Measurements Of Hf Electric Fields In Plasmas By Short-circuited Double Probes

Abstract

One way to measure the electric field in a plasma is to measure the potential difference between two probes which both are either at the local plasma potential or kept at the same potential difference from the plasma potential. The electric field is then directly obtained from the difference in probe potential and the probe separation. This method meets with difficulties at high frequencies when it is impossible to have a sufficiently high impedance in the measuring circuit. Also, probes kept at plasma potential can distort the plasma by drawing large currents from it. A new strategy for measurements of hf electric fields was introduced by Wohlin et al. Here two parallel wire probes, with their axis perpendicular to the expected direction of the E field, are kept at the common potential U{sub c} by connecting them by a small external resistor, usually the cable impedance 50 {Omega}. U{sub c} can be varied by an external circuit. The current I{sub ext.} through the resistor is the measured quantity. This current charges up the probes until they have the same potential, i.e., until the line integral of E inside the plasma from probe 1 to probe 2 is zero. The taskmore » is to relate I{sub ext.} to the field E{sub W}. The authors find that the interpretation using the vacuum capacitance originally proposed by Wohlin et al requires that the diameter r{sub W} of the probe wires is much smaller than their separation b, and analytic correction factors are given as function of r{sub W}/b.« less