Contamination Effects on Fixed-Bias Langmuir Probes

<p> 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 &filig;xed-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 &filig;xed-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 &fllig;uctuation spectrum as measured by the &filig;xed-bias Langmuir probe. We model thecontaminationasasimpleresistor-capacitorcircuitbetweentheprobesurfaceandtheplasma.We &filig;nd that measurements of small scale plasma &fllig;uctuations (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 &filig;xed-bias probes for different contamination parameters. The model results also show that a &filig;xed bias probe operating in the ion-saturation region is affected less by contamination as compared to a &filig;xed bias probe operating in the electron saturation region.</p>