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

The entry velocity of electrons into the electron sheath is a fundamental quantity that determines the electron saturation current into a Langmuir probe, and thus the respective calibration constant of a Langmuir probe’s plasma density measurement. Conventional wisdom believes that electrons enters the electron sheath at the random thermal flux speed v_e=(T_e/2πm_e)^1/2. A recent theory suggest that electrons enter electron sheaths at an electron Bohm velocity v_eB=(T_e/m_e)^1/2 instead of the electron thermal velocity as conventionally assumed. To test this theory, the flux density ratio Γ_e,se/Γ_i,se of electrons and ions entering their respective sheaths was directly measured in a multi-dipole confined hot cathode source via an almost continuous A_i/A_e area ratio scanning of an asymmetric double probe. Removing known perturbation to the experimental results, the measured value agrees the critical flux ratio of ~ (m_i/2.3m_e)^1/2 with the predictions assuming electrons entering the electron sheaths at their thermal velocity. The predictions associated with the electron Bohm criterion has 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.