HT-6B托卡马克参数下低杂波驱动电流的密度窗口

在简晰的物理模型下,采用数值方法,研究了HT-6B托卡马克参数下的低杂波射线轨迹,波谱移动,功率吸收和驱动电流分布。结果表明,存在着驱动电流的密度窗口,即仅在一定的等离子体密度范围内才能驱动电流(通常在理论和实验上仅注意到驱动电流的密度上限)。文中对这种密度窗口的存在给予了物理解释,并进而讨论了影响驱动电流效率的主要因素。 关键词：     

Radial transport dynamics studies of SMBI with a newly developed TPSMBI code

In tokamak plasma fueling, supersonic molecule beam injection(SMBI) with a higher fueling efficiency and a deeper penetration depth than the traditional gas puffing method has been developed and widely applied to many tokamak devices.It is crucial to study the transport dynamics of SMBI to improve its fueling efficiency, especially in the high confinement regime. A new one-dimensional(1D) code of TPSMBI has also been developed recently based on a six-field SMBI model in cylindrical coordinate. It couples plasma density and heat radial transport equations together with neutral density transport equations for both molecules and atoms and momentum radial transport equations for molecules. The dominant particle collisional interactions between plasmas and neutrals, such as molecule dissociation, atom ionization and charge-exchange effects, are included in the model. The code is verified to be correct with analytical solutions and also benchmarked well with the trans-neut module of BOUT++ code. Time-dependent radial transport dynamics and mean profile evolution are studied during SMBI with the TPSMBI code in both slab and cylindrical coordinates. Along the SMBI path, plasma density increases due to particle fuelling, while plasma temperature decreases due to heat cooling. Being different from slab coordinate, the curvature effect leads to larger front densities of molecule and atom during SMBI in cylindrical coordinate simulation.     

Parametric decay instabilities of lower hybrid waves on CFETR

The lower hybrid current drive is a potential candidate for sustaining plasma current in tokamak steady-state operations, which could be used in China Fusion Engineering Test Reactor (CFETR) with input power up to a few tens of megawatts. Such high input power could trigger the well-known parametric instabilities (PIs) at the plasma edge affecting the propagation and absorption of the lower hybrid pump waves. By analytically solving the nonlinear dispersion relation describing PIs, an explicit expression of the PI growth rate is obtained and analyzed in detail. It is found that pressure is the key parameter determining the PI characteristics. Ion sound quasi-mode is the dominant decay channel in the low-pressure regime, while the ion cyclotron quasi-mode (ICQM), as well as its harmonics, becomes dominant in the intermediate regime. In the high-pressure regime, only one mixed channel is found, which is related to Landau damping by free-streaming ions. Analytical expressions of growth rates of these decay channels are also obtained to show the parameter dependence at different pressure limits. The above analytical results are used to estimate the PIs on a typical profile of CFETR, and verified by corresponding numerical calculations. ICQM is found to be the strongest decay channel with a considerable growth rate for CFETR.     

捕获电子对低杂波与电子回旋波的协同效应的影响

电子回旋波和低杂波的协同效应可有效地提高两只波的电流驱动效率.本文数值研究了捕获电子效应对电子回旋波和低杂波协同的影响.结果显示,随着捕获角的增大,双波协同驱动电流会减小,且协同因子也会明显减小,即捕获角对两只波协同驱动流的影响要比其对单独驱动电流的影响更加敏感.通过加宽低杂波共振区可减弱电子回旋波电流驱动对捕获角的依赖,同时发现随着电子回旋波功率的增加,捕获角对电子回旋波电流驱动的影响也会变小.     

Sheath structure in plasma with two species of positive ions and secondary electrons

The properties of a collisionless plasma sheath are investigated by using a fluid model in which two species of positive ions and secondary electrons are taken into account. It is shown that the positive ion speeds at the sheath edge increase with secondary electron emission(SEE) coefficient, and the sheath structure is affected by the interplay between the two species of positive ions and secondary electrons. The critical SEE coefficients and the sheath widths depend strongly on the positive ion charge number, mass and concentration in the cases with and without SEE. In addition, ion kinetic energy flux to the wall and the impact of positive ion species on secondary electron density at the sheath edge are also discussed.     

Landau damping of electrons with bouncing motion in a radio-frequency plasma

One-dimensional particle simulations have been conducted to study the interaction between a radio-frequency electrostatic wave and electrons with bouncing motion. It is shown that bounce resonance heating can occur at the first few harmonics of the bounce frequency (nω_b,n=1,2,3,...). In the parameter regimes in which bounce resonance overlaps with Landau resonance, the higher harmonic bounce resonance may accelerate electrons at the velocity much lower than the wave phase velocity to Landau resonance region, enhancing Landau damping of the wave. Meanwhile, Landau resonance can increase the number of electrons in the lower harmonic bounce resonance region. Thus electrons can be efficiently heated. The result might be applicable for collisionless electron heating in low-temperature plasma discharges.     

基于微扰守恒模型的磁流体模拟并行计算

基于微扰形式的守恒磁流体方程,采用基于通量向量分裂(FVS)以及加权本质无振荡格式(WENO)的 有限差分方法,发展了平板位形下的非线性磁流体模拟程序。程序准确地模拟了开尔文-亥姆霍兹不稳定性和单 撕裂模不稳定性。采用 MPI 和 OpenMP 的并行计算框架,使得程序可以在多节点上运行。在并行计算过程中采 用负载均衡化,对各个节点计算量进行动态分配,提高了计算效率。