Kinetic Analyzes of Potential Formation and Transport in Boundary Plasmas

1. 通过动力学分析，阐明了磁性预鞘层的两种形成机理。冷离子等离子体中的磁性预鞘层是由离子极化漂移形成的，热离子等离子体中的磁性预鞘层是由有限离子回旋半径效应形成的。斜磁场有两种防止次级电子流入等离子体的方法。一种是二次电子在洛伦兹力作用下的反射，当德拜长度远远大于电子陀螺半径时，这种反射是有效的。另一种是由于磁性预鞘层的形成而增强了鞘层中的空间电荷效应。在空间电荷限制的条件下，磁性预护套代替护套充当热绝缘体。通过动力学分析，研究了空间扩展磁场的杂质控制效果和中性气体碰撞对导流室输电的影响。磁场强度的空间变化导致沿磁力线的静电电位下降，随着等离子体中性碰撞频率的增加而减小。如果电离速率不超过上限，则燃料和电离杂质的冷离子被静电势成功地捕获在导流器中。电荷交换用冷离子代替高能离子，离子通量的电离放大减小了静电电位的下降。因此，等离子体中性碰撞显著降低了到达分流板的离子的冲击能量。在托卡马克实验中测量到的功率传输因子低至2-3，主要是由于在分流器中通过等离子体-中性相互作用过程降低了离子能量。

1. Two formaiton mechanisms of the magnetic presheath are clarified by kinetic analyzes. The magnetic presheath in a cold-ion plasma is formed due to the ion polarization drift, and that in a hot-ion plasma is due to the finite ion-gyroradius effect. An oblique magnetic field has two means of preventing secondary electrons from flowing into a plasma. One is the reflection of secondary electrons by the Lorentz force, which is effective when the Debye length is much larger than the electron gyroradius. Another is enhancement of the space-charge effect in the sheath due to formation of the magnetic presheath. The magnetic presheath acts as a thermal insulator instead of the sheath under conditions of the space-charge limitation.2. Kinetic analyzes have been carried out to study impurity control effects of a spatially expanding magnetic field and effects of neutral-gas collisions on power transmission in the divertor chamber. The spatial variation of magnetic field strength causes an electrostatic potential drop along the magnetic field lines, which decreases with increasing plasma-neutral collision frequency. Cold ions of fuel and ionized impurities are successfully trapped in the divertor by the electrostatic potential if the ionization rate does not exceed an upper limit. Charge exchange replaces energetic ions by cold ions, and amplification of the ion flux by ionization reduces the electrostatic potential drop. Consequently, plasma-neutral collisions markedly reduce the impact energy of ions reaching the divertor plates. Low values of the power transmission factor as low as 2-3 measured in tokamak experiments can mainly be due to reduction of ion energy through plasma-neutral interaction processes in the divertor.