剪切磁场位形下电阻壁模不稳定性的数值模拟

数值研究了平衡磁场位形对电阻壁模稳定性的影响。研究发现,磁场剪切对电阻壁模有解稳作用,对于不同的剪切磁场位形,最不稳定的电阻壁模的环向模数和极向模数不同。等离子流对电阻壁模的增长有抑制作用,稳定住电阻壁模的临界流速度随着磁场剪切率的增大而增大。电阻壁模经线性增长后,进入非线性演化阶段,最后达到饱和状态,剪切磁场位形下的扰动磁能比均匀磁场位形下的扰动磁能饱和度高。     

剪切磁场位形下电阻壁模不稳定性的数值模拟

数值研究了平衡磁场位形对电阻壁模稳定性的影响。研究发现,磁场剪切对电阻壁模有解稳作用,对于不同的剪切磁场位形,最不稳定的电阻壁模的环向模数和极向模数不同。等离子流对电阻壁模的增长有抑制作用,稳定住电阻壁模的临界流速度随着磁场剪切率的增大而增大。电阻壁模经线性增长后,进入非线性演化阶段,最后达到饱和状态,剪切磁场位形下的扰动磁能比均匀磁场位形下的扰动磁能饱和度高。     

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研究了模与模之间的相互作用对电阻壁模(RWM)稳定性的影响.当存在多个模的相互作用时,最不稳定的(3,1)模的线性增长率有所下降.经过线性演化后,因磁力线在壁上的挤压,电阻壁模进入非线性饱和状态.与单个模的演化相比,多个模存在时,(3,1)模的饱和度会下降,(5,2)模的磁能会有相应的增长,而(2,1)模磁能饱和度变化不大.     

低杂波注入对剥离气球模的作用

基于BOUT++代码研究了托卡马克高约束模等离子体中低杂波(LHW)注入对边缘台基区剥离气球模(P-B模)线性和非线性特性的影响.模拟中分别考虑了LHW驱动的常规主等离子体电流和刮削层螺旋电流丝(HCF)产生三维扰动磁场对P-B模的作用.线性结果表明,LHW驱动的主等离子体电流通过降低平衡的归一化压强梯度和磁剪切,使得线性环向模谱整体向高模数和低增长率的方向移动.非线性模拟表明,由于线性模谱的展宽,LHW驱动的主等离子体电流对P-B模不同模式具有整体的抑制效果,可以降低边缘局域模(ELM)造成的台基能量损失;LHW驱动HCF产生的三维扰动磁场可以通过增强不同模式之间的耦合,促进主模之外的其他模式增长来降低ELM造成的能量损失.研究发现,HCF产生的三维扰动磁场促进增长的P-B模式集中在较高模数,当P-B模的主导模式远离此模数区间,ELM能量损失降低更明显.研究结果有助于深入理解LHW控制ELM实验中的物理机制.     

ITER装置中等离子体旋转和反馈控制对电阻壁模影响的数值研究

在托卡马克等离子体中,电阻壁模是非常重要的磁流体不稳定性,特征时间在毫秒量级.对长时间稳态运行下的先进托卡马克,电阻壁模限制着聚变装置的运行参数空间(放电时间和比压),影响经济效益,所以研究电阻壁模稳定性至关重要.本文使用MARS程序,针对ITER装置上9 MA先进运行平衡位形,研究了等离子体旋转和反馈控制对电阻壁模的...     

导体壳对电阻磁流体不稳定性的影响

结合HL-1装置的条件,采用撕裂模的准线性理论,研究了托克马克中导体壁对m=2/n=1扰动模的稳定作用。着重研究了导体壁位置,等离子体电流分布,等离子体位形对这种稳定效应的影响。结果表明,共振面的位置与壁的稳定作用有密切关系,存q_a接近于2的位形中,m=2的撕裂模扰动可以被靠近等离子体的导体壁完全抑制。导体壁的稳定效应与等离子体电流分布相联系,在一些现实的电流分布中,只要适当地压低等离子体边界区的电流密度,壁的稳定效应会更加显现出来。     

等离子体轴向运动对撕裂模不稳定性影响研究

从电阻磁流体模型出发,详细推导了柱形位形下低β等离子体中包括等离子体宏观轴向运动效应的电阻性撕裂模线性不稳定性理论。数值研究发现：等离子体轴向运动速度本身对撕裂模具有明显的稳定作用,而轴向运动速度剪切的作用并不明显。分析表明：轴向运动通过改变扰动势函数和磁通函数之间的相位差(偏离π/2)来降低撕裂模增长率,同时产生一个较低的撕裂模频率。     

激光非均匀性对内界面变形影响的数值模拟

使用LARED-S程序,参照NIF直接驱动DT点火靶,模拟研究了激光非均匀性对高收缩比球内爆内界面变形的影响。2维数值模拟计算表明：直接驱动高收缩比内爆对驱动激光非均匀性非常敏感,内界面流体不稳定性的发展严重破坏靶丸的对称压缩,使中心热斑的体积显著减小。以最大压缩时扰动增长幅度不超过热斑半径的1/3为限,模拟给出不同模数的低阶扰动模(模数小于等于12)对驱动激光均匀性的要求在2.5%～0.25%之间,其中模数在8～10之间的扰动模对激光功率均匀性的要求最严格,约为0.25%。     

激光非均匀性对内界面变形影响的数值模拟

 使用LARED-S程序,参照NIF直接驱动DT点火靶,模拟研究了激光非均匀性对高收缩比球内爆内界面变形的影响。2维数值模拟计算表明：直接驱动高收缩比内爆对驱动激光非均匀性非常敏感,内界面流体不稳定性的发展严重破坏靶丸的对称压缩,使中心热斑的体积显著减小。以最大压缩时扰动增长幅度不超过热斑半径的1/3为限,模拟给出不同模数的低阶扰动模(模数小于等于12)对驱动激光均匀性的要求在2.5%～0.25%之间,其中模数在8～10之间的扰动模对激光功率均匀性的要求最严格,约为0.25%。     

用初-边值方法研究具有固定边界的等离子体Kink不稳定性

本文解出了具有固定边界的平衡等离子体磁面方程的解析解和数值解。用简化的Lax-Wendroff方法求出了压力、速度和磁场的扰动值。得到了线性增长率、平均β和平均β_p以及安全因子。我们发现:1.具有凹向电流剖面的等离子体比具有均匀电流剖面的等离子体更稳定,而具有凸向电流剖面的等离子体是最不稳定的;2.具有逆磁电流的等离子体比具有顺磁电流的等离子体更稳定。 关键词：     

Numerical study of physical properties of resistive wall modes in tokamaks

The effect of plasma with toroidal rotation on the resistive wall modes in tokamaks is studied numerically. An eigenvalue method is adopted to calculate the growth rate of the modes for changing plasma resistivity and plasma density distribution, as well as the diffusion time of magnetic field through the resistive wall. It is found that the resistive wall mode can be suppressed by the toroidal rotation of the plasma. Also, the growth rate of the resistive wall mode decreases when the edge plasma density is the same as the core plasma density, but it only changes slightly with the plasma resistivity.     

Geodesic acoustic modes and zonal flows in toroidally rotating tokamak plasmas

The effect of equilibrium toroidal rotation on the rotational eigen-modes in large aspect ratio tokamak is studied. The case of equilibrium with uniform plasma density on magnetic surfaces is considered. It is shown that the toroidal rotation results in a frequency up-shift of ordinary Geodesic Acoustic Modes. A new unstable low frequency branch of the continuum modes is found. This mode appears as a consequence of the non-uniform plasma pressure created by the centrifugal force on the magnetic surfaces. This mode represents a linear eigen-mode counterpart of Zonal Flow modes. It is shown that the growth rate of such a mode increases with the increase of the angular velocity of toroidal rotation.     

Resistive wall mode in collisionless quasistationary plasmas

The stability analysis of the n=1 resistive wall mode is carried out for a simplified model of collisionless tokamak plasma. It is found that the trapped particle compressibility and the resonance between the mode and the precession drift frequency lead to a significant improvement of the beta stability limits. It is shown that, within the frame of the simplified model, the resistive wall mode can be fully suppressed and the plasma can be stable up to the wall beta limits for a slow plasma rotation.     

Interchange-Ballooning Mode in l = 2 Helical Systems

A high shear toroidal helical system such as Heliotron E gives a limiting beta comparable to tokamaks, by examining stability against interchange-ballooning modes with a low toroidal mode number. The limiting beta depends on equilibrium pressure profiles and a high beta value is found for the pressure having a flat profile in the central region and a large pressure gradient at the high shear peripheral region of the plasma column.     

Evidence for the importance of trapped particle resonances for resistive wall mode stability in high beta tokamak plasmas

Active measurements of the plasma stability in tokamak plasmas reveal the importance of kinetic resonances for resistive wall mode stability. The rotation dependence of the magnetic plasma response to externally applied quasistatic n=1 magnetic fields clearly shows the signatures of an interaction between the resistive wall mode and the precession and bounce motions of trapped thermal ions, as predicted by a perturbative model of plasma stability including kinetic effects. The identification of the stabilization mechanism is an essential step towards quantitative predictions for the prospects of "passive" resistive wall mode stabilization, i.e., without the use of an "active" feedback system, in fusion-alpha heated plasmas.     

Two-dimensional MHD simulation of isothermal plasma armatures

The authors report results on the use of a two-dimensional resistive MHD code to simulate the internal dynamics in a railgun plasma armature, starting from a slightly perturbed equilibrium initial state. The plasma temperature, conductivity, and ionization fractions are treated as uniform in space and constant in time. The rear of the plasma is in contact with a low-pressure nonconducting gas. A finite-difference, explicit, Eulerian, flux-corrected transport code is used to advance all quantities in time. The computation grid has 200 cells parallel to the rails, and 20 across the rails. The results show the growth and subsequent shedding of the plasma mass toward the nonconducting region at the rear. The mass lost is not replenished and the armature becomes shorter, with steeper pressure and magnetic field profiles. The bulk of the profiles is not thoroughly disrupted, but behaves rather sturdily and seems to maintain a shape fairly close to the equilibrium profiles