A Laboratory Simulation of the Ionospheric Plasma

Abstract

CORRECT simulation of the ionospheric plasma surrounding a satellite has been obtained in a large Vacuum chamber by means of a synthesized plasma stream produced by a low energy and high current density ion source. The plasma density and the beam velocity can be varied independently from 103 to 106/cm3 for the density and from 7 to 30 km/sec for the velocity. The electron velocity distribution is Maxwellian and the temperature can be varied between 500 and 2000°K. This plasma wind tunnel is being used for fundamental studies on the wake structure and for testing ionospheric probes. Contents The simulation of the spacecraft motion in the ionospheric plasma can be obtained by means of a synthesized plasma stream directed toward a model of the spacecraft. Several plasma windtunnels working on this principle have been built in recent years.1'2 Most of them have been used only for fundamental research such as satellite wake investigations or the diagnosis of streaming plasmas. Scaling laws are frequently used for these fundamental studies and the experimental conditions are then relatively easy to obtain. However the laboratory calibration of plasma diagnostic instruments which will be used on board a spacecraft does not permit any scaling. These instruments (and the plasma volume which is sampled) are generally of a large size, and the associated electronic circuits are adapted to the ionospheric conditions which must be reproduced exactly in the laboratory (in particular the plasma frequency, the electron gyrofrequency and the electron temperature). The correct simulation of the wake effect on these instruments also requires the ion source to be operated at very low acceleration voltage. Correct simulation is not indispensable for the fundamental studies of the wake structure, but if it could be obtained the use of the scaling laws would be no longer required. Moreover the use of a very low energy ion beam reduces to an insignificant amount the perturbation due to the electrons produced by secondary emission on the obstacles. For these reasons a plasma wind tunnel of large dimensions has been developed with a special effort to obtain, as much as possible, an exact simulation of the ionospheric pla'sma over large volumes. The two principal difficulties have been the development of an ion source working at low accelerating voltage and high current density, and the obtaining of electron temperatures in the range of 500 to 2000°K, which are lower than what is normally obtained in steady state laboratory plasmas. The plasma stream is produced in a vacuum chamber of large dimensions (3 m in diameter and 5 m long). The ultimate pressure is of the order of 10~ 5 N/m2 (1 N/m2 = 0.75 x HT2 Torr). The diagnostic instrument under test or the satellite