THE RESPONSE OF FREE MOLECULE CYLINDRICAL LANGMUIR PROBES IN A TURBULENT PLASMA

Abstract : A theoretical and experimental investigation of the response of a free molecule Langmuir probe in a turbulent plasma is made. The effect of arbitrary fluctuations in charged particle density, electron temperature, plasma potential, and ion temperature are considered. The results are applicable for (a) frequencies somewhat less than the ion plasma frequency, (b) moderate ratios of probe radius to Debye lenght, (c) moderate ratios of applied potential to electron temperature and (d) ratios of ion to electron temperatures less than one. The steady state characteristic is obtained by forming an empirical fit to a numerical analysis describing the collisionless probe current collection. Relations are then derived showing the effect of fluctuations on the average properties, and the connection between the r.m.s. probe current and the various correlations. Electron temperature, charged particle density, floating potential, single probe saturation ion and electron current and double-probe current measurements are studied in detail. It is found that the sensitivity of the probe to the various fluctuating quantities is a function of the ratios of probe radius to Debye length, applied potential to electron temperature, and ion to electron temperature. Subsequently it is shown that many of the correlations and average properties may be determined. Experiments are performed in an unsteady highly expanded low density flowing argon plasma. Both the average and r.m.s. currents are monitored using cylindrical Langmuir probes. (Author)