The Orbital Motion Limited theory (OML) is used to interpret ion
current and ion density. For OML theory to be valid, the sheath surrounding the probe
should be collision-less, which is generally valid for the condition = (rp=De) < 1 [66].
However, the OML theory has been found to hold very well even when the sheath
dimensions become smaller that the probe radius [67]. In our case where 0:2 < < 5:6,
the OML theory is still valid according to [67]. Using the formulation in [66,67] the ion
current Ii;OML (in the ion attracting region of the characteristic) follows the equation