Campanell M D
Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Sep;88(3):033103. doi: 10.1103/PhysRevE.88.033103. Epub 2013 Sep 9.
Most works on plasma-wall interaction predict that with strong electron emission, a nonmonotonic "space-charge-limited" (SCL) sheath forms where the plasma potential is positive relative to the wall. We show that a fundamentally different sheath structure is possible where the potential monotonically increases toward a positively charged wall that is shielded by a single layer of negative charge. No ion-accelerating presheath exists in the plasma and the ion wall flux is zero. An analytical solution of the "inverse sheath" regime is demonstrated for a general plasma-wall system where the plasma electrons and emitted electrons are Maxwellian with different temperatures. Implications of the inverse sheath effect are that (a) the plasma potential is negative, (b) ion sputtering vanishes, (c) no charge is lost at the wall, and (d) the electron energy flux is thermal. To test empirically what type of sheath structure forms under strong emission, a full plasma bounded by strongly emitting walls is simulated. It is found that inverse sheaths form at the walls and ions are confined in the plasma. This result differs from past particle-in-cell simulation studies of emission which contain an artificial "source sheath" that accelerates ions to the wall, leading to a SCL sheath at high emission intensity.
大多数关于等离子体与壁面相互作用的研究预测，在强电子发射情况下，会形成一种非单调的“空间电荷限制”（SCL）鞘层，其中等离子体电势相对于壁面为正。我们表明，一种根本不同的鞘层结构是可能的，在这种结构中，电势朝着被单层负电荷屏蔽的带正电壁面单调增加。等离子体中不存在离子加速预鞘层，离子流向壁面的通量为零。对于一般的等离子体 - 壁面系统，证明了“逆鞘层” regime 的解析解，其中等离子体电子和发射电子具有不同温度的麦克斯韦分布。逆鞘层效应的影响是：（a）等离子体电势为负，（b）离子溅射消失，（c）壁面没有电荷损失，（d）电子能量通量是热的。为了通过实验测试在强发射情况下形成何种类型的鞘层结构，对由强发射壁面包围的完整等离子体进行了模拟。结果发现，壁面形成逆鞘层，离子被限制在等离子体中。这一结果与过去包含人工“源鞘层”的发射粒子模拟研究不同，后者会将离子加速到壁面，导致在高发射强度下形成 SCL 鞘层。
