Fund Project:Project supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1831124), the Natural Science Foundation of Anhui Province, China (Grant No. 1908085MA28), the Natural Science Foundation of the Higher Education Institutions of Anhui Province, China (Grant No. KJ2018A0341), and the Follow-up Research Program of Fuyang Normal University, China (Grant No. 2018HXXM29).
Received Date:20 February 2019
Accepted Date:13 June 2019
Available Online:01 September 2019
Published Online:20 September 2019
Abstract:A one-dimensional fluid model is used to investigate the characteristics of secondary electron emitted by the interaction between electrons and the wall in plasma sheath with nonextensive electrons. The study focuses on the effects of electron nonextensive parameter on Bohm criterion, the wall potential, the critical emission coefficient of secondary electrons and the density of seconday electrons in plasma sheath through numerical simulation. Some conclusions are obtained. It is shown that secondary electron is significantly affected by electron nonextensive parameter. Whether the electron distribution is superextensive or subextensive, the critical Mach number at the sheath edge increases with the secondary electron emission coefficient increasing, but decreases with q-parameter increasing. The increase of q-parameter can cause the wall potential to increase and the critical emission coefficient of secondary electron at the wall to decrease. And for different types of plasmas, the effects of nonextensive parameter on the critical emission coefficient of secondary electron are different. The larger the mass number of ion in plasma, the smaller the influence of nonextensive parameter on the critical secondary electron emission coefficient will be. In addition, the increase of nonextensive parameter can result in the decrease of the sheath thickness and the increase of the number density of secondary electrons. It is found that the superextensive electron distribution has greater influence on the characteristics of secondary electron emission in plasma sheath than the subextensive electron distribution. Keywords:secondary electron emission/ nonextensive/ plasma/ sheath
图5呈现的是参量q分别等于0.8, 1.2和1.3时, 等离子体鞘层中二次电子数密度的不同分布曲线, 在等离子体鞘层中, 尽管电子分布呈现非广延性, 但从器壁发射的二次电子在鞘层中仍然是单调变化的, 这主要源于非广延性分布的电子并未改变等离子体鞘层离子鞘特性, 并且随着参量q的增加, 鞘边与器壁之间的距离(鞘层的厚度)有所减小, 鞘层中二次电子数密度跟着增加. 图 5 参量q对鞘层中二次电子数密度的影响($\gamma = 0.4$) Figure5. Normalized density of secondary electrons in plasma sheath versus x for different values of nonextensive parameter q ($\gamma = 0.4$).
从器壁发射的二次电子在鞘层电场的作用下, 将加速向鞘边运动, 从而到达中性等离子体区域. 图6显示的是参量q对不同二次电子发射系数下到达鞘边的二次电子数密度的影响. 从图6可以看出, 随着参量q的增加, 从器壁发射而到达鞘边的二次电子数增多, 对比$\gamma = 0.4$, $\gamma = 0.6$和$\gamma = 0.8$三条曲线, 可以发现, 参量q的变化对不同二次电子发射系数下发射的二次电子数密度影响程度不同. 对于$\gamma = 0.4$, $\gamma = 0.6$和$\gamma = 0.8$三种情形, 当$q = 0.5$时, 到达鞘边的二次电子数密度分别为${N_{{\rm{s0}}}} = 0.00248$, 0.00702, 0.02922; 当$q = 2.0$时, 由器壁发射而到达鞘边的二次电子数密度则分别为${N_{{\rm{s0}}}} = 0.01121$, 0.02743, 0.10616. 参量q的增加, 运动速度较低的电子数目增多, 导致器壁电势增加, 所以结合(21)式可得从鞘边的二次电子数密度增加. 图 6 参量q对不同发射系数下到达鞘边二次电子数密度的影响 Figure6. Normalized density of secondary electrons at the sheath edge versus nonextensive parameter q for different values of secondary electron emission coefficients ($\gamma = 0$, $\gamma = 0.4$ and $\gamma = 0.8$).