-Z plasma in non-local thermodynamic equilibrium

Opacity of high-Z element plasma in non-local thermodynamic equilibrium (NLTE) is calculated. In the calculation, a collisional radiative model in detailed-configuration-accounting (DCA) is applied to population calculations for NLTE plasmas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based on the first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate the electron wave functions. The spin-orbit splitting is included in the spectrum calculations. The unresolved transition array method is used in the opacity calculations. The calculated frequency-dependent opacity of high-Z plasma Lu is presented. The comparison shows that the present spectrum agrees with another theoretical result well except that the present one shifts a little to the lower energy due to its lower mean ionization stage. The effects of highly doubly excited states on ionization balance and on the opacity are investigated. The mean ionization stage increases more than three stages when doubly excited states 5l6l′ and 5l5l′ are not included in the population calculations and the absorption spectrum also shifts to the higher energy.     

A Diagnostic Method for Time Resolved Spatial Profile Measurements of Proton and Impurity Density and Temperature

A new diagnostic technique, based on elastic scattering of neutral atoms off the plasma ions, is proposed and analyzed. Space and time resolved measurements of the number densities and temperatures of the various components of the plasma are possible by energy analysis of the scattered neutral atoms. The ability to resolve ions of different masses is limited by ion thermal motion and the energy and angular dependence of the scattering cross sections. In hydrogenic plasmas with impurities, the scattering by the impurities is easily resolvable from the scattering of protons, even when the individual impurity contributions cannot be separately resolved. Detected particle count rates are calculated for a conceputal system for a tokamak plasma (n ~ 1013 cm-3, Ti ~ 300 eV). Time resolution of ~ 10 ms with spatial resolution of a few cm are predicted.     

VUV信号强度与等离子体宏观参数相关性分析

东方超环(EAST)上高速真空紫外(VUV)成像系统是一套选择性测量中心波长为13.5 nm的等离子体线辐射的光学成像系统。此系统具有高时空分辨能力,主要用于边界(包括台基区)等离子体行为研究。该系统已经投入EAST等离子体物理实验并获得了大量的实验数据。基于这些数据,分析了VUV诊断系统的信号强度与等离子体宏观参数之间的相关性,着重研究了EAST上中性束注入(NBI)加热功率、杂质(碳和锂)水平、电子密度等因素对VUV信号强度的影响。结果与预期基本一致:随着NBI功率的增加,VUV信号强度随之增强;VUV信号强度与电子密度、杂质水平呈现线性关系。此外,本文还评估了由于NBI注入引起的电荷交换复合产生的C5+离子对VUV信号的贡献,结果表明这部分贡献可以忽略不计。     

Charge perturbations around impurities in metals

The calculated properties of the charge perturbations around impurities in metals are summarized and contrasted with the experimental situation in this field.

The effects of coupling between the field gradient excited by the charge perturbation and the quadrupole moments of (host) nuclei on the nuclear magnetic resonance (N.M.R.) signal are summarized and various kinds of experiment based on this quadrupole effect are discussed.

The gross overall features of the charge perturbation are established by analysing N.M.R. data for copper and aluminium based alloys. This analysis leads to the conclusion that simple theories can account for the main experimental findings, but details, in particular the anisotropy of the charge perturbation, cannot be fully explained because band structure effects play an important role.

The relevance of phase-shift analysis is also assessed, and its applicability in cases of complicated Fermi surfaces is discussed. Effects connected with the energy dependence of the scattering potentials for three-dimensional transition metal impurities are reviewed and evidence for the depression of the charge perturbation near to the impurities is summarized for copper and aluminium based alloys. Temperature dependence, caused by the thermal smearing of the host Fermi surface, by the reduction of the mean free path and by the temperature dependence of the scattering potential (connected to the Kondo effect) is also discussed.

Other relevant experimental techniques are briefly summarized together with effects determined by the charge perturbation around the impurities.     

Role of bremsstrahlung radiation in limiting the energy of runaway electrons in tokamaks

Bremsstrahlung radiation of runaway electrons is found to be an energy limit for runaway electrons in tokamaks. The minimum and maximum energy of runaway electron beams is shown to be limited by collisions and bremsstrahlung radiation, respectively. It is also found that a massive injection of a high-Z gas such as xenon can terminate a disruption-generated runaway current before the runaway electrons hit the walls.     

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使用人工神经网络(ANN)对HL-2A装置破裂放电进行了离线预测研究。采用了两种方法训练网络,一种方法是采用原始实验数据作为网络输入训练网络,另一种是把训练样本中的Mirnov原始实验信号进行预处理,目的是突出Mirnov原始信号隐含的破裂信息。比较这两种方法,结果表明第二种方法获得的网络对破裂放电能够做出更加准确的预测。     

用人工神经网络预测HL-2A等离子体放电破裂

使用人工神经网络(ANN)对HL-2A装置破裂放电进行了离线预测研究。采用了两种方法训练网络,一种方法是采用原始实验数据作为网络输入训练网络,另一种是把训练样本中的Mirnov原始实验信号进行预处理,目的是突出Mirnov原始信号隐含的破裂信息。比较这两种方法,结果表明第二种方法获得的网络对破裂放电能够做出更加准确的预测。     

A New Criterion for Disruption Prediction on HL-2A

A new criterion has been proposed to predict the major disruptions caused by tearing mode instabilities. According to the HL-2A experimental results, the statistical analyses are employed to investigate the relationships between MHD activities and the plasma disruptions. Two kinds of the tearing mode activities can finally cause the disruption on HL-2A operations. By introducing a new parameter, i.e. an integral of poloidal magnetic field over time, as the criterion of disruption precursor, almost all of the disruptions can be predicted.