Fast-sweeping Langmuir probes: what happens to the I–V trace when sweeping frequency is higher than the ion plasma frequency?

Limited particle transit time is one of several limiting factors which determine the maximum temporal resolution of a Langmuir probe. In this work, we have revisited the known fast sweep Langmuir probe techniques in a uniform, quiescent multi-dipole confined hot cathode discharge with two operation scenarios: one in which the probe sweeping frequency f_sweep is much lower than the ion plasma frequency f_pi, another one where f_sweep is much greater than f_pi, respectively. This allows investigation into the effect of limited ion-motion on I–V traces. Serious distortions of I–V traces at high frequencies, previously claimed to be an ion-motion limitation effect, were not found unless shunt resistance is sufficiently high, despite a f_sweep/f_pi ratio of ~3. On the other hand, evidences of sheath capacitance on the I–V traces have been observed. Distortions of I–V traces qualitatively agree with predictions of sheath capacitance response to the sweeping voltage. Additionally, techniques in fast sweep Langmuir probe are briefly discussed. The comparison between the high-speed dual Langmuir probe (HDLP) and the single probe setup shows that the capacitive response can be removed via subtracting a leakage current for the single probe setup almost as effectively as using the HDLP setup, but the HDLP setup does remain advantageous in its facilitation of better recovery of weak current signal commonly in low density plasma.