Active and passive methods for the study of non-equilibrium plasmas using electrostatic waves

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Abstract
We study the possibility of measuring the properties of a non-equilibrium plasma by means of a double-dipole radio-frequency probe, consisting of two small dipoles immersed in the plasma, and separated by a distance one or two orders of magnitude greater than the Debye length. This type of probe can be used either in an active or in a passive mode. The theory of both modes is presented, taking the example of an isotropic plasma in which the electrons have a biMaxwellian distribution function; the first Maxwellian represents the major population of thermal electrons, and the second a minor population of suprathermal electrons. In the active mode, one dipole emits artificial signals which the other receives; thus we study the propagation of electrostatic waves between the two dipoles. In the passive mode both dipoles are used to receive random signals induced by the natural electric microfield in the plasma, and we compute the cross-spectrum of these signals. By combining the active and passive techniques, it is possible to measure the electron density and temperature of the thermal electrons, and also to get some information about the distribution function of the suprathermal electrons.