Langmuir probes are standard instruments for plasma density measurements on many sounding rockets. These probes can be operated in swept-bias as well as in ﬁxed-bias modes. In swept-bias Langmuir probes, contamination effects are frequently visible as a hysteresis between consecutive up and down voltage ramps. This hysteresis, if not corrected, leads to poorly determined plasma densities and temperatures. With a properly chosen sweep function, the contamination parameters can be determinedfromthemeasurementsandcorrectplasmaparameterscanthenbedetermined.Inthispaper, we study the contamination effects on ﬁxed-bias Langmuir probes, where no hysteresis type effect is seen in the data. Even though the contamination is not evident from the measurements, it does affect the plasma density ﬂuctuation spectrum as measured by the ﬁxed-bias Langmuir probe. We model thecontaminationasasimpleresistor-capacitorcircuitbetweentheprobesurfaceandtheplasma.We ﬁnd that measurements of small scale plasma ﬂuctuations (meter to sub-meter scale) along a rocket trajectoryarenotaffected,butthemeasuredamplitudeoflargescaleplasmadensityvariation(tensof meters or larger) is attenuated. From the model calculations, we determine amplitude and cross-over frequency of the contamination effect on ﬁxed-bias probes for different contamination parameters. The model results also show that a ﬁxed bias probe operating in the ion-saturation region is affected less by contamination as compared to a ﬁxed bias probe operating in the electron saturation region.
Review of Scientific Instruments
American Institute of Physics
Scholarly Commons Citation
Steigies, C., & Barjatya, A. (2012). Contamination Effects on Fixed-Bias Langmuir Probes. Review of Scientific Instruments, 83(). https://doi.org/10.1063/1.4764582