Fast Electron Temperature Measurements with Langmuir Probes: Considerations for Space Flight and Initial Laboratory Tests

Abstract

Measurements of ionospheric electron temperatures with Langmuir probes are usually done with a technique that sweeps a bias voltage to determine the current-voltage (IV) characteristic for the local plasma. We report on results in the use of a technique which is new to space experiments, but has been used in laboratory experiments. This technique should allow for faster electron temperature measurements than the typical swept probe while imposing less disturbance to the space plasma and to other instruments. The technique uses a small ac modulation of the Langmuir probe signal at a dc bias point, and the detection of harmonics in the probe current. Analysis shows that the ratio of the fundamental and second harmonic is dependent only on the electron temperature and the amplitude of the applied ac bias. We show results from tests of this technique in the NRL Space Physics Simulation Chamber (SPSC) and discuss the applicability for space measurements. The SPSC was designed to produce plasmas with parameters similar to those seen in the ionosphere. It appears that to make a reliable temperature measurement it is necessary to use a two-probe system. One probe is used to monitor the floating potential and, thus, allows for the correct dc biasing of the second probe. The laboratory results show that it should be possible to make fast (∼1 ms) temperature measurements using very simple electronics circuitry. Several suggestions are made for more sophisticated experimental configurations which could prove to be useful depending on the local ionospheric conditions.