Prototype Development of a Multi-Needle Langmuir Probe System
Tore André Bekkeng: Master thesis: Background for the main scientific instrument.
This thesis deals with prototype development of a novel fixed-bias multi-Needle Langmuir Probe system (m-NLP). The system has been invented at UiO for use on sounding rockets and satellites. Measurement principles, instrument design, and calibration and test results are reviewed. In addition, the first results from the ICI-2 rocket campaign are presented.
The instrument works by measuring the current collected individually from four needle probes, placed in front of the rocket/satellite's shock front. The collected current is converted to voltage, filtered, digitalized and then sent to the central telemetry (TM) system for down link to the ground station. By using data from four fixed-bias Langmuir needle probes, sampled at the same time, the plasma electron density can be derived with high time resolution without the need to know the electron temperature and the spacecraft potential. With the selected needle probe design and the estimated electron densities, the instrument had to be capable of measuring currents down to 1 nA. Final performance of the electronics was a lower measurement limit of about 0.5 nA with satisfying signal-to-noise ratio. The maximum sample rate is about 9 kHz, but for the ICI-2 mission configuration a sample rate of 5787 Hz was used. This gives a spatial resolution of about 11 cm at payload apogee. Due to the configuration with four probes, the smallest physical structures that can be measured are limited by the probe distance of about 1 meter. With the selected probe size, the instrument can cover an electron density range from Ne = 10^9 to 10^12 per cubic meter, and the four bias voltages for the probes can be set at any voltage between 0 V and 10 V.