Effect of finite electrode area ratio on high-frequency Langmuir probe measurements

Langmuir probe (LP) measurement generally requires a reference electrode with large surface area compared with that of a probe. Whereas a reference electrode with large surface area is available in laboratory experiments, surface area of the reference electrode is limited when the probe is installed on a small spacecraft such as a sounding rocket. Insufficient electrode area ratio of the reference electrode to the probe leads to distortion of the current-voltage (I-V) characteristics and therefore the derived electron temperature and density might be erroneous. In order to investigate the effect of finite electrode area ratio on LP measurements, we have carried out a laboratory simulation by setting up a model sounding rocket with whole apparatus for LP measurement in a vacuum chamber. By changing the frequency of sweep voltage applied to the probe from 0.1 Hz to 3 kHz, we have derived an equivalent resistance and capacitance of the reference electrode sheath, which was thought to be the principal cause of distortion in I-V characteristics. The result shows that when the electrode area ratio is 124, the electron temperature is unaffected by the impedance of the reference electrode while the electron density is underestimated by 50%. In addition, the high sweep frequency method introduced here turned out to be an effective approach to make measurements with LP possible even when a sufficient surface area ratio cannot be achieved.