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 determines the maximum temporal resolution of a Langmuir probe. In this work, we have revisited known fast sweep Langmuir probe techniques in a uniform, quiescent multi-dipole confined hot cathode discharge with two operation scenarios in which the probe sweeping frequencies is much lower and higher than the ion plasma frequencies respectively. This allows the investigation of the effect of limited ion-motion on I-V traces. Distortions of I-V traces at high frequencies, previously claimed to be ion-motion limitation effect, was not found unless shunt resistance is sufficiently high, despite achieving a ratio of ~ 3 between the probe sweeping frequency and the ion plasma frequency. This result essentially is a manifestation of the Langmuir probe as an electron collecting probe. Additionally, techniques in fast sweep Langmuir probe are briefly discussed. The comparison between the HDLP and the single probe setup shows that the capacitive response can be removed via subtracting a load line for the single probe setup almost as effective as using an HDLP setup, but the HDLP setup does remain advantageous in its facilitation of better recovery of weak current signal common in low plasma density situations.