High frequency probe for absolute measurements of electric fields in plasmas

At high frequencies (1 GHz), it is difficult to maintain a sufficiently high probe impedance to allow measurements of voltage differences using a double probe. A new type of double probe with terminated coaxial cables (50 Ω) is proposed and tested. The low impedance implies that the two probe wires are essentially short-circuited and only the probe current is measured. The relation between the displacement current 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 with the displacement current when the current due to plasma electrons is damped by an appropriate DC bias. Perturbations of the plasma by the probe field can also be checked by varying the DC potential of the probe.