Probe temperature effect on the curling probe and its correction technique

This article introduces the effect of the probe temperature existing in a curling probe, which enables one to measure the electron density in plasma and the thickness of deposited film on the probe surface. We have recognized the effect appearing on the resonant frequency in previous reports, but we made measurement conditions where no temperature affected the frequency shift. The practical use of curling probes does not always allow one to have such an ideal condition, so it is necessary to have a good understanding of the probe temperature effect. Toward the understanding, we firstly measured the resonant frequency shift for five different operating powers with a curling probe having a heater and thermocouple attached to the probe surface. The frequency shift measurement showed consistent trends with plasma off and on. With some careful analyses, we found that the correction factor, which is necessary to compensate for the electron density measurement due to geometry reasons, etc for curling probe, had a regularity as a function of probe temperature; the inversed square of the correction factor was proportional to the degree of resonant frequency shift. Furthermore, the proportionality depended on the probe temperature with the regularity, so we finally were able to include the probe temperature effect on the correction factor, which realized the correction of the electron density even when the probe has a temperature variation. The electron density measurement with this correction technique worked well and followed the density measured with the Langmuir probe well. In particular, this research revealed that the correct technique is effective when probing temperature increases.

Automated summary

This article introduces the effect of probe temperature on the measurement of electron density in a curling probe. Through experiments, the authors found that the resonant frequency shift is affected by the probe temperature, and they proposed a correction factor to compensate for this effect and achieve accurate electron density measurement.