Roles of Trapped Electrons on Turbulence Driven
Toroidal Momentum and Heat Transport
W.X. Wang
Princeton Plasma Physics Laboratory
Using global gyrokinetic simulation, a robust residual stress is found to be nonlinearly generated in both ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence via symmetry breaking in the parallel wave number spectrum, which is induced by turbulence self-generated flow shear. This residual stress represents a significant, universal, non-diffusive component of toroidal momentum transport, which may play an important role for the generation of intrinsic rotation in tokamak experiments. In the ITG marginality regime, trapped electron physics is shown to play a critical role in determining plasma transport, not only producing the proper ion heat flux in experiments but also largely enhancing the residual stress generation. However, trapped electrons do not change the qualitative phase space structure of ITG driven momentum and heat fluxes. On the other hand, TEM driven momentum transport is made by ions from different regions and in a different way in the phase space. Further, a momentum pinch is shown via simulations using different rigid rotations. Finally, very impressive observations of large eddy formation from earlier phase fine streamers, along with dramatic nonlinear energy cascades to longer wavelengths, in electron temperature gradient driven TEM turbulence is reported.
Study of density fluctuation during LHCD on HT-7 tokamak
章文扬，李亚东
Institute of Plasma Physics, Chinese Academy of Sciences and Center for Magnetic Fusion Theory, Chinese Academy of Sciences, Hefei 230031, China
在HT-7 tokamak上建造了CO2激光相干散射系统，用于测量密度涨落的信号。在低杂波加热的情况下，固定其他参数，进行低杂波的功率扫描。发现在没有形成e-ITB之前，TEM和ETG区间的微观湍流模的稳定和q的分布有关，并且在曲率pinch和热扩散效应的共同作用下，整个等离子体的密度分布是中空的。如果形成的e-ITB，发现TEM区间的微观湍流模趋于稳定，而在ETG区间的微观湍流模会增长。但即使e-ITB没有形成，但等离子体的电子温度分布函数的梯度有明显变化，TEM和ETG区间的微观湍流模也会变化。
Experiment progresses of zonal flow and GAM in the edge plasma of HL-2A tokamak
K. J. Zhao, J. Q. Dong, L. W. Yan, W. Y. Hong, Q. Li, J. Qian, J. Chen, Yi Liu, Y. Huang, Q. Li, X. M. Song, Q. W. Yang, X. T. Ding, X. R. Duan, Yong Liu
1) Southwestern Institute of Physics, P. O. Box 432, Chengdu, China
2) Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou, China
Measurements with a three dimensional set of Langmuir probe arrays have completely proven the co-existence of low frequency zonal flows (LFZFs) and geodesic acoustic modes (GAMs) in the edge of HL-2A tokamak plasma by verifying the spatial characteristics and the nonlinear couplings with ambient turbulence. Besides, their radial wave number of the LFZF and GAM demonstrated clearly linear dispersion relations. The analysis of the modulation in ambient turbulence envelopes have revealed that their spatio-temporal structures are similar to those observed in zonal flows and GAMs. The findings provide concrete evidences that the envelope modulation should be caused by the zonal flows, and strengthen the feasibility of the indirect method to detect the zonal flows through the density fluctuation envelopes that could be measured with various diagnostics. ECRH heating (300-700kW) and q (3.5-6.2) scaling experiments indicate that the intensity of zonal flow increases with higher ECRH heating power and lower edge safety factor.
Physics and control of the resistive wall mode
Yueqiang Liu
UKAEA Culham, UK
Resistive wall mode (RWM) is one class of the MHD instability, that may give severe limitation on the operational space of advanced tokamaks. Understanding the physics of this mode and the possibility of its active control using magnetic feedback coils has been an intensive research area during the recent years, both in experiments and in theory.
We study the RWM stability under the influence of the plasma toroidal flow, as well as certain drift kinetic resonances. The modelling results with a self-consistent, full toroidal MHD-kinetic hybrid code suggests a strong stabilisation effect of the kinetic damping on the mode.
The RWM interacts strongly with the conductors (the resistive walls) surrounding the plasma. A realistic modelling of the mode stability and control involves detailed 3D geometrical effects of the conductors. Simulation is carried out using a recently developed MHD-eddy current code, for various tokamak plasmas, such as DIII-D and ITER.
Reconnection properties in Hall MHD Plasmas*
MA Zhiwei (马志为)1,2, Lu Xingqiang (路兴强)1, and Hou Yawei (侯雅巍)2
1Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou 310027, China
2Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
Abstract: Dynamics and properties of magnetic reconnection with an asymmetry initial condition are studied by using Hall MHD simulation. In the early phase of the magnetic reconnection, the geometry of the magnetic field in the near reconnection region tends to form a Y-type structure under an asymmetry initial condition instead of to exhibit an X-type structure in the symmetry initial condition. The growth rate of the current density in the early stage is proportional to the square of the ion inertial length. In the late phase, the thin elongated current sheet associated with the Y-type geometry of the magnetic field breaks-up to form a magnetic island due to the secondary tearing instability. It is found that the onset time of the secondary tearing instability is inversely dependent on the square root of the ion inertia length. The generated Hall electric field and net charge are larger in the region inside than outside the magnetic island. The strengths of the Hall electric field and net charge density linearly increase with the increase of the ion inertia length.
*Supported by National Science Foundation of China under Grant No. 40536030, and the National Basic Research Program of China under Grant No 2008CB717806
Magnetic reconnection with pressure tensor in electron magnetohydrodynamics
Huishan Cai（蔡辉山）
Department of Modern Physics, University of Science and Technology of China, Hefei 230026, P. R. C
The effects of electron pressure tensor and electron inertia at tearing mode with guide magnetic field are analyzed systematically in electron magnetohydrodynamics. The behavior of tearing mode depends not only on the magnitude of pressure gradient, but also on the relative magnitude between the parallel and perpendicular pressure gradient. When electron inertia dominates over pressure gradient, the pressure gradient only modifies the growth rate of tearing mode slightly. When pressure gradient is dominant, pressure gradient drives reconnection if the perpendicular pressure gradient is larger than parallel pressure gradient. Otherwise, tearing mode is suppressed completely when pressure gradient is large enough.
Influence of circulating energetic ions on fishbone instabilities driven by trapped energetic ions
Deng Zhou
Institute of Plasma Physics, Chinese Academy of Sciences and Center for Magnetic Fusion Theory, Chinese Academy of Sciences, Hefei 230031, China
Fishbone modes were excited in experiments with nearly perpendicular neutral beam injection, but only in the co-injection cases. No fishbone was observed in a counter-injection experiment [K. McGuire K. et al. 1983 Phys. Rev. Lett. 50 891]. The previous theoretical work did not address this problem. In this paper, the role of the passing energetic ions is included. The influence of the passing ions on internal kink modes is different for co- and counter- passing ions when their orbit intersects the surface, where is the usual safety factor. The co-passing ions cross the region enclosed by the surface mostly on the high field side, where the direction of the ion magnetic drift is opposite to the mode propagation. So no resonance occurs between the mode and these co-circulating ions. While for counter-injection, the situation is reversed and a resonance may occur between the mode and these counter-circulating ions. This resonance causes a negative imaginary part in the potential functional which increases the threshold beta value of the trapped hot ions to excite a fishbone. The work leads to an important implication that the fusion alpha particles can not destabilize fishbone modes.
Study of Internal Kink Modes Excited by Barely Passing or Barely Trapped Energetic Particles in Tokamak Plasmas
Hongda He1, Jiaqi Dong1,2, G.Y. Fu3
1Southwestern Institute of Physics, Chengdu, Sichuan, 610041, China
2Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou, 310027, China
3Princeton Plasma Physics Laboratory, Princeton, NJ, USA
Internal kink (fishbone) modes, driven by barely passing or barely trapped energetic particles (EPs), are numerically studied and the preliminary results are presented. The spatial distribution of the EPs is taken into account. A new branch of the modes with frequencies comparable to toroidal precession frequency(~104) or diamagnetic drift frequency of hot ions(~105) is demonstrated to be driven by density gradient of barely passing energetic ions through resonant or non-resonant excitation. The unstable modes of different frequencies are found dependent on the spatial distribution of the EPs, or heating position of neutral beam injection (NBI). The low frequency (comparable to) modes can be resonantly excited in the case of positive spatial density gradient of the EPs, (i.e. off-axis heating), whereas a negative spatial density gradient (i.e. on-axis heating) can drive the high frequency (comparable to) modes. Interestingly, it is also numerically found that there exists a second stable regime for the modes, similar to that of the ballooning mode. The results show that a damping due to magnetic gradientand curvature () drifts as well as spatial density gradient of the hot passing ions results in the formation of the second stable regime.
Electron and Ion Acceleration by Ultrahigh Intensity Lasers
M. Y. Yu（郁明阳）12
1.Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou 310027, China
2.Institute for Theoretical Physics I, Ruhr University, Bochum D-44780, Germany
The fast-ignition schemes of inertial confinement fusion rely on the availability of very high energy electrons and ions, which can be produced by the interaction of ultrahigh-intensity lasers with plasmas. Several novel schemes have been proposed or discovered. Here the basic physical processes involved in the laser acceleration of charged particles shall be reviewed.
Controlling Strong-Nonlinear Dynamics: From Chaos to Turbulence
Xingang Wang （王新刚）
Institute of Modern Physics, Zhejiang University, Hangzhou 310027, China
In this talk, starting from the models of low-dimensional ODE systems, I will briefly introduce the basic ideas involved in controlling strong-nonlinear dynamics. Nextly, I will show in detail how to implement these ideas in PDE systems. Special attentions will be paid to the technique of pinning control, and demonstrations of this technique to the control of turbulence will be shown. Finally, I will conjecture/highlight the possible applications of pinning control to the MHD turbulence.
HL-2A装置高能粒子物理的实验研究
陈伟*, 丁玄同, 刘仪, 杨青巍, 宋宪英, 周俊, 雷光玖, 董家齐，段旭如，and HL-2A team.
Southwestern Institute of Physics, P.O.Box 432 Chengdu 610041, China
E-mail contact of main authors: ,
在未来的燃烧等离子体中，D-T反应会产生大量的高能粒子(3.5MeV)，同时也会存在大量的高能离子和高能电子。因此，研究这些高能粒子行为对聚变反应是及其重要的。高能粒子由于能量高，轨道半径比热离子或热电子的轨道半径大得多，因此它的轨道偏移也很大，在同样磁场的条件下它比热粒子更难约束，极易引起粒子和能量损失，致使D-T反应不能持续进行。同时，高能粒子会轰击聚变反应器部件，使反应器受到破坏而带来很大损失。
高能粒子物理的研究内容主要包括：1）高能粒子与磁流体不稳定性(MHD)的相互作用；2）高能粒子与等离子体湍流的相互作用等。国际上，对高能粒子的研究已经开展将近30年，在理论和实验上都取得了很多重要成果，但是目前还有很多现象没有认识清楚，比如阿尔芬本征模和鱼骨模的非线性激发及其在相空间中动理学问题等。同时，高能粒子对低频磁流体不稳定性（如动力气球模和局域内交换模等）的影响及与输运之间的联系也是重要的研究内容。在强辅助加热（ECRH+NBI）条件下，在HL-2A装置上进行高能粒子物理的研究，包括鱼骨模不稳定性和剪切阿尔芬本征模等。利用新的诊断系统CdTe进一步开展了电子鱼骨模不稳定性的研究。其中，包括电子鱼骨模频率的各种chirping行为，模频率的跳跃问题，模强弱场侧激发及模传播方向等问题，特别是模激发和高能电子之间的关系。在大功率的中性束加热的情况下，开展了离子鱼骨模不稳定性的研究。实验上观测到三种类型的离子鱼骨模，它们分别是锯齿混杂型，连续型和经典型。这些模的在频率6-20kHz，极向传播在离子的逆磁漂移方向上，它们具有强弱场不对称性，它们和m=3或m=4的模之间存在环向耦合，并且引起了很大的温度和密度扰动，这表明离子鱼骨模导致了很大的粒子和能量损失。另外，中性束产生的Sub-Alfvenic离子会通过Sideband共振激发AE，实验上也观测到了Pitch-fork 现象，这为相空间动理学的研究提供了实验基础。在欧姆加热的情况下，特别是在电流的下降段，观察到大的磁岛与测地声曲率耦合，提供足够高的自由能，进而激发BAE。实验上发现这种BAE一般成对出现，在相反方向传播，形成驻波结构。实验结果表明撕裂模频率决定了两支BAE模的频差。在ECRH加热情况下，观测高能电子也可以激发BAE，这可能与高能电子激发的TAE和EAE类似。
Nonlinear evolution of double tearing mode in Hall MHD
Zhang Chenglong (张城龙)，MA Zhiwei (马志为)
Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou 310027, China
Nonlinear evolution of a double tearing mode for different resistivities and ion inertial lengths is investigated using Hall magnetohydrodynamics simulations. In the Hall dominant regime, the magnetic field configuration in the reconnection region evolves from Y-type to X-type geometry, which leads to fast reconnection in the nonlinear growth phase. The maximum reconnection rate in the explosive growth phase is found to have a scaling and the maximum total kinetic energy scales as . In the regime with weak Hall effect, it is found that the elongated thin current sheet formed in the early phase is broken into two X-points, forming a magnetic island in the late stage, instead of shrinking to an X-type geometry.
Nonlinear evolution of resistive tearing mode with
sub-Alfvenic shear flow
Jiahui Li （李家辉）
Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou, 310027, China
Abstract: A two-dimensional incompressible MHD model is employed for studying the nonlinear evolution of tearing instability with equilibrium parallel shear flow. The equations are solved with numerical method in slap geometry. Only sub-Alfvenic shear flow is used in present work. We find that the flow can have either stabilizing effect or destabilizing effect on the tearing mode. When the flow shear length is larger than a critical value, the shear flow always has a destabilizing effect on the tearing modes.
Floating potential of ablating insulated probe in plasma
S.Mercurio
North University of China, Taiyuan, Shanxi
By floating potential is meant the electric potential of the isolatedbody surrounded by plasma. The insulated wall of the electric discharge chamber, the electrically isolated Langmuir probe and any injected pellet, globule o macro-particle in plasma are just few important examples. Since the plasma potential surrounding the probe is higher than that of the isolated probe potential (when the probe is negatively charged) there is a positively charged layer (sheath) between the (quasi) neutral plasma and the negatively charged surface of the solid globule. In this paper we are showing how the change of the solid layer of the probe (heated by energy flow from the plasma) into fluid phase, affects the electric field distribution in the sheath and therefore how the electric potential of the ablating solid be obtained.