Direct measurement of ion and electron flux ratio at their respective sheath-edges and absence of the electron Bohm criterion effects

Abstract

A recent theory suggests that electrons enter electron sheaths at an electron Bohm velocity given by (T e/m e)1/2 instead of the electron thermal velocity as conventionally assumed. To test this theory, the flux density ratio Γe,se/Γi,se of electrons and ions entering their respective sheaths was directly measured via an almost continuous A i/A e area ratio scanning. The measured value agrees with the predictions assuming electrons entering the electron sheaths at their thermal velocity. The predictions associated with the electron Bohm criterion have not been found. If the predictions of such theories are true, the electron or ion presheath density drops will be very different from conventionally expected values.