11:45 AM - 12:00 PM
[PCG20-11] Development of wideband impedance probe system for observation of the ionosphric ion composition
Keywords:Impedance probe, Lower hybrid resonance (LHR), Ionosphric ion composition
The effective capacitance of the probe in the magnetized plasma shows minimum not only at UHR frequency but also at another resonance frequency: Lower hybrid resonance (LHR). If we can measure LHR frequency with UHR frequency and electron cyclotron frequency, we can derive effective mass of ionospheric plasma and determine the ionospheric ion compositions. Because LHR frequency is about several kHz in the ionosphere, we have to extend the lower limit frequency of the current impedance probe system to 100 Hz.
Through the plasma chamber experiment in 2014 with bread-board model (BBM) of the new impedance probe system, we confirmed that it could measure (1) UHR in high frequency range as well as the current NEI could, and (2) equivalent capacitance profile from 100 Hz to 100 kHz, which indicates sheath capacitance of 120 pF and sheath resistance of 30 kohm. But it could not detect LHR as predicted due to high electron collision frequency in the chamber using backscatter-type plasma source. We are planning to perform another chamber test in 2015. In this test, we used large UV light source with propylene gas (C3H6) as plasma source in expectation of reduction of the electron collision frequency. However, although we found slight decrease of effective capacitance around 2 kHz, we could not confirm clear LHR depending on changes of background plasma density. The constant sheath resistance in low frequency range shows the existence of large sheath current due to potential difference between the probe and background plasma. Therefore, we are planning another chamber experiment in which we perform DC-potential control of the probe.