Studies of the Effect of Ion Current on Instantaneous Triple-Probe Measurements

A dimensionless parameter η is suggested to express the variation of ion current with probe potential in a probe characteristics which can be used to evaluate the influence of ion current on the ``instantaneous'' measurement of plasma parameters by means of triple-probe technique. Theoretical values of η for a cylindrical probe are obtained as a function of dimensionless parameter ξe(=rp/he), where rp is probe radius and he is the electron Debye length, both from classical sheath model and the more modern unified probe theory. η is small (<0.01) when ξe is large (>100), increases as ξe and mi (the ion mass) decreases and has a saturation value approximately equal to 0.1–0.2 at small ξe(≲1). The experimental η agrees with the theoretical η within a factor of 2. The possible errors associated with the direct-display system of plasma parameters using the triple-probe method by assuming η=0 are discussed in connection with η, and the results show that the maximum errors are within 40% for electron temperature determination and within a factor of 3 for electron density determination, respectively.