A HIGH-FREQUENCY PROBE FOR ABSOLUTE MEASUREMENTS OF ELECTRIC-FIELDS IN PLASMAS

At high frequencies (1 GHz), it is difficult to maintain a sufficientl y high probe impedance to allow measurements of voltage differences us ing a double probe. A new type of double probe with terminated coaxial cables (50 Omega) is proposed and tested. The low impedance implies t hat the two probe wires are essentially short-circuited and only the p robe current is measured. The relation between the displacement curren t due to surface charges and the electric field is derived and tested in a known field. The current measured in a plasma can be identified w ith the displacement current when the current due to plasma electrons is damped by an appropriate Dc bias. Perturbations of the plasma by th e probe field can also be checked by varying the Dc potential of the p robe.