Transient response of the Langmuir probe at low pressures

Quantitative measurements have been made of the spike in the positive-ion current which accompanies the application of a negative pulse to a Langmuir probe. The measurements have been performed over a wide range of applied voltage (1-400 v) and ion density (10 9 -10 13 ions/cm 3 ), and at a pressure (0·1 torr) where a collisionless model of the plasma sheath may be expected to apply. The results show reasonable agreement with calculations made by extending the previous theoretical work of Kamke and Rose. The implications of these results to swept-probe measurements have been investigated directly by sweeping a double probe at various different sweep rates in the same plasma. It is found, as expected, that above a certain sweep rate the probe characteristics are distorted. More unexpectedly, it is also found that very considerable errors (up to factors of 3 in derived ion density and 15 in derived electron temperature) occur when this limit is substantially exceeded.