Practical definition of the plasma sheath edge for modeling planar glow discharges

While the parallel-plate capacitive model is often employed to describe the electrical behavior of a plasma sheath, the simple question of how far apart the two plates are for a given plasma cannot be easily answered yet because of the lack of a workable definition of the sheath edge. The sheath edge estimated with the Child--Langmuir equation may misplace as much as 40% of the total space charge outside the sheath. An alternative practical definition of the sheath edge is proposed that includes almost all space charge within the sheath