Development of a high-time/spatial resolution self-impedance probe for measurements in laboratory and space plasmas.

Abstract

Plasma impedance probes are often used in laboratory experiments as well as in space to make measurements of important plasma parameters such as the electron density. Conventional impedance probe methods involve sweeping the frequency applied to the probe through a range containing the plasma frequency, which can take on the order of a second to complete. This acquisition time leads to very low spatial resolution when making measurements from sounding rockets in the ionosphere. A high-time resolution impedance probe is under development at the U.S. Naval Research Laboratory with the goal of increasing the spatial resolution of measurements in space. To achieve this, a short-time Gaussian monopulse with a center frequency of 40MHz and containing a full spectrum of frequencies is applied to an electrically short dipole antenna. Laboratory experiments were performed with the Gaussian monopulse triggered once every 10 µs and averaged over ten shots, equating to a spatial resolution of 13cm for a typical sounding rocket speed. This paper discusses the development of the new high-time/spatial resolution self-impedance probe and illustrates that the short-time pulse method yields results that match well with data taken using conventional methods. It is shown that plasma parameters such as the electron density, sheath frequency, and electron-neutral collision frequency can also be derived from the data. In addition, data from the high-time/spatial resolution impedance probe are shown to compare well with those from theoretical impedance models.