A new double probe system for studies of non-uniform plasmas

A theoretical and experimental study was developed about the applicability of a double probe system consisting of two directional Langmuir probes, both probes being located separately in a plasma column. The current- voltage characteristic of the double probe was obtained considering a plasma with a drifting Maxwellian electron velocity distribution function and stationary ion background. In deriving the characteristic of the double probe, the plasma parameters, namely, electron temperature (Te), electron density (Ne), electron drift velocity (Vde) and plasma potential (Vp) are assumed to be non-uniform. The double probe characteristic is also dependent on the angle between the axial direction of the electron drift and the normal to the collecting area of the probe. Each probe can be rotated such that this angle can be varied between zero and 180 degrees. Various probe characteristics were simulated using plasma parameters obtained by independent single probe measurements in the positive column of a low-pressure arc discharge in mercury vapor. Typical parameters of the positive column, used in the simulation, are: Te = 5 eV, Ne = 1017 m-3, vde = 8x105 ms-1. Experimental characteristics of the double probe were obtained and compared with the simulated results, showing good agreement. It is concluded that this directional probe system can be a reliable diagnostic tool especially for studies of non-uniform plasmas. (author)