The sheath at an electrode close to plasma potential

Abstract

It is shown that a sheath with a monotonic potential profile can exist at all negative voltages on an electrode in a plasma. When the electrode is close to plasma potential it is no longer valid to ignore those electrons which reach the electrode. The velocities with which ions approach the sheath satisfy a 'sheath criterion', which depends upon the potential drop acros the sheath and not just on the electron temperature in the plasma.