中文摘要：

结合热阴极电子发射理论和等离子体鞘层流体方程研究了热阴极附近存在虚阴极结构时的等离子体鞘层问题,采用Sagdeev势的方法讨论了鞘层解和广义Bohm判据.结果表明,不同于普通的Bohm鞘,由于热阴极附近存在大量发射电子,影响整个等离子体鞘层结构,使得进入鞘层的离子临界Mach数不是独立的常数,而是与鞘层电位降等参数有关的物理量.临界Mach数随着鞘层电位降（从鞘边缘到虚阴极）先增大后降低,并且随着热阴极温度的升高单调增大.此外,在平板模型下有相当可观的残余热发射电子越过虚阴极鞘层进入等离子体.

英文摘要：

The plasma sheath in the presence of virtual-cathode structure near a hot cathode was studied by combining the theory of thermionic emission and plasma fluid equations. Using Sagdeev potential method,the sheath solution and generalized Bohm criterion were discussed. It is shown that,different from usual Bohm sheath,the critical ion Mach number at the sheath-presheath edge is not an independent constant but depends on sheath potential drop as well as other parameters,owing to the fact that there exists large quantity of thermionic electrons near the hot cathode which has a significant influence on the whole sheath structure. The critical Mach number first increases and then decreases with the potential drop（from sheath-presheath edge to the virtual cathode） ,and monotonically increases with the temperature of the hot cathode. In the plane geometry,there exists an appreciable quantity of residual thermionic electrons which traverse the virtual cathode and sheath and enter into bulk plasma.